Drainage Design Manual - the City of San Luis Obispo

Construction of new Modified Channels should be avoided within the SLO watershed,and existing Modified Channels are to be reconstructed as Constructed Natural Channelswherever feasible, in association with new development or redevelopment projects, or asmitigation for upstream and downstream hydraulic/geomorphic impacts as required bySection3.4.The criteria, which makes a channel suitable for continued management andmaintenance as a Modified Channel, includes the following: Armored with concrete, grouted rock, or other hard, inflexible lining Not part of an existing continuous, riparian corridor; habitat is fragmented Very low biological value and low potential for enhancement/restoration withoutsignificant modification Little room at top of bank; top of bank urbanized Grossly inadequate channel capacity to meet Waterway Management Plans DesignFlow objectives, if altered.• Constructed Natural Channel. A Constructed Natural Channel (also sometimescalled a Landscaped, Constructed channel - see Figure 5-2) may be constructed from anAltered Naturalized or Modified Channel, (but not usually a Natural Channel) where ithas been shown that the channel lacks the Design Flow conveyance capacity, isgeomorphically unstable, or reconstruction is necessary to meet approval mitigationrequirements. The waterways are to be designed in accordance with all of the provisionsof these design criteria applicable to Constructed Natural Channels (see Section 5.3).Typically, this will include construction of a compound channel utilizing an in-channelbench or terrace whenever feasible, considerations of stable channel planform geometry,use of setbacks and buffer strips at top of bank, planting using native plants, slopestabilization using biotechnical erosion control methods, and incorporation of aquatichabitat enhancement elements within the stream bottom, such as boulders and root wads.The criteria, which makes a channel suitable for management and maintenance as aConstructed Natural Channel includes the following: Not extensively armored with concrete, grouted rock, or other hard, inflexible lining High potential for enhancement/restoration Adequate room at top of bank for alteration and management Existing channel instability or has inadequate capacity to meet flood conveyanceobjectives and requires substantial modification.• Closed Conduits. Closed conduits consist of pipe flow. Projects whose design dischargemay reasonably be placed in a 1,220 mm (48 inches) or smaller pipe shall be placedunderground, except for existing natural waterways, constructed natural waterways, andallowable gutter and roadside ditches. Generally waterways, which fall under thejurisdiction of the U.S. Army Corps of Engineers, cannot be undergrounded.SLO Creek Drainage Design Manual 40 February 2003