§ 5-88. Stormwater outlet protection.

(a)

Control of velocity. Persons shall plan and 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 by the table in paragraph (d) of this subsection; or

(2)

The velocity of the ten-year storm runoff in the receiving watercourse prior to development.

If conditions (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 ten (10) percent.

(b)

Acceptable management measures. Measures applied alone or in combination to satisfy the intent of this subchapter are acceptable if there are no objectionable secondary consequences. 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 area 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 dissipators 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.

(c)

Exceptions. This rule shall not apply in areas 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 of permissible velocity for stormwater discharges:

	Maximum Permissible Velocities
Material	(f.p.s.)	(m.p.s.)
Fine sand (noncolloidal)	2.5	.8
Sandy loam (noncolloidal)	2.5	.8
Silt loam (noncolloidal)	3.0	.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
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 channels, by 0.9 for moderately sinuous channels, and by 0.8 for highly sinuous channels.

(Ord. No. 93-2-22/O-1, § 9)