DESIGN OF FLUSHING FLOWS IN RAPID CREEK BELOW PACTOLA DAM, BLACK HILLS, SD

Design of a flushing flow in Rapid Creek immediately below Pactola Dam will clean fine silts and particles that are currently thought to be degrading trout spawning habitat. Pactola Dam was completed in 1956 and in the period since that time, flow has been regulated in the channel. Recent data obtained from the South Dakota Department of Game, Fish, and Parks has shown a decline of fish populations in an approximately 1 mile reach immediately below Pactola Dam. Sediment mapping of this reach has shown that high concentrations of silt and clay are present in the quiet water regions, near banks, behind eddies, and in slow-moving reaches. Stream profiles were measured at 9 locations below the dam and sediment size data were co-located at 4 of these sites. Several numerical models were evaluated including the Tennant method, the Dominant Discharge/Channel Morphology method, the Estes and Orsborn method, and a new fluid dynamics model that was developed for this project. The Tennant method utilized a discharge of 200% of the average annual flow and resulted in a flushing flow of 98 cfs. The Dominant Discharge/Channel Morphology method was based on the 1.5 year recurrence discharge of 110 cfs. Estes and Orsborn was based on the 2-year average annual peak discharge of 192 cfs. All these methods are referred to as office methods and are a first predictor of a design flushing flow. Additionally, a new model was developed based on the channel morphology and fluid shear generated at each site as a function of slope and sediment size. The new method utilized the Manning equation to determine flow velocity at normal stream conditions. This was then used to determine the discharge at a depth of approximately ¾ bankfull, a level required because of the fine-grained characteristics of the floodplain sediments. Results indicated that for this reach, an effective starting point would be 253 cfs which would effectively mobilize all sediment smaller than cobble-size, thus, cleaning the bed of fine silt and clay.