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Gravel Dunes

Summary of research observations at a gravel-bed river

Randal L. Dinehart, U.S. Geological Survey, Sacramento, California, USA

flowing river

Introduction

Just as wind-blown sand forms ripples and dunes, a flowing river will create dunes of many sizes from sediment on the streambed. Although most river bedforms are composed of silt and sand, some rivers with gravel or cobble beds can also create dune-shaped bedforms. The amount of force required to move river gravel as bedforms may occur during storm flows or floods.

When gravel dunes form in a riverbed, water flowing over the bedforms changes velocity because of the extra roughness. When gravel dunes migrate, they change the mean bed elevation by scouring sediment or forming gravel bars.

To show how gravel dunes in flooding rivers eventually affect river measurements by hydrographers, this document describes detailed observations of streambed and velocity over the gravel bed of the North Fork Toutle River.

Location and equipment

Despite being inundated with volcanic mudflows from Mount St. Helens in 1980, the North Fork Toutle River in Washington state soon developed a streambed with a gravel/cobble composition typical of many steepland rivers. Velocity and sonar sensors were deployed with prototype river equipment during storm flows in the North Fork Toutle River. Data were collected in both English and metric units because of differences in gaging-station data and research-oriented measurements.


Dunes, bars, and stage changes during storm flows

Streambed features observed during storm flow are shown at progressively larger scales:


Continuous velocity measurements

Electromagnetic velocity meters were used to measure velocity every 0.5 seconds at two or three points above the streambed. Velocity observations are shown from a variety of storm flows:


Hydrographic Effects

We can describe a river by the flow rate of water within its boundaries. The well-known formula, Q =VA, provides a discharge Q with an accuracy dependent on our ability to measure the mean velocity V and total area A. Continuous fluvial observations reveal variations in velocity and streambeds that are caused by large-scale turbulence and sediment transport. When computing daily discharges for streams, hydrographers may notice effects in their data such as:


Gravel dunes in California streams?

The minor fluctuations in stage induced by bedform migration at the North Fork Toutle River are sometimes found in stage records from other gravel-bed rivers. Examples of stage records from gravelly California streams are presented here:


References

Dinehart, R.L., 1989, Dune migration in a steep, coarse-bedded stream, Water Resources Research, v. 25, no. 5, p. 911-923.

Dinehart, R.L., 1992, Gravel-bed deposition and erosion by bedform migration observed ultrasonically during storm flow, North Fork Toutle River, Washington, Journal of Hydrology, v. 136, p. 51-71.

Dinehart, R.L., 1992, Evolution of coarse-gravel bedforms: Field measurements at flood stage, Water Resources Research, v. 28, n. 10, p. 2667-2689.

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