Cabin John Creek Watershed Study

Reference

Dan Harper, Senior Engineer (240) 777-7709

Project Highlights

  • Watershed Assessments
  • Rosgen Stream Classification
  • Geomorphic Assessment
  • Stream Restoration Design

Project Details

The objective of this project was to identify, prioritize, rank and develop conceptual stream restoration designs and aquatic habitat enhancement projects for 10 miles of stream in this highly urbanized watershed. An additional 25 stream miles were assessed for potential small-scale enhancement opportunities that could be undertaken by volunteer citizen groups or by those needing to provide mitigation for impacts associated with projects elsewhere in the watershed.

Initially this study was guided by a previous inventory of potential restoration/enhancement sites identified in the 1996 report, Rapid Stream Assessment Technique (RSAT) Survey of the Cabin John Creek Watershed, Montgomery County, MD, MWCOG and from Montgomery County Department of Environmental Protection’s Countywide Stream Protection Strategy, 1998. The feasibility of implementing restoration/enhancement projects that would provide water quality and habitat improvements were assessed. Based on a preliminary office ranking, a list of 40 stream reaches in 12 subwatersheds was reduced to 16 reaches.

CRI performed a field reconnaissance based on the Rosgen Stream Classification System on these 16 reaches to verify existing environmental conditions. The most frequently observed conditions included:

  • failing banks,
  • head cutting,
  • over widened channels,
  • straightened channels,
  • channel entrenchment,
  • buried or scoured habitat features, nonexistent habitat features,
  • poor sediment transport,
  • downed trees in channel,
  • debris jams,
  • loss of riparian buffers,
  • fish passage blockages and
  • exposed utilities.

In addition to evaluating stream stabilization/restoration opportunities, CRI identified, evaluated and ranked stormwater outfall repair needs. We recorded the design point extents using a GPS unit and imported the data into Arcview GIS as shape files. We shot photographs to document existing conditions and hand sketched detailed project design concepts. We generated a narrative describing the project concepts that employed a variety of natural channel design, soil bioengineering, and aquatic habitat enhancement strategies. CRI grouped the design point projects into major projects based on geographic proximity. Cost estimates for the major projects were developed.

Sixteen stream restoration projects were identified and developed to 30% design. When completed, these projects will provide streambank protection or habitat improvement. Some of the techniques used in the conceptual designs included:

  • streambank grading and stabilization with soil bioengineering treatments,
  • floodprone access enhancement,
  • rock vane and cross vane installation,
  • point bar restoration,
  • cross-sectional geometry improvement,
  • channel realignment and geometry modification,
  • step pool creation,
  • imbricated rock wall and bank toe protection,
  • debris jam and fish blockage removal,
  • outfall and headcut stabilization,
  • vernal pool construction,
  • riparian buffer enhancement, and
  • invasive plant species eradication.

An estimate of project construction costs was prepared. The average length of these projects is 4,400 feet with an average cost per linear foot of $64.00. Additionally, over 100 small-scale projects, suitable for volunteer groups, were identified and designed.