(A) Persons shall conduct land-disturbing activity so that the post-construction velocity of the ten- year storm runoff in the receiving watercourse to the discharge point does not exceed the greater of:
(1) The velocity established in Table 158.08-1 of this section; or
(2) The velocity of the ten-year storm runoff in the receiving watercourse prior to the land-disturbing activity.
(B) If the conditions of division (A) cannot be met, then the receiving watercourse to and including the discharge point shall be designed and constructed to withstand the expected velocity anywhere the velocity exceeds the "prior to land-disturbing activity" velocity by 10%.
(C) Acceptable management measures. Measures applied alone or in combination to satisfy the intent of this section are acceptable if there are no objectionable secondary consequences. The City recognizes that the management of storm water runoff to minimize or control downstream channel and bank erosion is a developing technology. Innovative techniques and ideas will be considered and may be used when shown to have the potential to produce successful results. Some alternatives are to:
(1) Avoid increases in surface runoff volume and velocity by including measures to promote infiltration to compensate for increased runoff from areas rendered impervious.
(2) Avoid increases in storm water runoff discharge velocities by using vegetated or roughened swales and waterways in lieu of closed drains and high velocity paved sections.
(3) Provide energy dissipaters at outlets of storm drainage facilities to reduce flow velocities to the point of discharge. These may range from simple rip-rapped sections to complex structures.
(4) Protect watercourses subject to accelerated erosion by improving cross sections and/or providing erosion-resistant lining.
(D) Exceptions. This rule shall not apply where it can be demonstrated that storm water discharge velocities will not create an erosion problem in the receiving watercourse.
(E) Maximum permissible velocity for storm water discharges shall be regulated in accordance with Table 158.08-1 below.
Table 158.08-1 Maximum Permissible Velocities.
Material Feet/Second Meters/Second Fine sand (noncolloidal) 2.5 0.8
Sandy loam (noncolloidal) 2.5 0.8
Silt loam (noncolloidal) 3.0 0.9
Ordinary firm loam 3.5 1.1
Fine gravel 5.0 1.5
Stiff clay (very colloidal) 5.0 1.5
Graded, loam to cobbles (noncolloidal) 5.0 1.5
Graded, silt to cobbles (colloidal) 5.5 1.7
Alluvial silts (noncolloidal) 3.5 1.1
Alluvial silts (colloidal) 5.0 1.5
Coarse gravel (noncolloidal) 6.0 1.8
Cobbles and shingles 5.5 1.7
Shales and hard pans 6.0 1.8
Source: Adapted from recommendations by Special Committee on Irrigation Research, American Society of Civil Engineers, 1926, for channels with straight alignment. For sinuous channels, multiply allowable velocity by 0.95 for slightly sinuous, by 0.9 for moderately sinuous channels, and by 0.8 for highly sinuous channels.
(Ord. O-2002-18, passed 4-2-02; Am. Ord. O-2003-33, passed 6-17-03)