§42-259.
Stormwater Outlet Protection
A.
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.
B.
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:
(1)
the velocity established by the Maximum Permissible Velocities Table set out within this
subsection; or
(2)
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.
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 channels.
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 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:
(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 stormwater discharge velocities by using vegetated or roughened swales and
waterways in place of closed drains and high velocity paved sections;
(3)
Provide energy dissipators at outlets of storm drainage facilities to reduce flow velocities to the
point of discharge;
(4)
Protect watercourses subject to accelerated erosion by improving cross sections and/or providing
erosion-resistant lining; and
(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 rate of
flow or increased velocity.
  
D.
Exceptions. This rule shall not apply where it can be demonstrated to the Soil Erosion and Sedimentation
Control Administrator that stormwater discharge velocities will not create an erosion problem in the
receiving watercourse.