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<br />Estimating Sediment Yield and Developing
<br />Sediment Storage Requirements for
<br />Reservoirs
<br />
<br />To assure the full effectiveness of a reservoir, storage
<br />capacity must be provided in it to offset the depletion of
<br />capacity due to sediment accumulation during its de-
<br />sign life. The sediment deposition in a reservoir is de-
<br />pendent on the sediment yield to the reservoir and the
<br />trap efficiency of the reservoir.
<br />
<br />Trap efficiency is the amount (in percent) of the sedi-
<br />ment delivered to a reservoir site that will remain in the
<br />reservoir. It is a function of detention storage time,
<br />character of the sediment, nature and character of in-
<br />flow and other factors. The general trap efficiency is
<br />usually estimated on the basis of the ratio of the capac-
<br />ity of the reservoir to the average annual inflow.
<br />
<br />The sediment yield to a reservoir is the product of the
<br />gross erosion and the sediment delivery ratio for the
<br />drainage area above the reservoir. The gross or total
<br />erosion in a drainage area of a reservoir is the summa-
<br />tion of all water erosion occurring in the drainage area.
<br />It includes sheet and rill erosion and channel-type ero-
<br />sion (gullies, valley trenches, streambank erosion,
<br />etc.). Procedures for determination of quantitative val-
<br />ues for each type of erosion are outlined in the refer-
<br />ences listed below. These procedures include (I) mea-
<br />sured sediment accumulation in existing reservoirs; (2)
<br />suspended sediment load measurements, and (3) soH-
<br />loss predictive equations. The sediment delivery ratio,
<br />the ratio of sediment yield to gross erosion, is estimated
<br />from a consideration of the various influencing factors,
<br />such as: (I) type of sediment sources; (2) magnitude and
<br />proximity of sediment sources; (3) transport system; (4)
<br />texture of eroded material; (5) depositional areas; and
<br />(5) watershed characteristics.
<br />
<br />References
<br />
<br />Brune, Gunnar M., Trap Efficiency of Rese11loirs,
<br />Trans. AGU, Vol. 34, No.3 pp. 407-418, June 1953.
<br />Gottschalk, L. C., Trap Efficiency of Small
<br />Floodwater-Retarding Structures, Conference Pre-
<br />print 147, ASCE Water Resources Engineering Con-
<br />ference, Mobile, Alabama, March 8-12,1965.
<br />U.S. Department of Agriculture, Soil Conse11lation
<br />Service, NationalEngineering Handbook, Sedimen-
<br />tation, Sec. 3, Chap. 4, 5, 6, and 10,1971.
<br />U.S. Department of Agriculture, Soil Conse11lation
<br />Se11lice, Technical Release No. 12, Procedure-
<br />Sediment Storage Requirements for Rese11loirs,
<br />January 1975.
<br />
<br />Urban Hydrology
<br />
<br />As population density and land values increase, the
<br />effects of uncontrolled runoff become an economic
<br />burden and a serious threat to the health and well-being
<br />of a community and its citizens. Planners must provide
<br />solutions to water problems caused by radical changes
<br />in land use. Estimating the magnitude and frequency of
<br />future floods makes planning and installation of con-
<br />servation measures possible.
<br />
<br />Urbanization of a watershed changes its response to
<br />precipitation. The most comon effects are reduced in-
<br />filtration and decreased travel time, which result in
<br />significantly higher peak rates of runoff. The volume of
<br />runoff is determined primarily by the amount ofprecipi-
<br />tation and by infiltration characteristics related to soil
<br />type, antecedent rainfall, type of vegetal cover, imper-
<br />vious surfaces, and surrace retention. Travel time is
<br />determined primarily by slope, flow length, depth of
<br />flow, and roughness of flow surraces.
<br />
<br />Management of runoff even from minor storms is
<br />rapidly becoming an engineering requirement of local
<br />and state governments. Counties and states are adopt-
<br />ing policies which limit the effects that changes in land
<br />use may impact on a stream. These policies cover such
<br />areas as (I) assisting in the planned management of
<br />water resources, including storm drainage, throughout
<br />the watershed; (2) promoting and encouraging the in-
<br />clusion offlood storage in all planned reservoirs; and (3)
<br />encouraging and assisting in planning for onsite reten-
<br />tion of runoff through the use of temporary storage
<br />structures and inftltration devices.
<br />
<br />To determine peak discharges for design of hydraulic
<br />structures or for delineation of the lOO-yearflood plain,
<br />see "Procedures for Determining Peak Flows in Col-
<br />orado" or other appropriate methods. The afore-
<br />mentioned publication is available for review at all Soil
<br />Conservation Service Field Offices.
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