CONTROLS ON SEDIMENT SUPPLY IN
COARSE-BEDDED CHANNELS
The particle size of sediments deposited in the lee of boulders should give an indication of the volume of material available for transport at different river stages. Pebble counts in various morphological areas are combined with bedload sampling to develop a quantification technique for sediment supply. The role of fine-grained wake and recirculating-eddy deposits behind boulders is an important area of investigation. These deposits may represent a large volume of sand-sized sediment that would be highly sensitive to river stage. Therefore, the volume of material available for movement would be reflected by differences in D50 sediment sizes in the wake zones relative to average channel D50 values. The basic project design is well suited to New England because most of the channels have coarse beds from glacial deposits and high artificial sediment loads from winter sanding activities. An abrasion-resistant Plexiglas flume 8.0 m long, 0.5 m wide with a slope range of 0-5 percent and a maximum flow rates of 2400 l/min is used to recreate channel conditions in a controlled setting. Bricks are used to create mid-channel obstructions on a gravel-bed flume substrate. Sand-sized sediments is fed into the flume until wake deposits are formed that maintain an equilibrium size and shape. The corresponding bedload-transport rates will be related to changes in stage relative to the mid-channel obstructions. We anticipate large differences in sediment-transport rates with activation of wake deposits as flow overtops the brick obstructions.
Sushil Bhattarai ('00), Sailesh Tiwari ('02), and Kim Hoffman ('00) determine bankfull-channel widths on the Blackledge River, CT (1999).
Sushil measures current velocity, Kim records times and data, and Sailesh measures bedload transport rates (1999).
Greg Stull (’01), Jaime Goode (’02) and Eliot Pitney (’01) conduct moderate-flow measurements on the Blackledge River, CT (2000).
Jaime Goode (’02), Melaine Gryboski ('01) and Lauren Hartzell (’03) conduct moderate-flow measurements on the Blackledge River, CT (2001).
Melaine Gryboski ('01) surveys using a laser total station on Dickenson Creek, CT (2001).
FLUME EXPERIMENT
Channel-bed armor can help to limit bedload-transport rates in coarse-bedded channels. In these systems, periodic large bed elements often exist with small deposits of fine material in the wakes of these boulders. A combined field and flume study was conducted to investigate the potential impact of these wake deposits on bedload-transport rates. Detailed sediment characterizations were performed on subsurface, surface and wake sediments in two coarse-bedded Connecticut rivers. Bedload measurements also were conducted at moderate and low flow in these two systems to determine the size distribution of bedload material. A 6-m long, 0.5-m wide flume was used to model these systems with fine sediment passing over a fixed-bed of sediment particles with uniform-sized, large bed elements. Sediment distributions of the wake deposits in the two Connecticut channels indicate the wake deposits may be produced from winnowing of sediments from the surface layer. Wake deposits also have sediment distributions similar to bedload-sediment distributions at low and moderate flow. Flume experiments show an increase in bedload-transport rate following changes in discharge versus steady-flow conditions. Even with decreases in discharge, bedload-transport rates show an increase versus the steady-flow state. The results demonstrate that sediment wakes establish a temporary dynamic equilibrium for a given discharge. Either increases or decreases in discharge disrupt this equilibrium condition and increase sediment delivery to the main flow. The study suggests that the influence of the rate of change in discharge may be as important as the absolute magnitude of discharge on sediment-transport rates at moderate and low discharges in sediment-limited systems.
This material is based upon work supported by
the National Science Foundation under Grant No. 9874751. Any opinions,
findings, and conclusions or recommendations expressed in this material are
those of the author(s) and do not necessarily reflect the views of the National
Science Foundation.
BACK