Modeling fluvial response to in‐stream woody vegetation: implications for stream corridor restoration

Abstract

River restoration and bank stabilization programs often use vegetation for improving stream corridor habitat, aesthetic and function. Yet no study has examined the use of managed vegetation plantings to transform a straight, degraded stream corridor into an ecologically functional meandering channel. Experimental data collected using a distorted Froude-scaled flume analysis show that channel expansion and widening, thalweg meandering and riffle and pool development are possible using discrete plantings of rigid, emergent vegetation, and the magnitudes of these adjustments depend on the shape of the vegetation zone and the density of the vegetation. These experimental results were verified and validated using a recently developed numerical model, and model output was then used to discuss mechanistically how rivers respond to the introduction of in-stream woody vegetation. Finally, a hybrid method of meander design is proposed herein where managed vegetation plantings are used to trigger or force the desired morphologic response, transforming a straight, degraded reach into a more functional meandering corridor. It is envisioned that such numerical models could become the primary tool for designing future stream restoration programs involving vegetation and assessing the long-term stability of such activities. Wu et al. (2005) developed a depth-averaged, two-dimensional numerical model to simulate flow, sediment transport and bed topography in mildly sinuous river channels with vegetation of various types. This model is based on the depth-integrated, Reynolds-averaged Navier-Stokes equations for shallow water flows as described by Wu and Wang (2004a, 2004b).Although the model described by Wu et al. (2005) is applicable to a wide range of hydraulic and geomorphic conditions with bedload and suspended load transport of sediment mixtures and with various configurations and types 894 S. J. Bennett et al.These effects can be seen from the topographic maps for the rectangular vegetation zone (Figure 3). Marked channel expansion occurs downstream and opposite the introduced vegetation zone in the form of bank toe erosion and bank failure (Figure 3(c)). Since flow stage is just below the threshold of sediment motion, this eroded and failed bank material is quickly deposited within the lower confines of the channel, and a migrating aggradational front is present in the mid-channel region upstream of the vegetation zone (see spatial coordinate x = 200 mm and y = 275 mm, herein denoted as [200,275], Figure 3(c)). As vegetation density increases, the magnitudes of bank erosion, channel widening and mid-channel aggradation increase. The greatest amount of bank erosion and channel widening occurs directly opposite and just downstream of the trailing edge of the vegetation zone. Given enough time, these zones of Figure 3. Contour maps of the measured bed surface topography for the rectangular vegetation zone with densities of (a) 0·77, (b) 2·94 and (c) 11·53 m −1 after run times listed in Table II. Flow is left to right. This fi...

Evidence of increase in woody vegetation in a river corridor, Northwest England, 1984–2007

“…River corridors are prime locations for maintenance of such connected habitats, and riverbanks can be refuges (Smart et al, 2005). The presence of riparian vegetation delivers many benefits, and much recent research has also been stimulated by a need for greater understanding of dynamics in order to implement protection and restoration (Bennett, Wu, Alonso, & Wang, 2008;Richter & Richter, 2000;Sandercock & Hooke, 2010;Schirmer et al, 2014;Tabacchi, Steiger, Corenblit, Monaghan, & Planty-Tabacchi, 2009). …”

A two-dimensional hydro-morphological model for river hydraulics and morphology with vegetation

“…In this section, the new hydraulics-morphology-vegetation modelling system is validated against 263 a number of laboratory-scale test cases, including steady flow over a compound channel with a 264 fixed bed (Pasche and Rouvé, 1985) and steady flow over a compound channel with a movable 265 bed (Bennett et al, 2008). 266…”

Experimental Investigations of Scour Pools around Porous Obstructions

“…In the past, channels were straightened to prevent flooding but it caused the destruction of riverine ecosystems [1]. Recently, stream restoration works have been extensively conducted to return degraded ecosystem and natural functions [1,2].…”

“…Recently, stream restoration works have been extensively conducted to return degraded ecosystem and natural functions [1,2]. One of the main objectives of these works is to create locally varied flow conditions, and promote scour pools and riffles.…”

“…Recently, to restore more natural streams, porous obstructions such as vegetation have been advocated for use, because they not only create scour holes but also improve water quality by removing contaminants [10]. Bennett et al [1] conducted laboratory experiments in a flume using fine sands within a compound channel. Porous obstructions were placed alternately along the channel, affecting the flow and bed morphology.…”