USGS

In cooperation with the Vermont Agency of Natural Resources,
Department of Environmental Conservation

Simulation of the Effects of Streambed-Management Practices on Flood Levels in Vermont

USGS Fact Sheet 064-00

INTRODUCTION

On July 14, 1997, an intense rainstorm resulted in rapid runoff and severe flooding in parts of Vermont. During the storm, streambed and streambank erosion and deposition were significant at several locations in the State. Residents in flooded regions questioned whether deposited sediment constricted water flow and elevated the 1997 flood levels. Since 1986, the State of Vermont's policy on streambed management is to restrict the removal of sand and gravel from channels; however, the extent to which the policy affects stream conditions during severe flooding is unknown. To answer this question, a sediment-transport study by the U.S. Geological Survey (USGS), in cooperation with the Vermont Agency of Natural Resources, Department of Environmental Conservation, began in October 1997 to evaluate the potential effect of various streambed-management practices on future flood levels (Olson, 2000).

Three stream reaches that had been affected by the flood of July 1997, and which covered a wide range of basin characteristics common to Vermont, were selected for the study (fig. 1). The reaches selected were a 4.3-mile reach of the Trout River in Montgomery, Vt., a 6.5-mile reach of the Wild Branch in Wolcott, Vt., and the entire 15.4-mile reach of the Lamoille River within Cambridge, Vt.

The BRIdge Stream Tube Model for Alluvial River Simulation (BRI-STARS) (Molinas and Wu, 1997), calibrated with data for the flood of July 14-16, 1997, was used to simulate channel erosion and deposition of the streambed and the peak water-surface profile during a 10- and 100-year flood for three streambed- management practices. The three practices included (1) no removal of streambed material, (2) "scalping", or removing bars and other alluvial streambed materials to increase channel capacity, and (3) dredging the entire streambed channel by 2 feet.

DESCRIPTION OF INVESTIGATED REACHES

The Wild Branch (fig. 1) flows south through Wolcott, Vt., in the north-central part of the state, and drains into the Lamoille River. Streambed material ranges from sand to boulders with several areas of exposed bedrock.

The Lamoille River (fig. 1) flows west through Cambridge, Vt., in the northwestern part of the state. Streambed material ranges from silt to coarse gravel with several reaches having some cobbles or exposed bedrock.

The Trout River (fig. 1) flows northwest through Montgomery, Vt., and is an upland stream in the north-central part of the State. Streambed material is primarily gravel and cobbles with some sand and exposed bedrock. Additional characteristics of Trout River and the other studied rivers are listed in table 1.

map showing the location of river reaches
			 in the New Hampshire study.
Figure 1. Location of river reaches in the study.

 

Table 1. Characteristics of studied reaches of three rivers in Vermont
Characteristic Trout River Wild Branch Lamoille River
Mean channel slope of study reach, in feet per mile 19 40 2.3
Approximate valley elevation at downstream end of study reach, in feet 470 670 460
Drainage area near downstream end of study reach, in square miles

71.6

39.5 520

 

SEDIMENT-TRANSPORT MODEL

BRI-STARS is a computer model that routes water through natural river channels and simulates streambed erosion and deposition. Because computer modeling of sediment transport is still in its developmental stages, the ability of models such as BRI-STARS to exactly simulate sediment-transport processes and effects is limited. For example, computer-based models currently available (1999) do not adequately account for the removal of fine-grained particles by streamflow, which leaves erosion- resistant large-grained particles to protect or armour the stream channel (Richardson and others, 1990). Likewise, stream-bank erosion and the formation of meander bends and bed forms cannot be adequately simulated.

MODEL SIMULATIONS AND (SIMULATION) RESULTS

Table 2. Magnitude of flood discharges used in the BRI-STAR simulations for three rivers in Vermont
River 10-year
discharge,
in cubic feet
per
second
100-year discharge,
in cubic feet per second
Trout River
9,400
18,000
Wild Branch
3,100
6,340
Lamoille River
16,000
29,250

Streambed-management practices simulated in this study refer only to the removal of streambed-channel materials; bank protection and other channel improvements were not considered. Three streambed-management practices were selected for evaluation. The first practice evaluated was based on current (1999) State policy, which restricts the removal of streambed materials from channels. The second practice evaluated was based on typical streambed-channel alterations and practices prior to 1986, when the current State policy took effect. Alterations under this practice included removing gravel bars and other features that may constrict flow. The third practice evaluated was based upon the frequent post-flooding argument that entire streambed channels need to be dredged periodically. The BRI-STARS model was used to determine the profile of the peak water surface and the final streambed elevation for a 10-year and a 100-year flood (table 2) in each river that would likely result from implementation of the three practices.

Channel bottoms from flood- insurance studies in effect prior to the 1997 flood are shown in figures 2-4 (Federal Emergency Management Agency, 1980, 1982a,b, and d). Also shown on these figures is the channel bottom after the 1997 flood (post-flood), and the 100-year water-surface profile from a fixed-bed model (Shearman, 1990).

See caption figure2
Figure 2. Pre-flood 1997 streambed and 100-year water-surface profiles from flood-insurance study and post-flood 1997 streambed profiles of the Trout River.
See caption figure 3
Figure 3. Pre-flood 1997 streambed and 100-year water-surface profiles from flood-insurance study and post-flood 1997 streambed profiles of the Lamoille River.
fig4a
fig4b
Figure 4. Pre-flood 1997 streambed and 100-year water-surface profile from flood-insurance study and post-flood 1997 streambed profile of the Wild Branch.

 

Modeled water-surface and streambed-elevation profiles of the three study reaches for the 100-year flood are shown in figures 5-7. These profiles show the streambed profile as surveyed following the flood of 1997, and the corresponding 100-year water-surface elevation. Results from the BRI-STARS model simulations also are shown on these profiles and include the streambed elevation following a 100-year flood and the peak water-surface elevation during a 100-year flood for the three streambed-management practices.

figure 5
explanation for figure 5-6
Figure 5. Simulated results of water-surface and streambed-elevation profiles of the modeled reach of the Trout River.
figure 6
 
Figures 6. Simulated results of water-surface and streambed-elevation profiles of the modeled reach of the Lamoille River.
fig7
Figure 7. Simulated results of water-surface and streambed-elevation profiles of the modeled reach of the Wild Branch.

For the Trout and Lamoille Rivers BRI-STARS model simulations, the average water-surface elevation decreased when streambed materials were removed; however, simulations did not show the same average decrease in water-surface elevations for the Wild Branch (table 3). Furthermore, flooding actually increased in some reaches of the maintained or dredged channels. This is because the dredged channel has a greater capacity to convey water and, in turn, transport sediment. The increase in sediment-transport capacity results in greater potential for erosion and deposition. Respective changes to the water-surface profile occur as the channel adjusts to re-establish equilibrium (Richardson and others, 1990). Simulations also showed increased streambed erosion beneath bridges following dredging.

 

Table 3. Model-simulated changes in peak water-surface elevations resulting from alterations to channels of three rivers in Vermont
[All measurements are in feet; - indicates a decrease; and + indicates an increase in water-surface elevation compared to that in simulation of unaltered channel]
  Trout River Lamoille River Wild Branch
Channel alteration Minimum Mean Maximum Minimum Mean Maximum Minimum Mean Maximum
Bars and obstructions removed, 10-year flood
-1.0
-0.1
+0.2
-0.2
-0.1
0
-2.0
+0.1
+3.8
Bars and obstructions removed, 100-year flood
-2.7
-0.2
+0.4
-0.1
-0.1
0
-3.8
0
+2.1
Channel dredged, 10-year flood
-4.7
-1.5
+1.1
-2.6
-1.4
0
-3.6
0
+4.2
Channel dredged, 100-year flood
-4.8
-1.1
+0.1
-1.7
-1.0
0
-3.1
-0.5
+2.2

 

Resulting water-surface elevations from BRI-STARS simulations also indicated that channel configuration has a greater effect on the water-surface elevation of a small flood such as a 10-year event than on a large flood such as a 100-year event or the 1997 flood. This result was expected because a large portion of the flood waters flow on the flood plains during a high flood regardless of the condition of the stream channel.

The model used in this study provides information on the short-term effect of streambed-management practices on the water-surface profile during a flood and on the streambed-elevation profile following a flood. The management practices evaluated in this study may have local effects on flooding, erosion, and deposition that are beyond the scope of this study. Investigations of streambed-channel stability by the Center for Watershed Protection (1999) and Rosgen (1996) have documented that containment of high flows within the channel increased erosion rates, generated large volumes of sediment, and ultimately reduced channel capacity. By Scott A. Olson

REFERENCES

Center for Watershed Protection, 1999, Impact assessment of instream management practices on channel morphology: Waterbury, Vt., Vermont Geological Survey, 160 p.

Federal Emergency Management Agency, 1980, Flood insurance study, Town of Montgomery, Franklin County, Vermont: Washington, D.C., June 1980, 17 p.

_____1982a, Flood insurance study, Town of Cambridge, Lamoille County, Vermont: Washington, D.C., December 15, 1982, 18 p.

_____1982b, Flood insurance study, Village of Cambridge, Lamoille County, Vermont: Washington, D.C., December 1, 1982, 15 p.

_____1982d, Flood insurance study, Town of Wolcott, Lamoille County, Vermont: Washington, D.C., February 2, 1982, 17 p.

Molinas, Albert, and Wu, Baosheng, 1997, User's primer for Bri-Stars (BRIdge Stream Tube model for Alluvial Simulation): Fort Collins, Colo., Federal Highway Administration Project No. DTFH61-93-C-00068, Hydrau-Tech, Inc., 43 p.

Olson, S.A., 2000, Application of a sediment-transport model to evaluate the effect of streambed- management practices in flood levels and streambed elevations at selected sites in Vermont: U.S. Geological Survey Open-File Report 00-55, 92 p.

Richardson, E.V., Simons, D.B., and Julien, P.Y., 1990, Highways in the river environment: Federal Highway Administration Publication FHWA-HI-90-016.

Rosgen, D., 1996, Applied river morphology, Pagosa Springs, Colo., Wildland Hydrology, 380 p.

Shearman, J.O., 1990, User's manual for WSPRO--a computer model for water-surface profile computations: Federal Highway Administration Publication FHWA-IP-89-027, 187 p.


For more information, please contact:

District Chief
U.S. Geological Survey
361 Commerce Way
Pembroke, NH 03275
(603) 226-7800 Phone
(603) 226-7894 FAX

Visit the USGS web site at URL:
http://www.usgs.gov
or http://vt.water.usgs.gov

 

The text of this report is presented here in Portable Document Format.
Adobe Acrobat Reader™ is required to view it. If you wish to download the latest version of Acrobat Reader free of charge, click here.

Fact Sheet PDF file (210 KB)


 

FirstGov button  Take Pride in America button