USGS LOBO

In cooperation with the Texas Parks and Wildlife Department
and the University of Texas Bureau of Economic Geology

Indications and Potential Sources of Change in Sand Transport in the Brazos River, Texas

By David D. Dunn and Timothy H. Raines

U.S. Geological Survey
Water-Resources Investigations Report 01–4057


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Contents

Abstract

Introduction

Purpose and Scope

Description of the Brazos River Basin

Suspended-Sediment Data-Collection Methods

Previous Studies of Sediment Transport in the Brazos River

Capacity of the Brazos River to Transport Sand

Indications of Change in Sand Transport

Changes in Channel Geometry

Changes in Suspended-Sediment Characteristics

Changes in Discharge

Potential Sources of Change in Sand Transport

Reservoir Construction

Changes in Land Use

Instream Sand and Gravel Mining

Summary

References Cited

Figures

1.   Map showing Texas Gulf of Mexico shoreline
2.   Map showing Brazos River Basin, Texas
3.   Photograph showing typical cut bank on the Brazos River, Texas
4.   Map showing location of selected streamflow-gaging stations in the lower Brazos River Basin, Texas
5–7.   Graphs showing:
  5.   Historical development of reservoir conservation storage capacity in the Brazos River Basin, Texas
  6.   Particle-size distributions of bed material samples near streamflow-gaging station 08114000 Brazos River at Richmond, Texas
  7.   Flow-duration curve of daily mean discharge for streamflow-gaging station 08114000 Brazos River at Richmond, Texas, 1903–95 (excluding 1907–21)
8–10.   Graphs showing stage-discharge relation at streamflow-gaging station:
  8.   08111500 Brazos River near Hempstead, Texas
  9.   08114000 Brazos River at Richmond, Texas
  10.   08116650 Brazos River near Rosharon, Texas
11–13.   Graphs showing water-surface altitudes at a discharge of 5,000 cubic feet per second at streamflow-gaging station:
  11.   08111500 Brazos River near Hempstead, Texas
  12.   08114000 Brazos River at Richmond, Texas
  13.   08116650 Brazos River near Rosharon, Texas
14–18.   Graphs showing:
  14.   Suspended-sand concentrations in samples at streamflow-gaging station 08114000 Brazos River at Richmond, Texas, 1969–95
  15.   Sampled discharges at streamflow-gaging station 08114000 Brazos River at Richmond, Texas, 1969–95
  16.   Relation of suspended-sand concentration to discharge at streamflow-gaging station 08114000 Brazos River at Richmond, Texas, 1969–95
  17.   Relation of percent sand in suspended-sediment samples to discharge at streamflow-gaging station 08114000 Brazos River at Richmond, Texas, 1969–95
  18.   Relation of mean channel velocity to discharge at streamflow-gaging station 08114000 Brazos River at Richmond, Texas, 1969–95
19–20.   Graphs showing flow-duration curves of daily mean discharge for streamflow-gaging station:
  19.   08111500 Brazos River near Hempstead, Texas, 1940–68 and 1969–95
  20.   08114000 Brazos River at Richmond, Texas, pre-1940, 1940–68, and 1969–95
21–23.   Graphs showing:
  21.   Suspended-sand transport curve for streamflow-gaging station 08114000 Brazos River at Richmond, Texas
  22.   Annual suspended-sand loads at streamflow-gaging station 08114000 Brazos River at Richmond, Texas
  23.   Total acres harvested in the 27 counties in the lower Brazos River Basin, Texas
24.   Map showing location of selected sand and gravel production sites in the lower Brazos River Basin, Texas
25–26.   Graphs showing:
  25.   Total annual weight of sand extracted and annual loads of suspended sand transported in the Brazos River between Hempstead and Rosharon, Texas, 1979–95
  26.   Cumulative annual weight of sand extracted and annual loads of suspended sand transported in the Brazos River between Hempstead and Rosharon, Texas, 1979–95

Tables

1.   Selected characteristics of streamflow-gaging stations on the main stem of the lower Brazos River, Texas
2.   Selected characteristics of the 13 largest reservoirs in the Brazos River Basin, Texas
3.   Ratios of shear stress to critical shear stress required for initiation of motion at streamflow-gaging station 08114000 Brazos River at Richmond, Texas
4.   Summary of statistical tests for differences in annually computed daily flow percentiles between the pre-1940 period and the 1969–95 period at streamflow-gaging station 08114000 Brazos River at Richmond, Texas

Abstract

Changes in the capacity of the Brazos River to transport sand can be identified within the context of Lane’s relation through changes in channel geometry, changes in the characteristics of suspended loads, and changes in discharge. The Brazos River channel has been undergoing continual adjustment since the 1940s. For a discharge of 5,000 cubic feet per second, the water-surface altitude has decreased 2 to 4 feet at the Hempstead and Richmond streamflow-gaging stations between 1940 and 1995. The characteristics of suspended-sediment samples at the Richmond streamflow-gaging station have changed between the periods 1969–81 and 1982–95. The amount of sand-size sediment transported in suspension has decreased. The distribution of both daily and annual-peak discharges has changed. However, the computed annual loads of suspended sand indicate no statistically significant change in the median annual load.

The transport of sand in the Brazos River depends on a complex set of factors, most of which are continually changing. Potential sources of change in sand transport in the Brazos River include the effects of reservoir construction, changes in land use, and instream sand and gravel mining. Extensive reservoir construction in the Brazos River Basin has reduced sand transport by trapping sediment and by reducing the magnitude of peak discharges. However, reductions in sand transport associated with reservoir construction apparently are compensated for by increases associated with tributary sediment inflow and localized bank erosion. The total area of harvested acres of non-hay crops in the lower Brazos River Basin during 1924–92 decreased more than 75 percent from about 32 percent to about 8 percent of the total area. Correspondingly, erosion potential has decreased substantially. Several sand and gravel mining sites are located on the Brazos River between Hempstead and Rosharon. The quantity of sediment extracted by instream sand and gravel mining operations could represent from 11 to 25 percent of the total sand transported by the Brazos River. The effects of mining on sand transport could not be quantified.




U.S. Department of the Interior, U.S. Geological Survey
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