(A) Persons shall prepare a plan that designs the post construction velocity of the 10-year storm run-off in the receiving watercourse to the discharge point that does not exceed the greater of:
(1) The velocity established by the table in division (D) of this section; or
(2) The velocity of the 10-year storm run-off in the receiving watercourse prior to development. If conditions (1) or (2) of this section 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 development velocity by 10%.
(B) 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 Commission recognizes that the management of Stormwater run-off to minimize or control downstream channel and bank accelerated 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 run-off volume and velocity by including measures to promote infiltration to compensate for increased run-off from areas rendered impervious.
(2) Avoid increases in stormwater 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.
(5) Upgrade or replace the receiving device, structure or watercourse such that it will receive and conduct the flow to a point where it is no longer subject to degradation from the increased volume, or increased velocity of the flow.
(C) Exceptions. This rule shall not apply where it can be demonstrated that stormwater discharge velocities will not create an erosion problem in the receiving watercourse.
(D) The following is a table for maximum permissible velocity for stormwater discharges: units are in feet per second (FPS) and meters per second (MPS).
Maximum Permissible Velocity | ||
Material | F.P.S. | M.P.S. |
Maximum Permissible Velocity | ||
Material | F.P.S. | M.P.S. |
Fine sand (non-colloidal) | 2.5 | 0.08 |
Sandy loam (non-colloidal) | 2.5 | 0.08 |
Silt loan (non-colloidal) | 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 (non- colloidal) | 5.0 | 1.5 |
Graded, silt to cobbles (colloidal) | 5.5 | 1.7 |
Alluvial silts (non-colloidal) | 3.5 | 1.1 |
Alluvial silts (colloidal) | 5.0 | 1.5 |
Coarse Gravel (non-colloidal) | 6.0 | 1.8 |
Cobbles and shingles | 5.5 | 1.7 |
Shale 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. 13-34, passed 11-26-2013)