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05 <br />a. avoid increases in surface runoff volume and velocity by including <br />measures to promote Infiltration to compensate for increased runoff <br />from areas rendered impervious; <br />b. avoid increases in stormwater discharge velocities by using vegetated <br />or roughened swales and waterways in lieu of closed drains and high <br />velocity paved sections; <br />c. provide energy dissipators at outlets of storm drainage facilities to <br />reduce flow velocities to the point of discharge; these may range from <br />simple rip-rapped sections to complex structures; <br />d. protect watercourses subject to accelerated erosion by improving cross <br />sections and/or providing erosion-resistant lining. <br />2. Exceptions <br />This rule shall not apply in areas other than the University Lake, Cane <br />Creek, and Upper Eno Protected Watershed Areas where it can be <br />demonstrated that storm water discharge velocities will not create an <br />erosion problem in the receiving watercourse. <br />3. Permissible Velocities <br />The following is a table of permissible velocity for stormwater discharges: <br />MATERIAL <br />Maximum Permissible Velocities <br />FPS MPS <br />Fine Sand (noncolloidal) 2.5 0.8 <br />Sandy Loam (noncolloidal) 2.5 0.8 <br />Silt Loam (noncolloidal) 3.0 0.9 <br />Ordinary Firm Loam 3.5 1.1 <br />Fine Gravel 5.0 1.5 <br />Stiff Clay (very colloidal) 5.0 1.5 <br />Graded, Loam to Cobbles (noncolloidal) 5.0 1.5 <br />Graded, Silt to Cobbles (colloidal) 5.5 1.7 <br />Alluvial Silts (noncolloidal) 3.5 1.1 <br />Alluvial Silts (colloidal) 5.0 1.5 <br />Coarse Gravel (noncolloidal) 6.0 1.8 <br />Cobbles and Shingles 5.5 1.7 <br />Shale and Hard Pans 6.0 1.8 <br />Source: Adapted from recommendations by Special Committee an <br />Irrigation Research, American Society of Civil Engineers, 1926, far <br />channels with straight alignment. For sinuous channels, multiply <br />allowable velocity by 0.95 for slightly sinuous, by 0.9 for moderately <br />sinuous channels, and by 0.8 for highly sinuous channels. <br />G:\EROSION\Amendments\AMENDS2001.doc <br />