§ 112-128. Design criteria.

The city's stormwater drainage design manual (drainage manual) latest version shall be the accepted design document. It is the responsibility of the developer and the engineer of record to verify that they are using the latest version of the drainage manual in the design of their proposed development. Unless otherwise provided, the following rules shall govern the design and improvements with respect to managing stormwater runoff:

(1)

Method of determining stormwater runoff. Developments where the upstream drainage area contributing runoff is less than 100 acres should be designed using the SCS, Type II 24-hour distribution, TR-55 Hydrograph Method for calculating runoff. Developments where the area contributing runoff is greater than 100 acres shall use the city basin analysis for calculating runoff. If not available, the FEMA Data shall be used for calculating runoff. If neither the basin analysis nor the FEMA Data is available, the SCS TR-55 Hydrograph Method for calculating runoff shall be used.

(2)

Differential runoff.

a.

For undeveloped property, peak flow rate, and velocity of stormwater runoff from the site, post-development, should to the extent practicable approximate that of the site prior to the development.

b.

For redeveloped property, peak flow rates, and velocity of stormwater runoff from this site, post-development, should be reduced by an amount equivalent to a 20 percent reduction in impervious area from the previously developed site.

(3)

Development design.

a.

Developments shall be located and laid out in such a manner as to minimize the velocity of overland flow, allow maximum opportunity for infiltration of stormwater into the ground, preserve and utilize existing and planned streams, channels, detention basins, retention basins, and include wherever possible, streams and floodplains within parks and other public grounds.

b.

Streets, curbs and gutters, parking areas, enclosed conveyance systems, detention basins, retention basins, and other generally accepted practices and methods for stormwater control may be a part of the overall stormwater runoff management systems for a particular site. To the maximum extent possible, these facilities shall be concurrently designed to effectively manage stormwater runoff in accordance with these regulations.

(4)

Enclosed systems and open channels. Enclosed systems and open channels shall be designed in accordance with the drainage manual.

(5)

Evaluation of downstream flooding.

a.

The engineer of record shall evaluate whether the proposed plan will cause or increase downstream flooding conditions within the drainage sub-basin in which the project is located or if it will otherwise increase peak flows from the drainage sub-basin. This evaluation shall be made on the basis of existing downstream development and an analysis of stormwater runoff with and without the proposed development. When it is determined that the proposed development will cause or increase downstream flooding conditions, provisions to correct such cause or increase shall be included in the overall design of the stormwater management system improvements. Such provisions may include downstream improvements or detention of stormwater runoff and its regulated discharge to the downstream storm drainage system, or both.

b.

When it is determined that a localized flooding condition exist downstream of a proposed development, the engineer of record for the project shall notify the city engineer of this discovery and include sufficient documentation in the project submittals to assist in the delineation of the identified special flood hazard area (SFHA).

(6)

Detention. The following design criteria shall govern the design of detention facilities:

a.

Release rate. The release rate from any detention facility shall be equal to or less than that of the site prior to the proposed development.

b.

Freeboard. Detention storage areas shall have adequate capacity to contain the storage volume of tributary stormwater runoff with freeboard in accordance with the drainage manual.

c.

Outlet control works.

1.

Outlet works shall not include any mechanical components or devices and shall function without requiring attendance or control during operation.

2.

Size and hydraulic characteristics shall be such that all water and detention storage is released to the downstream stormwater conveyance systems within 24 hours after the end of the design rainfall. Normal time for discharge ranges from three to 24 hours.

d.

Spillway. Emergency spillways shall be provided in accordance with the drainage manual.

e.

Design data submittal. In addition to complete plans, all design data shall be submitted as required in the detention design data submittal section of the drainage manual.

f.

Detention methods. Depending upon the detention alternative or alternatives selected by the engineer of record, the design criteria for detention shall follow those given in the drainage manual.

(7)

Alternatives to on-site detention.

a.

Alternative methods.

1.

Where on-site detention is deemed inappropriate, alternatives to on-site detention shall be permitted. The methods may include off-site detention or improvements.

2.

Determinations regarding the appropriateness of on-site detention shall be made by the city engineer based upon the impact of the proposed development on existing drainage networks and the location of the project in relation to existing floodplains, regional detention facilities, and other planned drainage or channel improvements. Disputes, if any, shall be resolved by the stormwater management board.

3.

Determinations regarding the appropriateness of off-site detention or comparable improvements shall be made by the city engineer based upon the impact of the proposed development on existing drainage networks and the location of the project in relation to existing floodplains, regional detention facilities, and other planned drainage or channel improvements. Disputes, if any, shall be resolved by the stormwater management board.

b.

Excess stormwater storage credit. A developer may receive credit for excess stormwater storage (in acre-feet) created on one site that may be applied to another site within the same watershed. The transfer of storage volume credit (in acre-feet) shall not be allowed when the site where credited storage is proposed to be transferred has an existing flooding condition downstream or the proposed development will cause downstream flooding.

(Ord. No. 08:099, § 2(4.1), 12-18-2008; Ord. No. 11:064, § 4, 10-4-2011)