Measures of river channel efficiency through cross-sectional form
DOI:
https://doi.org/10.12775/bgeo-2025-0012Keywords
hydraulic radius, width/depth ratio, the most efficient channel, cross-sectional form, Jalangi RiverAbstract
The efficiency of an open river channel refers to how effectively the channel conveys water with minimal energy loss due to friction with the perimeter. Cross-sectional form, roughness characteristics and slope play pivotal roles in governing a channel’s efficiency. Previous studies commonly used hydraulic radius (Rh) and width/depth ratio (ζ=w/d̄) to assess channel efficiency. Rh lacks a dimensionless representation for comparing rivers of different sizes, while the ζ value has no specific scale (e.g., 0 to 1) to fix a channel's efficiency level. This research specifically investigates the influence of channel cross-sectional form on efficiency, assuming a given slope and roughness, and presents novel tools for measuring channel efficiency based solely on cross-sectional form. It is widely acknowledged that a semi-circular cross-section represents the most efficient channel form due to its minimal wetted perimeter. However, little attention has been given to comparing an open channel with an ideal semi-circular form and establishing a fixed efficiency parameter. Hence, three handy tools are proposed to measure the efficiency of open-channel cross-sectional forms. The first tool (E1) compares the cross-sectional area of the observed channel with the area of a semicircle having an equivalent perimeter; we define channel efficiency, E1, as the “ratio of the cross sectional area (A) of the channel, to the area of a theoretical semicircle (Ac) having the same wetted perimeter P”. The second tool (E2) compares the wetted perimeter of the channel with the perimeter of a semicircle of the same area as the observed cross-section. Finally, the third tool (E3) assesses the width/depth ratio of the observed channel by comparing it to the width/depth ratio of a semicircle of the same area. The effectiveness of these tools was validated using a computed dataset of size 1080 and a field-based dataset comprising 45 cross-sections obtained from the Jalangi River in India. The major highlights of the methods are as follows.
• Three new indices of channel efficiency are proposed using the cross-sectional form.
• The proposed methods are unit-free, with known scale limits for comparison.
• The proposed indices are validated using field data from a tropical river.
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Copyright (c) 2025 Balai Chandra Das, Aznarul Islam, Uddalak Das
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