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Robust Design of a Riprap Basin Outlet
Subject area: Science,Engineering and Technology · Area of research: Water Engineering
DOI: 10.64388/IREV10I4-1723803
Abstract
One of the major causes of hydraulic infrastructure deterioration is scour around the structure and culverts suffer from this at the outlet, resulting in partial or complete collapse of the structure, roadway damage, and increased maintenance costs. Conventional outlet protection systems are commonly designed with a single design discharge, usually the 25- or 50-year return period depending on the size of the channel bed. This gives little consideration to their performance under larger flood events and thus questions the possible effectiveness under long term use. This study presents a hydrological and hydraulic assessment of scour and the robust design of a riprap basin outlet protection system by considering multiple design discharges using a reinforced box culvert affected already by scour in Ibadan, Nigeria as a case study. A 37.4 km2 catchment area was determined from the area around the box culvert using HEC-HMS to estimate peak discharges corresponding to the 25-, 50-, and 100-year return periods. HY-8 was used to perform the hydraulic analysis of the box culvert using the city’s master plan design discharge and the aforementioned return periods, while scour characteristics were calculated in accordance with the Federal Highway Administration Hydraulic Engineering Circular No. 14 (FHWA HEC-14) procedures. A riprap basin and apron were designed using the city’s master plan discharge and its performance was assessed under higher return periods to determine its robustness and effectiveness under long term use. Results showed that under the design discharge, the proposed outlet protection provides adequate stability but there is a significant increase in riprap size and basin depth under the return periods. However, the basin width, length and apron length remained consistent for all discharges. The required median stone diameter increased progressively with increasing flood magnitude, showing that conventional single event design underestimates the protection required for scour prevention and channel preservation. A simplified robustness-based design framework is proposed to facilitate the assessment of riprap basin adequacy under multiple flood scenarios. The study demonstrates the importance of incorporating robustness assessment into culvert outlet protection design to improve infrastructure resilience and reduce scour-induced failures in urban drainage systems.
Keywords
box culvert; scour; riprap basin; hydraulic modelling; HEC-HMS; HY-8; outlet protection; robustness assessment
How to cite this paper
@article{1723803,
author = {Bello Ahmad, Prof. G. M. Ayininuola},
title = {Robust Design of a Riprap Basin Outlet},
journal = {Iconic Research And Engineering Journals},
year = {2026},
volume = {10},
number = {4},
pages = {2980-3000},
issn = {2456-8880},
url = {https://www.irejournals.com/formatedpaper/1723803.pdf},
abstract = {One of the major causes of hydraulic infrastructure deterioration is scour around the structure and culverts suffer from this at the outlet, resulting in partial or complete collapse of the structure, roadway damage, and increased maintenance costs. Conventional outlet protection systems are commonly designed with a single design discharge, usually the 25- or 50-year return period depending on the size of the channel bed. This gives little consideration to their performance under larger flood events and thus questions the possible effectiveness under long term use. This study presents a hydrological and hydraulic assessment of scour and the robust design of a riprap basin outlet protection system by considering multiple design discharges using a reinforced box culvert affected already by scour in Ibadan, Nigeria as a case study. A 37.4 km2 catchment area was determined from the area around the box culvert using HEC-HMS to estimate peak discharges corresponding to the 25-, 50-, and 100-year return periods. HY-8 was used to perform the hydraulic analysis of the box culvert using the city’s master plan design discharge and the aforementioned return periods, while scour characteristics were calculated in accordance with the Federal Highway Administration Hydraulic Engineering Circular No. 14 (FHWA HEC-14) procedures. A riprap basin and apron were designed using the city’s master plan discharge and its performance was assessed under higher return periods to determine its robustness and effectiveness under long term use. Results showed that under the design discharge, the proposed outlet protection provides adequate stability but there is a significant increase in riprap size and basin depth under the return periods. However, the basin width, length and apron length remained consistent for all discharges. The required median stone diameter increased progressively with increasing flood magnitude, showing that conventional single event design underestimates the protection required for scour prevention and channel preservation. A simplified robustness-based design framework is proposed to facilitate the assessment of riprap basin adequacy under multiple flood scenarios. The study demonstrates the importance of incorporating robustness assessment into culvert outlet protection design to improve infrastructure resilience and reduce scour-induced failures in urban drainage systems.},
keywords = {box culvert; scour; riprap basin; hydraulic modelling; HEC-HMS; HY-8; outlet protection; robustness assessment},
month = {October},
doi = {https://doi.org/10.64388/IREV10I4-1723803}
}