Drainage Design Manual - the City of San Luis Obispo

maintenance to maintain the design capacity, due to siltation, bank erosion, and growth ofchannel bottom vegetation. The flattened bank slopes of the constructed or engineeredchannel also makes shading of the channel more difficult, and often eliminates theoccurrence of undercut banks, and associated pools which provide important fish habitat.This design-influenced condition is to be avoided in the San Luis Obispo Watershed:• Where detailed hydraulic studies indicate the need to modify or restore an existingAltered Naturalized Channel to improve flood conveyance, a Constructed NaturalChannel design shall be used.• Constructed Natural Channels, which emulate the form and function of natural channelsin design, are to be a part of the City’s and County’s open space and flood managementsystem, which place an equally high value on preserving natural systems and enhancingwildlife and fisheries habitat.• The stability and maintenance requirements of the Constructed Natural Channel shallbe considered in the design.• The design shall typically include a two-stage or compound channel with a separate lowflow channel within the overall flood channel.• The design shall include provisions for emulating natural channel cross-section, as wellas natural channel planform shape and bedform or channel bottom features, such as poolsand riffles.5.3.1.1 Cross-Section DesignFloodplain as used in this Design Manual refers to the generally flat area immediately adjacent toa channel, which is periodically flooded. Sometimes a specific recurrence interval is used todefine the floodplain, such as the 2- year, 10-year, 25-year or 100-year floodplain.The main channel occurs below the floodplain and is capable of carrying the “bankfulldischarge” which typically recurs at the approximately 2-year peak flow along a natural channel.Often the 2- year floodplain occurs as an easily recognizable but intermittent in-channel bench orterrace along the stream course, located about 1/3 to ½ of the way up the channel bank in theSLO Creek Watershed.Due to channel incision, channel modification and widening, and floodplain fill, the floodrecurrence interval for flooding which escapes the banks now ranges from less than 10 years toover 50 years along portions of SLO and Stenner Creeks upstream of Los Osos Valley Road(LOVR).The low-flow channel is typically well defined along most of the creeks in the SLO Creekwatershed, although it has not been clearly associated with any specific recurrence interval peakflow or flow duration. It can be considered as the lowest elevation part of the channel, includingthe channel thalweg, and for design purposes shall be assumed as the portion of the channel thatconveys the early summer (June) flow. The channel thalweg is defined as the line following thebed of a stream or river. Each of these subareas of the channel cross-section, the low flowchannel and thalweg, in-channel bench or 2-year floodplain, main channel, and overbankfloodplain plays a different role and must be considered in designing a Constructed NaturalChannel. Figure 5-2 shows a schematic channel cross-section of a Constructed NaturalChannel with a floodplain terrace.SLO Creek Drainage Design Manual 43 February 2003