Sediment Transport Capacity Under Different Subsurface Hydrologic Conditions.

Sediment transport capacity (Tc) is a conceptual term used in soil erosion modeling to identify whether erosion or deposition process is dominating.  Despite the wide use of this modeling concept, there are little experimental results to quantify Tc, particularly over an erodible surface under different subsurface hydrologic conditions. A box-flume system, consisting of a 1.8m long sediment source box and a 5m long test flume are designed to quantify Tc under simulated rainfall condition. A Miami silt loam from near Lafayette, Indiana was prepared under four subsurface hydrologic conditions: pre-wetted then drained, pre-wetted and saturated, pre-wetted and with an exfiltration or seepage rate of 60 g/m/min , and pre-wetted and with seepage rate 120 g/m/min. The source box allows a variable sediment feed, ranging from 0 to 150 g/min to the study flume.  Runoff and flow velocity were measured for the test flume.  By varying the sediment feed, the test box shifts from a net erosion regime at low sediment feed rates to a net deposition at high sediment feed rates.  If we define Tc as the equivalent sediment discharge (or feed) when neither erosion nor deposition occurs in the test flume, we found that the value of Tc increases as the subsurface hydrology was varied from free drainage to saturation to low and then high seepage rates.  For the same shift in subsurface hydrology from drainage to seepage, we observe an increased soil erosion and a decreased sediment deposition in the test flume.   The subsurface hydrologic condition not only affects soil erodibility and the sediment detachment process, it also affects sediment deposition and sediment transport processes.  These results demonstrate how an extrinsic condition, such as the subsurface moisture gradient, can affect soil erosion and sediment deposition.