Hydraulic effects of stormwater discharge into a small stream

The focus of this study is to describe the hydraulic effects of stormwater discharge, thus sediment transport occurring as a result of increased discharge from a stormwater detention pond, based on measurements made in a small high-slope Danish stream. In order to extrapolate the findings and predict the result of larger discharge flow rates from the detention pond in this study, 11 traditional threshold equations were tested, and results were compared to the sediment transport experiment with five formulas predicting the threshold based on shear stress and six based on stream power. The sediment transport experiment was constructed as a staircase pattern, step-wise increasing the discharge. During the experiment, measurements of sediment transport in the stream were made in two stations downstream from the point of discharge. Results from those measurements showed that there was no notable correlation between suspended sediment transport and bed sediment transport, and that suspended transport peaked during the periods of low flow conditions. Bed sediment transport peaked before the maximum flow, indicating that the available sediment for transport is a limiting factor. When comparing the calculated threshold of the collected sediment particle sizes to the shear stress and stream power calculated during the experiment, all 11 tested formulas overestimated the sediment transport and particle size moved by a specific flow. This result is in correspondence with results found in other experiments, and here the expected explanation is that the form roughness of the stream bed makes less energy available for sediment transport. This implies that the hydraulic impact from discharge of stormwater into small streams has to be evaluated on a case-by-case basis, rather than relying on general threshold sediment transport models.