UNDERSTANDING AND PREDICTING THE IMPACT OF AGRICULTURE ON THE ENVIRONMENTAL INTEGRITY OF MANAGED WATERSHEDS
Location: Water Quality and Ecology Research
Title: Towards a Basis for Designing Backwater and Side Channel Restorations
Submitted to: Congress of International Association for Hydraulic Research Proceedings
Publication Type: Proceedings
Publication Acceptance Date: April 1, 2009
Publication Date: August 9, 2009
Citation: Shields Jr, F.D., Knight, S.S., Stofleth, J.M., Wren, D.G. 2009. Towards a Basis for Designing Backwater and Side Channel Restorations. Water Engineering for a Sustainable Environment, Proceedings, International Association for Hydraulic Research 33rd Congress, IAHR, Madrid, CD-ROM.
Interpretive Summary: River ecologists have found that floodplain lakes and sloughs that are attached to the river during higher flows (backwaters) are extremely valuable habitats, but these areas are undergoing rapid degradation due to sedimentation and pollution. Several techniques have been proposed and demonstrated for restoring the ecological quality of these areas, but methods for evaluating the effectiveness of the techniques are lacking. An recently-published innovative approach for evaluation was applied to a three example backwater restoration projects using ten years of river stage records for each of the sites, and strengths and weaknesses of each project were quantified. This method will assist others in planning, design and evaluation of river backwater restoration projects.
Design criteria for river channel restoration is becoming highly developed with several handbooks and guidance documents available, despite notable differences among various schools of thought. Basic principles of stable channel design and riverine habitat simulation undergird channel reconstruction. On the other hand, guidance for rehabilitation, protection, and management of riverine backwaters is less well developed. These projects typically address problems associated with hydrologic perturbation due to levees, dams, main channel incision, and backwater sedimentation. Additional issues include water quality degradation, aquatic plant infestation, and extreme variation in water temperature and habitat volume. Designs include water control structures, pumping, dredging, and plant and animal manipulations. Possible approaches for selecting design targets include habitat requirements for selected species, emulation of hydrology or water quality at adjacent or historic reference sites, or a multidimensional assessment of hydrologic connectivity and variability. The later approach is based on assigning a position to the system in a four-dimensional space that represents temporal variability on one axis and connectivity in the three spatial dimensions on the remaining three axes. More than one approach may be used to assess a given restoration plan, and tradeoffs among possible outcomes may be made. For example, schemes that allow for greater hydrologic connectivity between the backwater and the main channel may incur higher risks of excessive sedimentation. Specific alternatives for implementing a given plan may be evaluated on the basis of construction costs, costs for operation and maintenance, uncertainty, sustainability, and flexibility. Three examples are presented: selected severed meander bends along the Tennessee-Tombigbee Waterway in Mississippi and Alabama, a severed meander bend along the Coldwater River, Mississippi, and a reconstructed side channel along the Sacramento River, California. Rehabilitation projects often improve connectivity but have little impact on variability, primarily due to large-scale hydrologic influences attributable to reservoirs.