Submitted to: Federal Interagency Sedimentation Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: December 15, 1995
Publication Date: N/A
Interpretive Summary: Drop pipe inlets, sometimes called riser pipes, are field drains formed by small dams with "L" shaped pipes. These structures are frequently used to drain water from fields into creeks that have very steep banks. These pipes prevent soil erosion that would occur if water was allowed to run freely over the stream bank. Many of these drop pipes have permanent or semi-permanent pools of water behind their dams. This study showed that pools formed by drop pipe inlets are good for water quality. Sediments and nutrients that would be bad for stream water quality are trapped in the small pools. This study also showed that the deeper the pools, the more they helped improve water quality. The results of this study will help farmers, natural resource conservation service personnel, and water resource managers prevent soil erosion and water pollution.
Technical Abstract: Field-scale grade controls are erosion control structures used extensively in the Demonstration Erosion Control Project in the Yazoo Basin. Common to many of these structures are shallow basins that are permanently or seasonally inundated. These pools play a role in stream water quality by processing nutrients from storm run-off and trapping suspended sediments. Sixteen drop pipe inlets were divided into four categories based on water depth and surrounding vegetation. Mean depth for categories 1 through 4 were 8.8, 17.6, 32.0 and 154.9 cm respectively. Water quality parameters were monitored to determine significant differences between the categories. Dissolved oxygen was significantly higher in both deep and shallow pools when compared to the intermediate category (2 and 3) pools. Conductivity was significantly higher in shallow pools. Filterable ortho- phosphate was significantly higher in category 1 and 2 pools than the deeper pools, and category 4 pools had the lowest mean total phosphorus concentrations. Category 1 and 2 pools were significantly higher in ammonia and nitrate than either category 3 or 4 and also had significantly higher coliform bacteria counts. As might be expected, chlorophyll concentrations were higher in the deeper pools. Suspended and total solids were not significantly different between pools. Based on these preliminary data, drop pipe inlets designed with deeper basins trap coliforms and process nutrients more efficiently than shallow basin designs. Overall, these small pools play a trapping and processing role in upland landscapes similar to small ponds on isolated wetlands. Efficiency is limited by size and water residence time.