Maximizing Project Value with Innovative Detention Sizing

POSTED April 8, 2019

Would you be surprised if we told you efficient design of drainage detention facilities is commonly overlooked? It happens quite often since the common practice is the use of a simplified spreadsheet to calculate the needed storage volume. Unfortunately, the method can be very conservative, and the larger the project area is, the more conservative it can become.  This “Easy Button” doesn’t always help the client. It can eat up valuable real estate and money resulting in an oversized detention system.  Better engineering tools and a little more design time is worth the effort to find the optimal pond size.  This can be done in conjunction with providing a proper overflow system that protects the site from extreme storm events.  Not only will you save money on construction but also increase the useable area of the site. Our method can add parcels to a residential development and increase building size or parking area for a commercial development.

There are a few factors and requirements to consider when sizing detention facilities. Parcel development increases impervious areas like parking lots and buildings as well as surface storm water runoff. Most cities require developers to limit surface storm water runoff during post-development conditions. They require developers to match pre-development runoff or a runoff release rate specified by that city. To comply with these runoff release rate requirements an aboveground/underground detention facility is designed. Storm water surface runoff is then temporarily stored until the total volume of runoff can be released safely over a controlled amount of time. As one can see the size of your detention facility is crucial if you want optimal functionality.  Oversizing will lead to greater cost in both construction and maintenance.

The “easy button” approach mentioned previously, uses a simplified equation to estimate peak runoff rates. The equation wasn’t created to determine surface storm water runoff volumes or the sizing of detention facilities. Yes, this simplified approach can quickly determine a storage volume but oftentimes results in a conservative storage estimation. And I bet you guessed it, you end up with an unnecessary, expensive, and oversized detention facility. If you were to also choose above ground detention then you also lose valuable real estate. Below is an example of an easy button calculation used to estimate the storage volume needed for an 11-acre parking lot project in Salt Lake City, UT.

We have great news though, Avenue has an optimal approach to get you the most bang for your buck. We use hydrologic software programs that accurately model city-specified rainfall events. We assign a rainfall distribution that accounts for variation of rainfall intensity throughout the duration of a storm. This precisely determines the surface storm water inflow and discharge rates as well as the required storage volume for a given site.  In short, our method determines the detention pond size with truly optimal accuracy. Below is a figure that shows the inflow and outflow rates as well as the required storage volume needed for the same parking lot site in Salt Lake City, UT.

A recent project for a parking facility in Salt Lake City had an 11-acre parking lot site. The easy button approach would have sized the detention facility with 54,000 cubic feet of storage and an installed cost of $260,000.  (see image below)

With Avenue’s approach our team used appropriate hydrologic modelling software to size a detention facility with 44,000 cubic feet of storage and an installed cost of $212,000. (see image below)

Our client saved approximately $48,000 in installation costs for their detention facility. The cost savings from using the optimal approach was more than our entire engineering design fee. Hiring the right team can quickly add value in every aspect.

Hitting the easy button might be quick but it’s not easy on the wallet. We can help your project avoid these hidden costs. See how we can help you optimize your project, please contact us at