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   Now that I have described fully the design, construction and use of one type of farm dam, this chapter will, in less detail, consider the design and construction of other dams, A full appreciation of these dams can be obtained, however, from comparisons with the keyline dam of Chapter XVIII.

   In Chapter VII I have classified farm valley dams into three types--the high or keyline dam, the reservoir, and the lower valley dam. Both the reservoir and the lower valley dam are larger usually than the high or keyline dam. The larger capacity farm dam water storages are generally not so much larger construction jobs as their capacities would suggest. The sections of the wall, as described for the keyline dam, are, with some exceptions, the same as these for the bigger storages. For instance, the same wall section may, in different circumstances of land shape, impound quantities of water varying from 2-1/2 million gallons to 150 million gallons (10 acre feet to 600 acre feet).

   A dam with a 20-foot depth of water at the inflow end of the lockpipe could be a keyline dam, or, alternatively, a reservoir, and have a capacity range of 21 to 100 million gallons. The maximum range of dam capacities varies widely, but there is a closer comparison in the minimum ranges of the various types of dam. The minimum capacity of a dam that is worthwhile for regular and effective irrigation is 2-1/2 million gallons, and this should be a limitation imposed only by land shape and run-off. The minimum size for a reservoir may be eight million gallons, and for a lower valley dam perhaps twelve million gallons.

   A keyline dam on one farm or grazing property may have a greater capacity than the largest practical lower valley dam on another property. There is a general tendency for the capacity range of dams, all with the same depth of water, to be much wider as the country slope is longer. This length of land was defined in Chapter VI as the distance from the main ridge to the watercourse below.

   Reservoir Sites: The reservoir in Keyline is classified as a reserve water storage located. at an intermediate elevation between the high or keyline dams and the lower valley dams. A reservoir may be located at the low end of a large primary valley and just above the point where the primary valley joins a secondary valley; or where a primary valley flows into a creek, a river, a lake, or flows to a flat plain. A reservoir is also suitably located in the upper area of a secondary valley and in height somewhere below one or all of the keyline dams in the primary valleys which flow to and together form the secondary valley. Circumstances of climate, land shape and the association of other storages, or sites for other storages, dictate the location of reservoirs.

   A reservoir (or several reservoirs), as part of a complete Keyline water conservation scheme, is designed to remain full during the time the higher and lower dams are used for irrigation. When these storages have been used, the reservoir is still full, despite some evaporation which may have taken place, and so the farmer or grazier in a dry time has at least one or more large reservoirs filled with water. He can plan his irrigation now from the reservoir and regulate stocking and management accordingly.

   A reservoir is seen then as the largest body of longest lasting water on the farm or grazing property. It may serve as the basis for the planning for such lesser but permanent water requirements as stock troughs supplied via pipelines directly from a constant flow take-off on the outlet equipment. It is also the one type of dam that is most suitable for fish stocking, and one on which other forms of pleasurable water activities may be planned. Dams, such as keyline dams, in which the water level is constantly changing, are not so suitable for fish stocking or as pleasure resorts.

   The design-for-use suggested for the various types of dams in Keyline (e.g., keyline dams and lower valley dams as general irrigation water and to be put into use at the same time, with reservoirs as reserve water during the use of these other dams), may be altered to suit individual requirements and convenience of management. However, the use-pattern for the stored water should be such that when some of the dams have storage capacity available by having been in use for irrigation, and run-off rain occurs, then no water will leave the property until all dams are again filled.

   The design of reservoirs follows very closely the general design features of the keyline dam already discussed. It is assumed that the reservoir is on the same property as our keyline dam. The wall height may be the same or less according to land shape and run-off conditions. Water capacity will be cheaper, the storage will be larger, and the valley floor slope will generally be flatter. The relationship between storage capacity and annual run-off can be up to double that for the keyline dam.

   I have suggested that the keyline dam should be designed to hold 1-1/2 times the annual run-off of its total catchment area, and this capacity is considered more a minimum than a maximum. Where the climate has a very uniform or regular rainfall characteristic the capacity should be somewhat less than this recommendation. The relationship of the reservoir size to its catchment area, other than in the climatic conditions just mentioned, should be from two to more than three times the average annual run-off. The actual ratio of storage to run-off should be related also to the cost of storage. For instance, if the storage cost in the keyline dam, which may range from under £20 to over £40 per acre foot, is double that obtainable in the reservoir site, then it could be good business to build reservoir capacity of double the storage capacity to run-off ratio of the keyline dam. If the storage capacity cost in the reservoir is more favourably related to the other dams, then it could be made even larger. In this case the reservoir's natural catchment may be increased by the use of a special water conservation drain. By keeping such points in mind, the circumstances of climate and land shape applying to each individual reservoir will clearly indicate the answers to these various questions.

   For purposes of illustrating design and construction features, one of my own reservoirs may form a background. The design of the reservoir is illustrated in the plan and section immediately following, and titled accordingly. (See Fig. 16.)

   The site had a general valley floor slope of 1 in 37. The features of land shape that determined the reservoir's precise location were, first, a saddle on the ridge on the left bank (the bank on the left of a valley when looking downstream) of the valley was suitable for the spillway for the reservoir and which would also allow all overflow water to fall into another dam site; secondly, the site was above a large area of land convenient for irrigation which would permit the water to be used for either early irrigation or reservoir use; thirdly, the contour shape of the valley was good and the reservoir would be in a position to receive the overflow of three other dams.

   The depth of the reservoir, because of these valuable features, was fixed at 24 feet; the height of the constructed wall was, therefore, with freeboard and settlement allowance, 29 feet 3 inches, but this was increased to 30 feet as a counter to wave erosion, which, while the dam was new, could be a consideration. The maximum width of the base of the wall was 122 feet, being calculated from batters I in 2; settled height 27 feet, a crest (wider than usual), 14 feet.

   The length of lockpipe laid was 130 feet and the baffle plates were eight feet apart in the forward half of the wall foundation. With these variations it was constructed as was the keyline dam of Chapter XVIII. (Pictures of the newly completed dam and stages of its construction are contained in the Pictorial section of this book.)

   The lockpipe was placed in position on one side of the valley centre line one foot below solid ground, and the end of the lockpipe on the inside of the dam was four feet six inches below the solid bottom of the valley. The whole of the earth for the wall was obtained within the dam site and at a satisfactory close distance to the wall, and also without excavating earth below the level of the lockpipe inlet.

   This reservoir has nine acres within its top water line. In addition, there is another acre or more of wall crest, back wall batter, drains and generally disturbed ground, which were all covered with the top soil we were able to strip from the original area. The land within the top water line was keyline cultivated and together with all areas of disturbed earth was sown with grass seed and superphosphate. The first flow of water into the dam did not move the newly-placed, sown and cultivated soil.

   The average depth of the reservoir is approximately 10 feet, and storage capacity is 90 acre feet. The cost of water storage was under £17 per acre foot, allowing £6 per hour for the 100-horse-power bulldozers which built it, and 20% of this cost represents the lockpipe equipment and drains for irrigation. The reservoir has a rather special asset value worth many times its cost, yet such is the variations of these factors in farm dams that the nearest dam to it on the same property was much less than half this storage price, while another adjacent dam has a storage capacity which cost double that of the reservoir. All three dams, however, are very much more valuable than their costs, particularly so since they include the whole of the equipment and drains for effective irrigation, at the lowest of all operating expenses.

   It will be seen that this reservoir has some special features, and this is so with most reservoirs. The recognition and the successful use of the individual and special features of each site depend largely on an appreciation of the relationship of climatic conditions to the shapes of the land. A facility in judging these matters is a great asset to the farmer and grazier. To give another instance of this individual value-of-site feature, we have a reservoir on another property which gains in usefulness by being precisely located according to the above principles. The effect of this location is that the spillway of the reservoir was made to coincide with the irrigation drain of a keyline dam located in the head or highest primary valley of the secondary valley formation in which the reservoir is located. Now we are able to turn water from the keyline dam into the reservoir, or we can direct the run-off past the keyline dam into the reservoir, or past the reservoir via its spillway into a lower keyline dam, or we may irrigate from the drain between the reservoir and the second dam. Flood overflow from the first or highest keyline dam can be controlled to flow either into the reservoir or into the second keyline dam, or, still further, if need be, into other lower keyline dams, or even the lower valley dam near the boundary of the whole region. Again, under drought conditions, after the top keyline dam is emptied by irrigating and its irrigation area. is requiring more water, this can be supplied by pumping water for the short distance from the outlet of the reservoir into its own spillway and thence it would flow along the irrigation drain of the first keyline dam.

   A reservoir is seen to be a very valuable and versatile type of dam in Keyline. It is usually a permanent supply, and the best type of dam for a pleasure resort. In a secondary land unit, one or more reservoirs may be employed according to climate, land shape and the layout of the rest of the dams. Reservoirs, then, are the best farm insurance for dry times, in that a drought would be well advanced by the time other water storages were depleted, and with stock held in top condition, there would still be a large supply of water remaining. At the stage of the drought where it would be necessary to commence the use of a reservoir for irrigation the landholder would be in an excellent position from which to view the prospects of continuing drought. If a farmer or grazier can sell when most others have nothing fit for sale he can profit considerably. If he is also in a position to buy when others must sell, he is even better off.

   Lower valley dams, in conjunction with the keyline dams and reservoirs, complete the series of valley dams that are available for construction on much of the undulating country. On the one property the lower valley dam will usually be the largest of the three types, although on occasions the reservoir may equal or exceed the storage capacity of the lower valley dam.

   In planning for the complete conservation of all run-off water, the lower valley dam is designed to hold the overflow of all dams above it in the secondary land unit as well as all the run-off from areas below these keyline dams and reservoirs. It represents the last opportunity of conserving all such run-off before it leaves the secondary land unit.

   The design of a lower valley dam is similar to that of the reservoir already described, but differing in that usually the lower dam is of larger capacity and has a flatter valley floor slope. Its capacity range on land of widely differing shapes may be from under 12 million to 100 million gallons or more. To illustrate its design and construction it may be assumed that a lower valley dam site has a valley floor slope of 1 in 55, and that it is to have a depth of water of 20 feet at the inlet into the lockpipe. Here the flatter valley floor slope necessitates a different design to that of the reservoir discussed. Such a lower valley dam is illustrated opposite this page, where both a plan and section of the dam are shown. The batters for the wall will be assumed to be 1 in 2, therefore, with a 20 foot depth of water, the final dimensions for the cross section shape of the finished wall will be the same as that of the Keyline dam construction of Chapter XVIII. The maximum width of the wall is 104 feet, and, with the flatter valley floor slope of 1 in 55, the variation in level of the valley floor inside the dam will be only two feet above that of the back or downstream toe. Therefore, after allowing for the lockpipe being positioned on one side of the valley and a foot into the solid valley floor, its depth below the valley on the inside of the dam will be three feet. The earth for the construction of the wall cannot be obtained from the valley down to this level, as was done in the construction of both the keyline dam of Chapter XVIII and the reservoir recently described. The distance away from the wall where some of the earth required would have to be secured would be too far for efficient bulldozer operation, so earth will need to be excavated from the inside area of the dam near the wall and below the lockpipe level. Other than the changed design of the excavation area occasioned by the flatter valley floor slope, the design and the construction are similar to that of the dams earlier described.

   The length of the wall of a lower valley dam may be considerably longer than the wall of a keyline dam having a similar depth of water, and so the construction may take a relatively longer time. The excavation of the material for the wall should be planned in two stages. In the first stage the earth is excavated only down to the level of the lockpipe and extending back from the wall toe at this level for approximately 100 feet. In this size dam and valley floor slope, earth down to four feet below the lockpipe level will need to be used, so an excavation is now made to this depth, and, to begin with, in the immediate vicinity of the inlet of the lockpipe. If heavy rain falls it would pond near the lockpipe in the most convenient position to be pumped out through the lockpipe. Also, if the area excavated to provide wall material is taken down to the full depth at this point and then extended outward from the lockpipe, water ponding in this area may be left there and will not hamper the completion of the wall.

   The spillway of a lower valley dam, because of its generally larger catchment area, needs to be wider than those of the other two types of dams. Even though the dams above will hold large capacity from heavy run-off, there may be rare occasions when the lower dams will, when all others in the same land unit are filled, be required to take all the run-off from the whole catchment area. Where water conservation drains augment the catchment area these can be blocked and breached to reduce the excess inflow. There are different means of calculating spillway size in relation to catchment area, but as these are based on other factors which themselves are difficult to determine, a more reliable guide is local or farmer information as to the height that previous floods reached in the valley under consideration. Against this type of information one method often quoted may serve as a check. The spillway, according to this method, should be a width in feet equal to the figure obtained from the square root of four times the catchment area in acres. Using simple figures, the spillway of a dam having a catchment area of 36 acres is equal to the square root of four times 36, which is 12 feet, and the spillway of a dam having a catchment area of 400 acres is equal to the square root of four times 400, which is equal to 40 feet.

   On my own properties there has been one occasion when these widths of spillway may have been inadequate, and the occasion was not during heavy general flood rains. However, subject to local knowledge, and also considering the greatly improved safety factors provided by an adequate outlet system, it is my opinion that this formula should serve as a satisfactory guide.

   Therefore, if the lower valley dam under discussion has a total natural catchment of 400 acres, including the catchments of the dams which would overflow into it, then the spillway will be 40 feet wide on the floor of the spillway. The section of the spillway is 40 feet wide on the level floor, with a slope to the wall of the dam of 1 in 3-1/2 to 1 in 4 and with a similar slope on the side away from the wall of the dam.

   Whenever a dam is flowing a large discharge through its spillway the lockpipe valve can be opened. "Sour" water, which sometimes lies at the bottom of a dam, is thus removed, and there is also an additional safeguard from this practice.

   The use of the lower valley storage is similar to that of the keyline dam, it being a continuous-working and quick-profit dam. Whenever it contains water above the level of the lockpipe the dam is used continuously if irrigation is needed and can be put to advantage. Lower valley dam storage is not a supplementary system but is part of the regular farming and grazing enterprise.

   The methods of irrigation from the lower valley dam depend on the circumstances of land shape and the type of pasture and crop to be grown. Although the dam is low in elevation in its situation in a secondary land unit, there may be much land below it on some properties, and on others it will be near the farm's lowest boundary. Our own lower dams illustrate these varying conditions. On one property "the Pond", or lower valley dam (16 million gallons) is within a few yards of our lower boundary fence, so in this case the water of the dam is pumped up to an irrigation drain directly from the lockpipe outlet via a pipeline to a point nearly 200 yards away and twenty-five feet vertically above the top water level of the dam. The pump delivers a little over 1,000 gallons a minute (60,000 gallons per hour), and the method of irrigation is again keyline pattern suited to our purpose, which is generally pasture growing, but on occasions may be the growing of other fodder crops. A lower valley dam on another property is near the lower limit of the land unit and very like the reservoir mentioned in the construction details except that it is somewhat larger. Whilst it is low in its own land unit, it is high above a large area of our own property, and so its utilisation follows the lines of those for the keyline dam of Chapter XVIII. In this particular dam, when all the water that will flow from the outlet for irrigating is used, there will still remain an acre and a half of water six feet deep which lies in the excavation area below the lockpipe, and from which it was necessary to dig earth for the wall construction. Although this water represents only 2% of the capacity of the dam, it also represents a large stock water reserve but one which we will rarely need.

   A lower valley dam, as I have said, is a real money-making dam. Apart from keyline pattern irrigation, the dam in various circumstances of climate, land shape and property management can supply water for almost any type of irrigation. In gently to steeply undulating country, the lower dam is usually adjacent to the flattest areas of the farm, and any type of watering now characteristic of the large irrigation district can be adopted here. These include border checks and bays (small crop and pasture), contour furrows (pasture), contour bays (rice, etc.), furrow irrigation (vegetable and orchards), and the basin system of irrigation (crops and pastures). All these systems and methods of irrigation are dealt with satisfactorily in official publications, so it is not necessary to include them here. Our lower valley type dam is ideal for any of the methods of spray irrigation (these also are described in official books), which may be suitable for particular purposes of cropping. It is suggested that the outlet of the dam is the most suitable point for a permanent or casual pumping set-up. Since the outlet is situated to one side of the low point of the valley behind the dam and with the water of the dam always above the level of the pump, pumping would not require a foot valve or any pump priming. The pump always works when the engine is started. It is also the best position for a permanent installation. Now-a-days, permanent pump and engine irrigation set-ups are placed on the wall of the dam, and after initial troubles have been ironed out, they operate satisfactorily. Personally, I much prefer to have the wall of a dam kept clear, so that it can be travelled and its soil and pasture aspects improved at the same time as adjoining paddocks are being worked. The arrangement whereby a pump and engine are placed somewhere around a dam, and, after pumping for a few hours the outfit has to be reset and often put down in the soft muddy area of the dam, is not a money-making arrangement, although in drought times the worst layout is better than nothing. Often after a dam has been constructed and has filled with run-off water and irrigating has been undertaken, the trouble of chasing the receding water into the mud begins; and it is a frustrating, time-consuming and money-wasting effort. Sometimes, too, a bulldozer is brought in to cut a trench from the deep part of a dam back to a spot on the water line to get at the deep water, so as to avoid changing the pump position. A small drag line excavator may be used for the same purpose. However, these things are evidence of not only inexperience, because the inexperienced can make a very good dam from a good plan, but the lack of proper forethought.

   The relatively large volume of water available from most lower valley dams of this Keyline layout are so valuable that the most economical working set-up for the use of the water becomes very good business. One method which I have used, and which in other instances may prove suitable, is the use of a pump-sump for the permanent site position of the pump and engine. In this arrangement a sump or hole two feet six inches square and three feet deep is excavated adjacent to the lockpipe outlet, a few inches lower in level and on one side of the valley. The sump is lined with wood, brick or concrete and a permanent pipe from the lockpipe enters the sump and a smaller pipe drains surplus water from the top of the sump. The larger pipe from the lockpipe includes a valve. In operation, the valve on the pipe is opened, the sump filled, and the pump started up. The water inlet control is then adjusted so that there is a continuous small flow of water out through the overflow from the sump.

   Some years ago I adopted a system which involved a series of lidded sumps 400 feet apart and located in a drain fed from the outlet valve of a high dam. Spray irrigation of pasture was the object and a readily portable pump and engine pumped the water from the sump adjacent to the land to be irrigated. Associated with the set-up was a mechanical means of moving the spray lines, which proved very successful. There were two dams, a larger one high enough for water to flow from it to a second and smaller dam. To use the system, irrigating commenced by using any water in the smaller dam first and then irrigating from the last sump (the one nearest the second dam) and moving towards the larger dam, using each sump in turn. For the sump to operate successfully there had to be a small overflow of water, which flowed via the drain to the smaller dam. Pumping from each sump only involved a lift of two feet and there was little trouble in pump priming. Although I did not continue with the use of this system, as our keyline flow method for pasture irrigating was very much quicker and cost almost nothing, the system may be of some value to others. However, the method applies more to the higher dams, and it could be valuable as a drought method of using water from reservoirs. It is mentioned here by way of illustrating the effectiveness of sump-pumping, but the obvious and best method of pumping is the direct one from the outlet of the dam.

   A lower valley dam may be constructed with a capacity equal to several years of the average annual run-off and still conserve water at low cost, because of its favourable site characteristics. It may remain empty, or nearly so in some instances, and therefore during this time its land area should not be wasted. On occasions two such dams, the second located in the same valley and immediately above the first, may fill, but then remain empty for a year or two, after using all the water for irrigation. So the valley shape below such dams should be considered during dam construction and a good natural shape made or preserved. If old washouts or eroded gullies lie below top water line, these should be reshaped to a suitable natural valley form, so that keyline cultivation (parallel downward from the top water line) when the dam is empty will be effective to the maximum extent. With a good draining shape so provided, the area of the dam when empty can be sown down to special pasture or other crops. When the area of such a dam is dry enough for cultivation it should be worked up quickly and sown as soon as the soil has sweetened. The empty condition of the dam will usually occur in a dry summer, and so can be sown and made to produce its special crop.

   I have cultivated this type of land and have found that a deep cultivation to gain quick aeration of the soil brings a rapid change and creates admirably suitable growing conditions; but the soil must not be touched while it is too wet.

   The art of land utilisation is largely a matter of making the best use of water, and these larger lower valley dams offer wonderful opportunities in this connection over a very wide range of climates and land types.

   Creek dams are classified as farm dams that are constructed in a natural watercourse in which the water flows over raw earth and between confined and defined banks. The stream may be flowing either permanently or intermittently. (See Fig. 18. )

   As the distance down the land from the keypoints of the primary valley is increased for the various types of dams, so will the construction problems that may be encountered become greater. In the secondary land unit mentioned earlier, of the three types of dams that can be constructed, the highest or keyline dam is usually associated with the best construction conditions. The earth and the foundations are often better and the stability of the structure can be obtained with the minimum of effort. The reservoir site may have a deeper built-up earth and require a deeper cut-off trench into. the more stable materials below. The cut-off trench may have to be excavated and filled with good earths as an independent job before the lockpipe trench is made. The cut-off trench of a lower valley dam in the same secondary valley may have further problems. However, the purpose of the cut-off trench and its filling with good material is always to prevent the water within a dam from seeping and then possibly later flowing under the wall of the dam. If this should happen the wall could collapse, and in any case the effectiveness of the dam is inevitably reduced. The creek dams, by being sited further down the land, may have these problems accentuated, and so not all creeks are suitable for farm dam construction. While there are ways and means of solving any of these problems, given the necessary finance, this discussion is concerned only with those dams whose construction is within the capacity of farmers and graziers generally, and with dams which will be very profitable to them. In the main, creeks with very large catchment areas are usually not so suitable for farm dams, as are likewise those with deep beds of loose sand below them and also the creeks of the very flat lands.

   If I had continued my classification of valleys in Chapter VI from the primary and secondary valley to the next larger system, then this valley would be the one with the most suitable creeks for the farm creek dams.

   The particular circumstances of the land shape and the characteristic of the creek itself should be studied before building any creek dam. For instance, the full benefit of the flow of the creek may often be obtained more economically by diverting the flow to an off-the-creek storage, which may be a dam of the type of the reservoir or lower valley dam, or possibly the contour dam to be later discussed. The kinds of diversion weirs that are suitable for such a purpose vary almost as widely as the circumstances of their use, but they have one common feature; they all must be able to take high flood flow of water over the top of the weir.

   Good natural sites for creek dams have to be searehed for, and not only for their largest water-storage capacities, but also for their absence of construction and management problems. On our new property at Orange we have a site for a creek dam which has a drainage area of under 600 acres, but if the drainage area was 4,000 acres the site would still be suitable and there would be no difficulties in providing cheaply the large spillway capacity that would be required. On the other hand we have a creek dam on the same property with less than 600 acres of catchment area, but the spillway for it had to be excavated, partly with explosives, into rock. A large creek dam may be constructed in some circumstances very cheaply, yet in almost similar conditions in the same area it could be an uneconomical task.

   The design and construction factors that apply in the construction of a keyline dam, reservoir and lower valley dam govern the construction of the creek dam. If the materials are similar, then wall batters may be the same as for these other dams. Site preparations are the same as set down for the keyline dam.

   The creek site is often associated with two features not generally encountered in the other dams; the creek may flow over rock and the earths adjacent may contain gravel and sands where the creek in earlier times flowed in a different course. It is then the examination of the material available for wan construction becomes more important; the cut-off trench in places may need to be deeper to get below the loose material, and there is more likelihood that some of the earth for the wall may need to be obtained outside the area of the dam.

   Where water is flowing continuously the whole of the marking-out and site preparation should be completed as far as possible before commencing in the water. If the bottom of the creek is on solid ground or firm rock the next stage would be to bulldoze the loose rock and sand straight downstream along the creek bed through the wall site, depositing it a little higher than the creek bottom and on one bank of the creek. Ripping and 'dozing out in the creek bed will form a suitable channel for laying the lockpipe, which is set with the baffle plates sunk into the creek bottom. With the lockpipe in position (Chapter XVIII) and completely bolted up, a small earth dam is quickly pushed up with the bulldozer, using the earth near the upstream ends of the line. The purpose of this small dam is to hold back the flow of the creek and to force all the flow water through the lockpipe. With the water under control, the forward half of the lockpipe channel (the creek bed) can be cleared by hand of rock and excess loose material, which may be deposited in the lower part of the channel towards the back of the wall. In the construction of any farm dam it is intended that the seal against the movement of water through the wall is effected in the upstream section of the wall foundation down to the outside (lower side) of the cut-off trench, and that if water reaches beyond this point it is allowed to get away. If this water were sealed in the wall it could build up enough pressure to blow-out or cause the collapse of a section of the back wall. Therefore, the forward end of the lockpipe channel is cleaned to back behind the cut-off trench and some loose rock beyond this point is not a disadvantage. The laying of the lockpipe and the filling of the trench proceeds as in Chapter XVIII.

   In a creek where flow is liable to continue for long periods a dam should have two spillways provided in the design of the dam. There should never be any question of the capacity of the main or flood spillway to dispose safely of the water. However, it is undesirable that any spillway constructed in earth and covered with soil and pasture should flow for long periods. The continuous flow eventually damages the pasture and soil. If damage is to be avoided where flow is prolonged, then provision should be made for the second spillway, a so-called mechanical spillway.

   The mechanical spillway, then, is simply a pipe through the wall, and it may be placed some three or four feet below top water level (flood spillway level), with a right-angle bend standing up vertically and having the inlet from six to twelve inches below the main spillway overflow level. The capacity of the mechanical spillway should be from three to six times normal creek flow. The inlet into it should be approximately three times the diameter of the pipe itself and be covered with a heavy-gauge one-inch wire mesh. The pipe, after coming through the wall, is turned down the wall of the dam and discharges directly into the creek onto a heap of stones. The pipe of the mechanical spillway is set in the wall in the same manner as a length or two of the heavy pipe sections of lockpipe. The down-pipe section to the creek bed below may be a pipe of lighter gauge.

   The main or flood spillway of a creek dam must be completely adequate for its purpose, that is the disposal of the run-off from the largest flood rains, so that the excess water cannot overtop the earth wall of the dam. The disposal area where flood run-off re-enters the creek then becomes one of the considerations of site selection. Very large quantities of water can be returned to the creek via a pasture ridge developed and pattern cultivated as already described. An adjacent valley form may be suitable, but whatever means is chosen, the development and preparation of the area should proceed as an essential part of the dam construction.

   No land adjacent to a dam should be kept solely for dam purposes, since the larger the flood spillway of a creek dam the more important it is that it be developed as a good pasture area. Flood overflow does not last long, while the mechanical spillway, which has been flowing during the flood, will then divert the decreasing flow and the main spillway flow will cease. Whenever work is being done in the paddock near any dam, improvement of the dam itself can always be given first preference.

   So we can see that again with creek dams, as with these other dams, they offer almost limitless possibilities for widespread improvement of the whole farming and grazing landscape, and, in addition, they yield those high monetary returns which are always a necessary final proof of good farming and grazing practices.

   I know of a farm dam so valuable that were its real worth assessed, and allowing for the deduction of its cost, the value of the dam would greatly exceed the purchase price of the property on which it is situated.

   We have a newly constructed creek dam on "Kencarley", at Orange, which serves to illustrate the possibilities associated with planning that is realistically based on climate, land shape and the farming requirements. The dam has a higher flat catchment, which continues to a lower but steep catchment. Hundreds of feet above this creek dam are other dams with a capacity of 150 million gallons of water, which will be increased later by another 50 million gallons. The water from the dams above may be used on adjacent areas in flow irrigation or turned into the creek above the creek dam, which we call "Control" dam. Control itself is of over 10 million gallons capacity, and full advantage was taken in the design and construction of all its natural features. It has a large flood spillway, soil covered and sown to pasture, also an 8-inch mechanical spillway to handle creek flow which is continuous, and with the large dams and irrigation areas above the dam will probably increase in flow within the next two years. For irrigation purposes Control has three lockpipe outlets, one a little to one side from the bottom of the dam, and two 10-inch systems, one on each bank of the valley of the creek and each located at the same level some three feet below the level of the intake into the mechanical spillway. The 10-inch lockpipe outlets connect to irrigation drains flowing along the top of large areas on both sides of the creek. These areas will soon become very valuable irrigated land. The steep areas of the dam's watershed are near-vertical slates and schists, which, when their soil fertility and pasture are improved, will absorb more of the rainfall into the rock below and practically all of which will no doubt add considerably to the high springs which are the present source of the creek's continuous flow. The developed water capacity of these steep hills, which will be consequential to the normal Keyline development, must be the equivalent of many millions of gallons of extra storage capacity, and will be used for irrigating from the higher 10-inch outlet systems. Lesser flows and the reserve capacity of the dam are available for irrigating from the lower 8-inch system. Any time we wish we could turn 300,000 gallons an hour extra water into Control dam. Since the entire water of its catchment is under complete control, we can use the catchment area for irrigation, which, in effect, also will replenish the ground water of the catchment and maintain and probably soon increase the normal flow of the creek below Control. The two 10-inch systems, apart from the irrigation, could be employed to fill dams which may later be constructed three-quarters of a mile away from the creek itself. (See Fig. 18. )

   In the foregoing discussions it will have been noted that each type of dam in turn has come further and lower down the landscape and from the keyline dam at the head of the primary valleys to the generally lesser slope land of the creek dam. In the flatter country, too, all valley dams can be useful if the valleys possess sufficient shape. A dam hereabouts will usually follow the design and construction methods of the previous ones, but the walls will not be as high and the dams themselves will have larger surfaces and be shallower in average depth. They will generally be more affected by evaporation, but this factor is offset by the lower cost storage capacities. If a keyline dam costing £120 per million gallons of storage capacity is a good business proposition, then a dam in flat country costing £30 per million and only filling once in two years and losing half its water by evaporation, can still be an equally good or even better proposition for the farmer and grazier. Apart from valley sites for dams, with their very wide suitability over all shapes of land, water can be conserved very economically on gentle or flattish slopes.

   A contour dam may be used to advantage on slopes which contain no valley form. This dam is essentially a long earth wall of medium height constructed from earth which is excavated from immediately above the dam, and with wing-walls made to taper up land to above the water level.

   In the flat lands all design features of the dams are flatter; the dams themselves are shallower; the water conservation and the irrigation drains are both flatter, but the irrigation drains are built up to flow water slightly above the level of the land. The land to be used for irrigation is also flatter and all the flat land methods of irrigation, already mentioned, can be used from the supply held in contour dams.

   The critical design feature of this type dam, other than the all-important one of climate and its associated run-off, is always that of slope. Contour dams can be constructed on slopes ranging from 1 in 25 (4% slope) to 1 in 100 (1% slope). They may be classed or named "straight", "inside", and "outside", according to their general contour shape. A straight contour dam is one whose wall follows a contour line which is reasonably straight; an inside contour dam follows a contour curving round a flat ridge shape, the dam being on the inside of the curved shape; and an outside contour dam is one associated with a flat valley formation where the water lies on the outside of the curve of the wall.

   :Features similarly associated with the location of the first valley dam are to be looked for in the site selection of a first contour dam for a property. It should be as high on the property as convenient; there must be run-off and sufficient catchment area above the water conservation drain to fill the dam. As to size, it may range from five million gallons (20 acre feet approximately) to 23 million gallons or more. To bring the matter to practical consideration, we may assume a contour dam is to be designed with a capacity of 80 acre feet of water; that the slope of the land is 1 in 50 (2% slope) ; that the depth of water at the inlet to the lockpipe is 12 feet; that the land shape contains large low forms only, and that the contour shape of the dam is "straight". This capacity would require a wall 900 feet long approximately. Water 12 feet deep on a 1 in 50 slope would place the water line up this slope 12 x 50 or 600 feet, and the dam would therefore have an area of a little over 12 acres. The average depth of a contour dam is somewhat over 50% of its full depth, or about seven feet in this case, so that the required capacity is satisfied by this general size.

   In the medium-size farm dam, a suitable freeboard height is three feet, but the circumstances of design in a contour dam suggest that this figure be reduced to two feet. There is no part of the wall of a contour dam that represents the main bulk of the earth, as is the case in the valley dam, and a failure of part of the wall is not nearly so serious a matter as in the valley dam; moreover, the inflow of water to the dam is readily controllable. These facts also suggest that the minimum or cheapest construction methods may be used in building the wall, and also with the lower wall height the wall batters may be nearly as steep as the most economical slope that the bulldozer equipment can construct.

   Wall height will then be 12 feet depth of water plus two feet freeboard, and as minimum construction methods are to be employed, an allowance for settlement and shrinkage will be increased to 10%. The constructed wall height is therefore 15.4 feet. The dimensions of the wall section are as shown on the plan and section opposite. The constructed height is 15.4 feet, the settled height 14 feet, the width of wall shape at the base is 52 feet, and the crest width is 10 feet. The lockpipe will be placed into solid ground and there will be approximately two feet of earth above the lockpipe level on the inside of the dam. At distances of 80 and 100 feet from the inside toe of the wall there will be four and five feet of earth respectively above this level and more than sufficient for the wall without digging earth below the inlet level of the lockpipe. (See Fig. 19. )

   The water conservation drain of a contour dam, like those of all these dams, does not fall directly into the dam, but is constructed right along and above the dam, and reaching water level height at the spillway of the dam. As mentioned, the drain may be flatter than the 0.5 % fall generally employed for the other dams. A flatter drain has less capacity, so that the drain needs to be of larger section. The drain should fall in the down land direction, vide Chapter VI.

   The position of the lockpipe may be in any portion of the length of the wall according to where the water is to be used. If the area of the gentle slope immediately below the dam is to be irrigated, the lockpipe is placed in the main wall at the end where the conservation drain first reaches the dam; in other circumstances it will be placed in the opposite end.

   The price per yard of earth moved in a contour dam of this wall height win be considerably less than in the higher wall dams. The average haul will be less, the push up the batter of the wall is shorter, and more of the operation, which is only shallow digging, can be performed in second gear. A reduction of 40% in earth-moving costs is to be expected.

   Marking-out and site preparation should proceed as for a keyline dam, with the clear marking-in of the wall shape on the ground with a furrow line. Top water line for the dam should also be marked. That part of the water conservation drain which is along the top of the dam could be first constructed to prevent any run-off into the area of the dam during construction. This drain may have a slope of from 0.2% to 0.5%, according to features of the land.

   A cut-off trench for the full length of the main wall and the two wing walls should be used, but may need to be only a few inches deep. Even where it may be considered that the cut-off trench is not required, it is still advisable to use a shallow trench, since it helps appreciably in controlling the job and in supervision. The area of the wall site is chiselled along the line of the walls to assist bonding as before.

   The cross section of the wall as illustrated (and it is not the minimum that could be used) represents an area of 48 square yards and (accepting the two wing walls as containing the equivalent of 200 yards of full wall section), the yardage in the walls is 24,000 cubic yards. The yardage of water storage capacity, with water depth an average of seven feet, is 135,500, or an earth to water ratio of 5.65. In other words, one cubic yard of very cheaply-moved earth creates about six cubic yards of water-storage capacity. The excavation area should be battered back into the land slope and covered with soil, cultivated and sown as suggested for the other dams. A contour dam of this or a larger size contains an area which, when uncovered, could be very valuable land.

   The particular purposes for which the water may be used from such a dam are legion and the value of the dam is considerable. We can consider a particular case by way of example. The rainfall conditions may have been such that with the augmented catchment provided by the water conservation drain, the dam will be filled each year from winter rains. The dam then could be the basis of the production of a special spring or summer crop on which an extra 12 inches of water from the contour dam would ensure a successful crop every year. A crop area of 60 acres may be used for the purpose or a series of three paddocks each of similar size and irrigated in rotation. The requirements for the lowest cost irrigation are thus provided and any of several methods already discussed for the other dams may be used, according to land form and slopes. If the dam will fill each winter, then the dam water should be used fully each spring or summer.

   We may suppose now that the water is all used in producing a spring crop and consider the empty dam. The dam area, which was provided during the construction with good draining slopes to the outlet, can then be cultivated, when dry enough, and sown to a summer crop. Because of the deep moisture of the dam area, a good summer crop could be produced in the driest of years. If, when this crop is nearly ready, heavy summer rains occur, the water conservation drain along the higher side of the dam, which filled the dam, can be used effectively to prevent run-off reaching the dam. The water conservation drain is blocked or breached before it reaches the dam and the section of the drain along the dam is put in order, so that run-off from the area immediately above the dam is made to flow out through the spillway. Only rain actually falling in the area of the dam would reach it, and, with the outlet open, even this water would drain away. On the other hand, the landholder may prefer to retain the water and so will allow the dam to fill up normally.

   Under the same climatic conditions, a grazier may decide to use the conserved water for flood or flow irrigation of a smaller area of pasture, say 20 to 30 acres, so as to enable him to have ample water available throughout the season. Likewise, the dam is specially advantageous for spray irrigation of valuable crops, such as vegetables.

   The costs of contour dams will vary considerably, but if I had such a suitable contour dam site on my own property I would expect my construction costs to be less than £1,800, including all the irrigation controls and drains. Under the climatic conditions of Richmond or Orange about 60 acres of irrigation could be provided and the area of the dam when emptied would be so much additional worth. Contour dams may be much smaller or larger than the one illustrated, but it seems from my own experience that the larger dam will return the higher capital cost just as quickly as the smaller dam returns the lower cost.

   We may consider now the storage of water on land that is flatter than those gentle slopes suitable for the contour dam. As our water conservation structures are to hold water above the level of some of the immediately adjacent land, then the only way this can be accomplished on land with slopes of only a few feet fall in a mile is by completely surrounding the water with a constructed earth wall. In these circumstances, also, the only way to get the water for storage into the dam is by lifting it into the storage area. This pumped type of storage is always the last to be considered for storing appreciable volumes of water, so before dealing with this new type, consideration will be given to slopes intermediate between those suitable for the various contour dams and those where water must be pumped for storage.

   It has been seen that a contour dam is an efficient water conservation structure where the land slope is 1 in 50. What changes are necessary where slopes are 1 in 100? A contour wall as described would, if employed on this slope, back water up to 12 feet (depth of water at outlet) multiplied by a slope of 100, i.e., 1200 feet, so that the main contour wall would be 900 feet long as before and each of the two wing walls would be 1400 feet long, after allowing for two feet of freeboard. The approximate relationship between the two contour dams, one on 1 in 50 slope and the other on 1 in 100 slope, is that for the same length of main wall the area and capacity of the second is double that on the 1 in 50 slope. Though the quantity of earth of the wing walls has been increased, the extra yardage of earth required has only increased by less than 50%. The ratio of earth moved to storage capacity now approaches 1 for 8 instead of the other 1 in 50 slope dam, which is 1 for 5.65.

   The second contour dam provides most efficient water storage capacity on this slope of land. The question now is, can any other type of structure be used satisfactorily for the same slope conditions? Of the various shapes which may be used to completely enclose a storage area, the ring shape is the most efficient in yardage of earth moved to capacity enclosed, so a ring dam may be considered for the above slope.

   Some confusion has arisen as to the names of these dams, so they are given the general descriptive name of "closed wall" dams, which indicates that a constructed wall completely surrounds the water storage area. In this text they will be given individual names based on the particular shape of the closed wall. Thus, the closed wall dam of ring shape is the "ring dam". This is sometimes incorrectly referred to as the turkey's nest dam. However, the turkey's nest dam, as is "the overshot", is a dam of long-established use in Queensland's country areas. It is constructed in the same manner as the nest of the scrub turkey, from which it takes its name. The turkey's nest dam is built with earth obtained outside the structure, which forms a circular wall, and it is filled from flowing or pumping bores. By being above ground level the water of the dam can be led into stock troughs equipped with automatic float valves, which replenishes the trough water as it is used. The ring dam, on the other hand, is constructed by digging earth from the inside to form a wall. The dam then is a larger diameter earth-wall. ring with a channel just inside the wall, the earth excavated from which forms the wall. Inside the ring-shaped channel there is usually a circle of unexcavated material or natural land surface. According to the depth of the dam, which may be from 11 to 16 feet in the channel, there will be from 8 to 12 feet of water over the land surface in the central area of the ring dam.

   A ring dam of the same maximum depth of water above ground on this slope and having a diameter of 900 feet would have the following characteristics. The wall at its maximum dimension would be identical to the cross section of the contour dam on the same slope. At the opposite side of the dam, 900 feet up the 1 in 100 slope, the ground level would be nine feet higher, so at this point the ring dam would hold three feet of water above ground level. The average depth of the ring dam, and disregarding the excavation, would then be the approximate average between its deepest depth of 12 feet and shallowest depth of three feet, or an average of seven feet six inches deep above natural ground level. The section of the wall at the shallowest part of the dam would be five feet high (three feet of water plus two feet of freeboard), 10 feet wide at the crest and 25 feet wide at the base. Calculations for both dams show that the ratio of earth moved to storage capacity is somewhat better in the contour dam when compared with the ring dam, but the ring shape provides more water per acre of land under top water level, its average depth is a little greater and it has a smaller area of shallow water. Both dams appear almost equally advantageous except when it comes to filling the dam with water. If the ring dam on this slope had to be pump filled it would not be chosen for the site and a contour dam would then be the only logical choice. However, a ring dam on such a site can be filled as can the contour dam on the same site, namely, by flow water. For the ring dam to fill, the flow water must enter the dam (or a part of the dam or a closed inlet to the dam) at or just above top water level. Top water level would be represented by a position 300 feet further up the slight slope and beyond the dam near its smallest wall section. Water will then need to flow to the dam from this point. There are two satisfactory ways of getting this done. One is by extending a pipe of suitable diameter through the shallow wall of the dam and continuing it up the incline to the height of top water level. A water conservation drain (as for the contour dam) would deliver run-off water to this point, whence it would flow via the large pipe (about 18 inches diameter) through the low wall into the dam. The pipe line, which would be a little over 300 feet long, would be underground and a small bank or bay at the height of top water level would need to. be formed around the higher end of the open pipe. The alternative method of filling the ring dam from the same water conservation drain is by extending a pair of parallel walls from the shallow wall section of the dam up to the water conservation drain. The walls are to be 30 feet apart and join the shallow wall side of the dam where a 30 feet section of the wall would not have been built to open into the waterway formed by the two parallel walls. The height of these little walls would be on the same level as the wall crest of the dam. This is now a "broken ring" dam or "pan-handle" dam, and the pan-handle walls at their water conservation drain end would be so arranged that when the dam reached its full planned depth no more water could flow in or overtop the dam. Spillway area for the disposal of excess water would be required and could be arranged on the lines already discussed for the other dams.

   Of the two methods of filling the dam, the pan-handle walls method would be the more economical and generally the one to be preferred. This type of dam, the "broken ring", may be of any suitable shape, because the full ring shape may be influenced by other features, such as a watercourse or a property boundary. The ring shape is depicted here, as it is the most efficient of all the closed wall types of dam. (See Fig. 20. )

   The above type of dam, i.e., a broken ring dam, coupled with the method of filling it, is suitable for slopes slightly steeper than 1 in 100, but is not likely to be preferred to the contour dam. While it may also be used on slightly flatter slopes than 1 in 100, obviously it eventually reaches its flat limit of slope where the pan-handle walls arrangement becomes too long and later completely impractical. The specially critical factor of these designs is the slope, and in designing a dam in these, or, for that matter, in any circumstances, no attempt should be made without first knowing the exact land slopes.

   Where the slopes are so flat that the contour dam and the broken ring become unsuitable and not worth consideration there is no way to store water above ground level other than with the closed wall dam and pump filling. All other dams may be filled by natural catchment, by water conservation drains, or by weir-diverted creek flows, but where a ring or other closed wall dam is the only choice the site should be near a watercourse of a particular type. The most usual source of water supply is an intermittently flowing stream, or, more often, one which flows only after heavy rainfall. The dam may need to be filled during heavy rainfall, therefore the above facts should be noted when locating the dam and when making the design of the dam and its related filling structures. In some circumstances, though a dam must be filled from a certain watercourse, it may be a disadvantage or even an impossible hazard to construct the dam close to the watercourse. In other circumstances and for the sake of efficiency and economy, it may be worthwhile to depart from the ring shape by having part of the wall following a section of the bank of a watercourse. (See Fig. 21. )

   However, only after carefully studying the possible locations for the closed wall dam and the filling site should the dam site be finally determined.

   The most satisfactory filling arrangement is that the dam can be filled through the lockpipe outlet, and to this end a bay from the watercourse could be excavated to a suitable common filling and outlet point. A small permanent flood weir in the watercourse may be arranged in such a way that during flow periods the weir causes the drain and bay to fill for pumping into the dam, but at the same time allows flood water to flow over the weir, with no inconvenience to the site and structure if pumping is not required from certain flood flows.

   There is an idea in the minds of some that water has to be pumped "over the top" of the wall of such a dam. On the contrary, this is a disadvantage against pumping through a suitable pipe beneath the wall. The higher water has to be lifted the greater is the power required, or, alternatively, less water will be delivered for a given power.

   The construction of the ring or other closed wall dam follows the general procedures already given. The size of the lockpipe may be increased according to the capacity of the pump which is to fill the dam, but generally a size of 10 inches is suitable in nearly all circumstances.

   The importance of making proper arrangements for the filling of ring dams (or any closed wall dam) cannot be overstressed. The full layout should be decided and included in the design of the dam itself; and the creek weir, the drain and bay, from which the water is to be pumped, should all be constructed and completed as part of the dam construction. Again, the elevation to which water has to be raised in the dam, the pump capacity and power requirement, must be logically determined in relation to the capacity of the dam. Generally, where water has to be pumped into a dam, time is so limited that large capacity low head pumps are invariably required. The dam for these reasons should be close to the level of creek flow, so that pumping will take place from only slightly below ground level. The likely length of time available after storm rains for pumping should be calculated against the capacity of the dam, that is to say, when the rate per hour of water delivery required has been estimated, a pump capable of this performance against the height of the total lift should be acquired. (See Fig. 21. )

   The most suitable dam filling arrangement is a permanent set-up of pump and engine that can be operated under the worst possible weather conditions. A lower initial cost method would be to arrange a permanent pump set-up so that the power of a farm tractor could be quickly coupled to the pump. With the pump in a permanent position and the suction line in place, the set-up is always ready for operation, i.e., the delivery pipe from the pump is left coupled permanently to the lockpipe at its outlet end. There is also coupled to the larger size lockpipe a Y piece which has a control valve on each leg of the Y piece. One leg is the permanent inflow or dam filling side, and the other the irrigation water outlet. Whenever there is water in the dam at the lockpipe level the pump may be primed very quickly by partly opening the valve on the pump delivery line, then the engine is started up and the valve opened fully. A large low-head high-capacity centrifugal pump selected to exactly suit the requirement is inexpensive and will have low running costs.

   The water from a closed wall dam may be used for many different irrigation systems and varying from flood to any type of spray irrigation.

   Generally these dams of the flatter lands are uneconomical when attempts are made to employ them outside their proper land suitability. One practical experience of the ring dam was in 1947, when I constructed one for the purpose of maintaining a head of water in a long underground irrigation main which was laid along a high boundary on "Yobarnie". An intended use for the water was the spray irrigation of a new orchard. The orchard idea was abandoned and the dam then was used only for pasture irrigation, which included the giant monitor type of spray irrigation just introduced for the first time into this country. The ring dam was filled by pumping, which involved a considerable lift, via a six-inch underground pipe line that included a crosspiece and thence under the wall of the dam. The crosspiece was fitted with valves, one of which opened for dam filling, and at other times it was used to flow water from the dam back along the delivery line, which in turn was equipped with numerous take-offs for irrigating along its length. Other valves on the crosspiece directed water to underground pipe lines in two different directions. The mechanics of the whole set-up worked admirably and the scheme might have been successful in other directions also if the orchard project had been continued successfully. However, as a pasture spray irrigation project for beef cattle production there was little likelihood that it could be economical or ever really profitable, and so was abandoned. A ring dam was constructed on a farm in N.S.W. as recently as last year, and in circumstances unsuitable for its use, although adjacent to this particular ring dam (which has a pump lift of about 100 feet), there are well-nigh perfect sites for keyline dams. If these were utilised similarly to our dams and irrigation system, they would be highly profitable. The mistakes that have been made, and are continuing to be made, in all aspects of farm water storage and water use, would be very educational to farmers and graziers if they were made known. Many branches of agricultural science have been improved considerably by utilising knowledge gained from mistakes, and it is hardly an exaggeration to say that science generally is largely the accumulated knowledge gained from innumerable mistakes.

   All dams of whatever type or kind and for any purpose must fit in with and became part of the landscape. They all must have a means of getting water into them, but they also need the best and cheapest means of getting water out of them and into effective use.

   The foregoing discussion will have been most successful if it draws attention, much-needed attention, to the need and great value of farm dams of these various types and usages in their application to the development of the whole Australian landscape. It is a development that is not only beneficial to the individual farm and grazing property but one which adds merit to the work of every Australian landman.

   The problems of farm dams do not end with their design and construction, so these dams we have been describing need proper care and attention, and this aspect, along with some associated problems, will now be considered.

   After Care of Farm Dams: All newly-constructed farm dams, including those described in this book, are subject to change, The covering of all raw earth with soil and the sowing of grasses on every part of the dam and its immediate surroundings must be considered a part of the construction of the dam itself. Grass may grow and quickly cover the wall and the surroundings, but, even so, the wall will shrink and crack, and so needs inspection, and especially so during the first year of its useful life.

   The earliest and best check on the general performance and accuracy of the newly completed dam takes place with the first occurrence of heavy rainfall. If it is heavy enough to promote considerable run-off, so much the better. To learn all that rain can teach, the farmer should get out in the rain with a longhandled shovel. He should walk the wall of the dam and look for little ponding areas on the wall crest, ponds which will later break out in one particular spot and flow water in a small but concentrated stream down one or other batter of the wall, and cutting little gutters. These first little gutters, if they are left, will form real flow paths for the continuing rain and so increase quite rapidly in size. It is therefore necessary to fill up the little ponds on the crest of the wall with earth from higher spots. The shape of the wall preserved at this very early stage ensures an even and harmless flow of water in the heaviest of rainfalls. Next, the four areas should be inspected where the constructed wall joins the banks or sides of the valley. Often water may flow along these junctions in small concentrated streams, and so should be diverted and spread away from the wall. The water flowing into the dam will not cause even a slight movement if the cultivation pattern below the waterline is properly done. A small soil movement is not of much consequence, but an inspection is a good check on the work, since it will illustrate, as nothing else can, the effectiveness of keyline cultivation. Now the water conservation drain should be inspected. If there are low spots where water is threatening to overflow or is likely to overflow with heavier rain, the low places should be repaired. A low section in a drain indicates that the drain at that place is slightly downhill and off its proper line. The low place is therefore repaired in such a way that the drain position is moved slightly uphill to its correct position. This is done very simply by shovelling earth from the uphill batter of the drain and placing it, not on the very top line of the bank of the drain, but just inside this line, so that the bank line is moved uphill slightly. If the low place is merely raised by the placing of new earth on the top of the drain bank, the slightly incorrect position of the drain is preserved and more earth will be needed to raise it to the appropriate height. Since the water conservation drain is to be one of the really permanent man-made structures on the property, earth should not be shovelled indiscriminately in adjusting the section of the new drain; the cross section shape of the drain should be preserved in the shovelling and repair work.

   When water is beginning to break out of a water conservation drain, first, a high spot is looked for on the dam side of the break, which may be directly causing water to pond back and overflow. The high place should be fixed before the low spot. A high place in a drain indicates that the whole drain position at that point is slightly uphill and off its true position. Repairing the high place is done by digging earth at the downhill edge of the stream of water and spreading the earth either well uphill or down behind the drain bank, whichever is indicated by the circumstances. With the high spot adjusted, the overflow can be treated. Sometimes a bad break in a water conservation drain cannot be controlled directly. A suitable spot should then be selected upstream where the drain is in strong section. Here the drain is blocked with earth, and so, by causing water to overflow, relieves the bad break, which may then be repaired to its full section. The block is then removed and the drain. adjusted.

   When heavy dam-filling rain occurs a new dam is worth watching from many points of view. I have watched a man who, seeing a new and larger dam than he had experienced before filling rapidly, rush to break the water conservation drain to stop the flow into the dam and to open the outlet valve to let water out of the dam. But the larger dam is built only to hold more water and so should be allowed to fill as quickly as it can. If the dam was built reasonably well, and wall weight and shrinkage properly allowed for and with a suitable spillway, there is little cause for apprehension. If anything should go wrong, then the measures suggested in this book can be taken to correct matters. However, the new wall of a dam constructed of earth which was too dry should be controlled to fill more slowly, since the material of such a wall often lacks cohesion until it becomes slightly moist right through. If the first cracks on the water side of the wall of a new dam discloses dry, powdery earth in the wall, the water level may need to be lowered immediately. The very wet earth can slip off the dry, deeper material into the dam, and in doing so fracture the full section of the wall and result in the loss of a large part of it.

   After or during the first heavy rain on a new dam, the first notable shrinking and cracking of the wall may take place. These movements are normal in a farm dam. They are a part of the design and construction of the dam, since the wall costs have been reduced by about half, because, instead of going to the expense of using sheepfoot rollers or pneumatic-tyred rollers to get complete compaction of the wall material, the natural compacting forces of settlement and shrinkage are allowed to operate. There are two types of wall cracking, and they are named according to their mode of occurrence--longitudinal cracking and cross cracking. The early longitudinal cracks usually occur near the outer edge of the crest of the wall and are often associated with the paths the tractor made along the wall. The looser earth on the outside of the path will pull away or shrink away from the more settled earth where the weight of the tractor compressed it. Such cracks are rarely a hazard, but they should be treated by raking earth to fill them a day or two after rain, when the wall dries out a little. just sufficient earth is raked to fill the crack. Neglected longitudinal cracks become larger and could, after further heavy rain, hold water in such quantities, which, in finding its way out through the wall, could cause a slip in the wall.

   Cross cracking or cross-the-wall cracking is not usually associated with early settlement and shrinkage of a new wall. It may occur only after a dam has been first filled, and then all the water used and the wall has started to dry out. Though rarer, it is a more serious form of cracking if neglected or overlooked. These cracks may form a continuous split across the wall or be in the form of short cracks from the front and back of the wall to a longitudinal crack, and so form a crooked path through the wall. The cross cracks never or very rarely reach down the wall to the water level. Danger lies in cross cracks forming when the water level is reduced and the crack reaching down near to water level. Should flow water cause the dam to rise above the cracks, water will flow through the wall. If this flow occurs below spillway level, and is undetected, quite considerable damage to the wall can occur. Prevention of damage lies in filling in the cracks as with longitudinal cracks, but paying particular attention to the crack on the inside of the wall where it appears above the present water line of the dam. Here the crack should be rammed after filling, then filled again after ramming. Dry, more so than moist earth, is always to be used for filling cracks. Fine dry earth is probably the best.

   I have experienced occasions where the cracks, by first flowing as a very small stream, have become closed on the surface by stock walking the wall in wet conditions and the leak later developing into a pipe-type of flow. To repair such a flow after it has occurred, adjacent earth is shovelled into and around the opening under water, and no attempt is made to fill the full break across the wall until water flow has been stopped. Later the crack is filled with dry earth.

   The ordinary settlement and shrinkage cracking of a new wall may be effectively filled by a 2-inch to 3-inch cultivation with a chisel plow. The cultivation may be a part of the soil and pasture development of the area. However, occasional inspection of the wall will ensure that all is well.

   The often-recommended procedure of planting special wall-binding grasses can act against the safety of a wall. The generally coarse nature of such vegetation may hide dangerous cracks. Personally, I favour planting only the usual pasture mixture on the wall and fencing the dam off, or, alternatively, fencing the dam into a smaller paddock. Stock can be put on to graze the wall and the surroundings of the dam as part of the improvement programme, and the grazing should be controlled properly to these ends.

   A well-designed and constructed dam such as any of those discussed is a very safe and permanent asset. The methods of construction are low cost, and these after-care considerations are for the purpose of seeing that natural forces in compacting and consolidating the dam do their work without creating damage.

   Although things do not go wrong when all the above methods and procedures are adopted with good supervision, we may, however, consider some of the problems that are the result of less effective design and construction methods or that arise out of the use of the less suitable earths.

   Failures in farm dams are generally presumed to arise from three main causes. The first cause is inadequate spillway size, which fails to convey the overflow water and forces the water over the wall of the dam. Soon a channel will be cut in the wall by the water, which, once it has got down below the spillway height, causes all the water entering the dam to flow through the break in the wall. The second presumed cause is from a low spot near the central area of the wall crest caused by inadequate or no allowance for shrinkage in the construction of the wall. The effect is the same as before; water flows over the low place in the wall, cutting a channel and destroying the wall. The third cause is presumed to be inadequate compaction of the wall, material, which allows heavy seepage to build up into a strong flow through the wall. All the water may be lost and the wall remain in position, or the flow through the wall may cause the wall above the hole to collapse into the flow and leave a break in the wall from the bottom to the top. Overtopping may destroy a new wall in 20 minutes and an older wall in two hours or more.

   Many wall breaks resulting from the first two causes, and inspected after the failure, are attributed to the latter cause, because the material inside the now broken wall does not appear to be well compacted. The breaking of the wall itself often makes this aspect appear bad, but in my opinion the failures resulting from this cause are very few when compared to the other two causes of failure. I have inspected broken walls of dams that were said to have failed because of poor compaction, but in each case the breached wall remaining and the obviously inadequate spillway provisions clearly showed that overtopping of the wall had occurred. Many of these failures were in dams of some age which were holding water previously and had been washed out in later flood rains.

   Dams may fail as mentioned earlier by cross cracking in the wall. Faulty dam construction or a batter design that may be too steep in relation to the stability of the wall material, may cause slumping or slipping of the wall and reduce the wall height and so overtopping may then breach the wall. Of all the failures in farm dams, the large majority are caused by faulty design. The earth of the shale area of the County of Cumberland--Sydney and hinterland area of N.S.W.--holds water well even with the most unsound construction methods, yet in one day during the heavy rains of 1956 between 40 and 50 dams were stated to have breached. Although the rain was heavy, the fault in all or at least the large majority of cases must have been poor design, since none of the new dams were constructed in drought conditions when earth walls could be too dry to stand sudden filling. No doubt many of these failures were due to the type of advice given to farmers on farm dams, which invariably emphasises good construction as "most important", when, quite obviously, good construction can only follow better design. All aspects must be given their rightful attention, and then good construction becomes a counterpart of good design.

   We had one leaky dam on "Yobarnie" and another recently reconstructed dam that still could leak. As we are discussing problems in dams, the story of these two dams may serve as practical illustrations.

   The first of these two dams was constructed in an unfavourable site in that the valley was small and steep and the available material for the wall was a medium hard blue shale. The site was chosen for a variety of reasons; it fitted in well with the other dams, and there was a very good-shaped piece of land adjacent at the right height and suitable for pattern flow irrigation. Also, I wanted, to build a dam wall with the hard blue shale as a test of this material under the worst possible conditions, namely, to construct a high wall as steeply as the bulldozers could operate. The planned depth of water at the lockpipe was 34 feet, freeboard was three feet and shrinkage allowance three feet, giving a total height of 40 feet to the new wall at its largest section. The wall was built, but before the feeder drain could be made very heavy rain filled the dam and a large flow of water was running out through the spillway. There was also a considerable flow from five different places through the back of the wall. We raked up and down the inside of the wall with a rake attached to a pole and found one leak by the simple process of walking along the wall batter in the water in gum boots and finding oneself suddenly sinking into a semi-fluid hole. Earth shovelled from the crest of the wall was tramped into the hole with the boots and closed this high leak. However, the raking had no apparent effect on the rest of the considerable flow, although this method of stopping seepage is often successful. The leaking areas were evidently too deep below water. So I prepared explosives. Six plugs of 60% gelignite were cut in half and each fused. I decided to explode the charges in the water about 20 feet from the wall water line and two feet above the bottom, which was a part of the underwater batter of the wall. A piece of board was tied as a floater to each half plug with a length of string about 12 feet long, and then, holding the plug, string and board together, each fuse was lit in turn, and together with the charges and floater, thrown out into the water about 10 feet apart. There was a series of dull thumps and rather convincing vibrations through the wall on which I was standing, as the charges exploded about 10 seconds apart. The result was a considerable reduction in the flow of all but the deepest leak. Two more shots were used, each a full plug, and thrown further out and attached with a longer piece of string to a floater. (The length of string and floater checks the depth to which the charge sinks.) The dam was still leaking a little but held the water, which was used in the summer for irrigating. There had been also some slumping of the back batter of the wall, and as the leaks were not completely stopped, a bulldozer was put into the dam when it was empty to work on the leaking area and to trim the job up. By this time there had been a considerable breakdown of the shale material with now sufficient clay to make the wall impervious.

   Explosives are often valuable for farm purposes and are simple to use, but no one lacking experience should touch them without first studying the instruction book issued by the suppliers of explosives.

   The recently reconstructed dam mentioned was built in 1945 and had a four-inch outlet under the wall. The dam held about seven million gallons (28 acre feet or 42,000 cubic yards) and had been used for various types of flow, flood and spray irrigation for many years.

   After the first three years of the development of Keyline as a planning guide for, among other things, the siting and location of farm irrigation dams, it was decided that a keyline dam would be built above it. This old one would be reconstructed and enlarged with an eight-inch lockpipe placed under the wall and the enlarged dam would then become a reservoir and be kept filled as a reserve while the water of the three keyline dams and the large lower valley dam in this one catchment area was used first for irrigation.

   The wall of the dam was cut with a big V-shaped excavation after all the water had been used and the dam stood empty. The keyline dam was then constructed higher up the valley. The eight-inch lockpipe, 120 feet long, was placed in dead level in the V cut of the old wall and the closing of the V and the enlargement of the wall started. A height of nine feet was reached above the lockpipe when continuous and heavy flood rains commenced.

   I saw the work a week later and realised that a lot of trouble and waste of time confronted us. The bulldozer operator had left the job with waves of loose earth everywhere when he finished for the day. Rain was not expected, but now ponds of water were in the waves of loose earth and the work became super saturated. Fortunately, the lockpipe was left open, and this prevented the flood water from filling the dam to the point where it could overflow the unfinished V cut fill. Some millions of gallons flowed out the lockpipe in the flood rains which continued, yet no earth was washed away and lost.

   Months later, anxious to finish the wall and seeing the surface of the earth dry and cracking, the bulldozer operator started work again. The whole wall was like jelly underneath and all he succeeded in doing was pushing a few yards of earth on to the wall at one part, which promptly settled back to its original height, and blocking the lockpipe with a fluid mud. Lengths of two-inch pipe were joined together and pushed through the lockpipe from the back of the wall to clear it. Again it rained, and once more the lockpipe carried more millions of gallons of water away, saving the earth and preventing a bigger mess. Later again, with the wall dried out somewhat, although still too wet, the bulldozer operators with a lot of perseverance succeeded in raising the wall to its planned height. Sticking to the bulldozer blade, the clay material would not spread evenly and went into the wall in lumps. Everyone by now was tired of the dam; however, it was completed. The operator relaxed for lunch before moving off, then when he looked at the wall again there was a great bulge on the inside batter; the whole wall section had slumped about five feet. Work was delayed while it dried out further, and eventually the wall was raised again to the finished level. The lumpy clay material which was placed in this wall looked bad enough for us to prophesy that the wall was likely to leak.

   The sad story of this dam's troubles arose from the fact that the construction was not supervised. By neglecting to trim the wall, water got into and jellied the clay. The story adds point to my suggestions on supervision and the trimming of work before finishing each day. Yet it could have been even worse. If the lockpipe had not been used, the partly constructed dam would have been lost and the whole valley floor for 800 yards below would have been covered with the washed-out mud and earth which would have resulted from the heavy and continuous overflow of the loose bank. Instead, we now had a good dam and during the dry period which followed the final work the wall had shrunk, settled and cracked. The wall crest was cultivated on three occasions to fill the cracks, and no doubt the long dry spell helped the wall a great deal by allowing settlement to take place before much water entered the dam.

   With regard to heavy rainfall on a new dam, it is to be remembered that a new dam, when filled rapidly in continuous heavy rain, imposes the worst conditions on the back (downstream) batter of the wall. The front wall batter is safe, being assisted by the water in the dam, but with the dam filled and the back saturated, the critical condition for the back batter of the wall occurs. Slips may follow. Small slips that start on the crest, well away from the water line, usually reach a stage where they stabilise or balance themselves. They present no particularly urgent problem, for they can be repaired later by filling the slump-hollow with earth. The bulge formed by the slip near its lower end down the batter of the wall can be left, as it acts as a wedge against further movement. Such smaller slips are repaired from above and not by pushing up from below with a 'dozer. The opposite extreme of a back-of-the-wall slip is the major one and the worst possible. Here the slips start from under water inside the dam, moving a section of earth right across the crest of the wall and bulging out lower down the back of the wall. Pushing earth onto the wall crest and down into the water is dangerous. The earth falling in the water stands up very straight, and when the water level drops, or even before, the concentrated weight of the new earth on the saturated wall below water will invariably start a slip of the material inside the dam. Earth that is pushed into the water must be spread out under water. In the large back-of-the-wall slip or slump water can immediately flow around one or both edges of the break, and, because the material in the path of the water is loose and fractured from the movement of the slip, the earth. moves rapidly down the wall. This most serious of all slips is treated by immediately opening the lockpipe and then breaching the feeder drain. The lockpipe flow is continued until the water level drops below the break. When the wall and slip area dries out, the slip may stabilise itself and not move any further. If the slip is never as soft again as it was when the slip occurred, it will usually remain stable. Repairs are done in dry weather by filling the cavity of the slip area back to the original wall profile. The bulge area of the slip, if it is not too unsightly, could be left undisturbed.

   While the critical stage in the back batter occurs in heavy rain with the dam filled, the same stage for the front batter occurs when the dam has been filled and is later emptied. The inside batter of the wall can slump or slip even before the dam is quite empty. Usually, inside slumping is a slower process. The first signs may show up as a series of little slumps and cracks over a sizeable area about as much as one-third of the whole inside batter of the wall above water. The little cliffs formed by the movement may be only an inch or two high, but in the following week or two can become cliffs three or more feet high.

   The prospect of serious inside slumping is greatly increased if, during the construction of the dam, material has been excavated from under the inside toe of the wall, as mentioned in the discussion on dam design in Chapter XVIII.

   The complete repair of a serious inside slump can only be made satisfactorily when the dam is empty and the slump area has dried out. The earth under the slump is usually very wet, even jellied and unstable, though it may appear dry on the surface. A pole should be pushed into the deeper earth through the cracks to test the material below. If it is wet and sloppy it will need further time to dry out properly. All the water of the dam should be released as irrigation water, and, if there is a pond below lockpipe level, this will need to be pumped out. A bulldozer will be required if the slump is large and serious. The wet sloppy material on the bottom of the dam should be pushed back and away from the inside toe of the wall and an area of more stable material selected in the bottom of the dam that is suitable for pushing up into the slump area of the wall. The bulldozer operator should test the slump area by backing the tractor from the solid bottom of the dam straight up the wall and moving slowly and carefully while watching for excess sinking of the tracks. It is very easy to bog a bulldozer in the slump, so the effect of the tracks should be inspected to see if the slump is stable enough for safety. If there is any doubt as to the stability of the wall supporting the weight of the bulldozer, a load of drier earth should be pushed forward up the wall and the blade raised gradually to spill earth under the tracks as the machine travels. If the slump area is at all boggy, constant care is necessary to see that there is always new earth being dropped ahead of the tracks of the bulldozer. The path of travel up the wall is constantly changed slightly, so that a uniform compaction of all the unstable material of the slump is produced.

   The repair of an inside slump should produce the planned profile of the wall with a finished height of the crest about 5% higher to allow for settlement.

   An inside slump may be repaired by smaller equipment or by hand work and working from the crest of the wall. This type of repair aims at using the bulge of the slump as a stabiliser. The slump is allowed to dry out as much as possible and the cracks and cliffs of the slump filled with new earth. The bulge is not cut down, but is left as it is, only being covered with earth where it is cracked. The bulge at the bottom of the slump acts against further movement. Such a repair may slump a little further after the dam has been filled and then emptied again. However, the slump will be much smaller and may soon become stable. It is important in hand repair work to allow the bulge to act against further slumping, and not to place any more earth than is necessary to reform the wall profile by filling the hollow of the slump.

   Throughout these discussions the bulldozer has been considered as the type of equipment most suitable for farm dam construction. Although I believe this to be so, contrary opinions are often expressed which favour the use of large scoops or scrapers. The reasons given in preference for the latter machines are that this equipment imparts better compaction to the wall material. In my own experience of building dams of the type and size of the useful farm dam with both scoops and bulldozers, I believe the only worthwhile advantage of one over the other depends on the length of haul. Where the earth has to be transported distances beyond 200 feet the advantage lies with the scoop and continues so as this distance is increased. As for the scoop providing the best compaction, there is little significance to this view. What generally happens is that the use of the scoop allows the earth to be spread in thin layers (also a feature of good bulldozer operation), which aids uniformity of texture in the material; but the path of travel, being longitudinal along the wall, provides compaction from the tracks and scoop wheels only in this direction. In the early stages of building a dam wall the scoop and tractor equipment is working on a wide and rising wall crest and compaction can be made uniform by regulating the various paths of travel of the equipment. As the wall approaches its full height it becomes impossible to travel the material uniformly, and in the final stages, with the wall crest 10 to 14 feet wide, only narrow bands of earth across the long section of the wall are affected by the wheels. Hard compacted earth then is adjacent to loose fill and the contact between the different textured material forms larger longitudinal cracks. Later, the wall will still be satisfactory, but not because of the type of equipment or for the reasons given in its favour.

   I have seen the loosest walls settle into good impervious walls and I believe that in the relatively small earth wall what is called proper compaction is necessary only on comparatively rare occasions. However, this aspect of farm dam building will be further tested over a range of different materials, as we propose shortly to build experimental farm dam walls with loose earth. "thrown" into the site. In these experiments no equipment will travel over the earth wall except possibly to trim the wall to shape after all the earth has been placed. We have now a special machine for the tests. Another aim of this new series of experiments is to devise ways of building large farm dams efficiently with smaller equipment. Even now smaller equipment can build sizeable farm dams by using the designs of this book and the lockpipe system to prevent constant flooding by providing good drainage for the much longer period of construction. Our experiments with the new piece of small equipment which will operate in conjunction with on-the-farm machines, should, if successful, extend the scope and expedite the work of building farm dams.

   With the coming of the large bulldozer many men are unaware or have. forgotten what they can do without the bulldozer. While the bulldozer is a wonderful tool, farmers, by becoming too bulldozer conscious, tend to let a job wait, when the smaller equipment already available on the farm or even their own hand work can be used cheaper and more profitably. There is too great a tendency for farm workers to let a job or an earth works repair await the arrival of a bulldozer, a job which, considering the high hourly cost of a £10,000 to £15,000 bulldozer, they can do cheaper without it.

   Wave erosion in the larger farm dam is yet another problem of maintenance; that is, if it has not been allowed for as a factor of design. Wave erosion does not operate evenly along the length of a wall of a dam filled with water, but tends to concentrate its larger waves on smaller sections of the wall. Wave action can destroy a wall on occasions by this larger wave concentration cutting right through the crest, or, with the waves breaking across the wall, causing a flow of water over it. Wind and heavy rainfall together would accentuate the danger. Then the continuous flow of water across the wall would have the same effects as any other destructive overtopping. The dam could now be lost unless effective action is taken. However, preventative action is simple and requires only the opening of the outlet valve fully and the blocking or breaking of the water conservation drain to prevent further inflow into the dam. With farm dams of the designs of this book this action would quickly reduce the water level below the breach, when it can be repaired later. A temporary repair in such a breach, once the earth is brought to the full height of the wall, will hold back all the water of the dam even if the repaired section of the wall is only one or two feet wide at the top.

   However, the best cure for destruction from wave erosion lies in preventing it or providing effective counter measures in the original design and construction of the dam. Wave erosion as a serious and destructive force in the larger farm storages is reasonably predictable and can be duly considered and planned for in the design of the dam.

   The accepted engineering counter to wave erosion is the provision of a riprap (heavy loose stone or rock) covering over the inside batter of the wall. However, the provision of riprap for a farm dam is usually an expensive item and more often than not it would cost more than the earth wall itself. This cost would no doubt be fully justifiable if no less expensive counter to wave erosion could be obtained.

   In our own case, in designing the larger farm irrigation dams, we counteract wave erosion by quite simple and effective measures. There are three main considerations. One, the prevailing winds in relation to the shape and lie of the dam will indicate the portion of the wall where the highest waves will strike. The wave crests do not usually advance as a straight line or as a wave line with all parts at the same height, but rather as a curved line, curved according to the drag on the ends of the wave caused by the shallow bottom near the shores of the dam in relation to the wind direction. Winds from various points over a 90-degree are of the compass, will drive the high crest of the waves over a relatively short length of the wall and rarely more than one-third of its total length. It is possible to estimate approximately the length of wall that will be affected by wave erosion and to provide for it in design. In the earth wall farm dam the material that is most readily available and cheapest to handle is earth, not rock; therefore, earth in suitable quantities at the right place becomes the obvious counter to wave erosion. If a large farm dam built without considering wave action in the design, is being damaged by wave erosion, no doubt the farmer would repair the damage, which we may assume is reducing the effective width of the wall, by placing further earth at the reduced section. The cheapest means of providing extra earth is surely placing it there during the building of the wall. The design should therefore include an allowance of extra earth in the wall as an increased wall section to counter wave erosion. While there is a great deal known about the action of waves, there does not seem to have been any experiments conducted to determine effective counter measures to wave erosion of farm dams by the use of earth.

   We built a farm dam which had a water surface of 350 yards extending out in front of the wall, and we made an arbitrary adjustment to our design to counter wave erosion. The width and height of the wall crest was increased by one foot along that section of the wall where, from the shape and lie of the dam in relation to winds, we considered wave erosion would be most damaging. The wall of the dam was treated by covering the crest and rear batter, but not the waterside batter, with soil and planting the ordinary pasture mixture. Wind shortly after the dam filled caused waves, which soon cut into the wall and formed perpendicular cliffs of earth with a long, almost flat, terrace on and just below the water line. Meanwhile, the grass was growing on the crest and back batter of the wall. Later, the level of the dam was reduced by about three feet in irrigating, and waves again formed the perpendicular cliff and the nearly flat terrace below on the new water line. This time the wave erosion was moving primarily some of the earth of the first terrace formed by the earth eroded in the formation of the highest cliff. Again the water level was reduced by irrigating and a new line of cliff with its associated line of terrace was formed. By this time grass had started to cover the highest terrace and soon appeared on the next lower terrace.

   Many people had seen the experiments, and although its purpose had been explained, some sent special grasses to us to help stabilise the wall. However, we made no attempt to influence the course of the development of the cliffs and terraces, and the use of the water and the consequent lowering of the water level was dictated only by our irrigation requirements.

   It was quite obvious to us from this experiment that we could, by releasing water via the lockpipe, determine the course of the development of the cliffs and terraces. However, a definite stability formed naturally against the damaging action of the waves, and as seen in the grassing of the terraces. It is now apparent that, even without attention, wave erosion would have to act for a very long time before it could affect the efficiency of the wall. There would be a gradual movement of earth from the crest of the wall down towards the bottom of the dam, which would occur each time the water of the dam was reduced from the filled to the nearly empty stage.

   A recommendation in the design of dams that are of a size where wind erosion is likely to be a factor is to design the wall with an extra foot of height and an extra foot of width for the one-third distance of the wall where the waves are most likely to affect the wall. Thereafter, cover over, as, with all dams, the crest and both the front and rear batters with a couple of inches of soil and sow down with pasture grass seeds with fertilisers. The first cliff should be allowed to form, but when the level of the water is next reduced, the cliff is smoothed off by hand work, allowing the earth to fall on the terrace below, where more fertiliser and grass seed are then scattered. When further earth is required to maintain a good top section of wall, earth would be taken from the extra foot of wall height provided in the design for this purpose. Only the top cliff and terrace will need to be so treated.

   These suggestions should be effective in dams with a water surface 400 yards in front of the wall; and the allowance of extra width and height could be doubled for those very rare occasions of a farm dam having half a mile of water surface extending out in front of the wall.

   In bringing these discussions to a close, it is not to be assumed that the subject of suitable structures for the farm conservation of water has by any means been exhausted. Indeed, there are so many opportunities in farm dam construction it is a marvel that we have not taken more advantage of them. There are other interesting types and kinds of structures, for instance the log dams mentioned earlier, which will be suitable for particular occasions. There will no doubt be other problems of materials and sites and circumstances which are not touched on here. Yet it is hoped and expected that a study of the shapes of land in relation to agricultural pursuits generally and to the efficient conservation of water on rural properties in particular, together with the detailed design and construction methods of Chapter XVIII and this present Chapter, will assist the farmers and graziers in doing this work for themselves. I hope my experiences will lead to many farmers attacking these problems more confidently and that my mistakes and experiments will obviate mistakes in their own work. I hope also I have said enough to justify my earlier contention that the farm dam is a job worthy of as specialised attention for its own sake as that of the "big" dam.

   In agricultural areas where there are no existing farm dams, or farm dams have a reputation for failing to hold water, then I would suggest that the best procedure for the landman would be to construct his first dam high up on the property or at the keypoint of a valley where, I have found, the best dambuilding materials are likely to be discovered. When it becomes necessary to employ special anti-seepage techniques the method known as mechanical stabilisation could be investigated first. It is generally desirable that the design of a dam in which these methods are to be used should provide for flatter batters, so that all the area of the inside of the dam can be worked with suitable equipment. Mechanical stabilisation involves several cultivations of the inside of the dam with chisel implements to fine-up the material. Several such workings should be followed by harrowing and rolling. The fine materials tend to move with seepage water to seal the leaking areas. The solution of the problem of the first dam may quickly lead to a wide use of other sites and a wonderful development of valuable land. But before all this farm development can take place on a wide and national scale there is this further aspect to be considered. It is quite certain that if the work of providing all the farm dams which the country needs is not to be done largely by our landman himself as the chief supervisor and expert, then the job will not be done soon enough. When one realises that the farmers and graziers are the largest force of executives in the nation, employing a huge and capable army of helpers who work always closely with them, and that the land of these directors of our rural industries is the nation's greatest capital, then one may be confident that the great challenge of the Australian landscape--the need for more water--will have many acceptors; and, moreover, in accepting this individual challenge, no farmer or grazier needing help, whether from governmental or private sources, should ever have it denied him.