I-495 I-270 Managed Lanes Study CA-5 (Unnamed Tributary to Great Seneca Creek) Stream Restoration Mitigation

Coastal Resources, Inc. (CRI) provided environmental assessment and restoration design services for the Unnamed Tributary to Great Seneca Creek (referred to as CA-5) stream restoration design project located in Montgomery County within Quince Orchard Valley Neighborhood Park. The Park is owned by Maryland National Capital Park and Planning Commission (M‑NCPPC). To mitigate for impacts to waters of the U.S. associated with the I-495 & I-270 Managed Lanes Study under the I-495 & I-270 Public-Private Partnership (P3) Program, the P3 Program identified the CA-5 stream restoration project as partial compensatory mitigation.  The overall stream restoration of this site is 3,868 LF.

CRI completed a wetland and watercourse delineation at the project site. The study area consists of approximately 17 acres which includes the mainstem, two intermittent tributaries, and another perennial stream at the downstream end of the project along with adjacent undeveloped riparian areas directly adjacent to the stream valley wall.  Wetlands were identified using the USACE Atlantic and Gulf Coastal Plain Regional Supplement. CRI collected and recorded detailed field data on vegetation, soils, and hydrology for representative wetlands on Wetland Determination Data Forms.  Each wetland was categorized by type (e.g., forested, scrub-shrub, emergent, etc.). Stream characteristics were recorded for each identified watercourse.  The boundaries of each wetland and watercourse were flagged and each flag was then surveyed using a handheld Trimble GPS unit.  The two perennial streams and most of the valley were surveyed. A detailed wetland delineation report was prepared summarizing all identified resources and included datasheets, mapping, and photographs of each resource.

All specimen trees [30-inch or greater diameter at breast height (DBH)] within the study area were identified. All trees were tagged and numerically labeled.  The location of each tree was GPS surveyed and picked up by the land survey team for inclusion on the design plans.

CRI performed a detailed geomorphic assessment including a longitudinal profile and cross sections, BANCS streambank analysis, site features and mapping for the project. Existing conditions were documented and provided the basis for the concept stream restoration design.  CRI conducted a detailed hydrologic study of the stream and tributary to determine the design discharge based on GISHydro and agreement with the Maryland regression equations and the geomorphic data. CRI developed a HEC RAS 2D model. The results of this model are used to show the stability of the in-stream structures and the water surface elevations for the 2, 10, and 100-year storms.

CRI produced Concept (15%), Preliminary (30%), and Semi-Final Plans (65%)/Phase II Mitigation Plan and submitted to MDOT SHA, M-NCPPC and the regulatory agencies for comment.  CRI also completed the US Army Corps of Engineers Maryland Stream Mitigation Framework (MSMF) to determine the functional feet of stream restoration mitigation the site will provide.

The overall goals of the restoration were to: provide 3,637 LF of stream mitigation, providing 918 functional feet of mitigation credit; increase floodplain connection; provide a stable channel design; increase bank stability; stabilize groundwater seep and tributary headcut channels; minimize the impact to adjacent trees and other natural resources; provide consistent unit stream power to convey sediment through the stream reach; stabilize the existing pond outlet and lower the overall pond elevation to create a functioning wetland habitat; and provide diverse habitat for wildlife and other aquatic species. The overall design approach is to fill in the channel to reconnect the stream to the existing floodplain, this will reduce the need for mass grading within the healthy forest. The tributaries were designed to stably convey storm flows and provide an appropriate tie in elevation to the main channel. The outlet from the farm pond will be lowered and stabilized with step pools to maintain a lower standing water level and allow for the establishment of wetland plant and animal species.


Montgomery County, MD


MDOT State Highway Administration, I-495 & I-270 P3 Office

Key Services:

  • Wetland Delineation
  • Tree Survey
  • Agency Coordination
  • Stream Restoration Design
  • Geomorphic Assessment
  • BANCS Assessment (BEHI/NBS)
  • Permitting Support and Submittal
  • Maryland Stream Mitigation Framework