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386 <br /> <br />conditions permit a steeper slope or where the slopes are stabilized <br />by using mechanical devices, structural devices or other acceptable <br />ditch liners. In any event, the angle for side slopes shall be <br />sufficient to restrain accelerated erosion. <br />Must be provided for any portion of land-disturbing activity in a <br />HQW zone within 15 working days or 60 calendar days following <br />completion of construction or development, whichever period is <br />shorter. <br /> <br />Section 9 Storm Water Outlet Protection <br />a. Persons shall conduct land-disturbing activity so that the post- <br /> construction velocity of the 10-year storm runoff in the receiving <br /> watercourse to the discharge point does not exceed the greater of: <br /> <br /> 1. The velocity established by the table in Paragraph (d) of this <br /> Section; or <br /> 2. The velocity of the 10-year storm runoff in the receiving <br /> watercourse prior to development. <br />If conditions (1) or (2) of this Subsection cannot be met, then the receiving <br />watercourse to and including the discharge point shall be designed and <br />constructed to withstand the expected velocity anywhere the velocity exceeds the <br />"prior to development" velocity by 10%. <br />b. Acceptable Management Measures - measures applied alone or in combination <br /> to satisfy the intent of this section are acceptable if there are no <br /> objectionable secondary consequences. The NCSCC recognizes that the <br /> management of stormwater runoff to minimize or control downstream channel <br /> and bank erosion is a developing technology. Innovative techniques and <br /> ideas will be considered and may be used when shown to have the potential <br /> to produce successful results. Some alternatives are to: <br /> 1. 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 /> 2. Avoid increases in stormwater discharge velocities by using <br /> vegetated or roughened swales and waterways in lieu of closed drains <br /> and high velocity paved sections. <br /> 3. Provide energy dissipaters at outlets of storm drainage facilities <br /> to reduce flow velocities to the point of discharge. These may range <br /> from simple rip-rapped sections to complex structures. <br /> 4. Protect watercourses subject to accelerated erosion by improving <br /> cross sections and/or providing erosion-resistant lining. <br /> c. Exceptions - this rule shall not apply where it can be demonstrated that <br /> stormwater discharge velocities will not create an erosion problem in the <br /> receiving watercourse. <br />d. The following is a table for maximum permissible velocity for stormwater <br /> discharges: <br /> Maximum Permissible Velocities <br /> <br />Material <br />Fine sand (noncolloidal) 2.5 <br />Sandy loam (noncolloidal) 2.5 <br />Silt loam (noncolloidal) 3.0 <br />Ordinary firm loam 3.5 <br />Fine gravel 5.0 <br />Stiff clay (very colloidal) 5.0 <br />Graded, loam to cobbles (noncolloidal) 5.0 <br />Graded, silt to cobbles (colloidal) 5.5 <br />Alluvial silts (noncolloidal) 3.5 <br />Alluvial silts (colloidal) 5.0 <br />Coarse gravel (noncolloidal) 6.0 <br />Gobbles and shingles 5.5 <br />Shales and hard pans 6.0 <br /> <br />Feet/ Meters/ <br />Second Second <br /> <br />0.8 <br />0.8 <br />0.9 <br />1.1 <br />1.5 <br />1.5 <br />1.5 <br />1.7 <br />1.1 <br />1.5 <br />1.8 <br />1.7 <br />1.8 <br /> <br />Source - adapted from recommendations by Special Committee on <br />Irrigation Research, American Society of Civil Engineers, 1926, for <br />channels with straight alignment. For sinuous channels, multiply <br />allowable velocity by 0.95 for slightly sinuous, by 0.9 for <br />moderately sinuous channels, and by 0.8 for highly sinuous channels. <br /> <br />Section 10 Borrow and Waste Areas <br />When the person conducting the land-disturbing activity is also the person <br />conducting the borrow or waste disposal activity, areas from which borrow is <br /> <br /> <br />