3.13 Application of a Sediment Transport Program

Sediment transport programs are generally used to estimate the overall degradation or aggradation trends for a reach of stream. Major channel modification projects are prime candidates for these studies, with either qualitative or quantitative sediment studies being appropriate, depending on the magnitude of the sediment problem. The Harding Ditch sediment analysis (Dyhouse, 1982) is a representative case study of sediment transport modeling.

Harding Ditch is a man-made channel first constructed early in the 1900s to take runoff from hillside watersheds draining to the Mississippi floodplain east of St. Louis and transport the flow south to exit through a levee to the Mississippi. Urbanization of the hillside watersheds in the second half of the twentieth century resulted in significant deposition in portions of Harding Ditch. As the Harding Ditch area was just a small portion of an 86,000 ac (34,800 ha) urban floodplain under study by the USACE, it was selected as a test area to quantify the sediment problems. Twenty years of discharge data were developed with a hydrologic model of the two main watersheds tributary to Harding Ditch. These discharge data were further modified into a histogram representing a series of steady flow discharges for use in the USACE's HEC-6 sediment transport program. Sediment-inflow data were developed with HEC-6 to estimate the amount of material in transit for a full range of discharges from each watershed. Bed and bank material samples were collected along the 11.5 mi (18.5 km) of Harding Ditch for specification of the sediment bed material grain size distribution in HEC-6. Channel and overbank cross sections were surveyed and coded for HEC-6 and supplemented with additional interpolated cross sections. The model was calibrated to the limited actual data available.

Base conditions were established for comparison to other land use and sediment conditions. Water and sediment runoff from estimated land use conditions for the year 2020 were evaluated using HEC-6, with findings that the major effect on channel deposition will occur during the urban development phase. Following the urbanization process, the sediment transported to Harding Ditch and the subsequent deposition decreases significantly, primarily due to the increased amount of impervious area that limits land surface erosion and transport of the eroded materials. The effect of a Harding Ditch diversion around some state park lakes on the sediment regime was also modeled, finding that deposition in the downstream channel greatly increased now that the lakes were no longer a sediment sink. Dredging intervals were studied with HEC-6 to determine the frequency of channel dredging required to maintain the channel flood capacity. Intervals of 5, 10, and 20 years between dredging were simulated using HEC-6, with a finding that dredging every 10 years is necessary to maintain adequate channel capacity. A detailed description of the full study process, analysis, and findings is given in the reference.