Long-term bed degradation in Maryland streams (phase 2) : Blue Ridge and Western Piedmont provinces.
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Long-term bed degradation in Maryland streams (phase 2) : Blue Ridge and Western Piedmont provinces.

  • 2012-03-01

Filetype[PDF-2.22 MB]

  • English

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    • Edition:
      Final report.
    • NTL Classification:
      NTL-HIGHWAY/ROAD TRANSPORTATION-Bridges and Structures ; 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 has been to improve predictions of this potential long-term bed degradation (LTBD) in Maryland streams through the measurement and analysis of stream bed and waterway structure survey data and bridge plans. Long-term bed degradation was defined as the vertical change in the channel profile other than that caused by local or contraction scour. A total of 30 sites—23 bridges, 2 culverts, 2 utility crossings, 2 embankment walls, and 1 concrete ford—in Frederick, Carroll, and Montgomery counties were selected for data collection. Drainage areas of these sites in the Blue Ridge and Piedmont physiographic provinces ranged from 1.7-25.9 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 outlet of a culvert or a bridge with a paved or riprap-protected invert or down-stream apron. Six factors that may influence a site’s risk of LTBD in the three western Maryland provinces were also investigated. These include (1) the valley slope, (2) the effective floodplain width, (3) discharge, (4) downstream channel entrenchment, (5) bed material size, and (6) downstream grade controls. The possibility of developing re-gional relations between watershed area and LTBD was evaluated for each physiographic province, but the data was inconclusive. Three relations between LTBD and five of the risk factors were examined: LTBD and valley slope; LTBD and an index combining Factors 1-4; and LTBD and an index combining Factors 1-5. A comparison of the resulting equations revealed that valley slope was as good a predictor of the susceptibility of a site to LTBD as the two indices that required additional data and considered more parameters. The relation between valley slope and LTBD was recommended to estimate LTBD for streams with slopes of less than 0.027 ft/ft. The relation will not apply, 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. The development of rate relationships for LTBD was also considered, but the number of available structure plans was insufficient to develop a rate relation. Future research on LTBD in Maryland should include the development of a method to include the effectiveness of downstream bed controls in limiting degradation, and the development of a rate relation should be explored further.
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