Analyzing Your Home's Stormwater Flow Analyzing Your Home's Stormwater Flow

Stormwater flow originates during precipitation events and may include snow-melt or runoff from overwatering. If left uncontrolled, it can present several problems.

Potential Problems of Storm Water

  • Rotted Wood - Even with treated wood, flowing (and standing) water can cause wood rot. It can infiltrate behind fascia and soffits to untreated wood. 
  • Insects - Water that is not transported away can serve as a breeding ground for insects and other pests.
  • Mold - Combine wood, wet and warmth, and you have the recipe for potentially dangerous mold.
  • Damage to Concrete - Foundations, walkways and other concrete is porous. Running or standing water can seep into the concrete or undercut it, causing settling or cracking. This is especially true in colder climates, where absorbed water turns to ice.
  • Pollution - Water picks up dirt, leaves and even herbicides as it moves through your land. This flows downstream.

Stormwater Management

With good drainage design, you can direct the stormwater drainage into appropriate channels using gutters, berms, swales, catchments or basins. You can use it to water your landscaping, or harvest your stormwater into a cistern for on-demand use. In order to manage your stormater effectively, you must first determine the amount of rainfall on your land and where it flows.

Determining Total Rainfall

To figure out how much rainfall you will have to manage, take the total square footage of your property and multiply it by the average daily rainfall (ADR) for your area. This information is available online from the National Weather Service. This gives you the daily average of total volume of stormwater runoff that you will have.  

Next,determine the total amount of stormwater for each area of your property you wish to manage. For each downspout, determine the impermeable (non-porous) surface area that drains to it. Multiply by the ADR, and you have the volume. Or, if you have 4 downspouts, take the total footprint of your house (length times width) and divide by 4 (roof slope does not impact this). This is the drainage area of each downspout. Multiply this by the ADR.

For the rest of your property, you will need to actually walk the land. Find the high points—anywhere the land slopes down, stormwater will drain. Pay careful attention to built-up areas such as landscape beds, walkways and even your garage. Draw a small map of your property and indicate each high point, with arrows showing where water will flow. If you can’t visualize this, get outside during a rain and watch where the water goes. Alternatively, you can simulate the effect with a hose.

Once you have a map of the areas, measure them. This will give you hard data on not only where the water flows, but how much you will have to deal with. This volume will impact everything from the size of a swale to the diameter of piping you use.

After you have determined the flow and the volume of stormwater, you can move to the next step: planning on how to harvest, use or transport the stormwater most effectively.

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