Evaluation of Surface Skimmer Flow Rates and Size Selection

Abstract

A floating surface skimmer is a device used to dewater a sediment basin as it fills. The skimmer floats on the surface, draining the least turbid water as sediment falls out of suspension. An adjustable orifice on the skimmer regulates the filling and draining rate of the basin. After significant runoff events, skimmers slowly drain the basin over several days to maximize settling, while draining less turbid water from the top of the water column. Manufacturers have published data for products that customers can use to decide on the skimmer type, size, and orifice opening, but these design parameters tend to be very rough estimates with numerous assumptions. This study details a methodology for testing skimmers, including the materials, data collection process, and data analysis approach required to obtain the most accurate skimmer flow rate data. Testing was performed on a 15.2 cm (6.0 in.) post-construction stormwater skimmer prototype provided by J.W. Faircloth & Son, Inc., in an approximately 30 m³ (1000 ft³) evaluation tank. This skimmer used an adjustable sluice gate to control flow rate and had two barrel lengths. Six sluice gate opening sizes were tested three times for both barrel lengths, resulting in 36 tests performed. Experiments revealed that the skimmer had a capacity ranging from 0.03 m³/s (0.5 ft³/s) with a 2.54 cm (1.0 in.) opening and a capacity of 0.071 to 0.085 m³/s (2.5–3.0 ft³/s) with an opening of 15.2 cm (6.0 in.). Experimental results were then used to create a user-interactive skimmer sizing tool to provide more accurate information on sediment basin storage and drawdown times based on skimmer selection.