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<br />/ <br /> <br />i <br /> <br />( <br /> <br />( <br /> <br />I <br />, <br /> <br />OJlv57 <br /> <br />30.5--1 <br /> <br />WATER INFORMATION MANAGEMENT SYSTEM HANDBOOK <br /> <br />CHAPTER 30 - PROCEDURE FOR QUANTIFYING CHANNEL MAINTENANCE FLOWS <br /> <br />This chapter describes the methods for determining which streamflows are <br />necessary for the self-maintenance of river, stream, or creek channels to <br />retain their capability for passing flood-flow discharges. The retention <br />of channel capacity is accomplished ~hrough, among other things, <br />maintaining the stream's transport of sediment and minimizing the growth of <br />vegetation within the channel. The procedure described in this chapter <br />focuses on determinating which streamflows are required to maintain <br />stability of stream channel systems. <br /> <br />30.3 - Policy. Use the procedures described in this handbook to <br />extrapulate stream flow data when minimum on-site data exists and its <br />collection is impractical. <br /> <br />30.5 - Definitions. <br /> <br />Active Channel. The channel cross section at the stage corresponding to <br />bankfull discharge. <br /> <br />Aggradation. The geologic process by which .stream beds, floodplains, and <br />{fie bottoms of other water bodies are raised in elevation by the deposition <br />of material eroded and transported by water from other areas. It is the <br />opposite of degradation. <br /> <br />Bankfull Discharge. The flow which just fills the channel to the top. of <br />the banks. <br /> <br />Bankfull Stage. The level of the active flood plain and, therefore, is the <br />stage at which overbank flooding occurs. <br /> <br />Baseflow. The sustained or fair-weather runoff, composed of groundwater <br />runoff and delayed subsurface runoff. In this procedure baseflow is the <br />discharge that just fills the thalweg channel. <br /> <br />Channel Maintenance Flows. A range and duration of in-channel flows <br />necessary to maintain the stability and effective function of the stream <br />channel. This includes those flows which are necessary to provide for the <br />self-maintenance of the stream channel network so as to retain its <br />capability for passing flood dischar~es, minimize channel erosion and/or <br />sediment deposition associated with lnstability or disequilibrium <br />conditions. The consequences of not maintaining channel stability include <br />channel aggradation, channel erosion, floodplain encroachment, vegetation <br />encroachment, changes in hydraulic geometry, and reduced channel capacity <br />with resultant increased risk of flooding and associated resource damage. <br /> <br />~. An index point describing the beginning of the rise in the required <br />~enance flows hydrograph. It generally occurs on or after the day <br />during snowmelt runoff when the mean 24-hour flow equals or exceeds mean <br />annual discharge for some user defined period of time. . <br />1D No. 1 <br />9/10/85 <br />