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<br />e <br /> <br />e <br /> <br />e <br /> <br />DRAINAGE CRITERIA MANUAL (V. 2) <br /> <br />HYDRAULIC STRUCTURES <br /> <br />because of potential piping. Problems with rock density, durability and hardness are of concern and can <br />vary widely for different locations. The rock should be inspected at regular intervals to meet minimum <br />physical dimensions, strengths, and weights as defined in the specifications. <br /> <br /> <br />Photograph HS-11 <br /> <br />Urban grass channels can experience bank erosion and degradation when established design <br />criteria are not used. The invert of pipe used to be at invert of channel before degradation. <br /> <br />2.9 Low-Flow Check and Wetland Structures <br /> <br />Urbanization causes more frequent and sustained fiows, and therefore the trickle/low-f1ow channel and <br />wetlands become more susceptible to erosion even though the overall floodplain may remain stable and <br />able to resist major flood events. <br /> <br />Erosion of the low-flow channel, if left uncontrolled, can cause degradation and destabilization of the <br />entire channel. Low-flow grade-control check structures are designed to provide control points and <br />establish stable bed slopes within the base flow channel. They should be used as needed to limit slopes <br />to about 0.5% and as described in the MAJOR DRAINAGE chapter. The check structures can be small <br />versions of the drop structures described elsewhere in this chapter. Low-flow check structures are not <br />appropriate in instances such as completely incised floodplains or very steep channels. <br /> <br />06/2001 <br />Urban Drainage & Flood Control District <br /> <br />HS-39 <br />