Section 250-RICR-150-10-8.29 - Pretreatment - Sediment ForebayA. The required surface area of the sediment forebay shall be determined using the following equation that is based on Camp-Hazen. Click here to view
Where
As "sedimentation surface area (square feet)
Q = discharge from drainage area (cubic feet/second = %WQv/86,400 sec
W = 0.0004 feet/second particle settling velocity recommended for silt
E = sediment removal efficiency (assume 0.9 or 90%)
The percent of the water quality volume used for the sediment forebay design depends on which treatment BMP is being used.
Therefore, for the purposes of this Part, use:
As = 5,750 * Q
B. The forebay shall have a minimum length to width ratio of 1:1 and a preferred minimum length to width ratio of 2:1 or greater. Designers shall calculate scour potential and provide riprap sizing calculations (diameter required to effectively dissipate erosive velocities).C. The forebay shall be sized to contain at least 10% of the WQv (depending on the requirements of the treatment BMP) and be of an adequate depth to prevent resuspension of collected sediments during the design storm, often 4 feet to 6 feet deep. Shallower depths shall be evaluated such that flow-through velocities do not exceed 2 feet/second for all design storms up to the 100-year storm. The goal of the forebay is to at least remove particles consistent with the size of medium sand.D. Direct access for appropriate maintenance equipment needs to be provided to the forebay and may include a ramp to the bottom of the embankment if equipment cannot reach all points within the forebay from the top of the embankment. The forebay can be lined with a concrete pad.250 R.I. Code R. 250-RICR-150-10-8.29
Amended effective 11/13/2018