Stormwater Management

Managing Stormwater for Your Project

Chapter 26 of the City Code addresses water discharge, specifically the impact of drainage onto neighboring properties or public rights-of-way. According to Section 26-44, when a property increases its impervious surface area (any surface that prevents water from soaking into the ground) the property owner must manage the resulting runoff. This includes storing or redirecting the water so it can either be absorbed on-site or used later for purposes such as irrigation. Examples of impervious surfaces include but are not limited to roofs, driveways, decks, patios, sheds, pools, and other paved or hardscaped areas

The purpose of this regulation is to prevent stormwater from becoming a nuisance. Uncontrolled runoff can lead to property damage, erosion, and other issues. Per Crestwood Code Section 16-56, water discharge must not create a nuisance for adjacent properties or public spaces.

If your upcoming project adds an impervious surface to your property—like a shed, patio, deck, or driveway extension—you are required to offset the additional stormwater runoff it may generate. As established by Ord. 5119 in January 2021, the goal of this requirement is to prevent excess water from flowing onto neighboring properties or public rights-of-way.

Step 1: Determine Your Project Size

Start by identifying the square footage of the impervious surface you're adding. For example:

• A 10' x 10' shed = 100 sq ft
• An 18' x 3' driveway extension = 54 sq ft

Knowing this number will guide your offset requirements.

Step 2: Calculate How Much Water You Need to Offset

Most storms in the St. Louis region produce about 1.14 inches of rain. That’s the amount used to estimate how much runoff your project could create. Here’s how to calculate it:

• Square footage × 0.095 = Cubic feet of runoff
• Cubic feet × 7.5 = Gallons of water to offset

Or use this shortcut:

• For every 100 sq ft of impervious surface, you need to offset approximately 71 gallons of water.

Step 3: Choose an Offset Method

There are several ways to manage stormwater runoff on your property. While the best option may depend on your available space, budget, and landscape, here are some of the most effective:

• Rain Gardens: Rain gardens use native plants and shallow depressions to collect and absorb water within 12–48 hours.
  • Rule of thumb: For every 100 sq ft of project area, install 20 sq ft of rain garden (5:1 ratio at 6 inches deep).
• Rain Barrels: Rain barrels capture water from roofs and store it for future use or slow release. Most off-the-shelf barrels hold 50–80 gallons. 
  • Rule of thumb: For every 100 sq ft, store at least 71 gallons.
• French Drains: French drains use gravel-filled trenches with pipes to redirect water. Because gravel displaces space, capacity is reduced by about 60%.
  • Rule of thumb: For every 100 sq ft, a 1' x 1' x 1' trench must be at least 24 feet long.
• Dry Wells: Dry wells are underground barrels that temporarily store water and allow it to slowly absorb into the ground. They are usually surrounded by gravel to help with drainage. If filled with rock, the well’s capacity can drop by 60%, so it’s best to keep the barrel itself empty.

Step 4: Submit for Review

Before beginning construction, contact the Department of Public Works to review your plan and ensure it meets City requirements. Staff can help you finalize your offset method and answer any questions.

• Questions? Contact the Department of Public Works at 314-729-4720.