The Assisted Living Project at Whittier in Frederick, M.D. is a 6.3 acre retirement community situated next to Whittier Lake. Planned by Fox & Associates, it included special design considerations for a cost-effective and attractive means of treating stormwater.
To detail the Whittier site, it is comprised of a retirement assisted living facility which is 19,151 sq. ft., a retirement medical care facility which is 26,950 sq. ft., a handicapped accessible walk system, courtyards and exercise paths, and a 47 space parking lot.
The project fell under the EPA’s Stormwater Management Phase I requirements. To meet those requirements and the special design considerations, initial stormwater management plans included the use of grading, swales, catch basins, and an underground single manhole gravity separator as pre-treatment to runoff through a system that would eventually reach the lake. However after some research, Fox & Associates found the underground stormwater treatment system offered by BaySaver leveraged the advantages of a two manhole system and provided more effective treatment at a better cost per treated cfs, so the original single manhole system was replaced on the plans.
Ensuring that pre-treatment of the storm water met the minimum 80% removal requirement of total suspended solids (TSS), the BaySaver Separation System was sized based on flow rate. Rainfall data used during the design process for the selection of the appropriate BaySaver model was provided by local National Weather Stations. This ensured that local data was being used to determine design. And by using flow rate, all site conditions were taken into account, such as slope, percentage of impervious area, acreage, and of course rainfall.
Based on site layout plan and calculated flow rate it was determined that two separate underground treatment systems would be used to pre-treat the storm water runoff. The east side of the site includes a system which treats a drainage area of 1.30 acres, and the west side includes a system which treats a drainage area of 1.60 acres.
Both underground systems included installation of the BaySaver Separator Unit with two standard pre-cast manholes for each. The primary manhole for each system is inline to collect and retain larger particulates, while the second or “storage” manhole is situated offline to collect and retain fine particulates, oils, and floatables.
The effectiveness of storm water pretreatment is usually evident upon inspection. Pulling the manhole covers in this case, the proof could be seen immediately. As a witness to the inspection, Todd Davis, P.E., with BaySaver said, “When the manhole covers were removed I was amazed to see a 2 ½ foot long 2x4 (lumber) in the storage manhole. Tests have shown the systems are at least 80% effective, but every once in a while we see something new and fascinating that has been prevented from polluting our waters”.
In this case study, the underground BaySaver Stormwater Treatment System was used as a pre-treatment in the stormwater plan. These systems are also used as a stand alone BMP in a variety of applications. Whether pre-treatment or stand alone, underground storm water treatment systems are becoming necessary to improve surface water quality. Although a number of structural BMP’s are available only some can manage the runoff volumes from large and small developments alike while treating 100% of the storm flow and meet regulatory requirements.