A channel evolution model to guide sustainable urban stream restoration

Abstract

Channel evolution models (CEMs) are used to structure the interpretation of observed channel morphology to support long-term restoration of these systems. However, channels reflect the variety of their watersheds' climatological, ecological and physiographic contexts, and so no single CEM can be truly 'global' . Unrecognised differences between the assumptions and the reality of evolutionary trajectories of particular streams can subsequently lead to restoration actions that neither fully achieve their intended objectives nor successfully self-maintain even limited improvements. Despite the daunting variety of biophysical settings, however, urbanisation imposes distinctive, homogenising influences on virtually all watercourses, suggesting that even a relatively small set of evolutionary pathways can embrace much of the diversity of critical watershed drivers on urban channels. CEMs describing single-thread channel response to incision are most common in the published literature, but not every urban disturbance yields this classic sequence, initiated by excess transport capacity followed by incision, bank erosion, widening and ultimately a lowered re-equilibrated channel. A comprehensive urban CEM must also include responses under less common (but locally ubiquitous) conditions, such as excess sediment relative to transport capacity (the 'inverse' of the classic CEM), imposed constraints on vertical and/or lateral adjustment, and multi-thread channels or those influenced by instream or riparian vegetation. An urban CEM also requires a hierarchical framework that acknowledges fundamental differences in the process drivers within any given watershed, because a single observation of channel form can rarely pinpoint the context or evolutionary trajectory of every stream. We present a geomorphic framework for diagnosing and predicting the evolution of urban streams, potentially guiding the selection of restoration targets that are achievable within an urban context and sustainable without ongoing maintenance.

Enlargement and evolution of a semi‐alluvial creek in response to urbanization

“…For example, the lower reach of Wilket Creek has an enlargement ratio (E r ) of 2.6 in comparison with the 1958 channel (Figure 8), which is within the expected range for temperate streams (Chin, 2006). The significant increase in depth (0.4-0.5 m or~75%) is also in line with expectations from other studies (Chin, 2006), and together with the exposure of till in lower parts of the the channel banks confirms that the geology is not acting to create overly widened channels, as would be the case for a bedrock channel (Booth and Fischenich, 2015), and that the channel can in fact incise into the bed material and till. Over 200 mm of rain fell on saturated ground over a 48-hour period in this storm, and damage in the larger Don River catchment was well documented, with a number of bridges failing and considerable damage to buildings in the floodplain.…”

“…In contrast with the classic concave profile of a graded channel (Mackin, 1948;Schumm and Lichty, 1965), many channel profiles in the region are convex or 's' shaped (Vocal Ferencevic and Ashmore, 2012;Thayer et al, 2016) as channels continue to rework the lag material from relatively recent glacial melt channels (Church and Slaymaker, 1989;Newbury, 2013;Phillips and Desloges, 2015). The model builds on previous work (Schumm et al, 1984;Simon, 1989;Hawley et al, 2012;Booth and Fischenich, 2015), but also includes the observed dynamics of particle size, till exposure, and planform adjustments in Wilket Creek. The current study thus provides a test case of enlargement and evolution in a channel with a resistant bed.…”

“…In the 'classic' model, channel straightening creates an imbalance of the sediment capacity and supply, which leads to a series of more or less discrete phases of adjustment characterized by incision, mass failure of the banks, and re-establishment of a new floodplain (Schumm et al, 1984;Simon, 1989). In some cases the channel may widen (Pizzuto et al, 2000) rather than immediately incising (Booth, 1990;O'Driscoll et al, 2009), probably due to differences in geology or other constraints (Henshaw and Booth, 2000;Bledsoe et al, 2012;Booth and Fischenich, 2015). In some cases the channel may widen (Pizzuto et al, 2000) rather than immediately incising (Booth, 1990;O'Driscoll et al, 2009), probably due to differences in geology or other constraints (Henshaw and Booth, 2000;Bledsoe et al, 2012;Booth and Fischenich, 2015).…”

“…The extent of imperviousness and the degree to which watershed drainage is accelerated appear to be critical factors (Hammer, 1972). It is therefore important to establish regional models of channel evolution Booth and Fischenich, 2015) and to examine the spatial variability in channel adjustments (Gregory et al, 1992;Gregory and Chin, 2002). It is therefore important to establish regional models of channel evolution Booth and Fischenich, 2015) and to examine the spatial variability in channel adjustments (Gregory et al, 1992;Gregory and Chin, 2002).…”

Quantifying River Channel Stability at the Basin Scale

“…Additional complex responses to channel instability include tributary rejuvenation through headcutting, accelerated rates of lateral mobility in meandering rivers [13,14] and channel widening as flow is deflected around maturing sediment bars, both of which could threaten linear flood defences not set back from the channel. While Lane's relationship provides a useful concept to describe the capability of an alluvial stream to adjust its morphology through sediment erosion, transport and deposition processes [15] and has provided a platform to develop more rigorous models of qualifying (e.g., [16]) and quantifying (e.g., [6]) river channel change and framing river restoration methods (e.g., [17,18]), critically it does not state explicitly how channel geometry and/or planform might change, or whether the bed and/or banks are likely to adjust in response to a given sediment imbalance scenario. Also, in some cases, the channel boundary might not be free to adjust or might be responding to other influences not accounted for in the relationship [19] and therefore, despite efforts to extend the relationship to include morphological variables [20][21][22], Lane's approach remains a qualitative, 'indicative' treatment of potential channel instability (degradation vs aggradation) that is not necessarily a reliable predictor of the 'type' of morphological response, e.g., bed scour, bank erosion, bar formation or some combination thereof.…”