Channel Rehabilitation: Processes, Design, and Implementation (original) (raw)
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River channel adjustment to hydrologic change
2001
The size of an alluvial river channel can adjust in response to changes in the pattern of flows that it carries. An important case of such channel change occurs downstream of dams or water diversions where flow regulation has been observed to cause morphological and ecological impacts.
Environmental River Engineering
Water and Environment Journal, 1990
In the past, river engineering works have often caused channel instability and adversely affected the river's conservation and amenity value. Recent guidelines have advocated a more natural approach to river engineering practice which retains habitat diversity within the river system. While a more natural approach is desirable, geomorphological guidance is required to ensure that the advocated changes are feasible and sustainable, both in the long and short term. The key requirement for sound environmental river engineering is a basic understanding of the natural processes controlling channel shape and dimensions. Examples are given in the paper to illustrate how such knowledge can be used to (a) stabilize rivers, (b) design environmentally-acceptable and stable flood-alleviation schemes, and (c) restore previously canalized rivers. The basis of the geomorphological input in the assessment and design process is a river survey which determines the factors controlling channel characteristics and how it will respond to planned changes.
Channel responses and management strategies in disturbed channels: a numerical simulation approach
The Yalobusha River watershed underwent extensive channelization and channel repositioning during the 1960s. The newly channelized system experienced channel degradation, rejuvenating tributaries and increasing bank heights above stable conditions, causing bank failures and the addition of vegetation and sediment to the channels. The amount of sediment added to the river basin due to bank failures and bed degradation alone has been estimated to be 833,000 tonnes/yr, while the input of vegetation due to bank failure in the vicinity of major knickpoints has been estimated to be around 100 trees/yr. This has promoted the development of a large logjam at the downstream terminus of channelization works. This debris 'plug' has caused increased stages and flood frequencies in the vicinity of Calhoun City, 5 km upstream. The U.S. Army Corps of Engineers have identified a number of remediation strategies including plug removal, numerous grade-control structures to arrest headward migration of knickpoints following plug removal, and flood-retarding structures. The one-dimensional, unsteady, gradually varying open channel flow model, CONCEPTS (CONservational Channel Evolution and Pollutant Transport System) is being used to model channel responses to channelization, including bed degradation, bank failures and hence sediment inputs and loads from 1968 to 1997. CONCEPTS has been shown to accurately depict in-channel and bank processes and hence can correctly predict the effects of channelization together with future rehabilitation measures.
Channel adjustments of alluvial channels and implications for river management and restoration
Supplementi di Geografia Fisica e Dinamica Quaternaria
Most Italian rivers have experienced widespread channel adjust- ments over the last 100 years, mainly in response to a range of human activities. The aim of this paper is to show how knowledge of channel ad- justment and reconstruction of evolutionary trajectory are or can be used in river management and restoration. The first part of the paper deals with channel adjustments and summarizes the results of recent studies carried out on twelve rivers in northern and central Italy. The second part illustrates three examples of application. The selected rivers have undergone almost the same processes in terms of temporal trends. Initially, river channels underwent a long phase of narrowing (up to 80%) and incision (up to 8-10 m), which started at the end of the 19th century and was very intense from the 1950s to the 1980s. Then, over the last 15-20 years, channel widening and sedimenta- tion, or bed-level stabilization, have become the dominant processes in most of the rivers, though cha...
Environmental Earth Sciences, 2009
Site assessment and monitoring data were analyzed for 26 stream restoration projects in North Carolina where the channel was reconfigured. Post-project changes in channel capacity were highly variable from site to site, but more than 60% of the projects underwent, on average at a given site, at least a 20% change in channel capacity. An analysis of site and basin geomorphology revealed that large post-construction adjustments were associated with highly dynamic stream channels characterized by a combination of high sediment transport capacity, large sediment supply, and/or easily eroded bank materials. In-stream structures along dynamic, reconfigured channels also exhibited high incidences of damage. Thus, the design and construction of channels in a state of equilibrium, which do not exhibit excessive erosion or deposition along highly dynamic rivers is currently problematic. In light of these findings, a conceptual framework based on geomorphic parameters is put forth to assess the likelihood of project success early in the design process to (1) eliminate high risk sites from consideration of channel reconfiguration and (2) improve upon the implemented management strategies that are ultimately used. It is also argued that where space permits an enhanced natural channel, adjustment approach is likely to be more effective than projects based on natural channel design.
2008
knowledge of the potential impacts of river diversions on channel morphology, especially induced sedimentation in the river channel. Processes considered in this note are those most pertinent to riverine, as opposed to estuarine, aspects of diversions. In particular, this note provides general guidance on the physical process issues, outlines strategies for more effective application of existing tools, and summarizes the needs for developing better tools to address the issues. These issues become critical in the planning and design of diversions when the operation of these structures could adversely impact other authorized uses of the river. For example, the management of water and sediment resources in coastal Louisiana for the mitigation of land and habitat loss must be balanced with the needs for flood control and navigation. In one management scenario, plans are being formulated (U.S. Army Corps of Engineers (USACE) 2004) to divert flow from the Mississippi River to distribute w...
River Channel Relocation: Problems and Prospects
Water
River relocation is the diversion of a river into an entirely new channel for part of their length (often called river diversions). Relocations have been common through history and have been carried out for a wide range of purposes, but most commonly to construct infrastructure and for mining. However, they have not been considered as a specific category of anthropogenic channel change. Relocated channels present a consistent set of physical and ecological challenges, often related to accelerated erosion and deposition. We present a new classification of river relocation, and present a series of case studies that highlight some of the key issues with river relocation construction and performance. Primary changes to the channel dimensions and materials, alongside changes to flow velocity or channel capacity, can lead to a consistent set of problems, and lead to further secondary and tertiary issues, such as heightened erosion or deposition, hanging tributaries, vegetation loss, water...
Impediments to River Rehabilitation
Rehabilitation of river ecosystems is generally initiated when a river is no longer able to provide benefits to humans and other living beings. The major purposes of river rehabilitation projects are improving water quality, conserving nature, rehabilitating fisheries and riparian habitats, flood mitigation, and creating recreational opportunities. For a river rehabilitation project to be successful, it must have specific and clearly explained objectives, although these objectives will vary for each project depending upon factors such as economics, public demand, ecosystem benefits, and the like. In this article, we provide a critical analysis of the purposes that drive river rehabilitation projects and we identify and discuss impediments that can cause a project to deviate from its intended goals. We also discuss major constraints on rehabilitation efforts that can result in failed projects.
2004
Highly modified channels drain extensive portions of productive agricultural land in the U.S.A. In many of these areas, most natural channels have been deepened and straightened to facilitate the flow of water from agricultural subsurface drainage outlets and to maximize conveyance. Work done periodically to maintain the drainage function typically includes removal of woody vegetation and deposited sediment. Ancillary work includes stabilizing bank slope failures and toe scour. Ditch form is a result of not only construction and maintenance but also, to verifying degrees, due to fluvial (flowing water) processes. The purpose of this project was to demonstrate through applied research and outreach education an alternative drainage channel form approach that incorporates naturalized fluvial features to enhance stream integrity and maintain or improve drainage capacity. The objectives of this project, addressed below, focused on understanding the conditions that favor such evolution an...