This depends on state or local requirements. Generally states require that the skimmer be sized to drain the basin volume over a specified time frame. For example, South Carolina requires that the sediment basin be drained from near the surface and over a time period of 2-5 days, with 3 days being preferred. A simplified method is to determine the volume of the basin to be drained and then using the average flow rate for each skimmer determine which one(s) meet the required drawdown time range. If a conservative approach is used then this is a generally accepted method for selecting skimmers.
Although skimmers are often referred to as “constant-flow” devices, testing has shown the flow rates actually vary by basin depth. Because the flow rates do vary, a more accurate way to incorporate the device into the design is by inputting the actual discharge rate at given elevations into stormwater modeling software. We provide flow rate data based upon our third party test results for use by engineers when a more precise design is preferred or required.
We have also developed an easy to use design tool for use by engineers that takes into account the actual flow rates of the Marlee Float, as well as basin size and depth. Those tools can be found here »