Designing CMP Detention Systems: Part One – The Value of Going Underground

The two essential functions of a stormwater management system are to control the quality and quantity of runoff leaving a site. There are various ways to do this; a common method is with an above ground system in the form of a detention pond.

The problem with ponds is that they take up usable land space therefore not allowing one to maximize their return on investment. Perhaps this is not a major issue in rural areas, but in urban environments land space is expensive, and the loss of land space results in lost revenue.  The question then becomes how to quantify the value of putting the pond underground in a detention system? One method is to calculate the value of additional leasing space gained if a pond is placed underground, allowing the developer to maximize their return on investment by using all available land space.  The easiest way to explain this is with an example.

A typical parking space is 9’ x 18’ or 163 square feet. For every 1,000 square feet of developed space, the developer is typically required to have 2.6 parking spots. In other words, 1,000 square feet of developed space requires 421.2 square feet for parking. The more developed space you have, the more parking spaces you will need. You just need to ensure the amount of developed space plus the amount of parking space does not exceed the lot size.

Now let’s look at a parcel that has two, one acre ponds. One developer may look at the parcel and decide he could never maximize the return on his investment, as they cannot fully use all the land space. Another developer could look at the same parcel and see it as a great opportunity. Those two ponds represent 87,120 square feet of footprint. By putting the detention system underground, the second developer will have an additional 87,120 square feet for parking which will allow for an additional 206,000 square feet of office space. With office rates ranging from $13-$23 per square foot annualized, this represents $2.7MM- $4.7MM of additional revenue.  Extrapolated over a ten year timeframe, the developer could make an additional $25MM-$50MM. In areas such as New York City and San Francisco, the return will be even higher, with rates ranging between $65 -$74 per square foot. 

The main point is owners, developers, and engineers can all benefit by exploring the use of below ground stormwater detention systems to maximize land space which maximizes revenue.

In part two of our series we look at cost efficient design and layout of CMP detention systems.

In part three of our series we look at detention design red flags.