Stormwater Detention Policy

Stormwater Detention Policy

STORMWATER DETENTION POLICY (Revised November 23, 2015) 
The purpose of this document is to provide policy direction for the provision of stormwater detention systems for new developments .
The objective of this Policy is to ensure that developments, where required, provide stormwater detention that temporarily stores the difference in volume between the City’s pre-development and post-development design storms.The design storms to be analyzed are the 25-year, 50-year and 100-year return periods. Each design storm must be analyzed for the 1 hour, 2 hour, 6 hour, 12 hour and 24 hour durations.The proposed detention system must limit the post-development runoff rate from the development for each return period/duration to the respective pre-development runoff rate for the same return period/duration. All computer modeling of stormwater detention must be done with the latest version of XPSWMM. All detention designs should ensure the preservation of the environment and fish habitat.
The Policy applies to all new developments within the City of St. John’s with the exception of:

  • Downtown St. John’s - subject to City infrastructure having sufficient capacity as determined by the Department of Planning, Engineering and Regulatory Services;
  • Infill development can be exempted from stormwater detention provided that the Developer demonstrate that there will be no downstream issues associated with capacity, flooding, erosion control, and velocities;
  • Cemeteries, grassed playing fields, and vegetated areas of public sports and recreational facilities;
  • Where there is a written agreement between the Developer and the City to provide stormwater infrastructure improvements that remedy the downstream flooding problems in lieu of constructing a stormwater detention system. The Developer would be required to provide the City with a certified cheque or an acceptable Irrevocable Letter of Credit for the value, as determined by the City, of the downstream flood remediation work; and
  • Other areas where the Department of Planning, Engineering and Regulatory Services determines, based on hydrologic/hydraulic  analysis,  that  stormwater  detention  is  not necessary, or may be permissible at a reduced level.


The Policy came into effect January 1, 2013, and this revision is effective immediately.  
It is the responsibility of the Developer(s) to submit for City approval a stormwater management plan which meets the requirements of this Policy. The City reserves the right to accept or reject the stormwater management plan, or propose amendments to the plan. Where requested by  the Developer, the City may provide guidance as to the type of stormwater detention which might be acceptable for a particular development. The latest version of the City’s Subdivision Design Manual provides the design methodology that the Developer must use to design and construct the stormwater detention system.
The City may, where it is considered more effective, direct Developers to cooperate in, and fund the cost of, a regional detention system as a condition to a development(s) proceeding. A regional detention system would establish large scale stormwater detention structure(s) to meet this Policy’s requirements for several developments within a geographic region. Similarly, a Developer(s) may also propose a regional stormwater detention system to the City.

Developers  will  fund  all  costs  of  stormwater  infrastructure  constructed within the borders of their property. In the case of a regional stormwater detention system, where the detention infrastructure serves more than one development, the regional detention infrastructure costs will be shared among developers in proportion to the amount of stormwater volume each development is expected to detain. Where the City must upgrade its infrastructure outside the borders of the development, the City may recover its costs, including interest and financial charges, through assessment charges/fees against developable properties served by, or to be served by, the stormwater detention system.
Stormwater detention systems in residential developments may be accepted for  ownership  and  maintenance  by  the  City. Detention  systems  in Commerical, Industrial, or Institutional developments will not be accepted for ownership by the City.
Stormwater detention systems whose ownership is to be conveyed to the City are subject to the following requirements:

a)  The Developer must construct a stormwater detention system in accordance with the approved engineering plans and must convey the system, and associated lands, at no cost to the City as a condition of Final Approval.

b)  The system must be 100% complete (in accordance with the approved plans), operational, and commissioned in the presence of the Water & Wastewater Division (or their designate).The Developer must continue to own and maintain the detention system until accepted by the City.
c)  The City, at its discretion, may require a field test to demonstrate the maximum discharge rate from the detention facility for a designated water level/head.