Innovative Stormwater for a Small Flood Resiliency Project
The Slash Cottage Drainage Improvement project exists to solve localized flooding issues that had been occurring behind some homes in a neighborhood in the town of Ashland, Virginia. Eight homes in the community are connected through an existing drainage ditch that has been under-performing and was later determined to be inadequate to control nuisance flooding (1-yr storm). Additionally, the downstream culvert was found to be broken and the downstream invert had been un-set from the manhole it connected to. The primary challenges to solving the drainage problems on this project are the very shallow and flat grades that impede drainage flow and the dilapidated infrastructure that the system ties into. Additional concerns that increase the project complexity include the size and use of each home’s backyard, the presence of underground utilities, the sheds and other homeowner items within the drainage easement, and the secondary goal of providing water quality treatment to meet the Town’s Chesapeake Bay TMDL Action Plan.
The ultimate solution to the drainage problem involved splitting the drainage into two separate systems to reduce the required depth of the ditch and to utilize a concrete lining to maximize the flow rate to the outfall. Moreover, the stormwater management facility that was added to the project to achieve some water quality benefits is noteworthy for its hybrid design. Due to the extremely shallow grades of the entire project site, the BMP depth was limited and underdrain was not possible. Therefore, the stormwater management facility is a hybrid design consisting of two grass channel BMPs that wind to the same outlet riser that is located within a ponded area. The grass channels will handle the flow and treat the runoff in the nuisance storms (up to a 2-yr storm) while the ponded area will act as a shallow “dry pond” to provide detention of higher return period (up to 10-yr). This hybrid design maximizes water quality treatment and water detention in an extremely limited space and could serve as a useful case study on facility design in shallow and flat conditions.
Author Bio
With over 6 years of experience in the civil engineering field since earning a bachelors degree in Civil and Environmental Engineering at the University of Maryland, College Park, in 2018, Ryan has been involved with many innovative water resources projects across Maryland, D.C. and Virginia. Ryan began his career working in the land development field and designed subdivisions around Fredericksburg, VA. He joined JMT in 2020 and has since grown and led the D.C. Water Resources team for the last 4+ years. Ryan earned his PE and a Masters of Engineering degree in Project Management in 2023.
