In Chapter 26 of the Code, the City addresses water discharge, particularly drainage towards an adjoining property or right-of-way. Within Section 26-44, if a property increases their impervious surface footprint - any surface in which water cannot be absorbed - they must offset that increase in potential discharge by storing/offsetting it and allowing it to absorb into the ground or be used for watering at a later date.
The main goal of this section is to make sure that water does not become a nuisance throughout the City. Water that is not properly kept or maintained upon a premises can cause a public nuisance. Therefore, water cannot cause a nuisance to a neighboring property or public right-of-way (Crestwood Code 16-56).
These types of impervious surfaces include, but are not limited to, roofs, decks, driveways, patios, sheds, pools, and other paved surfaces.
Step 1: Determine the scope of the project
- The first step in this process is to decide what the size/scope of the forthcoming project will look like. Am I going to put in a 10'x10' or 10'x12' shed? Am I going to redo my deck or patio? From there, go to step two.
Step 2: Calculate how much water (by volume) needs to be offset
- The second step in this process is to calculate how much water, by volume, needs to be offset. There is a tab to the left that further explains it but a general rule of them is that approximately 71 gallons of water will need to be offset for every 100 square feet of project.
- For example - If you are adding a 10'x10' shed in the back yard, you would need to offset approximately 71 gallons of water (100 sq ft = 71 gallons). If you are adding a 18'x3' extension of your driveway, you would need to offset approximately 38 gallons of water (54 sq ft = 38 gallons).
Step 3: Determine which offset to use for the project
- The third step in the process is to determine which offset to use for the project. There is a tab to the left - effective offsetting options - that gives a little more details and a picture of what that might look like.
Step 4: Get approval from the Department of Public Services
- The fourth step is to get approval from the Department of Public Services. These are the experts of stormwater management and will help you to combine your calculations and offsets to effectively finish your project. For more information, please contact the Department of Public Services at 314-729-4720.
How to calculate stormwater discharge?
The Missouri Botanical Garden notes that 90 percent of rainstorms in the greater St. Louis region are 1.14 inches or less of rainfall.
To calculate the volume that would be needed to offset these potential changes, one would need to multiply the square footage of the project by 1.14 inches to determine the cubic feet. One cubic foot is approximately 7.5 gallons of water.
For example: for every 100 sq. feet of project, a property owner would have to offset 9.5 cubic feet (71 gallons) of water.
While there are many options available to property owners, here is a list of some of the most effective offsetting options for stormwater runoff:
Rain Gardens use native shrubs, perennials and other plants/flowers to temporarily hold and soak in rain water runoff, allowing it to drain within 12-48 hours. The 5 to 1 ratio is set up for rain gardens, at a standard depth of 6 in. (For every 100 sq ft of project, 20 sq ft of rain garden needed).
French Drains use a gravel-lined or rock-filled ditch with an embedded pipe to collect runoff and reroute the flow of water for less water runoff to a specific area. Since rock is used in a French drain, water capacity is reduced by roughly 60 percent (For every 100 sq ft, a 1' x 1' x 1' drain, must have a trench of approximately 24 feet in length).
Dry Wells use a buried, perforated barrel to collect and slowly disperse water into the ground to be absorbed, causing significantly less runoff. The barrel should be buried empty, but surrounded by rock to aid with water discharge. If rock is used inside the barrel for any reason, that could reduce water capacity by up to 60 percent.