These preliminary studies in Virginia indicate that the vinyl liners for ponds must be installed on smooth surface areas. On unstable areas where voids are likely to occur, special reinforcement methods must be worked out. Maintenance will be needed where wave action and ice cause problems at the water's edge. This calls for a protective cover of soil, sand, or stone over the liner, and this cover may need to be replaced periodically. Cost.-Cost of the vinyl liner is about 30 cents per square yard, or about $1,450 for enough material to cover a 1-acre pond. Material is available from many sources and should continue to be available at the above price. MEMBRANES FOR CUTOFF WALLS, INFLATABLE DAMS, AND RELATED CONTROL DEVICES Plastic film to control seepage Plastic film has been used as a curtain wall to control seepage from canals and through earth dams. Control can be effected in this way, particularly if the film can be carried deep enough to intercept a profile with low permeability. Even when this can't be done, the flow length is increased and some control is effected. Butyl sheeting or plastic film in dam construction When heavier soil materials are used in constructing the dam, they have a tendency to crack upon drying. In small earth reservoirs that dry up periodically, the use of butyl sheeting or plastic film buried in the dam structure does two things: (1) It makes possible the use of relatively porous materials, and (2) provides good insurance against breaching before cracks have time to seal. Although no tests have been conducted, it appears the membranes particularly the heavier gage butyl sheet-could well be used to advantage as a substitute for the core to obtain watertightness, and thereby effect great savings by removing the necessity of importing selected earth materials. Plastic and butyl dams Plastic dams have long been used advantageously as a substitute for canvas dams. The excellent durability of butyl makes checks and diversion structures of this material well adapted to structures of this type. A butyl diversion structure could well take the place of concrete structure. It would be inexpensive, removable and reusable, quick to install, completely watertight, and it can be connected to gated pipe or tubing. However, shoots of vigorous perennial weeds such as johnsongrass, nutgrass, and woody plants may rupture the lining. Chemical treatments to eliminate these weeds and the application of soil sterilant herbicides before installation of the linings has partially solved the problem. Water control on ricefields is also an important problem for the agricultural engineer. The earth levee has been the standard structure for maintaining uniform depth and control of flooding water. Studies were made on 10 ricefields in the area around Biggs, Calif., by the University of California. These fields ranged from 40 to 400 acres in area, the average being 181.2 acres. The area devoted to levees averaged 7.4 acres (3.8 percent) per field, the equivalent of 205 feet of 8-foot levees per acre. Such levees are costly to construct and to maintain. Serious weed growths develop on the levees and harvesting around the levees is so difficult that grain losses are high. Studies of plastic film lerees Preliminary studies on the use of plastic film levees have been conducted at Biggs, Calif., and Beaumont, Tex. It appears that they have these possible advantages: Require a minimum of land area. Will not support weed growth. Permit harvesting large areas because plastic levees can be removed before harvest. May eliminate or reduce the large levee-forming equipment. A few of the problems yet to be solved in this application include: Selection of a plastic formulation that can fully resist prolonged exposure to sun and temperature variations. Development of an improved method of supporting the film on stakes. Mechanization of both the placement and removal operations. Development of a levee with a wire, nylon, or fiberglas cord embedded in the top ridge that would aid in applying tension to hold up the film. Thinner, lower cost films might be utilized with this type of support. Cost. There is little doubt that the placement and pickup of these plastic levees can be fully mechanized. Based on 205 feet of levee per acre and careful estimates of material, stake, and labor costs, the university studies showed an annual installation and removal cost of the plastic levee as $14.39 per acre. Similar studies showed that the cost of constructing, maintaining, and removing conventional earth levees was $6.01 per acre. Savings. Savings in harvest time through removal of the levee was estimated to be one-half the harvest cost, or $4.34 per acre. Based on the area of land that is returned to rice production when earth levees are replaced by the plastic levee, the studies showed an additional $4 return from the land so returned to production. It was estimated that approximately two sacks of rice per acre may be gained if a plastic levee is used to replace the soil levee, because the weed seed population one eliminated by spraying or tillage may possibly be reduced by half. This contributes an additional $8 return per acre. Economic value. The university summarized the economics of the situation by comparing the cost of 4-mil black polyethylene film, stakes, and manual installation at $14.39 per acre with the approximate cost that could be spent on such a levee. This figure is the sum of the following items: Per acre production from land formerly occupied by earth levees - Per acre savings from weed damage.. Total Cost $4.00 4. 34 6. 01 8.00 22.35 Hence $22.35 less $14.39 results in a profit of $7.96, or approximately $8 per acre, or about 8-percent increase in gross income. TUBING Relatively rigid plastic, aluminum, asbestos cement, bituminous, or steel tubing has been used successfully for a number of years. However, continuous search is being made for new materials to fill the need for durable and flexible tubing for irrigation purposes. Plastic water pipe The largest single farm use of polyethylene resins has been in the form of plastic water pipe. This pipe, when less than 2 inches in diameter, is shipped in coils. The larger diameters are shipped in straight pieces. The smaller diameters can be installed with a pipe layer attached to a conventional tractor. Many miles of plastic pipe have been used for delivering water to plantbeds, livestock tanks, and small irrigated areas that are a considerable distance from a water supply. Some high-density resin is now being used in this pipe application. Flexible tubing "Lay-flat" or low-pressure flexible tubing, another type of plastic pipe or tubing, is also gaining in demand. Made largely of black polyethylene, it is available in diameters from 2 to 18 inches. Film thickness varies from 8 to 20 mils, depending on size and operating heads. These tubes may either be continuous for transporting water, or may be equipped with grommets or plastic sleeves spaced at intervals for discharging water into individual furrows. Large-diameter flexible tubes Studies are underway at several locations on the use of largediameter (up to 36 inch) flexible tubes (fig. 10). These tubes are constructed from two or more layers of plastic between which is sandwiched a reinforcing fabric of nylon, Fiberglas, or other high tensile strength fibers. Polyethylene, vinyl, butyl, and other materials are being tested for this use. In these large diameters a very effective low head, high-volume water transmission tube can be effected. Note the advantages: no seepage loss, no evaporation loss, no loss to phreatophytes, excellent hydraulic characteristics, and a minimum of earth moving to provide a base. Some of the disadvantages are restricted heads, and liability to puncture or damage by rocks, bullets, or animals. Some of the early problems of separation and blisters forming between the layers seem to have been solved. Costs A full evaluation of cost and hydraulic characteristics of largediameter lay-flat tubes is being developed by the ARS at Utah State University. Preliminary results indicate that with further development these tubes will become an important item in irrigation water transmission systems. PIPE Plastic pipe of large diameters is becoming more and more common for irrigation pipelines. The use of concrete continues and the developments in this field consist of improved methods of installation. Cast-in-place concrete pipe has proved successful at a number of locations. The procedures are patented and result in a continuous pipe with no joints (No-joint and Fullerform). Outlets, elbows, and other fittings can be installed wherever needed. FIGURE 10.-Thirty-six-inch-diameter lay-flat tubing used to convey water over porous areas without loss by seepage. PHREATOPHYTES AND AQUATIC WEED CONTROL Solution of the phreatophyte problem necessitates elimination or control of heavy water-using phreatophytes. Their elimination or control with present methods is extremely difficult and expensive. Chemical control has not been explored adequately. For instance, no one-spray treatment with the common brush killer chemicals will kill saltcedar. Chemical methods for aquatic and bank weeds Since 1950, chemical methods have largely replaced mechanical methods in controlling aquatic and bank weeds in and along irrigation and drainage channels and farm ponds. The chemical methods usually are more effective, more convenient, less time consuming, and often less expensive. |