Stormwater lessons from Southern California – Performance Assessment

Concrete box culverts, like this one being installed in Washington state, are a standard way of managing stormwater. Green infrastructure can offer cost-competitive alternatives. Photo credit Washington State Dept. of Transportation.

This is the third in a three part series by MPC research assistant Matt Nichols.

As explained previously in the first and second installments of this series, southern California regulations now require that all new construction and major renovations incorporate elements of low impact development (LID) to manage stormwater better.  While a do-it-yourself project to install rain barrels or plant a rain garden – as we’ve been helping residents in Blue Island, Ill. do this month – can make a big difference for an individual homeowner, corporations and municipal entities with large facilities need to think bigger when it comes to managing stormwater.

In those cases, LID often takes the form of on-site measures that can be readily added to residential or commercial developments, such as permeable parking lots, bioswales, or underground cisterns to hold rain water. We’re helping residents and business owners in Chicago’s Logan Square neighborhood use grant money from the Illinois EPA to implement many of those types of projects. We’ve also reported on Chicago area businesses that have adopted such measures. Alternatively, off-site regional projects offer another effective way to manage run-off, typically at a lower cost. Examples of these larger, regional projects include adapting existing parks or open spaces to double as targeted groundwater recharge areas, as we saw at the Coca-Cola plant in Niles, Ill.

Because they closely imitate natural processes and do not require much construction, regional projects can cost as little as $0.40 per gallon of runoff treated, whereas more intensive urban methods of green infrastructure, like permeable paving and green roofs, can often cost $13 per gallon or more.  Likewise, constructing wetlands for stormwater retention and infiltration usually costs less than $3,000 per acre, versus nearly 10 times that cost for an equal area of coverage on residential green roofs.

In examining case studies from the San Fernando Groundwater Basin, medium- and large-scale stormwater management projects ranged in cost from $0.07 to $1.53 per gallon of water treated. (These figures refer to the initial construction costs of the projects; annual maintenance and operations costs are typically pennies on the dollar, or less.)  Predictably, the two most expensive projects from the sample group were on-site, while regional projects, like converting an old gravel processing plant to a water retention pond, were much more cost-effective.

Implementing green infrastructure in all new construction to manage rain where it falls is an admirable goal – but it should not stop regional planners from taking advantage of relatively “low-hanging fruit,” such as parks or reclaimed brownfields that achieve the same benefits at lower cost.

When development requirements, like the MS4 permit conditions mentioned earlier, require an accounting of the full costs of development, including impacts on the water cycle, green infrastructure often proves to be the least expensive mitigation strategy.  For example, in order to construct its new headquarters in Chino, Calif., the Inland Empire Utilities Agency (IEUA) would have needed to build a 10’x10’ culvert to channel run-off from the property in such a way that did not further contribute to erosion problems along Chino Creek.  This lengthy system of underground pipes would have cost more than $3.2 million.

The site developers presented two alternatives: 1) an on-site permeable parking lot and detention area with a volume of 8.1 million gallons and a cost of $1.8 million; or 2) a regional Chino Creek Park employing wetlands to store 29.6 million gallons at a cost of $3 million.  Both options were considerably more cost-effective – in terms of dollars-per-gallon – than the traditional culvert would have been, and in the end, IEUA was able to afford to fund both measures. The robust accounting procedures in the MS4 permitting requirements made it possible for IEUA to compare the costs and benefits of each option in quantitative terms, rather than attempting to measure the performance of a fluid, dynamic system after the fact based on imperfect downstream approximations.

As green infrastructure becomes a more prevalent part of water planning discussions around the country, it’s important to keep in mind that not all project types are created equally.  Some are more cost-effective or simply more appropriate for a given site due to practical concerns, as the case studies in southern California demonstrate.  Additionally, stormwater management can be targeted toward achieving one or more specific goals, including reduced neighborhood flooding, increased aquifer recharge, and decreased run-off and surface water pollution. Regional priorities regarding these goals can help direct the project types implemented at the local level.  Southern California has proven that a pro-active approach to water planning can both drive the use of innovative, cost-effective technologies and restore balance to natural systems.

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