Efficacy of Longitudinal Training Walls to Mitigate Riverbed Erosion

Abstract

The Waal Branch of the Rhine River has eroded over the last 150 years following channel straightening and narrowing. In 2014–2015 a pilot project replaced existing groynes over an 11 km long reach with three longitudinal training walls (LTWs) to mitigate channel bed erosion, among other purposes. Walls are lower than the river bank and split the flow between a primary and an auxiliary channel, which are hydraulically connected during floods. Water enters the auxiliary channel at three elevations (from bottom to top): via an entrance weir, through inter‐wall notches, and over the wall. Bathymetry and discharge data were collected for 5 years after construction, which is a first indication that longitudinal dams can help mitigate channel bed erosion and analyzed to understand how the walls partition water and sediment and whether erosion is mitigated by LTWs. As the river discharge increases, a larger fraction of flow is diverted from the primary channel into the auxiliary channel. After a flood, sediment is deposited in the primary channel near the upstream end of each wall and localized scour occurs where the auxiliary channel rejoins the primary channel. Between floods, the accumulated sediment disperses and scour pits tend to fill. We observe a net‐accumulation of sediment in the study domain 5 years after construction. Erosion is best mitigated when weir flow is minimized to keep bed material in the primary channel, but weir flow remains important at lower flows for ecological purposes.

Large‐Scale Channel Response to Erosion‐Control Measures

“…Figure 4a shows a succession of incising and aggrading waves around Sint Andries after 2015. These waves are not related to the fixed bed, but rather a response to the construction of longitudinal training walls over river km 912–918 in the period 2014–2015 (Czapiga et al., 2022a).…”

“…Current domain-wide incision rates range between 0 and 2 cm/a (Ylla Arbós et al, 2021a). To mitigate this incision and to maintain the navigation channel, various interventions have been carried out, ranging from sediment nourishments, to scour-filling measures, bendway weirs, longitudinal training walls, and fixed beds (Czapiga et al, 2022a(Czapiga et al, , 2022bFrings et al, 2014;Havinga, 2020;Quick et al, 2019;Ylla Arbós et al, 2021a).…”

“…Consequently, the side effects of interventions may negatively affect areas elsewhere in the basin. Examples range from sediment starvation in deltas due to upstream dams (e.g., Bussi et al., 2021; Hu et al., 2009; Rao et al., 2010; Syvitski et al., 2005), to local erosion pits of different depths downstream of erosion‐control measures (Czapiga et al., 2022a; Korpak et al., 2021; Kostadinov et al., 2018; Lenzi et al., 2003).…”

Evaluation of Local Scour along the Base of Longitudinal Training Walls

“…As technology and science progress, river courses without human intervention are becoming rare, while different engineering projects focused on improving navigation and reducing flood risks are more commonplace [1]. Scour represents a primary mechanism within the realm of water erosion, whereby it engages in the excavation of bed materials and induces a reduction in the structural integrity of fortifications safeguarding riverbanks or margins [2][3][4].…”