Improving hydraulic capacity with inlet modifications to box and circular culverts

Abstract

Culverts convey water underneath roads or through embankments. Culverts can become inadequate over time due to increasing flood peaks caused by higher-intensity rainfall events and urban development. When roads are upgraded, the culverts on these roads might also require upgrades to comply with new road standards. Modifying the culvert inlet to increase discharge capacity can negate the need to rebuild the entire structure, providing a cost-effective solution with minimal disruption to traffic. This study used physical modelling to evaluate different inlet modifications for box and circular culverts under inlet control. The tested modifications included adjustments to wingwall and headwall angles, rounded or tapered inlet edges and the addition of an air vent. Different angles for the wingwall and headwall, when combined, significantly improved the discharge capacity. For box culverts, a 15º headwall with a 30º wingwall increased the flow capacity by up to 34% at a headwater depth of twice the culvert opening (2D). Headwalls and wingwalls had a smaller influence on circular culverts than on box culverts. However, rounded edges that smoothly direct water into the circular barrel shape improved flow by up to 34% at a headwater depth of 2D. The installation of an air vent had an insignificant influence on the discharge capacity under inlet control. A flow improvement coefficient, CTG, in the form of a set of equations, has been developed to quantify the improvement for each type of inlet. This coefficient can be used in South Africa and globally with existing equations to calculate the improved discharge capacity for specific inlet modifications. This practical solution enhances culvert performance and, if applied in practice, will reduce flood risks, especially within South Africa’s expanding road network. It is envisaged that implementing inlet improvements for both new and existing culverts will contribute to a sustainable drainage infrastructure for the country.