An indication of the trapping efficiency (TE) of a construction site4 stormwater management basin can be provided by measuring toal inlet and outlet sediment load throughout individual storm events. Water samples and flow measurements taken continuously during and following five rainfall events at a site in Northern Ohio yielded representative TEs between 56precent and 79precent, with an average value of 68precent. The watershed is dominated by soils with a high percentage of silt and clay, and no particles larger than 0.250 mm were found in basin inflow or outflow. Grain size analysis performed on a storm with a total load TE of 67precent using an x-ray particle analyzer gave TEs for silt and clay of 90precent and 63precent, respectively. This suggests that during typical storms the basin is adequate for trapping silt-size particles and larger sediment, but does a relatively poor job in trapping clay-sized particles. The same basin was then modified using a baffle, recommended in many stormwater management handbooks as a means to improve trap efficiency by reducing inflow velocity. Four rainfall events monitored after basin modification revealed a dramatic decrease in average TE to 2.7precent for storms during which flow overtopped the baffle, and a slight increase in TE to 80precent for a storm during which the basin water level remained below the top of the baffle. As an explanation for this surprising result we suggest a reduction of vertical mixing in the basin resulting from inflow flowing across the basin because of either the baffle or thermal stratification. This surface sheet flow effect leads to increased dead storage and reduced residence time for stormwater. Surface sheet flow and thermal stratification can be prevented by using a baffle with a height greater than the outflow elevation, and perforations to allow flow to pass through the baffle at a rate sufficient to prevent overtopping.
展开▼
