Long-Term Bed Degradation in Maryland Streams (Phase IV) : Western Shore of the Coastal Plain Province
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Long-Term Bed Degradation in Maryland Streams (Phase IV) : Western Shore of the Coastal Plain Province

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Details:

  • Creators:
  • Corporate Creators:
    RK&K
  • Corporate Contributors:
    Maryland. State Highway Administration. Office of Policy & Research
  • Subject/TRT Terms:
  • Publication/ Report Number:
    MD-18-SP609B4H
  • Resource Type:
    Tech Report
  • Geographical Coverage:
  • TRIS Online Accession Number:
    01675087
  • Edition:
    FINAL REPORT
  • Corporate Publisher:
    Maryland. State Highway Administration. Office of Policy & Research
  • NTL Classification:
    NTL-ENERGY AND ENVIRONMENT-Environment Impacts
  • Abstract:
    Estimation of potential long-term down-cutting of the stream bed is necessary for evaluation and design of bridges for scour and culverts for fish passage. The purpose of this study was to improve predictions of this potential long-term bed degradation (LTBD) in Maryland streams through the measurement and analysis of stream bed and water-way structure survey data and bridge plans. LTBD was defined as the vertical change in the channel profile other than that caused by local or contraction scour. A total of 22 sites in Baltimore City and Harford, Baltimore, Anne Arundel, Prince George’s, Calvert, Charles, and St. Mary’s counties were selected for data collection. Drainage areas of these sites in the Western Shore of the Coastal Plain province ranged from 0.7−30.5 mi2. At each sampling site, the vertical drop at the outlet of the structure was measured with a pocket rod and a hand level. These rapid measurements were conducted where a step, a series of steps, a steep section, or a riprap-protected streambed was at the out-let of a culvert or a bridge with a paved or riprap-protected invert or downstream apron. Five of the six factors that may influence a site’s risk of LTBD were investigated. These include (1) the valley slope, (2) the effective floodplain width, (3) discharge, (4) downstream channel entrenchment, and (5) bed material characteristics. The possibility of developing regional relations between LTBD and percent impervious area was evaluated for the physiographic province, but the data were inconclusive. Three relations between LTBD and the risk factors were examined: LTBD and valley slope; LTBD and an index combining Factors 1-4; and LTBD and an index of bed mobility. Data indicated no trend in LTBD with either of the two indices. The relation based on valley slope was compared to a relation based on drainage area. The comparison revealed that valley slope was a better predictor of the susceptibility of a site to LTBD than drainage area. The relation between valley slope and LTBD was recommended to estimate LTBD for streams with slopes of less than 0.014 /ft. The relation should not be applied, however, to structures located in deep deposits of sediment created by backwater from dams or other structures or to structures located in streams with evidence of active channel degradation or recent downstream channelization.
  • Format:
    PDF
  • Funding:
    SP609B4H
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