Irrigation Sciences and Engineering

Irrigation Sciences and Engineering

Debris Blockage Risk of Modified Stepped Spillways: A Laboratory Study on Obstacle Configuration

Document Type : Research Paper

Authors
Seyed Amin Asghari Pari
Seyed Ali Asghari Pari
Mohammad Amin Mokarram
Mojtaba Kordnaeij
Department of Civil Engineering, Faculty of Engineering, Behbahan Khatam Alanbia University of Technology, Behbahan, Iran.
10.22055/jise.2025.50358.2173
Abstract
Stepped spillways are critical hydraulic structures for energy dissipation, but their performance can be significantly compromised by floating debris. This study experimentally investigates the hydraulic behavior of floating debris on a stepped spillway under three distinct configurations: (1) an unobstructed baseline, (2) continuous on-step obstacles, and (3) discontinuous on-step obstacles. Experiments were conducted in a 1.2-meter-wide laboratory flume under three flow regimes—nappe, transition, and skimming—corresponding to relative critical depths (dc/h) of 0.47, 0.85, and 1.09, and with varying relative obstacle heights (ho/h) from 0.19 to 0.76. Results demonstrate that, in the baseline configuration, debris passed with only a minor delay of 1–3 seconds. For the continuous obstacle configuration, increasing obstacle height significantly enhanced debris conveyance by promoting a "pooling effect" that reduced trapping probability by up to 90% compared to lower heights, virtually eliminating trapping for ho/h ≥ 0.38. Conversely, discontinuous obstacles, particularly under nappe flow conditions, induced stable and extensive blockages with trapping probabilities exceeding 70% for large debris. The presence of debris in the skimming flow regime led to a minor reduction in energy dissipation (up to 5–10%) due to altered surface flow patterns, but no significant changes in discharge coefficient were observed. The findings reveal a critical design paradox: the hydraulic mechanisms that make discontinuous obstacles effective for energy dissipation (up to 39% increase over baseline) are the same mechanisms that cause catastrophic debris trapping. This research provides crucial insights for risk-based design, suggesting that for catchments with high debris loads, continuous obstacles are a more resilient solution despite their lower energy dissipation efficiency (5–15% decrease compared to baseline).
Keywords
Stepped Spillway
Floating Debris
Blockage Risk
Dam Safety
Energy Dissipation
Hydraulic Structures
Experimental Hydraulics
Dam Failure
Subjects
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Irrigation Sciences and Engineering
Volume 48, Issue 4
Winter 2026
Pages 105-115

  • Receive Date 07 August 2025
  • Revise Date 24 November 2025
  • Accept Date 14 December 2025
  • Publish Date 04 January 2026