Great Seneca Creek – Floodplain Restoration as a Compensatory Mitigation Alternative
Andrew W. Donaldson
Johnson, Mirmiram & Thompson, Inc. (JMT)
York, PA
Brian Cox, Project Manager
Maryland Department of Transportation
Office of Environmental Design
Baltimore, MD
The Great Seneca Creek floodplain restoration was designed to mitigate for palustrine and riverine impacts associated with the Maryland State Highway Administration’s I-270 at Watkins Mill Road Interchange project. The project restored approximately 5,500 feet of degraded stream within a suburban Montgomery County, Maryland community to offset aquatic resource impacts to approximately 1,000 feet of the tributary to Great Seneca Creek and approximately 2.4 acres of palustrine resource impacts The final restoration design focused on a comprehensive and holistic stream corridor restoration that created an environmentally diverse and self-sustaining stream, wetland, and riparian corridor that re-established the natural valley bottom ecosystem functions and services; promoted improved aquatic and terrestrial habitats; and provided long-term stability to meet project mitigation performance standards. In addition, the floodplain restoration project provides stormwater volume and rate control for project roadway redevelopment areas for increases in the 10-year storm event.
Project design elements include a detailed soil investigation that identified paleo-geologic hydric soils (7,856 ± 35 yrs BP), valley bottom quartz (angular-shaped) basal gravels, and the historical overburden sediments. The stream corridor restoration focused on reattaching the channel to the native basal quartz geology and reconnecting the buried hydric soils throughout the valley bottom to the proposed and restored lower floodplain elevation. This was accomplished through the removal of the overburden of highly erodible modern-day sediments that are typically associated with 17th to early 20th-century anthropogenic impacts.
Project goals were achieved through the re-establishment of a lower, broader, highly connected, and functional floodplain that allows the stream and wetland valley bottom to adjust naturally over time. Alternative approaches, such as bank-armoring and/or a narrower confined floodplain corridor, would not have allowed this low-energy system the ability to adjust in the face of climate change and higher extreme rain events while safely conveying the full range of flow events. This well-connected floodplain provides the necessary geomorphic characteristics and hydraulic connections to maintain channel stability and improve ecological functions. In all, the stream, wetland, and riparian floodplain restoration design restored approximately 10 acres of palustrine wetlands and 5,500 feet of riverine restoration to the Great Seneca Creek.
About Andrew W. Donaldson
Andrew Donaldson is a Senior Environmental Scientist at JMT and has 27 years of experience in stream, floodplain, and wetland restoration and ecological design projects throughout the eastern and southeastern United States. Andrew’s expertise is in the assessment and design of stream, floodplain, and wetland restoration projects using the principles of fluvial geomorphology to reestablish appropriate riverine and valley bottom ecological functions and services. Andrew has a B.S. in Environmental Sciences from Drexel University and a Master’s in Environmental Engineering from The Pennsylvania State University. He has designed numerous successful ecological restoration projects throughout his professional career and also has extensive experience in construction inspection and construction oversight for the implementation of stream, floodplain, and wetland restoration and mitigation projects.
https://www.linkedin.com/in/andrew-donaldson-b1936043/
About Brian Cox
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