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

INTRODUCTION 100-year flood for three streambed- management practices. The three prac- On July 14, 1997, an intense rain- tices included (1) no removal of stre- storm resulted in rapid runoff and severe ambed material, (2) “scalping”, or flooding in parts of Vermont. During the removing bars and other alluvial stre- storm, streambed and streambank ero- ambed materials to increase channel sion and deposition were significant at capacity, and (3) dredging the entire several locations in the State. Residents streambed channel by 2 feet. in flooded regions questioned whether deposited sediment constricted water DESCRIPTION OF INVESTIGATED flow and elevated the 1997 flood levels. REACHES Since 1986, the State of Vermont’s pol- icy on streambed management is to The Trout River (fig. 1) flows north- restrict the removal of sand and gravel west through Montgomery, Vt., and is an from channels; however, the extent to upland stream in the north-central part of which the policy affects stream condi- the State. Streambed material is prima- tions during severe flooding is unknown. rily gravel and cobbles with some sand To answer this question, a sediment- and exposed bedrock. Additional char- transport study by the U.S. Geological acteristics of Trout River and the other Figure 1. Location of river reaches in the study. Survey (USGS), in cooperation with the studied rivers are listed in table 1. Vermont Agency of Natural Resources, computer-based models currently avail- Department of Environmental Conserva- The Wild Branch (fig. 1) flows south able (1999) do not adequately account tion, began in October 1997 to evaluate through Wolcott, Vt., in the north-central for the removal of fine-grained particles the potential effect of various streambed- part of the state, and drains into the Lam- by streamflow, which leaves erosion- management practices on future flood oille River. Streambed material ranges resistant large-grained particles to pro- levels (Olson, 2000). from sand to boulders with several areas of exposed bedrock. tect or armour the stream channel Three stream reaches that had been (Richardson and others, 1990). Like- affected by the flood of July 1997, and The Lamoille River (fig. 1) flows wise, stream-bank erosion and the for- which covered a wide range of basin west through Cambridge, Vt., in the mation of meander bends and bed forms characteristics common to Vermont, northwestern part of the state. Stre- cannot be adequately simulated. were selected for the study (fig. 1). The ambed material ranges from silt to coarse reaches selected were a 4.3-mile reach of gravel with several reaches having some MODEL SIMULATIONS AND the Trout River in Montgomery, Vt., a cobbles or exposed bedrock. (SIMULATION) RESULTS 6.5-mile reach of the Wild Branch in Wolcott, Vt., and the entire 15.4-mile SEDIMENT-TRANSPORT MODEL Streambed-management practices reach of the Lamoille River within Cam- simulated in this study refer only to the bridge, Vt. BRI-STARS is a computer model that removal of streambed-channel materials; routes water through natural river chan- bank protection and other channel The BRIdge Stream Tube Model for nels and simulates streambed erosion improvements were not considered. Alluvial River Simulation (BRI-STARS) and deposition. Because computer mod- Three streambed-management practices (Molinas and Wu, 1997), calibrated with eling of sediment transport is still in its were selected for evaluation. The first data for the flood of July 14-16, 1997, developmental stages, the ability of practice evaluated was based on current was used to simulate channel erosion and models such as BRI-STARS to exactly (1999) State policy, which restricts the deposition of the streambed and the peak simulate sediment-transport processes removal of streambed materials from water-surface profile during a 10- and and effects is limited. For example, channels. The second practice evaluated

U.S. Geological Survey USGS Fact Sheet 064–00 U.S. Department of the Interior May 2000

Table 1. Characteristics of studied reaches of three rivers in Vermont

Trout Wild Lamoille Characteristic River Branch 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 was based on typical streambed-channel bottom after the 1997 flood (post-flood), adjusts to re-establish equilibrium alterations and practices prior to 1986, and the 100-year water-surface profile (Richardson and others, 1990). Simula- when the current State policy took effect. from a fixed-bed model (Shearman, tions also showed increased streambed Alterations under this practice included 1990). erosion beneath bridges following dredg- removing gravel bars and other features ing. that may constrict flow. The third prac- Modeled water-surface and tice evaluated was based upon the fre- streambed-elevation profiles of the three Resulting water-surface elevations quent post-flooding argument that entire study reaches for the 100-year flood are from BRI-STARS simulations also indi- streambed channels need to be dredged shown in figures 5-7. These profiles cated that channel configuration has a periodically. The BRI-STARS model show the streambed profile as surveyed greater effect on the water-surface eleva- was used to determine the profile of the following the flood of 1997, and the cor- tion of a small flood such as a 10-year peak water surface and the final stre- responding 100-year water-surface ele- event than on a large flood such as a ambed elevation for a 10-year and a vation. Results from the BRI-STARS 100-year event or the 1997 flood. This 100-year flood (table 2) in each river that model simulations also are shown on result was expected because a large por- would likely result from implementation these profiles and include the streambed tion of the flood waters flow on the flood of the three practices. elevation following a 100-year flood and plains during a high flood regardless of the peak water-surface elevation during a the condition of the stream channel.

Table 2. Magnitude of flood discharges 100-year flood for the three streambed- used in the BRI-STAR simulations for management practices. The model used in this study provides three rivers in Vermont information on the short-term effect of For the Trout and Lamoille Rivers streambed-management practices on the 10-year 100-year discharge, discharge, BRI-STARS model simulations, the water-surface profile during a flood and River in cubic feet in cubic feet average water-surface elevation on the streambed-elevation profile fol- per per decreased when streambed materials lowing a flood. The management prac- second second were removed; however, simulations did tices evaluated in this study may have not show the same average decrease in local effects on flooding, erosion, and Trout River 9,400 18,000 water-surface elevations for the Wild deposition that are beyond the scope of Wild Branch 3,100 6,340 Branch (table 3). Furthermore, flooding this study. Investigations of streambed- actually increased in some reaches of the channel stability by the Center for Lamoille River 16,000 29,250 maintained or dredged channels. This is Watershed Protection (1999) and Rosgen because the dredged channel has a (1996) have documented that contain- Channel bottoms from flood- greater capacity to convey water and, in ment of high flows within the channel insurance studies in effect prior to the turn, transport sediment. The increase in increased erosion rates, generated large 1997 flood are shown in figures 2-4 sediment-transport capacity results in volumes of sediment, and ultimately (Federal Emergency Management greater potential for erosion and deposi- reduced channel capacity. Agency, 1980, 1982a,b, and d). Also tion. Respective changes to the water- shown on these figures is the channel surface profile occur as the channel —By Scott A. Olson

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 +2.2 510 TROUT RIVER, MONTGOMERY, VERMONT 500

490 Covered Bridge

480 T Route 118 Bridge E E

F 470

N

I Vincent Bridge

, 460 L E V

E 450 L Route 118 A Bridge E 440 S

E

V 430 7,000 8,000 9,000 10,000 11,000 12,000 13,000 14,000 15,000 16,000 17,000 18,000 19,000 O B A 540 N O I TROUT RIVER, MONTGOMERY, VERMONT T 530 A V E

L 520 E 510 Route 118 500 Bridge

490 Section Q

480 Section M 470

460 19,000 20,000 21,000 22,000 23,000 24,000 25,000 26,000 27,000 28,000 29,000 30,000 31,000 RIVER DISTANCE, IN FEET, FROM DOWNSTREAM CORPORATE LIMIT Pre-flood streambed elevation 100-year water-surface Post-flood streambed elevation 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.

460 LAMOILLE RIVER, CAMBRIDGE, VERMONT 450

440 T E

E 430 F

N

I Route 15 Route 15

,

L 420 E

V Section C E L

410 A E S 400 E

V 5,000 10,000 15,000 20,000 25,000 30,000 35,000 40,000 45,000 O

B 480 A

N LAMOILLE RIVER, CAMBRIDGE, VERMONT

O Railroad Bridge I 470 T A V

E 460 L E 450

440 Section W Section AA 430 Covered Bridge 420 Route 108

410 45,000 50,000 55,000 60,000 65,000 70,000 75,000 80,000 85,000

RIVER DISTANCE, IN FEET, FROM DOWNSTREAM CORPORATE LIMIT

EXPLANATION Pre-flood streambed elevation 100-year water-surface Post-flood streambed elevation 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. 750

740 WILD BRANCH, WOLCOTT, VERMONT

730 Gulf Road Bridge

720 T

E 710 E F 700

N Railroad Bridge I

, 690 L E

V 680 E L

670

A Route 15 Town Highway 2 Bridge E 660 S

E 650

V 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 O

B 800 A

N 790 WILD BRANCH, WOLCOTT, VERMONT O I

T 780 A

V 770 E L 760 E Section Q 750

740 Section R

730

720 Private Bridge 710

700 9,000 10,000 11,000 12,000 13,000 14,000 15,000 16,000 17,000 18,000

850

840 WILD BRANCH, WOLCOTT, VERMONT 830

820 T Section AA E 810 E F 800 N I

, 790 Section Z L E

V 780 E L

770

A Tamarack Road Bridge

E 760 S

E 750

V 18,000 19,000 20,000 21,000 22,000 23,000 24,000 25,000 26,000 27,000 O

B 930 A

N 920 WILD BRANCH, WOLCOTT, VERMONT O I

T 910 Private Bridge A

V 900 E L 890 Section AH E 880 870 Private Bridge 860

850 840

830 27,000 28,000 29,000 30,000 31,000 32,000 33,000 34,000 35,000 36,000 RIVER DISTANCE, IN FEET, FROM MOUTH Pre-flood streambed elevation 100-year water-surface Post-flood streambed elevation

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. 510 100-YEAR FLOOD ON THE TROUT RIVER, MONTGOMERY, VT 500

490

480 Route 118 470 Bridge L E

V 460 E

L Vincent Bridge

A 450 E S Route 118

N 440 Covered Bridge

A Bridge E

M 430

E 7,000 8,000 9,000 10,000 11,000 12,000 13,000 14,000 15,000 16,000 17,000 18,000 19,000 V O B

A 540

T E

E 100-YEAR FLOOD ON THE TROUT RIVER, MONTGOMERY, VT

F 530

N I

, 520 N O I T

A 510

V EXPLANATION E

L 500 E Route 118 490 Section Q Bridge Current streambed 480 Post-flood streambed--no bed material removed 470 Section M Post-flood streambed--bed material scalped 460 19,000 20,000 21,000 22,000 23,000 24,000 25,000 26,000 27,000 28,000 29,000 30,000 31,000 Post-flood streambed--channel dredged RIVER DISTANCE, IN FEET ABOVE CORPORATE LIMIT 100-year water surface--fixed-streambed model Figure 5. Simulated results of water-surface and streambed-elevation profiles of the modeled Peak water surface--no bed material removed reach of the Trout River. Peak water surface--bed material scalped

470 Peak water surface--channel dredged 100-YEAR FLOOD ON THE LAMOILLE RIVER, CAMBRIDGE, VT Route 108 460 Route 15 450

440

430

L 420 Route 15 E V E

L 410 Section C A E S

400 N A

E 390

M 10,000 15,000 20,000 25,000 30,000 35,000 40,000 45,000

E V O B A

480 T

E 100-YEAR FLOOD ON THE LAMOILLE RIVER, CAMBRIDGE, VT E

F 470

Railroad Bridge N I

, 460 N O I

T 450 A V E

L 440 E Section AA 430 Section W

420 Covered Bridge 410

400 50,000 55,000 60,000 65,000 70,000 75,000 80,000 85,000 RIVER DISTANCE, IN FEET, ABOVE CORPORATE LIMIT

Figures 6. Simulated results of water-surface and streambed-elevation profiles of the modeled reach of the Lamoille River.

REFERENCES ———1982a, Flood insurance study, Molinas, Albert, and Wu, Baosheng, Town of Cambridge, Lamoille 1997, User’s primer for Bri-Stars County, Vermont: Washington, (BRIdge Stream Tube model for Center for Watershed Protection, 1999, D.C., December 15, 1982, 18 p. Alluvial Simulation): Fort Collins, Impact assessment of instream Colo., Federal Highway Adminis- management practices on channel ———1982b, Flood insurance study, tration Project No. DTFH61-93-C- morphology: Waterbury, Vt., Ver- Village of Cambridge, Lamoille 00068, Hydrau-Tech, Inc., 43 p. mont Geological Survey, 160 p. County, Vermont: Washington, D.C., December 1, 1982, 15 p. Olson, S.A., 2000, Application of a Federal Emergency Management sediment-transport model to Agency, 1980, Flood insurance ———1982d, Flood insurance study, evaluate the effect of streambed- study, Town of Montgomery, Fran- Town of Wolcott, Lamoille County, management practices in flood lev- klin County, Vermont: Washing- Vermont: Washington, D.C., els and streambed elevations at ton, D.C., June 1980, 17 p. February 2, 1982, 17 p. selected sites in Vermont: 740 730 100-YEAR FLOOD ON THE WILD BRANCH, WOLCOTT, VT 720 Gulf Road 710 Bridge 700 Railroad Bridge 690 680 670 Town Highway 2 660 Route 15 Bridge 650 640 630 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000

790 780 100-YEAR FLOOD ON THE WILD BRANCH, WOLCOTT, VT 770 760 Section Q

L 750 E 740 V Section R E 730 L

A 720 EXPLANATION E 710 S

Private Bridge N 700 Current streambed A 690 E

M 680 Post-flood streambed--no bed material removed 9,000 10,000 11,000 12,000 13,000 14,000 15,000 16,000 17,000 18,000 E

V Post-flood streambed--bed material scalped O

B Post-flood streambed--channel dredged A

T 860 100-year water surface--fixed-streambed model E

E 850 100-YEAR FLOOD ON THE WILD BRANCH, WOLCOTT, VT F

Peak water surface--no bed material removed 840 N I

, 830 Peak water surface--bed material scalped N 820 O I

T 810 Section AA Peak water surface--channel dredged A

V 800 E

L 790 Section Z E 780 770 760 Tamarack Road 750 Bridge 18,000 19,000 20,000 21,000 22,000 23,000 24,000 25,000 26,000 27,000

940 930 100-YEAR FLOOD ON THE WILD BRANCH, WOLCOTT, VT 920 910 900 890 Private Bridge Section AH 880 870 860 Private Bridge 850 840 830 27,000 28,000 29,000 30,000 31,000 32,000 33,000 34,000 35,000 36,000 RIVER DISTANCE, IN FEET ABOVE MOUTH Figure 7. Simulated results of water-surface and streambed-elevation profiles of the modeled reach of the Wild Branch.

U.S. Geological Survey Open-File water-surface profile computa- For more information, Report 00-55, 92 p. tions: Federal Highway please contact: Administration Publication Richardson, E.V., Simons, D.B., and FHWA-IP-89-027, 187 p. District Chief Julien, P.Y., 1990, Highways in the U.S. Geological Survey river environment: Federal High- 361 Commerce Way way Administration Publication Pembroke, NH 03275 FHWA-HI-90-016. (603) 226-7800 Phone Rosgen, D., 1996, Applied river mor- (603) 226-7894 FAX phology, Pagosa Springs, Colo., Wildland Hydrology, 380 p. Visit the USGS web site at URL: http://www.usgs.gov Shearman, J.O., 1990, User’s manual for WSPRO—a computer model for or http://vt.water.usgs.gov