RCA III
Sedimentation in Irrigation Water Bodies, Reservoirs, Canals, and Ditches

Working Paper No. 5

Frank Reckendorf
Natural Resources Conservation Service

July 1995

In 1993 there were 59,077,159 irrigated acres in the United States, of which 40.6 percent were sprinkler irrigated, 55 percent were surface irrigated, and the balance were trickle irrigated. There is some irrigated acreage in every state and in Puerto Rico, but most of the irrigated acreage is in the 17 western states. California was the leading state in 1993, with 1,589,351 acres sprinkler irrigated and 6,146,103 acres irrigated by surface gravity systems (Irrigation Association 1993).

Many sources of sedimentation activities are associated with irrigated agriculture. For surface water systems with direct diversions from watercourses, there can be sedimentation in the watercourse downstream from irrigation stream diversions. This occurs because of the reduced discharge to transport the sediment load, especially the stream's bedload. Along the diversion canal there can be both channel bottom and bank erosion, causing associated offsite sedimentation, or there can be deposition in the canal because of decreased stream power to transport the sediment load.

The next source of erosion and associated sedimentation is erosion along the irrigation canals, including that which occurs at the turnout. Downstream from the lateral there is irrigation-induced erosion (see SCS 1993 for erosion definitions) from furrow erosion, especially in the upper one-third of the irrigation run. Frequently a portion of this eroded sediment is deposited in the lower one-third of the irrigation run and in the tailwater recovery area. From the tailwater area there may be additional erosion and associated sedimentation along the return flow to the watercourse or canal. This is frequently expressed as areas of classic gully (SCS 1993) erosion.

For sprinkler systems, particularly center-pivot sprinkler systems, there may be sheet and rill erosion and associated sedimentation, as well as ephemeral gullies (SCS 1993). The sediment in the concentrated flow of ephemeral gullies can be partly deposited in fans outside the irrigation circles, with the remainder moving as overland flow that may reach a watercourse. The degree of sheet and rill and ephemeral gully erosion and associated sedimentation in sprinkler-irrigated areas may be caused in part by pre-existing moisture conditions.

The relative importance of erosion in irrigated areas to offsite sedimentation is reflected in the recent paper by Koluvek et al. (1993), which focused primarily on flooding of small channels called furrows or corrugations. The authors state that a number of studies have reported sediment yield from furrow-irrigated fields to exceed 9 tons per acre per year, with some fields exceeding 45 tons per acre per year. Under center-pivot sprinkler irrigation they reported sediment yields as high as 15 tons per acre per year. In addition, they reported sediment yields as high as 2 tons per acre per year from erosion along the tracks of irrigation equipment.

A recent paper reported on high-magnitude, low-frequency runoff events during summer thunderstorms as a cause of sediment yield from irrigated areas (Shepherd 1994). This paper states that significant erosion is common as thunderstorms move across sprinkler-irrigated cropland that contains low-order (1(2) channels of the drainage net. In addition, the paper notes examples of the thunderstorm effects along the Front Range of Colorado and on uplands in the South Platte and Arkansas drainages.

A 1993 evaluation (Atwood 1994) of 1,819 reservoirs and lakes, with 1,625 usable records and 3,940 valid sediment survey records, showed an average storage loss of 5 per cent from sediment depletion. However, 40 percent of these reservoirs were projected to be half full by 1993. In addition, there is a substantial geographic variation in loss of reservoir storage from sedimentation, so the low percentage of storage loss may be misleading. For example, in one study of 42 reservoirs in Iowa, Nebraska, and Missouri, 18 reservoirs lost 25 percent of their storage capacity in 11 years or less (Clark et al. 1985). About 60 percent of the reservoirs in the data base (Atwood 1994) have drainage areas of 5 square miles or less.

Of the 1,625 reservoirs, 111 had irrigation as a purpose. In these reservoirs there is a storage depletion of 8.5 percent from sedimentation, but this loss is based on the most recent survey covering the period 1932 to 1989 (Atwood 1994). If just the records with sediment surveys more recent than 1978 are examined, the average storage depletion for irrigation reservoirs is 15 percent, with a range of 1 to 42 percent loss. This higher rate of sedimentation can be attributed to more intensive agriculture and to population growth with associated land uses. In addition, the loss of irrigation storage from sedimentation varies significantly with drainage area and reservoir capacity. Additional data on irrigation reservoir storage loss are currently being collected but were not available for this report.

As previously indicated, sedimentation problems are also encountered in canals and ditches. There are about 110,000 miles of irrigation canals in the United States (U.S. Department of Commerce, 1978). There is no comprehensive study of capacity losses in irrigation conveyances from sedimentation. Additional data are being collected on the loss of capacity in irrigation ditches and canals but are not yet available. However, data from some specific studies show that sedimentation effects in ditches and canals can be substantial.

For example, a study of the I Canal in San Joaquin Valley in California noted frequent sediment problems since 1968 (Arthur and Cederquist 1976). Removal of sediment and filter-feeding organisms averaged 24,000 cubic yards per year prior to 1966 and 20,000 cubic yards per year from 1968 to 1976. The study reported that, because of sediment and organic accumulation in the canal, every 2 years the canal was dewatered for 8 to 10 weeks to remove sediment. This caused a delivery shortfall of 400,000 to 450,000 acre-feet of water. Since irrigators cannot, in general, tolerate restricted capacity caused by sediment deposition in their canals, they clean out their systems accordingly.

In an Idaho study (Carter 1993) the annual sediment loss from an 161,500-acre furrow-irrigated area was 1.78 tons per acre. This sediment was deposited in drains and canals, requiring removal. For this tract approximately 287,000 tons were annually removed from relatively flat sloping canals (Koluvek 1993).

In central Washington, two typical irrigation districts had deposition along 8,040 and 9,407 feet of canal or lateral, with sediment deposition of 7,695 and 9,910 tons respectively, at an annual average removal cost of $9.26 per ton. An older evaluation by Carlile (1972) reported cost of $50,000 per year to remove sediment from canals and drains in the Yakima-Tieton irrigation district. He also stated that farmers along the Yakima River were reluctant to convert from furrow to sprinkler or trickle irrigation because of the heavy sediment loads in the canal from irrigation return flows.

The specific extent to which the sediment from various forms of erosion reaches offsite areas, such as canals, drainage ditches, streams, or reservoirs, depends on a number of physical as well as management factors. From a physical point of view, the fall velocity of the sediment particle is the prime variable that determines the interaction between the sediment and the water in the conveyance (stream, canal, or ditch) or impoundment. The size of the bed material in the conveyance, as measured by sieve size or fall diameter (Malone, Richardson, and Simon 1975), is the primary factor determining fall velocity. It is generally believed that the settling velocity of a particle decreases as the sediment concentration increases.

Relationships have been developed between particle diameter and fall velocity for several shape factors of naturally rounded quartz particles. The information on the settling velocity of the particle size being considered, along with depth and width of flow in the conveyance or basin, can be used together with the velocity of flow to determine the length of the conveyance or settling basin needed to deposit a specific particle.

In certain instances, small particle sizes such as silts and clay may be beneficial by sealing conveyances such as canals and thus reducing seepage in the coarser materials of the conveyance. The fine sediment load that intrudes into the conveyance bottom and sides may also provide some increase in bank stability. In addition, the application of irrigation water high in fine sediment load to coarser soils may increase the soils' water-holding capacity and fertility.

The removal of sediment in impoundments may on occasion have a detrimental effect on the fine sediments that seal coarse-textured canals. The clean water releases from the structure have the potential to scour the bed and sides of conveyances to such an extent that past benefits from sealing of canals with fines are removed.

Another aspect of sedimentation in irrigation conveyances is the effect of introduced polymers. In the last 4 years, tests have been run using polyacrylamide (PAM) as a flocculating agent to deposit sediment in furrows, ditches, canals, and laterals. Tests have shown reductions of 85 to 95 percent in sediment yield offsite during the first irrigation as the norm, using PAM concentration of 10 ppm or less in irrigation water (SCS, WNTC 1994). A recent study in California indicated that at current prices polyacrylamide application could range from $15 to $20 (McCutchan et al. 1993).

This report has presented the results of available studies of the loss of capacity of irriga-tion water conveyances and of water bodies (reservoirs and lakes) that have an irrigation purpose. An ongoing Irrigation-Induced Erosion study for the third RCA Appraisal will use a process that identifies areas by levels of irrigation-induced erosion. The study will collect data to enable projection of the amount of sediment leaving the field. The sediment deposition will be partitioned judgmentally by the data gatherer into the percentages of sediment deposited on-farm and off-farm. Off-farm sediment deposition will be further partitioned into: (1) other agricultural land; (2) irrigation canals, laterals and drains, etc.; (3) roads and culverts; (4) water bodies (reservoirs and lakes); and (5) wetlands.

The data sets will be kept on an 8-digit Hydrologic Unit Area (HUA) basis with data collected on selective resource settings for each HUA, to determine the predominant resource setting in respect to soil K, T, and S and to crops grown.

Data sets from various studies currently being developed will later be used as follow-up to this study, to project on-farm and off-farm effects for the categories discussed.

Bibliography

Arthur, J. and N.W. Cederquist. 1976. Sediment transport studies in the I Canal and the California Aqueduct. Proceedings of the Third Federal Interagency Sedimentation Conference, pp. 4-88 to 4-99.

Atwood, J. 1994. RCA reservoir sediment data reports 1-5. SCS, Washington, D.C.

Carlile, B.L. 1972. Sediment control in Yakima Valley. In Proceedings of the National Conference on Managing Irrigated Agriculture to Improve Water Quality, pp. 77-82. Fort Collins, Colorado: Colorado State University.

Carter, D.L., C.E. Broadway, and K.K. Tanji. 1993. Controlling erosion and sediment loss from furrow-irrigated cropland. Journal of Irrigation and Drainage Engineering 119(6):975-988.

Clark, E.H., II, J.A. Havercamp, and W. Chapman. 1985. Eroding Soils: The Off-Farm Impacts. Washington, D.C.: The Conservation Foundation.

Dendry, F. 1968. Sedimentation in the nation's reservoirs. Journal of Soil and Water Conservation 23:137ff.

Irrigation Association. 1994. 1993 irrigation survey. Irrigation Journal 44:25-41.

Koluvek, P.C., K.K. Tanji, and T.J. Trout. 1993. Overvieew of soil erosion from irrigation. Journal of Irrigation and Drainage Engineering 119:929-946.

Malone, A.M., E.V. Richardson, and D.B. Simons. 1975. Exclusion and ejection of sediment from canals. Fort Collins, Colorado: Colorado State University.

McCutchan, H., P. Osterli, and J. Letey. 1993. Polymers check furrow erosion, help river life. California Agriculture 47:10-11.

Shepherd, R.G. 1994. Some fluvial systems' impacts of irrigation-induced erosion. Proceedings of the Summer Symposium of the American Water Resources Association (Effects of Human-Induced Changes on Hydrologic Systems). In press.

Soil Conservation Service. 1993 (1990 with !993 revision). Conservation Management Systems Formulation: A User's Guide for Implementing the Conservation Practice Physical Effects Process and Concepts. Portland, Oregon: USDA, SCS.

Soil Conservation Service, West National Technical Center. 1994. Irrigation erosion control (Polyacrylamide), Draft Interim Conservation Practices Standard. Portland, Oregon: USDA, SCS, WNTC.

U.S. Department of Commerce, Bureau of the Census. 1978. Census of Agriculture, volume 4, p. 266. Washington, D.C.: U.S. Government Printing Office.

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