Stormwater Management Guidelines
The Planning Department and the National Roads Authority have been working together to draft revised procedures for addressing stormwater management on Grand Cayman. These policies are available for public review, as well as an online form to assist applicants with determining their stormwater infrastructure requirements. Questions or comments can be directed to [email protected]. Please click here.
Guidelines for Stormwater Management Plan Preparation
Developments are encouraged to implement state-of-the-art techniques that manage stormwater runoff through redesigned streets, open spaces that take into account the natural drainage characteristics, and overall designs which require less corrective measures in the long term. When a site is developed, the design should maintain drainage characteristics of the site as much as is feasible through innovative design and use of alternative construction techniques. Development sites should be designed so that post-development stormwater runoff is less than pre-development runoff.
For major developments, applicants shall demonstrate that the Stormwater Management system can be designed to include storm water runoff produced from a rainfall intensity of 2 inches per hour for one hour of duration and ensure that surrounding properties that are lower and nearby public roadways are not subject to stormwater runoff from this site.
In preparing a Stormwater Management plan, the following is required
>> Applicant information such as Name, legal address, and telephone number, common address and legal description of site as well as contact name of designer of the SWM Plan
>> Mapping of proposed development site and surrounding area (recommended scale of 1" = 50.) which illustrate at a minimum:
- Topography (minimum of 1-foot contours recommended)
- Predominant soils from soil surveys
- Boundaries of existing predominant vegetation and proposed limits of clearing
- Location and boundaries of resource protection areas such as wetlands, lakes, ponds, and other setbacks (e.g., stream buffers, drinking water well setbacks, septic setbacks)
- Location of existing and proposed roads, buildings, and other structures
- Existing and proposed utilities (e.g., water, sewer, gas, electric) and easements
- Location of existing and proposed conveyance systems such as grass channels, swales, and storm drains # Surface Run-Off flow paths
- Location of floodplain/floodway limits and relationship of site to upstream and downstream properties and drainages
- Preliminary location and dimensions of proposed channel modifications, such as bridge or culvert crossings
- Preliminary location, size, and limits of disturbance of proposed stormwater treatment practice
>> Hydrologic and hydraulic analysis including:
- Existing condition analysis for runoff rates, volumes, and velocities presented showing methodologies used and supporting calculations
- Proposed condition analysis for runoff rates, volumes, and velocities showing the methodologies used and supporting calculations
- Preliminary analysis of potential downstream impact/effects of project, where necessary
- Preliminary selection and rationale for structural stormwater management practices
- Preliminary sizing calculations for stormwater treatment practices including contributing drainage area, storage, and outlet configuration
Preliminary landscaping plans for stormwater treatment practices and any site reforestation or re-vegetation.
For smaller developments, should the applicant elect to address site’s drainage conveyance system by vertical wells, a minimum of 1 well per every 4,500 square feet of impervious surface area shall be provided, which is the combined square footage for sidewalks, parking and roof areas. Stormwater drains shall be located in the parking lot and soft landscaped areas with parking areas sloped towards the drains. The minimum specifications of the wells hall be one hundred (100) feet deep and eight (8) inches in width.
Prepared by the Transportation Planning Unit
National Roads Authority
Revised June 2006