Skimmer Flow Rate Testing and Size Selection

A skimmer is a device used to drain a sediment basin as it fills. The skimmer floats on the surface, draining the cleanest water as sediment falls out of suspension. An adjustable orifice on the skimmer helps to regulate the filling and draining rate of the basin. After significant rain events, skimmers will slowly drain the basin over several days to maximize settling, while draining less turbid water from the top down. There are several options when choosing a skimmer product. Manufacturers have published data for their products that customers can use to make a decision on skimmer type, size, and orifice opening, but are very rough estimates with numerous assumptions. This presentation details a methodology for testing skimmers including materials, data collection process, and data analysis approach. Testing was performed in a 7 ft deep tank with a volume of approximately 1,000 cubic feet. The skimmer tested was a 6 inch prototype provided by Faircloth Skimmer which used an adjustable gate to create openings from 0.5 to 6 inches in 0.5 inch increments. Pumps were used to fill the tank and a Solinst Levelogger recorded water levels in 5 second intervals as the skimmer emptied the tank. From these tests, models were created to approximate the flow rate of the skimmer at various water depths. These models were then used to create an interactive Excel tool. The user inputs values such as basin elevations and cross section properties to calculate the basin volume. Then, the user can select skimmer parameters to obtain data and graphs on flow rates at each depth, basin storage, and the total design drawdown time. This method of testing and data analysis provides the designer with more accurate information on sediment basin storage and drawdown times based on skimmer selection.