Erosion and Sediment Pollution Control Program Manual.pdf

Embedded section of aluminum or aluminized pipe shall be painted with zinc chromate or
equivalent.
Freeboard
A minimum of 24 inches of freeboard is required above the elevation at which the 2 cfs per acre
discharge capacity is provided — or the maximum water surface elevation while routing the anticipated
peak flow from the 25-year, 24-hour storm event. If the emergency spillway is being used to discharge
part of the design discharge, the 24 inch freeboard should be provided above the design flow elevation
through the emergency spillway. An acceptable alternative is to provide a spillway combination
with the capacity to pass the peak flow from a 100-year, 24-hour storm event with 12” of
freeboard.
Emergency Spillway - Every sediment basin should be provided with an emergency spillway.
PA DEP
The elevation of the emergency spillway crest should be at least 6” above that of the principal spillway.
Emergency spillways should be located on undisturbed ground, wherever possible. Otherwise, the
emergency spillway should have a TRM, riprap or other hard armor protection which extends at least
15 feet beyond the toe of the embankment to the receiving waterway (Dl), channel or other non-erosive
outlet. Minimum width at the end of the armor protection (Dw) should be 0.4 times the length of the
apron unless discharging directly into a channel leading away from the apron. Where a TRM is used to
line the spillway down the face of the embankment, the channel at the toe of the embankment should
be lined with riprap a sufficient length in both directions to prevent scour.
Wherever riprap is used as a protective liner in an emergency spillway, a compacted and stabilized
earthen plug should be left in place on the inner side of the spillway to prevent water from discharging
through the emergency spillway until the design water elevation is reached, as shown in Standard
Construction Detail #7-12). Note: Provide supporting calculations for the outlet channels of all
emergency spillways.
363-2134-008 / March 31, 2012 / Page 192