§ 46-105. Minimum requirements for major developments.

The following minimum requirements for stormwater quantity control shall apply to all land developments that meet the definition of a major development:

(1)

All surface water and stormwater entering the development site in its pre-development state shall be received at the naturally occurring or otherwise legally existing locations. All surface water and stormwater leaving the development site shall be discharged at all times during and after development at the naturally occurring or otherwise legally existing locations so as not to be diverted onto or away from adjacent downstream properties, except diversion which will correct an existing manmade downstream problem may be permitted by the director. For the purposes of this chapter, the term "naturally occurring location" shall mean the location of those channels, swales, and pre-existing and established systems as defined by the first documented topographic contours existing for the subject property, either from maps or photographs, site inspections, decisions of a court of law, or other means determined appropriate by the director.

(2)

The volume of water to be detained and the rate at which water may be gradually released from detention shall be calculated in a manner consistent with sound engineering practices and shall be based on the characteristics of the drainage basin in which the proposed construction is located.

(3)

The post-development peak stormwater discharge rates from the development site for the 25-year, 24-hour duration storm events shall at no time exceed the pre-development peak stormwater runoff rates for the same design storm events, except as expressly permitted by this chapter.

(4)

When a proposed project is contributory to a closed depression located off site, the volume of runoff discharged may not be increased for the 25-year, 24-hour duration. The exception to this requirement is in the case where discharge would not result in an increase in water surface elevation of greater than 0.01 foot for the 25-year storm events.

(5)

Major developments shall provide stormwater quantity control facilities designed to meet, as a minimum performance standard, the requirements of this section, except in the following circumstances:

a.

The development site discharges directly into Red River.

b.

The development site discharges to a regional stormwater facility approved by the director to receive the development site runoff.

c.

The development site discharges to a receiving body of water (lake, wetland, etc.) where it can be demonstrated by the applicant, to the satisfaction of the director, that stormwater quantity control is not warranted.

d.

The director may require that runoff from a development site be controlled for additional design storm events.

(6)

In the event that conditions downstream from a proposed development site are determined by the director to be exceptionally sensitive to potential stormwater discharges from the subject site, the director may require a factor of safety be applied to the total retention/detention storage volume and/or a reduction of allowable stormwater release rates.

(7)

Submittals for all proposed development projects shall include an analysis of downstream water quantity impacts resulting from the project and shall provide for mitigation of these impacts. The analysis shall extend a minimum of one-fourth of a mile downstream from the project. The existing or potential impacts to be evaluated and mitigated shall include, but not be limited to, excessive stream bank erosion, flooding, surcharging of existing closed drainage conveyance facilities, discharge to closed depressions, and discharge to existing off-site runoff control facilities.

(8)

Retention facilities and open stormwater quantity control facilities shall not be located in dedicated public road rights-of-way.

(9)

Reasonable access for maintenance, as determined by the director, shall be provided to all stormwater facilities.

(10)

As the first priority, streambank erosion control BMPs shall utilize infiltration to the fullest extent practicable, only if site conditions are appropriate and ground water quality is protected. Streambank erosion control BMPs shall be selected, designed, and maintained according to good engineering judgment. Stream bank erosion control BMPs shall not be built within a natural vegetated buffer, except for necessary conveyance systems as approved by the director.

(Code 1985, § 27-41; Ord. No. 4717, 11-21-2007)