Carriage Hills

Carriage Hills


Biohabitats Collaboration Project

Located in the community of Carriage Hills, this outfall had caused erosion and carried an estimated 7,000 cubic yards of sediment that was conveyed downstream to Clements Creek, contributing to the need for repeated dredging.   Extending approximately 425 linear feet, the restoration site began at the rip-rap near a recently repaired concrete stormwater outlet and plunged into an eroded ravine twenty-five feet wide and twenty-two feet deep.  

The project was led by the Severn Riverkeeper, with Biohabitats, Inc. as the designer and U&A as both design partner and contractor.  This project was the first American Recovery and Reinvestment Act restoration project to be implemented in Maryland. It was completed in approximately four weeks in early 2010 ahead of schedule. The contract amount was $355,000 and final the construction cost was the same.  This project encompasses 43,473 square feet (approx. 1 acre). As the project designer and construction lead, Keith Underwood was onsite every day during construction to both manage the project and operate the machinery.  

The key objectives of the project were:

  1. Decrease stormwater discharge volume and velocity/energy by increasing infiltration into the filled channel, storing stormwater in each of the pool-weir complexes, extending the time of concentration of the runoff event by slowing the rate of water movement through the channel due to increased material roughness (e.g., rock) and form (riffle and pool sequence) that interrupts the formation of a simplified drainage system;
  2. Dramatically reduce sediment supply and trap sediment from the pipe discharge in the pools, increase nutrient and pollutant processing through increased contact time and movement through the engineered hyporheic sand and organic carbon (i.e., wood chip) substrate matrix;
  3. Restore the local shallow groundwater table by facilitating wetting and infiltration of the fill material used to restore the old channel dimensions and seepage into the connected sandy soil strata underlying the restored channel; and
  4. Reconnect the stream and its flow to the existing forested floodplain so that future large storm discharges are not concentrated in the stream channel and can spread out into a shallow, non-erosive flow that rehydrates the forest floor.

We accomplished the restoration by:

  1. Filling the eroded channel to mimic pre-erosion morphology with low cost, locally available bank-run sand and gravel mixed with 20% (by volume) wood chips;
  2. Installing rock and riffle grade control structures to create a system of shallow pools and riffles; and
  3. Establishing a native plant community to the restored channel’s riparian zone and shallow pools.

In order to minimize intrusion and damage to the stream corridor and floodplain during construction, we worked within the gully and eliminated the need for additional access.  The planting was done by hand using wheelbarrows and shovels rather than heavy equipment.

Our design had several innovative features.  The basic approach was a regenerative step-pool conveyance.  We also modified the existing stormwater pipes and drop structures to create a bubbler, which attenuated discharge energy and increased the flowpath, allowing greater treatment area. In order to reduce the velocity and distance of the piped water, we rerouted the water from the downstream outfall and filled the pipe with cobble.  Instead of rushing out of the original outfall, the water instead escaped through a former manhole upstream.

The fill for the incised gully was a mixture of sand and shredded hardwood. We created a repeating system of riffles and pools on the fill to provide non-erosive conveyance for the larger stormwater runoff events. We started filling at the top of the channel and progressed downstream, then constructed each of the riffle-pool sequences from the downstream end of the fill, working back upstream to exit the site.

The relatively large volume of carbon-rich and porous fill (3200 cubic yards) allows all but the largest stormwater runoff events to infiltrate into the channel bed and flow downslope through carbon-rich material, losing runoff volume by wetting  the fill, vertical and downslope lateral groundwater recharge, and evaporation along the path.  The remaining surplus water moves via hyporheic flow further downstream and seeps out of the fill into the receiving stream with reduced sediment and nutrient loads because of the physical filtration and biochemical treatment associated with the media.  In addition to the improved water quality, the contact time in the fill during the hyporheic flow lowers the water temperature.

The biological uplift of the system is significant because the pre-restoration site was an ephemeral gully that was restored to a stable seepage wetland ecosystem with a strong connection to its historic floodplain, maintaining constant flow on the restored site. We planted native vegetation, and the new hydrology created habitat for plants, aquatic insects, frogs and salamanders, which now are able to forage, breed, and find refuge within the restored site. This project was constructed with the utmost attention to preserving the riparian forest; U&A successfully constructed the project with no tree removal.

Numerous field trips for students and representatives from public agencies, research institutions, non-profit groups, and environmental restoration organizations have used Clements Creek as an example of a successful and innovative approach to addressing stormwater in the urban stream corridor. In fact, this project won the Smart Green and Growing Award for Sustainable Infrastructure and Innovation in Stormwater Management from the Maryland Department of the Environment in 2010.

The Carriage Hills RSC project was constructed with the utmost attention to preserving the riparian forest; U&A successfully constructed the project with no tree removal. The Carriage Hills RSC is highly treasured by the local community and has become a showcase project for the Severn Riverkeeper. The project has won the Smart Green and Growing Award for Sustainable Infrastructure and Innovation in Stormwater Management from the Maryland Department of the Environment in 2010. Carriage Hills consists of stream/pipe daylighting, stream restoration, and stormwater management, with the goal of preserving the riparian forest, maximizing aquatic resource habitat, aesthetics, and passive access for visitors. Formerly a deeply eroded ravine resulting from discharge of stormwater into a natural ravine from a pipe & convey system installed prior to stormwater management requirements. Increased volumes & velocities exiting this pipe caused the level spreader/apron to fail and cut a channel up to 25’ in depth through the ravine, lowering groundwater tables and causing significant sedimentation to downstream habitats. Now, with a 440 foot Regenerative Stormwater Conveyance system in place designed and built by Underwood & Associates, the water has been brought back up to the surface and the one hundred year storm event is controlled easily without further erosion and delivery of sediments and pollutants to the natural stream below.