Field Assessment of Culvert Impacts on Stream Channel Morphology

Abstract

ROBERTS, JESSICA TALLEY. Field Assessment of Culvert Impacts on Stream Channel Morphology. (Under the direction of Gregory D. Jennings.)

Culverts are an integral part of the transportation network and thereby are unavoidable additions to most stream networks. As the regulations for culvert design continue to advance, the tools used to examine the impacts that culverts have on streams also must evolve. Due to the substantial quantity of culverts continually being installed and the associated cost of their design and construction, there is a need to increase the frequency and efficiency of channel stability assessments. As such, a comprehensive, field-tested rapid culvert and channel assessment tool was developed to relate culvert condition and observed channel stability to the culvert’s design characteristics. The Culvert Assessment Tool (CAT) was tested by teams of trained professionals to characterize culvert and stream channel conditions and properties within the piedmont of North Carolina, focusing on Wake County.

Eighty-seven sites were tested, including one hundred thirty-nine culverts and ten floodplain culverts, including round, box elliptical and arch shapes of at least 24†(0.61m) in diameter and made of various materials (reinforced concrete pipe, corrugated metal pipe, and bottomless or natural channel material). The purpose of this research is to determine if culverts have a significant impact on stream channel morphology as it pertains to stability. The CAT requires measurement of various culvert characteristics. Stream channel condition and instability at the culvert are also quantified using a rating for each specific observation and includes the identification of the potential causes of various deficiencies. The study aims to identify relationships between CAT score and culvert cross sectional area, primary slope, shape and/or material as well as surrounding land use, stream channel mean particle size and the ratio of the culvert area to the stream channel cross-sectional area (Acul/Axs), or CAT area ratio.

Potential CAT scores can range from 0 to 85 upstream and downstream of the culvert. A lower score indicates greater channel and culvert condition and stability. For eighty-two sites, the CAT scores are normally distributed with upstream CAT scores ranging from 2 to 39 with a median score of 24, downstream CAT scores ranging from 5 to 49 with a median score of 25, and total CAT scores ranging from 9 to 80 with 50 as the median score. Upstream and downstream CAT scores are not significantly different. Culvert cross sectional areas range from 3.1 ft2 (0.94 m) to 433.5 ft2 (132 m) and have a median area of 28.3 ft2 (8.6 m). Arch culverts had the largest range of cross sectional area and the largest median cross sectional area. Culverts with cross sectional area of 30 ft2 or greater have significantly lower CAT scores than culverts with less cross sectional area (α=0.05), indicating that larger culverts are more stable. Culverts with cross sectional areas greater than stream channel cross sectional area, or a CAT area ratio greater than 1, also exhibited more stable stream and culvert condition with significantly lower CAT scores (α=0.05). Culverts with greater than 1% slope were determined to produce less stable conditions than culverts with slopes less than 1%, as they reported significantly higher median CAT scores (α=0.05) . Results show there is not a significant difference in median CAT scores between corrugated metal pipe and reinforced concrete pipe; however, both are significantly different from bottomless culvert material, which was determined to be more stable. Results show that bottomless arch culverts or box culverts with cross sectional areas greater than 30 ft2 and mild slopes (less than 1%) demonstrate the best associated culvert and channel stability condition based on the CAT.