3.2.1 Sediment flux

Sediment flux (suspended load or bedload discharge per unit time), estimated from measurements of sediment load in mainstem or tributary channels, is arguably the most reliable metric for evaluating sediment supply to project reservoirs and project-affected reaches. In addition to providing estimates of sediment supply, measured bedload and suspended load flux rates can be used to estimate coarse-to-total load ratios. Bedload sampling technologies include instream installations (e.g., pit traps, net-frame samplers, and detention basins or weir ponds), portable devices (pressure difference samplers, bedload collectors, tracer particles [Figure 2], and scour chains [Figure 3] or cores), and surrogate technologies (acoustic Doppler, hydrophones, gravel impact sensors, magnetic tracers, and topographic differencing). Suspended sediment sampling technologies include single and multi-frequency acoustics, laser defraction, optical sediment flux, digital image analysis, pressure differential, and bulk optics.

 

The usefulness of existing sediment gaging records for characterizing the longitudinal pattern of sediment supply and extrapolating flux rates to other areas depends on the length of time sediment flux has been monitored and the distribution of monitoring sites relative to geologic terrains, tributaries, and other areas important to sediment production. Correlation of long-term sediment gaging records to flow gaging records is necessary to characterize variability in flux rates over different water year types, especially when projects are located in geomorphic settings characterized by stochastic erosion processes that deliver sediment to channels and mobilize stored channel sediment during large, infrequent triggering events (e.g., large storms, fires, earthquakes, and volcanic eruptions). Short-term gaging records can be correlated to other longterm sediment gaging records if sediment supply is controlled by similar geology, climate, hydrology, and land use.

 

3.2.1.1 Selected references

Edwards, T. K., and G. D. Glysson. 1999. Field methods for measurement of fluvial sediment. U. S. Geological Survey, Techniques of Water-Resources Investigations, Book 3, Chapter C2.

Gray, R. D., editor. 2003. Proceedings of the Federal Interagency Sediment Monitoring Instrument and Analysis Research Workshop. 9-11 September. Flagstaff, Arizona.