EVALUATING THE EFFECT OF LAND-DEVELOPMENT ON SEDIMENT TRANSPORT USINGA PROBABILITY DENSITY-FUNCTION

An important problem in sedimentation analysis is the development of a channel section that preserves, as best as possible, the current sedi mentation regime even though the flood frequency tendencies have been altered due to land development within the catchment. In order to acco mplish this task, a methodology is needed that estimates sediment tran sport capacity for various channel configurations. Such a procedure is described which allows the computation of the total sediment transpor t capacity for each of several T-year return frequency runoff hydrogra phs. This information is used to obtain an approximate probability dis tribution for the total sediment transport capacity, and the mean and standard deviation of this distribution are computed. Comparing the re sults for the catchment in its present state with a future developed s tate, using a selection of new channel parameters, indicates how to im prove the channel to control changes in sedimentation due to developme nt. The analysis procedure provides a basis for estimating a new chann el configuration such that the new flow conditions retain, as best as possible, the existing condition sedimentation effects, and hence reta in the natural sediment supply and transport trends even though runoff flow rates have changed due to land development within the catchment. The results of Wilson Creek are typical of the several sites examined , see Table 3 below. The T = 2, T = 5, T = 25, and T = 100 year values for total sediment transport capacity, in kilotons, are 6.9, 39.4, 61 .3, and 96.7 with a mean of 17.1 and standard deviation of 19.3. After development with no change in the channel the respective values incre ase to: 17.9, 84.6, 128.1, and 258.0 with a mean of 39.1 and standard deviation of 44.3. A new channel can be constructed which will reduce these sediment transport capacity values, after development, to 5.2,41 .0,62.0, and 124.8 with a mean of 17.4 and standard deviation of 22.0.