Representing natural and artificial in‐channel large wood in numerical hydraulic and hydrological models

Abstract

The influence of naturally occurring in‐channel large wood (LW) on the hydraulics, hydrology and geomorphology of rivers is well documented. To inform management and better understand naturally occurring or artificially placed LW, hydraulic and hydrological models are applied to predict the possible benefits and drawbacks for habitat, sediment management and flood risk mitigation. However, knowledge and guidance on appropriate representation in models, needed to underpin realistic predictions, is lacking. This could lead to unrealistic expectations of the effectiveness of LW for different river management goals. To date, seven types of LW representation in hydraulic and hydrological models have been applied, the range partly reflecting the variety of LW, model types, scales and purposes. The most common approach is by altering channel roughness to represent flow resistance. Although qualitatively the effects of LW have been captured using models, to date quantitative validation, as well as transferable knowledge to help a priori parameterization of LW representations, remain limited. Therefore, additional empirical investigations and robust model validation are required to inform defensible LW representations for specific purposes and scales in numerical models coupled with better accounting of input uncertainty to improve confidence in predictions. Future studies should also consider a greater range of artificial and natural LW features, settings, larger spatial scales and better account for temporal variability of flow, morphology and LW configuration. This article is categorized under: Water and Life > Methods Science of Water > Methods Water and Life > Nature of Freshwater Ecosystems

Modelling the Impact of Leaky Barriers with a 1D Godunov-Type Scheme for the Shallow Water Equations

“…The results suggest that representing leaky barriers as hydraulic structures is a valid method, provided that the barrier resembles the hydraulic structure, the energy losses are incorporated into the momentum part of the internal boundary condition, and possible submergence from downstream backwaters is considered. In particular, this investigation of the hydraulic structure method addresses a specific research gap put forward in a recent review paper on modelling leaky barriers [56]. Given the current interest discussed above in installing multiple leaky barriers across catchments for flood prevention purposes, greater understanding of potential synchronisation issues is critical.…”

“…For example, online barriers are often represented as increased values of Manning's n, both in hydrological models [36,53] and hydraulic models [54,55]. However, even if these parameter values come from the field, it is inappropriate to apply them to other sites or flow conditions [56]. Moreover, an increased Manning's n will impact low flows as well as high flows, and so this would not be a good model of barriers that let low flows through unimpeded.…”

“…To get a clearer idea of how leaky barriers behave in isolation, they have to be investigated in isolation. In particular, a recent review paper [56] has recommended that the hydraulic structure method be investigated in more detail. The aim of the present study is to develop a methodology to model leaky barriers in catchments, using a combination of physical and mathematical models.…”

A risk-based, network analysis of distributed in-stream leaky barriers for flood risk management

“…, these studies do not consider performance failure, and there remains much trial-and-error installation of different designs which could be improved upon for more efficient risk-reduction strategies at the large scale . 5 There have been many attempts at representing the effects of leaky-barriers on flow, with methods ranging from increasing roughness in 1d models to full 3d representation, but relatively few have been able to test the accuracy of the physical representation (see Addy and Wilkinson, 2019). The NERC project, Q-NFM 2 has developed a set of small, accurately monitored 'micro-catchments' in Cumbria to attempt to quantify the effect of different nature-based interventions.…”

“…The dimensions of the slot height are also not clear, and vary in grey literature between 0.1-0.3m, but in practice, are less (on average across a cross section) than this in locations like Penny Gill. A compounding factor is that there are many forms of leaky barrier or large woody debris dams (Addy et al, 2019) including placing large woody debris in channel, to the horse-jump type barriers in use in Penny Gill, combined with engineered log jams which will also reduce passage of debris should a structure fail. 20…”