Erosion and Sediment Pollution Control Program Manual.pdf

Perforated Riser Dewatering - This device has MODERATE sediment removal efficiency and is an
ABACT for HQ, but not EV, watersheds unless used in conjunction with sediment forebay with
flocculation or turbidity barrier.
PA DEP
For sediment basins with perforated risers, designers may use the following rule of thumb. Provide
1 square inch of opening per acre of drainage area with all perforations one inch in diameter and
equally spaced vertically along the riser. NOTE: A 1” diameter perforation provides 0.785 sq. in. of
opening. Therefore, the number of perforations needed may be determined by dividing the total
number of acres tributary to the basin by 0.785. The lowest row of perforations is at the sediment
storage zone elevation (i.e. sediment clean-out elevation). No further computation of dewatering time is
required if the rule of thumb is used correctly. The rule of thumb may not be used in special protection
(HQ or EV) watersheds where a longer dewatering time may be required than that provided by the rule
of thumb.
Wherever total dewatering of the basin is necessary, and perforations are located below the top of the
sediment storage zone, the rule of thumb may not be used. In these instances, the discharge through
the perforations in the sediment storage zone as well as perforations in the dewatering zone must be
considered when computing dewatering time. In most cases, only one or two perforations will be
needed below the top of sediment storage zone to facilitate total dewatering (Standard Construction
Detail # 7-11).
The rule of thumb is not a precise procedure. If an accurate dewatering time is required, Standard E&S
Worksheets #15 and # 16 should be used.
Determine the principal spillway discharge capacity.
The combined capacities of the principal and emergency spillways should be at least 2 cfs/acre for the
entire watershed of the basin. An acceptable alternative is to route the anticipated peak flow from the
25-year, 24-hour storm event without encroaching on freeboard.
363-2134-008 / March 31, 2012 / Page 174