Water Stewardship Information Sources
| Citation | Church, M. 2010. Sediment management in lower Fraser River, Criteria for a sustainable long-term plan for the gravel-bed reach. Prepared for Emergency Management BC, Flood Protection Program. |
|---|---|
| Organization | UBC |
| URL | http://ibis.geog.ubc.ca/fraserriver/reports/Sed_manage_Fraser.pdf |
| Abstract/Description or Keywords | A program of planned sediment removals from the gravel-bed reach of lower Fraser River was initiated in 2004 with the objective to maintain or lower flood water levels in the reach. To date, annual licensing difficulties have prevented the program from achieving gravel removal targets in all but one year. This report is intended to define criteria for a program that might be licensed for multi-year sediment removals in a sustainable long-term sediment management program. “Sustainable”, in this context, means achieving the goals for flood hazard mitigation without creating deleterious consequences for the riverine ecosystem. A number of lessons have been learned as the result of 2004-2009 and earlier experience. These include: o Sediment removal at a rate that approximates gravel influx has no immediately major effect on river processes and morphology; o Sediment removal by bar top scalping has little effect on local water levels; o The actual sediment removals to date appear to have had no lasting impact on fish or invertebrate populations in the vicinity of the removals, but monitoring effort has been insufficient to qualify this as a robust conclusion; o Sediment removal that is not properly planned and/or executed can have immediate and serious effects on fish populations; o Effective environmental monitoring of fish and invertebrate organisms requires a large continuing effort because of the very large natural variability in their occurrence, both spatially and temporally; o We still lack sufficiently robust baseline information of both the sediment budget and the aquatic ecosystem to be able to confidently establish an assuredly sustainable long-term sediment management program. A program of sediment removals from the river to counteract the effects of aggradation might be based on either of two distinct strategies: 1) routine removal, at convenient places along the river, of a volume of sediment that, over a period of years, approximates the bed material influx to the reach, with the effect of eliminating net aggradation. Such a strategy might be termed “profile maintenance”, meaning maintaining the flood water surface profile elevations; 2) targeted removal, from specifically identified sites, of volumes of sediment in order to counteract the effects of local sediment accumulation with consequent rise of water levels immediately upstream. Such a strategy might be termed “profile control”, meaning the management of flood water levels locally along the river. Experience to date suggests that only the first objective is feasible in a program that is sustainable – that is, a program that respects ecological values along the river. Such a program also requires measures of effectiveness, which currently are lacking. Direct observation of water levels along the river – possible because there is an extant network of manual gauges – is the most effective measure of the desired objective, but will be indicative only over a period of several years. Measures of habitat quantity and quality along the river are a second measure, also assessed over a period of years. Site monitoring remains important to ensure no direct damage to the aquatic ecosystem. All of these measures are most effectively appraised on a time scale comparable with the time scale for morphological and ecological changes along the river, which is of order 10 years. Hence, a program planned for a 10-year horizon and operated in an adaptive and precautionary fashion appears most appropriate. Accordingly, a sustainable long-term program of sediment removal from Fraser River in the gravel-bed reach will observe the following circumstances and criteria. o It will recognise that a sustainable long-term program of sediment removal may limit or eliminate general bed aggradation in the gravel-bed reach over a period of years, but that it cannot be used to eliminate local water level problems. o It will recognise that for the program to operate in the long-term in a sustainable way additional information is required, in particular more precise knowledge of the sediment budget needs to be developed and knowledge of the annual pattern of fish activities within the reach needs to be detailed. The program should proceed only if there is a commitment to conduct the research to acquire this knowledge. o The program should be planned and approved on a 10-year period, with more frequent review, as monitoring outcomes dictate. o Sediment removals will be focused in those sub-reaches where chronic sediment accumulation occurs. Four such zones can be defined today but these may change over a period of years; indeed, there is some evidence that some changes may currently be under way. o Within these zones, sites for sediment removal should be identified on a year-toyear basis, according to recent sediment deposition, the recent history of removals, and perceived (lack of) impact on current activities of fishes. o Volumes removed should conform with recommendations given in Church et al. (2001), as discussed in Appendix B of this report, using 230 000 m3 a -1 as the current estimate of average annual bed material recruitment. o Site-scale monitoring should follow the prescription given in G3 Consultants (2009) with emphasis on benthic invertebrates and on physical habitat measures. o Reach-scale monitoring should be undertaken on an annual or biennial basis using aerial photography, as described in this report. o The program must be adaptive and precautionary, with provision to change any elements of the program as soon as monitoring activities identify unfavourable changes to river morphology and/or the riverine ecosystem, or as soon as it becomes clear that secular changes in flow and sediment influx are affecting the river. o It will be recognised that there may be reasons to make sediment removals from particular sites along the river on a special basis (e.g., to maintain side channels; to facilitate navigation). Such needs should be considered outside the long-term sediment management program, but sediment removals should be considered as part of the volume removed for purposes of the program. |
| Information Type | report |
| Regional Watershed | Lower Fraser |
| Sub-watershed if known | Fraser River |
| Aquifer # | |
| Comments | |
| Project status | complete |
| Contact Name | Michael Church |
| Contact Email | [email protected] |