Horsepen Creek Crossing
Otterdale Road in Chesterfield County, Virginia, is notorious for flooding during heavy rainfall, trapping residents in the area and creating an emergency access concern for the community.
Under contract with the County, JMT performed hydraulic and hydrologic analysis and completed surveys at five stream crossings along Otterdale Road. Three of these crossings–at Blackman Creek, Otterdale Branch, and Horsepen Creek–were selected for redesign, and JMT’s role was to assemble the request for proposals package for the Otterdale Road drainage improvement design-build project.
However, time constraints and the urgent need to address travelability in the region during heavy rainfall events resulted in JMT’s increased involvement. Our team fully designed the award-winning Horsepen Creek Crossing, coordinating with numerous stakeholders to complete analyses and assemble all the necessary approvals and permits ahead of time, which allowed the winning contractor to start building on day one.
Surveying on a short timetable
JMT had just 12 months to redesign Horsepen Creek Crossing, which is located within a larger floodplain. The road’s low-lying location raised unique design challenges for the team to tackle in this short timeframe.
For one, simply raising the road would have created a dam effect, exacerbating flooding rather than improving conditions. Our team determined that replacing the existing under-road drainage pipe with a bridge would be the most effective solution for the area.
Yet spanning the entire floodplain would have been intrusive to the surrounding land–the crossing abuts residential areas–and too expensive. JMT’s technical experts in hydraulics, roadway engineering, and bridge engineering collaborated closely to design a smaller bridge that would serve the community’s needs while minimizing the impact on the surrounding environment.
The team’s careful evaluation of the area allowed them to minimize impacts to the surrounding wetlands, meaning that the project did not require additional environmental approvals. However, some increases in flood elevations were unavoidable. The design thus required a Conditional Letter of Map Revision (CLOMR) from FEMA before construction could begin. JMT coordinated with FEMA to vet our modelling approach and techniques early in the process; through proactive communication, JMT navigated this time-consuming process, which typically takes 18 months to complete, in just seven-and-a-half months.
Other delays were avoided through timely communication when the JMT Cultural Resources team discovered Native American artifacts along the creek. This was unexpected and required the team to work closely with the County and with the Virginia Department of Historic Resources. The team determined that an additional archaeological survey was required; the open communication channels between JMT and other project
stakeholders ensured the survey was completed on time and did not delay the project’s overall schedule.
Surveys also showed that the current conditions of the stream did not match FEMA’s existing models; the streambed was four feet higher than expected. The stream had incurred significant sedimentation, where additional particulates had settled along the streambed for thousands of feet above and below the project site. The stream also featured an unusual reverse grade where larger particles of sediment had settled near the top, burying smaller particles, rather than the other way around. These complications meant that the area needed additional stormwater management alterations that went far beyond the scope of the project.
Addressing stormwater concerns
To address these challenges in a timely and resource- and space-conscious manner, the team opted to redesign the floodplain grading around the bridge and realign the stream’s course to ensure water flowed more efficiently through the area under the bridge while also mimicking the natural shape of the rest of the stream.
These measures were intended to improve the movement of water through the floodplain, preventing pooling and reducing standing water. The improved grading also reduced erosion risk for the abutments on either end of the bridge, improving long-term wear and stability.
The site also required a full stormwater management plan, including additional best management practice (BMP) features like dry swales and a bioretention basin to contain stormwater. Our team coordinated with Chesterfield County’s Department of Environmental Engineering to establish the management plan and place BMPs.
The team identified a small parcel of County-owned land for the bioretention basin. However, the chosen location was too small for the required water quality pretreatment facility, which would help filter out pollutants and particulates before water entered the basin.
This pretreatment is a part of the state’s strict stormwater treatment requirements, so JMT collaborated with the County, a consulting reviewer, and the Virginia Department of Transportation (VDOT) to design a special dual-chamber forebay–a smaller pool through which the water must travel before entering the basin–that features a gravel diaphragm to filter out larger particles. This innovative solution allowed the team to use the smaller plot of land more efficiently and avoid right-of-way issues with nearby residential land.
The team also strategically placed earthen dry swales to guide stormwater into the new basin. These swales featured check dams–structures attached to the swales that help slow the water–made of Flexamat, an erosion control material made of concrete blocks tied together with a strong woven mat.
All these innovative design elements help control water flow around the bridge, allowing it to remain traversable even in heavy rain.
The final design
With the necessary permits and approvals secured, and the innovative design completed ahead of contractor bids, the JMT team was able to hand the project off to the winning contractor for immediate construction.
The final design of the bridge raised the road by eight feet and featured wider shoulders and adjusted bridge approach angles to make crossing the bridge safer and accommodate bicyclists and pedestrians. The bridge provides recreational opportunities alongside improved traversal for Otterdale Road residents.
Having designed the bridge and its associated stormwater drainage improvements, JMT also provided construction engineering and inspection services to support the contractor. Our team reviewed all contract change orders and managed requests for information to ensure the project remained on schedule. Close coordination between JMT and the contractor led to excellent time savings, and the bridge was completed two months ahead of schedule.
Communication was key throughout the project, and JMT’s close collaboration with County officials, consultants, and residents of the Otterdale Road area ensured that the project met its aggressive timeline, quickly improving the amenities and safety for the community.
JMT received a 2024 Transportation Engineering Award for this project from the Virginia Transportation Construction Alliance, recognizing the design team’s achievements in the “Less than $10 Million Design Bid Build – Non-VDOT” category.
Future crossings in the project area will continue to build on these successes, and JMT is proud to have completed this important piece of the puzzle.