42-258. Stormwater Outlet Protection
- Intent. Stream banks and channels downstream from any land-disturbing activity shall be protected from increased degradation by accelerated erosion caused by increased velocity of runoff from the land- disturbing activity.
- Performance Standard. Persons shall conduct land-disturbing activity so that the post construction velocity of the ten (10) year storm runoff in the receiving watercourse to the discharge point does not exceed the greater of:
- the velocity established by the Maximum Permissible Velocities Table set out within this subsection; or
- the velocity of the ten-year storm runoff in the receiving watercourse prior to development.
If condition (1) or (2) of this Paragraph 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 percent.
The following is a table for maximum permissible velocity for storm water discharges in feet per second (F.P.S.) and meters per second (M.P.S.):
| Table 8.1. Maximum Permissible Velocities | ||
|---|---|---|
| Material | Feet Per Second (F.P.S.) | Meters Per Second (M.P.S.) |
| 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 channel
3. 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 Henderson County Soil Erosion and Sedimentation Control Administrator recognizes that the management of stormwater 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, while not exhaustive, are to:
- Avoid increases in surface runoff volume and velocity by including measures to promote infiltration to compensate for increased runoff from areas rendered impervious;
- Avoid increases in stormwater discharge velocities by using vegetated or roughened swales and waterways in place of closed drains and high velocity paved sections;
- Provide energy dissipators at outlets of storm drainage facilities to reduce flow velocities to the point of discharge;
- Protect watercourses subject to accelerated erosion by improving cross sections and/or providing erosion-resistant lining; and
- 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 rate of flow or increased velocity.
4. Exceptions. This rule shall not apply where it can be demonstrated to the Henderson County Soil Erosion and Sedimentation Control Administrator that stormwater discharge velocities will not create an erosion problem in the receiving watercourse.
