Effect of Surface Inlet Type On Suspended Sediment Transported Through a Subsurface Drain Tile System.

Open surface inlets in subsurface drainage systems provide a direct conduit for sediment and nutrient export from cropped fields to surface waters. This study was conducted at the large-field scale to compare sediment transport of systems with open inlets prior to, and after, installation of buried inlets. Subsurface drain flow and suspended sediment concentration (SSC) were measure for two fields (67 and 104 ha) for three years prior to and two years after open inlets were converted to buried inlets in which perforated tile was placed in a trench of gravel (i.e., French drains). Results showed that when open inlets were present, SSC averaged 310 mg/L for the snowmelt period (February through April) and 170 mg/L during the non-snowmelt period (May through January). Maximum concentrations averaged 1450 mg/L during snowmelt and 5980 during non-snowmelt. In the first year after inlet conversion, SSC averaged 428 mg/L (maximum 4770 mg/L) during snowmelt and 34 mg/L (maximum 590 mg/L) during non-snowmelt. Disturbance of the soil during the inlet construction may have resulted in no reduction in average or maximum SSC during snowmelt in the first year. After the snowmelt period, maximum and average SSC were only 10% and 20%, respectively, of the average of the three years prior to inlet conversion. Conversion of open inlets to buried inlets reduces SSC at the large-field scale.