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Open channels can be used to divert runoff around the face of vulnerable slopes and to direct the flow to larger drainage courses. Various linings can be used depending on the steepness of the channel. Porous linings offer the ad- vantage of allowing grad- uai infiltrarion while water is being transported. 62 ~~ ~ _-'- ~ -;~.-~~}t{f_~ . ~-:~~ij/j~=' :?L"'--~ . R1_ Channel. ~ --.~::~{~~~~,~~ ...----.-_Z;:,-,-~~' /_KA_ .:-:..~w ~~~S~~~~~?~~;;~ Gra-.Uned Channel. ^ ~-:'''2r, -"iywr.J ':':".--7/71 J" '-'"J=07'./~ ~ ..,...__-.:t.X"e;;-=7' /7'lJ;~ ~~1~~ -/-7 .:c.c_ __c--. 1111: ,41 II .. -I.-~I-' Concrata Channel. can be used to detain water, but this requires special at- tention to the roofs structural ability to support the weight of collected water and reliable waterproofing measures. Both of these methods can be helpful in offset- ting the effects of existing impervious surfaces in urban areas where there is little open space to absorb runoff. Permanent water storage in the form of ponds or lakes can be used in circumstances where a consistent supply of water and sufficient space exist Ponds and lakes can add to site amenity and offer added potential for rec- reation and conservation habitats, though they do require regular maintenance. Open Channels. Open channels can serve as both small-scale storage devices and a means of directing run- off away from vulnerable areas. Their primary purposes are to divert water away from areas likely to erode, such as large, gently sloping areas and shorter steep slopes, and to collect and transport water runoff to larger drainage courses. Channels with grass cover are appropriate where the channel gradient is low; they can serve as percolation trenchs by allowing gradual infiltration while water is being transported Linings are necessary in channels where vegetation cannot be established because flow is of long duration, runoff velocities are high erodable soils exist or slopes are steep. Concrete and asphalt paving or riprap are the most commonly used channel linings. However, such linings can increase the velocity of runoff and thus should be designed with velocity checks to con- trol the rate of water flov,c Stm3/S and Curbs Streets and curbs are frequently added during development, and the layout and gradient should be designed to complement runoff control sys- tems. Streets, by decreasing the area of permeable surface, can create excessive runoff and contribute to localized flash flooding Their design should avoid these problems, and should work to collect and convey water runoff at controlled velocities and to safe outlet points. Storm Sewer !iYstem. Installation of a storm sewer sys- tem is often part of site development in large projects, usually parallel to the street and curb system Storm sew- ers should interconnect with other drainage devices to Channel running across the slope to divert water.