The Uses of Sediment Transport and Morphodynamic Modeling in Stream Restoration

Abstract

(number)Many streams have become morphologically and ecologically compromised because their sediment budget is out of balance. Examples include a) stream reaches upstream and downstream of dams, b) streams subject to gravel/sand mining, c) streams subject to urban encroachment, d) streams the drainage basins of which have been made largely impermeable and e) streams subject to sediment disposal, toxic or otherwise. Restoration of such streams can be greatly aided by considerations of sediment balance. Depending on the type of project under consideration, the calculations involved can be as simple as those implemented with a spreadsheet or sufficiently complex to require a custom-made numerical model. An example is given in the presentation.

Experimental study of the effect of grain sizes in a bimodal mixture on bed slope, bed texture, and the transition to washload

“…Parker [] pointed out that understanding the sediment budget of a system—size distribution as well as quantity—should be a major part of river restoration projects, noting that anthropogenic alterations to both the watershed and water course often have significant effects on the sediment budget. Anthropogenic changes that influence the sediment budget can be driven by changes in either water or sediment supply and include urbanization; dam construction, removal, or flushing; timber harvest and forest fires; sediment mining; and near‐channel development with loss of floodplain storage [ Wohl et al ., ].…”

“…The effects of this uncertainty can be large; for example, a modest difference in the size cutoff for the slope‐determining size range can change the predicted slope by a factor of two [ Paola et al ., ]. Additional examples are found in applications related to environmental restoration: stream restoration often requires consideration of the response of a gravel bed to the addition of fine sediments partly because of its influence on bank stability and bed slope [ Parker , ] but also because of its influence on habitat quality related to the presence of fine particles in pore spaces [e.g., Carling and McCahon , ; Suttle et al ., ; Power , ], as we discuss shortly.…”

Assessment Approach for Identifying Compatibility of Restoration Projects with Geomorphic and Flooding Processes in Gravel Bed Rivers

“…In the former case, we noted that while the approach can work at times, the results are susceptible to failure because they do not account explicitly for larger-scale processes such that the river simply ignored the specific imposed channel form (Kondolf 1998;Kondolf et al 2001;Parker 2004;Simon et al 2007), and the choice of which class a stream belongs to may be equivocal (Roper et al 2008). In the second case, while landscape or watershed scale characterizations provide important information for basin-scale restoration planning and prioritization, the associated statistical and GIS correlation results cannot be used to discern physical feasibility of a specific project type at a given location in the river network.…”

“…We heeded the restoration research literature calling for process-based decisions, and decided our conceptual framework for performing assessments should instead reflect the idea that a project that lets the river do most of the geomorphic work to effect channel and habitat change can be expected to have a higher potential for success because there is no attempt to impose some predetermined form on the channel and floodplain (Parker 2004;Klingeman et al 2004). This requires assessing the overriding hydraulic and sediment transport characteristics controlling habitat form and function.…”

“…In our framework, a strategy was employed for identifying the extent to which shorter, distinct, contiguous segments within a river reach are geomorphically and hydrologically active or inactive in terms of sediment transport imbalances, channel migration, and floodplain inundation, which are the key factors influencing restoration design (Kondolf 2000;Shields et al 2003;Parker 2004;Wilcock 2004). This strategy is consistent with Wohl et al's (2005) key criterion that methods of restoration should be effective under existing constraints, because it leads directly to specifying project types that are compatible with the existing local and reach-scale processes.…”