Drainage Design Manual - the City of San Luis Obispo

Discharge. A volume of water flowing past agiven point per unit time. In its simplestconcept, discharge means outflow;therefore, the use of this term is notrestricted as to course or location, and it canbe applied to describe the flow of water froma pipe or from a drainage basin. If thedischarge occurs in some course orchannel, it is correct to speak of thedischarge of a canal or of a river. It is alsocorrect to speak of the discharge of a canalor stream into a lake, stream, or ocean.Diversion. The change in character,location, direction, or quantity of flow of anatural drainage course.Drains. A pipe, ditch, or channel forcollecting and conveying water. Sometimesused in AStorm Drains@ when describing anurban storm drainage system to carry theinitial runoff.Drainage. (1) A general term applied to theremoval of surface or sub-surface waterfrom a given area either by gravity or bypumping. The term is commonly appliedherein to surface water. (2) The area fromwhich water occurring at a given point orlocation on a stream originates. In suchcase, the term is synonymous with DrainageArea and Watershed. (3) The term is alsoused in a general sense to apply to the flowof all liquids under the force of gravity.Drainage Area. See Catchment Area.Drainage Way. Those natural depressionsin the earth=s surface, such as swales,ravines, draws and hollows, in which surfacewaters ten to collect, but which do notconstitute a watercourse in the definedsense.Drawdown. The vertical distance the freewater elevation is lowered or the reductionof the pressure head due to the removal offree water.Dredge material. Soil that is excavatedfrom a stream channel.Drop. A vertical structure in a conduit orcanal installed for the purpose of droppingwater to a lower level.Drop Inlet Culvert. A culvert installed with adrop inlet on one end and daylighted at theother end.Eddy current. A circular water movementthat develops when the main flow becomesseparated from the bank. The eddy currentmy then be set up between the main flowand the bank.Encroachment. The advance orinfringement of uses, plant growth, fill,excavation, buildings, permanent structuresor development into a floodplain which mayimpede or alter the flow capacity of afloodplain.Energy Dissipater. A structure for thepurpose of slowing the flow of water andreducing the erosive forces present in anyrapidly flowing body of water.Enhancement. Improvements to theexisting conditions of the aquatic, terrestrial,and recreational resources.Erosion. In the general sense, the wearingaway of the land by wind and water. As usedin this pamphlet, the removal of soil particlesfrom a bank slope primarily due to wateraction.ESA. Endangered Species ActFailure. Collapse or slippage of a largemass of bank material into a stream.Fascines. Sausage-like bundles of plantcuttings used to stabilize streambanks andother slopes. Also known as wattlings.FEMA. Federal Emergency ManagementAgency. The agency which administers theNFIP at the federal level.Fill material. Soil that is placed at aspecified location to bring the groundsurface up to a desired elevation.Filter. Layer of fabric, sand, gravel, orgraded rock placed between the bankrevetment or channel lining and soil toprevent the soil from moving through therevetment; to prevent the revetment fromsinking into the soil; and to permit naturalseepage from the streambank, thuspreventing buildup of excessive groundwaterpressure. If a filter is used by a landowner orlocal government, technical assistanceshould be obtained to properly match thefilter with the soil.Fine particles (or Fines). Silt and clayparticles.Fish habitat. The aquatic environment andthe immediately surrounding terrestrialenvironment that meet the necessarybiological and physical requirements of fishspecies during various life stages.Flanking. Streamflow between a structureand the bank, possibly occurring becausethe structure was not property tied into thebank.