Evaluating Floating Surface Skimmers

In 2010, the Georgia Soil and Water Conservation Commission (GSWCC) received funding to revise the Manual for Erosion and Sediment Control in Georgia. One of the parameters was to incorporate new BMP’s into the Manual. This was done by characterizing full-scale, installed performance of commonly used best management practices (BMPs) for sediment control. Some of the specific BMPs tested included what the GSWCC refers to as floating surface skimmers.

A floating surface skimmer, or floating pond skimmer, is a buoyant device that releases/drains water from the surface of sediment ponds, traps or basins at a controlled rate of flow. It “skims”, or dewaters, from the water surface where sediment concentrations are at a minimum in the water column instead of draining from the bottom where sediment concentrations are their highest. The skimmer and associated piping drains to a riser or the backside of a dam.

Critical elements of performance are the ability of the floating surface skimmers to:

• To discharge cleaner, less turbid water from the surface of a sediment pond, trap or basin at a relatively uniform rate. This practice is in contrast to a traditional perforated riser which discharges more turbid and sediment-laden water from lower depths of a sediment pond, trap, or basin.
• To reduce the retention time associated with meeting a desired water quality standard for discharge
  from a sediment pond, trap or basin.

Each skimmer product (and each product size) has a unique performance, including the associated hydraulics, which is affected by the floatation, inlet, and drain design chosen. The discharge rate is dependent on the specific product design and can only be determined through product-specific testing. This paper describes the testing of several sizes of each of 4 unique designs of floating pond skimmers and presents the associated results. The wide range of results is discussed and a straight forward process to choose the skimmer that best matches the required “time-to-drain” specified for a project is proposed. The required volume (or dimensions) of the sediment pond, trap, or basin must be known, as well as, the number of hours/days to drain the basin. With this information, a drawdown rate calculation is made for each product and size using product-specific flow rates.

Keywords: floating pond skimmer, floating surface skimmer, BMP, pond outlet, GSWCC, flow rate