Improvements to the hydraulic performance of culverts under inlet control conditions through the optimisation of inlet characteristics

Abstract

The design of culverts, a fundamental aspect of engineering hydraulics, has traditionally followed standard methodologies that may not fully consider the potential benefits of optimizing inlet characteristics, particularly under inlet control conditions. This oversight, coupled with an often overly conservative design approach, has led to the overdesign of culverts, resulting in unnecessary expenses. However, there is a renewed interest in exploring innovative design modifications to enhance culvert hydraulic performance. The South African National Roads Agency (SANRAL) is facing challenges as it integrates provincial roads into its national network, where the existing culverts may not meet the design criteria outlined in the SANRAL Drainage Manual. Additionally, projections of climate change suggest an increase in rainfall intensity, potentially rendering original culvert capacities insufficient. While new culverts can be designed with these considerations in mind, the greater challenge lies in increasing the capacities of existing culverts without resorting to costly and inconvenient road closures for replacements or for installing additional culverts in parallel. This research studies the enhancement of culvert hydraulic performance through novel design modifications. The study focuses on the addition of angled ingwalls and headwalls to culvert inlets, a combination that has not been extensively researched before in South Africa. Ventilation devices are also considered to introduce air into culverts. An experimental study was undertaken, involving a model consisting of a single-barrel culvert with three headwall / wingwall combinations (90 degrees, 45 degrees, and 30 degrees), with the latter two being 3D-printed. Results indicate that the proposed modifications significantly improve culvert hydraulic performance. Adjustments to coefficients for flow under inlet control conditions, such as increasing the Cb and Ch factors, is proposed to align with experimental data. The 45-degree and 30-degree models demonstrate flow rate improvements of 18% and 16% at H1/D = 1.2, respectively, compared to the 90-degree model for specific conditions. Additionally, the inclusion of a ventilation device alters the flow from outlet control to inlet control in the experimental model under specific conditions. The research suggests that precast headwall/wingwall elements can be easily connected to existing culverts, offering a quick, cost-effective solution that also provides added road width for pedestrian use. Despite the structural advantages of the proposed angled headwall/wingwall element over traditional designs, the research acknowledges that cost savings might not be realized in terms of construction. However, the avoidance of road closures, excavations, culvert replacements or the installation of additional culverts in parallel and backfilling and resurfacing a road will contribute significantly to overall cost-effectiveness and cost savings. In conclusion, this research emphasizes the feasibility and benefits of innovative design modifications for culverts to enhance hydraulic performance. The proposed solutions not only offer improved functionality but also present sustainable and cost-effective alternatives to conventional approaches, particularly in the face of infrastructure challenges and evolving environmental demands. This research contributes valuable insights to the field of hydraulic engineering, promoting adaptability and resilience in infrastructure development.