Effects of driftwood on backwater rise at submerged culverts

Abstract

Accumulations of floating debris at culverts and inverted siphons cause partial blockage of the hydraulic structure’s inlet, leading to an increase in head loss that is also referred to as backwater rise. This effect is of high importance in flood management, because backwater rise associated with floating debris accumulations can cause or intensify flooding in the area upstream of the hydraulic structure, yet it is often not adequately addressed in practice or in design guidelines due to a lack of fundamental understanding. A prime example of this was observed at the Geul inverted siphon located near the village of Bunde in the south of Limburg, the Netherlands. During the 2021 floods the inverted siphon experienced a reduction in conveyance capacity due to accumulation of large wood at its inlet, intensifying flooding of nearby villages.

This thesis studied the development of floating debris accumulations at fully submerged and outlet-controlled culverts and inverted siphons, and analysed the effect that floating debris accumulations have on backwater rise. Experiments were performed in a flume under varying conditions of Froude number and submergence rate, achieved by adjusting the upstream flow depth compared to the culvert’s height. The scale model was representative of inverted siphons typical of the Limburg province. Upstream and downstream water levels were measured after addition of fixed batches of debris to the flume. The debris mixture consisted of six size classes of natural wood and was based on logs observed at the Geul inverted siphon. A grate was placed at the culvert’s inlet to prevent logs from passing through. In addition, flow velocities were measured at various locations using Laser Doppler Anemometry.

Based on visual observations and a feasibility study three mechanisms were proposed that describe the development of floating debris accumulations, and the feasibility of these mechanisms was confirmed in preliminary analyses. Furthermore, the experimental results were analysed to determine the influence on backwater rise of canopy drag due to skin friction within and below the accumulation, as well as form drag due to partial blockage of the inlet. It was found that in all cases inlet blockage was the dominant cause of head loss. Using this knowledge a theoretical framework based on a momentum balance equation was derived that relates the inlet blockage ratio to backwater rise, separating the contributions of the grate and debris. A regression analysis was then performed to obtain a design equation for the inlet blockage ratio caused by debris. In conclusion, this thesis provides improved understanding of the development of floating debris accumulations and their influence on backwater rise at submerged culverts and inverted siphons. The results enable more accurate predictions of water levels that can occur during floods and thus offer a solid foundation for flood management strategies.