A synthetic unit sedimentgraph for ungaged watersheds

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Virginia Polytechnic Institute and State University


The concept of the unit sediment graph is important and useful in the study of non-point source pollutant transport, in the estimation of sediment yield and in the design of sediment basins. At the present time, a physically sound method of deriving unit sediment graphs for ungauged small watersheds is not available. Based on synthetic principles as well as linear and time-invariant principles, applied to the systems approach of hydrology, a synthetic model has been developed to derive the unit sediment graph and to generate the sediment graph for an ungauged watershed. The model is limited to the generation of single peak sediment graphs where the sediment particle sizes of interest range from 0.002 mm to 1.0 mm. Seven small watersheds located in the lower Potomac River Basin were selected for this study. For each watershed about 12 storm events were included in the study. Available hourly rainfall and streamflow data were collected and used for model calibration. Results of both"spatial" and"temporal" verification show that agreement between the synthetic and actual sediment graphs is fairly good.

A new rigorous definition regarding the unit sediment graph has been established. The study is based on a one-hour unit sediment graph which is defined as the direct sediment graph resulting from 1 unit of effective sediment yield of a storm of 1-hour duration generated uniformly over the basin at a uniform rate. Thus, the one-hour sediment graph of a storm for a specified watershed can be generated by convolving the one-hour unit sediment graph with the effective sediment erosion of one hour duration provided that the rainfall record and characteristics of that watershed are known.