3.4 Sediment Storage
Channel sediment storage can be altered by sediment trapping in project impoundments, flow regulation, alterations to LWD volume, anthropogenic increases in erosion rates, and other natural events that affect sediment supply and transport. The dynamics of within-reach channel sediment storage, however, is often difficult to assess due to limited historical information, spatial and temporal variability in sediment storage, and uncertainties in sediment activity level and residence time. Changes in stored sediment volume in project-affected reaches may be quantified using several approaches, including reoccupying historical channel cross sections, repeated surveying of channel topography to determine volumetric changes through time, and by assessing the relative activity and response times of sediment in storage based on reservoir theory. Important considerations when planning a study of storage changes include:
- type and distribution of potential sediment storage sites;
- accommodation space for sediment at storage sites;
- grain size distribution and activity of sediment in storage;
- potential for storage changes based on sediment supply to the reach, transport rate within the reach, and yield from the reach;
- processes responsible for sediment delivery to and transport within the reach (e.g., debris flows vs. bedload transport during floods vs. long-duration transport of fine sediment); and
- time scale over which measurable storage changes may occur.
Often no historical cross-section or topographic data exist to document initial sediment storage and therefore storage changes. The relative importance of sediment storage can be assessed by estimating the maximum potential for scour and fill at sediment storage sites based on the maximum and minimum surfaces exposed to scour and fill (e.g., typical high water surface elevation and thalweg, respectively). Comparison of potential mass scour and fill estimates to average annual sediment mass balance (supply vs. transport) gives an indication of the potential for average annual storage changes. Marked tracer rocks in gravel bed channels can be monitored over multiple flood events to determine particle transit time, and therefore the age distribution and residence time of stored sediment.
Listed below are advantages and disadvantages of sediment storage approaches:
- Changes in the distribution, volume, and grain size of stored channel sediment are commonly a significant impact of hydroelectric projects that influence the ecological integrity of regulated rivers.
- Quantification of the activity level of stored channel sediment allows development of a more complete and representative sediment budget.
- Quantification of changes in channel sediment storage requires historical data or repeated topographic surveys after multiply flood events capable of transporting bedload and altering channel morphology.
- Selective transport of different grain sizes leads to variable residence times (i.e., finer fractions may be stored for shorter periods that coarser material).
- Steep, rough channels have variable transit times that are difficult to predict.
The following references are recommended for additional information on sediment storage approaches:
Gaeuman, D. A., J. C. Schmidt, and P. R. Wilcock. 2003. Evaluation of in-channel gravel storage with morphology-based gravel budgets developed from planimetric data. Journal of Geophysical Research 108 F1 6001: doi:10.1029/2002JF000002.
Hazel, J. E., D. J. Topping, J. C. Schmidt, and M. Kaplinski. 2006. Influence of a dam on fine sediment storage in a canyon river. Journal of Geophysical Research 111 F01025: doi:10.1029/2004JF000193.
Kelsey, H. M., R. Lamberson, and M. A. Madej. 1987. Stochastic model for the long-term transport of stored sediment in a river channel. Water Resource Research 23: 1738-1750.
Lisle, T. E., and M. Church. 2002. Sediment transport-storage relations for degrading, gravel bed channels. Water Resources Research 38: doi:10.1029/2001WR001086.
Wathen, S. J., T. B. Hoey, and A. Werritty. 1997. Quantitative determination of the within reach sediment storage in a small gravel-bed river using transit time and response time. Geomorphology 20: 113-134.