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Storm Water Retention Policy

The City of Fargo has developed a storm water master plan. A major part of the plan addresses the need to control storm water runoff from developed and developing areas. Storm water retention is required for all new development and should be included as part of the developer's master plan. Storm water retention is also required for new sites in existing developments.  

Local retention is classified as a retention pond or facilities that provide storm water retention on an individual property or site. Local retention facilities are constructed and maintained by the individual property owner.

Regional retention generally refers to larger facilities that are easily maintained and, generally, more cost effective to build. The City of Fargo believes that regional retention provides the most effective way to control localized runoff. The City of Fargo encourages developers to provide regional, rather than local, retention. Regional or area-wide retention facilities are classified as publicly constructed retention ponds installed to serve multiple properties. They may also be ponds privately built to city standards within an individual development not exceeding a quarter-section of land and meeting the requirements listed below.

The City of Fargo’s storm water retention policy is a supplement to the North Dakota Department of Health requirements.  All proposed development and redevelopment will be required to, at a minimum, meet the North Dakota Department of Health permit NDR04-0000, effective date July 1, 2009.  In scenarios where the City of Fargo’s storm water retention policy exceeds permit NDR04-0000 requirements, the City’s policy shall apply.

The City of Fargo is currently in the process of updating the storm water retention policy.

Retention requirements for development: 

A.       For sites under one acre:

It is not practical to require retention on very small sites. On-site or individual retention will not be required; however, retention is required on a regional basis. Regional retention is based on the acreage being developed and shall meet the requirements shown below.

B.       For all sites between one acre and less than two acres:

On-site retention is not required for sites with less than or equal to 65% imperviousness; however, retention is required on a regional basis.

On-site retention is required for sites with greater than 65% imperviousness.

Retention based on equation method: 10–year and 100–year requirements

Discharge is two–stage: 10–year allowable and 100–year allowable

Note: On–site retention requirements will be waived if regional retention is provided.

C.       For developments or sites greater than two acres and less than 25
          acres:

On–site retention is required for all lots.

Retention based on equation method: 10–year and 100–year requirements

Discharge is two–stage: 10–year allowable and 100–year allowable

Note: On-site retention requirements can be waived if regional retention is provided and storm water discharge can be delivered to the regional retention facility without impacting existing drainage conditions. 

1. This can be accomplished if the site is in close proximity to the regional facility.

2. This can be accomplished by providing increased capacity of mainline storm sewers between the site and the regional facility.

3. If the runoff coefficient is greater than the design coefficient of the regional retention, on-site storage providing the volume difference between the two methods will be required, with the discharge pipe size limited to the 100–year pre–development standard.

Retention/Detention Requirements Calculator   
for less than 25 acres (pdf)

D.       Sites and developments greater than 25 acres:

Retention is generally required for all developments or sites in this category unless regional retention is provided.

Retention based on modified equation method

(Retention requirements are increased by requiring 0.2% additional capacity for every acre in the drainage area.)

Discharge calculated by the large–basin equation method: two–stage, 10–year and 100–year allowable

If regional retention is provided, and if the runoff coefficient is greater than the design coefficient of the regional retention, on-site storage providing the volume difference between the two methods will be required with the discharge pipe size limited to the 100–year pre–developed standard.

Retention/Detention Requirements Calculator   
for greater than 25 acres (pdf)

If you have further questions regarding required retention, allowable discharge and typical development requirements please contact the Engineering Department at (701) 241-1545