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When Detention Regulations Don’t Meet Detention Expectations

Runoff from a new development is often conveyed to a detention pond that temporarily stores water and reduces the increase in flows. Since detention is intended to prevent localized flooding, questions begin to arise when downstream residents complain of new flooding issues after construction. Does the detention design meet criteria? Was there an abnormally large storm event? Does the criteria need to be reevaluated?

Recently, the Town of Sunnyvale was confronted with several questions from residents when two recently constructed schools possibly created new downstream flooding issues, despite installing detention ponds. FNI assisted the Town in reviewing and analyzing the detention ponds to provide some answers.

History and Background of the Detention Ponds

The Sunnyvale Middle School was built in 2004 followed by the High School in 2008. Separate detention ponds were constructed for each new school to help reduce flows and prevent localized flooding, as shown in Figure 1. Soon after construction, residents noticed higher than normal water surface elevations in the channel downstream of the two ponds. In an effort to alleviate the increase in water levels, the Town of Sunnyvale reduced the size of the pond outfall structures, as shown in Figures 2 and 3. The smaller openings were intended to store more water during smaller storm events and reduce the amount of runoff downstream. Higher-than-pre-construction water levels were still noted during the May 2015 storm events even after installing the smaller outfalls. The Town then asked FNI to investigate further.

Detective Work

The results of FNI’s investigation showed that the original pond designs appropriately followed Sunnyvale criteria to detain the 100-year storm event. The smaller outfall structures would detain the two-year storm event; however, with these smaller structures, the ponds do not have enough storage to hold water during larger events without overtopping the spillway.

FNI then reviewed the rainfall data in an effort to better understand the size storm event experienced in May 2015. It was determined that two of the larger events experienced were less than the one-year return event, as shown in Figure 4. The results support the public perception that the channel was experiencing higher water levels during the May 2015 rain events because the ponds were not designed to hold water during events less than a two-year storm.

How do we make it better?

The scheduled addition to the middle school created an opportune time to recommend detention pond modifications. After discussions with the Town Manager, it was decided that FNI would study the feasibility of installing an outfall that would release the one-year storm event over a 24-hour period, while still meeting pre-development peak flows for the 25-year and 100-year storm events. The final recommended design required combining the two detention ponds to provide adequate storage for the 100-year storm event while concurrently providing detention for the one-year storm event to pre-development conditions. The recommended design for the one-year storm event is an 8-inch PVC pipe opening. The 25-year and 100-year storm event flows will be reduced by a 25-foot weir or equivalent opening to match flows before construction of the two schools.

Criteria Evaluation

The Town of Sunnyvale is currently updating their drainage criteria and took this experience into consideration when deciding to modify their new development detention requirements. While the standard criteria will continue to require detaining the 100-year storm event to pre-development flows, the Town is adding an optional requirement of detaining the one-year storm event over 24 hours at the Town Engineer’s discretion, based on downstream impacts.

Moral of the Story

While common practice for drainage criteria is to detain and design for a 100-year storm event, it is important to understand that residents experience smaller events more regularly. Although detention may have been provided for a new development, residents downstream of a pond designed for the 100-year storm may experience increased flows, velocities, and water surface elevations during smaller rain events due to development. It is important to understand a detention pond’s performance for different storm events, especially in areas with sensitive downstream conditions. Detaining for a smaller storm event over a 24-hour period may increase the required storage volume of the pond, but can provide the benefits of reducing flows downstream of the project, protecting against erosive conditions, and increasing the water quality performance of the pond.

For more information on sizing a detention pond for the one-year, 24-hour storm event, reference the iSWM Hydraulic Technical Manual, or contact Freese and Nichols Stormwater Engineer Lesley Brooks.

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