Erosion and Sediment Pollution Control Program Manual.pdf

Design Discharge -----1. The anticipated rate or volume of flow as a result of a specified storm intensity,
usually expressed in cubic feet per second (CFS). 2. The discharge produced by impounded water
at a specified stage.
Design Elevation -----In dams, it is the height above a defined datum describing the required elevation of
pool that will provide the required temporary storage.
Design Flow (Qr) -----The anticipated peak flow from a Design Storm that a BMP must be able to convey
while providing sufficient freeboard.
Design Frequency -----The recurrence interval for hydrologic events used for design purposes (e.g. a
design frequency of 10 years means a storm of such magnitude that it would occur an average of
once every 10 years).
Design High Water Elevation -----1. The water surface elevation in a basin, pond, or dam at which the
required discharge rate and/or storage capacity is provided. 2. The flow depth in a channel at
which the required channel capacity is provided.
Design Life -----The period of time for which a BMP or other facility is expected to perform its intended
function.
Design Standards -----Detailed engineering drawings and/or specifications promulgated by public or
private organizations that leave little choice to design engineers and technicians.
Design Storm -----The rainfall duration and return frequency used to size a BMP (e.g. for E&S purposes,
Temporary BMPs not located in Special Protection Watersheds are generally designed to have
sufficient capacity for the anticipated runoff from a 1-hour storm that has a return frequency of
once every 2 years, i.e. a storm of that intensity occurs on average once every 2 years). (See
Chapter 5)
Designated Uses -----Uses of surface waters specified in 25 Pa. Code §§ 93.4(a) and 93.9a-93.9z for
each water body or segment whether or not they are being attained.
Detachment -----The removal of transportable fragments of soil material from a soil mass by an eroding
agent. Through detachment, soil particles or aggregates are made ready for transport (soil
erosion).
Detention -----The process of temporarily collecting and holding back storm water for later release to
receiving waters.
On-Site Detention -----Temporary Storage of runoff on the same land development site where the
turnoff is generated.
On-Stream Detention -----Temporary storage of runoff within a principal drainage system,
receiving stream or conduit.
Off-Stream Detention ----- Temporary storage accomplished off-line — not within a principal
drainage system.
Detention Facility -----A storage pond or tank (above or below ground) that temporarily stores stormwater
runoff and releases it at a slower rate than it was collected. Typically, there is little or no infiltration.
Detention Pond -----An impoundment used for stormwater management to delay the peak runoff from a
site long enough to minimize the potential for downstream flooding.
Detention Storage -----Temporary storage of excess runoff in surface ponds, or underground tanks, for
the purpose of attenuating the peak flow rate.
Detention Time -----The time required for a drop of water to pass through a detention facility when the
facility is filled to design capacity.
Determination Letter -----A letter given by PHMC to a project applicant indicating PHMC’s determination
of the historical or archaeological significance of the site of the project applicant’s intended activity.
Detritus -----Rock, gravel, sand, silt or other material carried by flowing water.
Dewater -----To remove water from all or part of a storage facility in a controlled fashion within a specified
period of time.
Dewatering Facility -----A BMP used to dewater an impoundment. In erosion control, it usually refers to
the facility used to dewater the sediment storage zone of a Sediment Basin or Trap.
Dewatering Time -----The time required to dewater a Sediment Basin or Trap from the top of the
Dewatering Zone (Top of Riser) to the top of the Sediment Storage Zone (Lowest Row of
Perforations). Dewatering Zone -----Formerly called the Settling Zone, the part of the storage
capacity of a sediment basin or trap extending from the top of the sediment storage zone to the
elevation at which the required storage capacity is provided.
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