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United States Department of National Agriculture Soil Engineering Conservation Service Handbook Section 3 Sedimentation Chapter 6 Sediment Sources, Yields, and Delivery Ratios Contents Page Introduction .............................................................................. 6-1 L' General ................................................................................6-1 Interrelationship of processes .............................................................. 6-1 Sedimentsources ..........................................................................6-2 General ................................................................................6-2 Determining the relative importance of various sources ......................................... 6-2 Mapsandaerialphotographs ............................................................. 6-2 Distinctiveminerals .................................................................... 6-2 Colluviation ...........................................................................6-2 Procedure ............................................................................. 6-2 Sediment yield ............................................................................ 6-3 General ................................................................................6-3 Climaticfactors .........................................................................6-3 Watershedfactors ........................................................................ 6-3 Size .................................................................................. 6-3 Topography ...........................................................................6-4 Channeldensity ....................................................................... 6-4 Soilandcoverconditions ................................................................. 6-4 Landuse .............................................................................. 6-4 Methods of determination ................................................................. 6-5 Gross erosion and the sediment delivery ratio ................................................ 6-5 Measuredsedimentaccumulation ......................................................... 6-5 Suspended load records .................................................................. 6-6 Predictiveequations .................................................................... 6-6 Informationsources ...................................................................... 6-6 Sediment delivery ratio ..................................................................... 6-8 L1Influencing factors ...................................................................... 6-8 Sediment source ....................................................................... 6-8 Proximity of sediment sources ............................................................ 6-8 Transportsystem .............................................................i. ........ 6-8 Texture of eroded material ............................................................... 6-9 Depositionalarea ...................................................................... 6-9 Watershed characteristics ............................................................... 6-9 Procedures for estimating the sediment delivery ratio ........................................... 6-9 Size of drainage area .................................................................... 6-10 Relief-lengthratio ......................................................................6-10 Source-texture analysis .................................................................. 6-10 Source deposition ....................................................................... 6-11 Summary ................................................................................ 6-12 References ...............................................................................6-13 Figures Page 6-1. Asedimentratingcurve ............................................. ................ 6-7 6-2. Relationship between drainage area and sediment delivery ratio ............ ................ 6-11 Tables Page 6-1. Sediment yield from various sources ..................................................... 6-3 6-2. Sediment source and the delivery ratio ................................................... 6-11 Chapter 6 Sediment Sources, Yields, and Delivery Ratios Introduction General watershed and on the transport of eroded material out of the watershed. Only part of the material C Sediment yield depends on the erosion processes eroded from uplant areas in a watershed is carried at the sediment source and on the efficiency of the out of the watershed. Variation in the proportion of system, that transports the sediment to the point of the eroded material deposited as colluvium at the measurement. The sediment yield usually differs at base of slopes and in swales, as alluvium on flood different locations in a stream system. plains and in channels, and as lacustrine deposits Many interrelated factors affect sediment yield. in natural or artificial lakes usually results in Knowledge of each of these factors is important in: variation in the yield rate for different parts of a 1. Evaluating downstream sediment damages. watershed. 2. Determining the location and extent of sedi- Field determination of sediment yield may require ment sources so that effective controls can be plan- long-term sampling and measuring procedures. A ned and installed. short-term procedure is to extrapolate (and adjust as 3. Recognizing the relative contribu Eon of the appropriate) known sediment yield from measured various sources to present and future sedi lent similar watershed in the same physiographic yield. section. 4. Determining the sediment storage require- ment for designing proposed structural works of improvement. This chapter presents several procedures for deter- mining sediment sources, sediment yields, and delivery ratios. Interrelationship of Processes L Sediment yield depends on gross erosion in the Sediment Sources General sediment deposits helps in identifying and evaluating sediment sources. Because a watershed Sources of sediment must be delineated to plan an may contain contrasting rock formations, the adequate program for reducing downstream sedi- distinctive erosion products of these rock formations ment yield. Sediment sources include agricultural may clearly indicate the location of the sediment land, range and forest land, road banks and ditches, sources. These distinctive minerals are quartz, stream channels and banks, flood plains, spoil micas, iron oxide, feldspar, chert, and calcite; some banks, and gullies. In planning a program to reduce can be easily identified and traced to their original sediment yield, the relative importance of the source. Other watersheds may lack geologic variety various sources and the methods for treating them and hence may not provide such specific clues to must be determined before the physical and the location of significant erosion. economic feasibility of the program can be deter- mined. Sediment derived from sheet erosion can Colluviation usually be reduced by land treatment measures, Another aid in evaluating the sediment sources is whereas that derived from channel-type erosion the extent and location of colluvial deposition. If a usually requires structural works. coarse-grained material such as sand or gravel is A sediment source study is made to determine: (1) being actively eroded, it may produce large volumes the origin of the sediment; (2) the rate of erosion of sediment, little of which moves very far from the from each source; (3) the proportion of the sediment site of erosion. Substantial deposits may form at the derived from each source; (4) for program planning foot of the first slope. Fans and valley deposits may or structure design, the kinds of treatment that form in small tributary valleys or in the next lower should be recommended for reducing sediment valleys downstream. yield; and (5) the relative effect that reducing ero- sion from the various sources will have on reducing Procedure sediment yield and damage. Any procedure requires study of the various types The relative importance of the sediment source of erosion apparently producing sediment. Sorting may differ at different locations in a watershed. the types of erosion according to the treatments J Therefore, the treatment measures may also vary, that could be recommended to reduce erosion and depending on the location in the watershed where a thus sediment yield will make the effectiveness of reduction in sediment yield is desired. the various treatments much easier to evaluate. Several procedures can be used to determine the relative importance of the various sediment sources. Determining the Relative Importance of A recommended procedure is to gather information Various Sources on that part of the sediment yield which can be at- tributed to each of the various sources. Erosion and The following items must be considered in the the sediment delivery ratio should be estimated early stages of any study made to determine the above each reach or other point of interest for the location, extent, and relative importance of the sedi- drainage area. ment sources. The sediment yield at the point of interest must be allocated to the recognized sources. Analyzing Maps and Aerial Photographs the available data, studying the watershed, and Careful review of aerial photographs often reveals considering the sediment delivery ratios
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