Excerpts from a conversation with Bennett Engineering Services’ Tony Ozanich, PE, Director of Land Development Services and the firm’s Hydraulics and Hydrology specialist, Donald Jones, PE.
What are detention basins and how are they used?
Detention basins are surface storage basins or facilities that provide flow control through attenuation of stormwater runoff. They also facilitate some settling of particulate pollutants. Detention basins are normally dry and in certain situations the land may also function as a recreational facility. Therefore, co-locating sports facilities with large turf areas and other recreational features on these sites is an effective way to gain maximum use of the property.
In master plans, municipalities may designate multi-use basins for both recreational uses and to address stormwater flooding issues. Private developers often use multi-use facilities to meet stormwater hydromodification and park development requirements simultaneously.
Several issues and opportunities can arise when engineering multi-use stormwater detention basins. These may include sizing, site access, water quality and treatment, and managing nuisance water.
What are some of the sizing issues?
Specifying the site’s ultimate use is essential when determining dimensions during design development of a detention basin. The design may need to be larger than necessary for stormwater capacity to adequately accommodate the desired sports fields.
“In the City of Biggs (where Bennett is the consulting city engineer) the master drainage study determines the need for detention capacity and the approximate locations for these new detention facilities,” said Tony Ozanich. “Further planning of projects determine whether detention basins will be multi-use facilities.”
To further the development of a multi-use design the following questions must be answered:
- Will the facility be used for soccer and/or other field sports? If so, what level of play?
- Can the basin be oriented in a way that works for the intended recreational sport?
Depending on age groups, soccer fields can range from 15 x 30-yards to 80 x 120 yards while other recreational activities have significantly different requirements for sizes and configurations. Likewise, soccer fields should be oriented so that the sun does not set behind a goal, causing an unfair advantage to one team.
Public and/or private golf courses can also provide an expansive opportunity for detaining stormwater in water features. During a larger stormwater event, it may be an option to inundate the fairways.
“Municipalities could encourage the use in private course development and request the designers to look at this opportunity to mitigate potential flooding,” said Ozanich. “Designers would need to elevate tees, greens, restrooms, and other facilities so that they are above the inundation zone.”
Redeveloping older parks and golf courses to accommodate stormwater comes with more challenges than new development but also offers an opportunity for municipalities to address current flooding issues.
What are some of the challenges when designing detention basins?
Site design issues start with access design. That includes adequate parking or alternative transportation options for players, families, and friends. Americans with Disabilities Act (ADA) compliant access to the field needs to be addressed. Players and spectators require space to sit and walk around the fields without disturbing play.
Sports facilities also frequently include irrigation, restrooms, drinking fountains, and lighting that require utility design and need to be strategically placed.
Safety can be a major issue with detention basins when inundated. Side slopes need to be designed so that people and animals can safely exit the basin. Typically, a 5:1 side slope is adequate but sometimes slopes that alternate between slope and flat areas are desired.
How is stormwater flow and nuisance water managed?
Each project is evaluated and optimized, but as an example, the team is designing a multi-use facility in Sacramento that includes soccer fields. The facility has multiple inlets and outlets that allow two-year/24-hour storms (85% of all storms) to drain quickly without inundating the field for long durations. During larger storm events the recreation aspect of the facility will not be used, and the detention basin will attenuate the peak flows. Nuisance water, either from sprinklers or short-duration events, is drained with ribbon drains that are strategically placed at the edges of the fields. The ribbon drain system is tied into the park’s drainage inlet structures.
What are some unusual engineering challenges and solutions?
“Sacramento and San Joaquin Valley communities near rice fields may have water tables only four to seven feet below ground, which is an issue that needs to be addressed,” notes Ozanich. “The easiest solution is to make the field shallow and large.”
If additional space is unavailable or unaffordable or there is no natural outfall, pump stations can be used to direct water into canals or pipelines that flow to a natural outfall.
“A few years ago in a Modesto area development, I designed stormwater flow into an irrigation district’s canals,” said Don Jones, PE. “Agricultural uses may have limits to the amount of water or when the water can be pumped, but they are an optimal user in droughts. Dedicated wetlands can also receive some water and provide water quality benefits.”
Are there water treatment and quality issues?
Detention basins can be a key element of a treatment train (treatment or removal of stormwater volume or pollutants) that improves water quality as it goes through the system. Runoff is sometimes directed through low impact design (LID) measures, such as smaller water quality basins, tree planters, and grassy swales along roadways and parking lots before reaching the detention basin. The scale of multi-use detention basins allows wide, diluted settling of sediments and pollutants that remain.
If you would like to discuss how detention basins might help your project, contact Tony Ozanich, PE, Director of Land Development Services, or our Hydraulics and Hydrology specialist, Don Jones, PE, or (916) 783-4100.