Evaluation and Enhancement of the Performance of Drop Inlets

There are numerous drop inlet (DI) structures installed along U.S. Highway 50 in the Clear Creek Drainage Basin near Carson City, Nevada which are maintained by the Nevada Department of Transportation (NDOT). The maintenance requirements can be significant since large quantities of traction sand are routinely applied to this section of highway during winter storm events. This sand accumulates in DIs and may clog the outlet culvert piping because the configuration of the DI and the culvert piping does not allow an adequate scour velocity to be maintained, thus allowing the sand particles to settle. The main objective of this project was to identify a geometric configuration of the bottom of the DI that minimized the accumulation of sand. The performances of various modified configurations of the bottom geometry of a standard Type 2 DI currently used by NDOT were systematically evaluated in the hydraulics laboratory. Several DIs were constructed by varying the bottom slope towards the outlet pipe and/or the side slopes toward the center of the DI. The results of performance testing indicated that as the slope of the bottom towards the outlet pipe was increased from 50:1 to 5:1, the amount of sand retained dramatically decreased. Further, as the degree of the side slopes were increased from zero to 1:1, the amount of sand retained decreased. The results obtained through testing various bottom configurations over a range of water flow rates and loading rates of sand were evaluated and used to develop the proposed modified bottom configuration. The proposed modified bottom configuration incorporated a relatively steep 5:1 bottom slope and a tapered wing sloped at 2:1 along one side of the DI. The testing performed using this configuration demonstrated optimal performance with minimal amounts of sand retained within the DI. Existing DIs could be retrofitted using the proposed modified bottom configuration to reduce the maintenance requirements associated with the removal of accumulated road sand. This configuration should be used in locations where the sand conveyed could be captured in an adjacent detention pond, sedimentation basin, or other low impact development BMPs to minimize detrimental effects to the environment.