1

Abstract

We developed procedures for installing prefabricated trapezoidal flumes in

deep (10 to 12 feet) drainage ditches to monitor hydrologic functions and

provide gauge locations for sampling discharge. Flows from the instrumented basins were generally low, but the ditches were occasionally

subject to high flows caused by rain events of 2 to 3 inches or more. These

high flow events caused severe erosion and undercut the flumes when they

were installed in the usual manner, washing two of the flumes downstream.

Our modifications to the installation procedure included four main

elements: (1) securing the flume to a concrete pad; (2) bolting entrenched

wooden barriers to the flume’s inlet and outlet; (3) placing a plastic apron

beneath the flume’s outflow opening; and (4) surrounding the flume with

sandbags. Flumes installed using these elements have been significantly

more stable and resistant to undercut and bank erosion than those we

installed using the normal methods. As a result, the modified installation

procedures have reduced necessary maintenance and data loss, making the

additional cost and increased initial effort well worthwhile.

United States

Department of

Agriculture

Forest Service

Southern

Research Station

Research Note

SRS-11

Charles A. Harrison, Forestry Technician, Soil Scientist, USDA Forest

Service, Southern Research Station, Charleston, SC 29414; and Susan E.

O’Ney, Resource Biologist, National Park Service, Grand Teton National

Park, Moose, WY 83012.

January 2002

Design and Modification of an

Installation Method to Stabilize

Small Trapezoidal Flumes in

Drainage Ditches

Charles A. Harrison and Susan O’Ney

Introduction

Precalibrated flumes have been used for decades as control

structures for measuring water flow in streams and drainage

ditches (Brakensiek and others 1979). Such flumes are

particularly convenient because the relationship of water

level to flow volume (flow rating curve) is known. They

also allow very accurate measurement, as well as easy

access.

The trapezoidal flume was developed to measure a wide

range of flows in irrigation channels and has been especially

accurate in measuring low flows (fig. 1). The trapezoidal

shape conforms to channel shape, and the straight bottom

permits self-cleaning, thus reducing silt build-up.

Flumes are chosen to accommodate a full range of drainage

flows. Although time and money constraints may not allow

it, runoff should be measured at low, medium, and high

stages before installation. Our objective was to place

prefabricated flumes in deep (10 to 12 feet), existing

drainage ditches to monitor hydrologic functions and

provide gauge locations for sampling drainage flow.

Generally, flows were low where we installed our flumes,

but the ditches were subject to occasional high flows caused

by rain events of 2 to 3 inches or more.

Because of difficulties encountered in stabilizing the

structures as well as in anticipating the effects of erosion,

flume installation is not easy. Modifications we made to

commonly used installation procedures, which resulted in

the successful placement of this valuable hydrological tool,

are described in this paper.

Project Background

In May 1997 we installed three extra large, 60ο V trapezoidal

flumes, which were designed to handle flows from 0.0001 to

1.55 cubic feet per second, and one 2-inch, 45o

 WSC

(Washington State College) trapezoidal flume, which was

Figure 1—Typical trapezoidal flume.
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designed to handle flows from 0.029 to 2.59 cubic feet per

second. Both were constructed of fiberglass-reinforced

polyester (FRP) and manufactured by Plasti-Fab, Inc. of

Tualatin, Oregon. Temporarily interrupting stream flow, we

placed and leveled the flumes in the channel beds. To direct

and confine water flow, we used wooden posts and planks

that were sealed to the flume with caulking. We packed

native soil around the base and sides of the flumes, which

also served to stabilize the structure.

That summer, two large rainfall events (2.89 inches on June

5 and 2.88 inches on June 27) caused excessive flows that

undercut the flumes. Severe erosion occurred around the

sides and base, and two of the flumes washed downstream.

We modified the installation procedure in order to stabilize

the structures. Our modifications have been successful, and,

with minimal maintenance, the installations have survived

numerous intense rain events.

Modified Installation Method

Modified flume installation was completed in October 1997.

The tools and materials required for installation of a single

flume are listed in table 1.

Our modifications included four major components:

1. Securing the flume to a concrete pad.

Using 2- by 4-inch lumber, we designed and constructed a

concrete form about 3 inches shorter than the flume but

wide enough to overlap its upper surface. We lined the form

with felt paper and poured premixed concrete to a depth of

3.5 inches. After pouring about half of it, we embedded a

strip of “hog wire” fencing (cut to fit within the form) to

reinforce the finished pad. While it was still wet, we inserted

two 18-inch lengths of 5/16-inch threaded stainless steel rod

2 inches into the concrete, each about halfway along the

Table 1—Tools and materials required for installation of a flume by the modified procedure

Item Quantity Comments

Flume 1 —

5/16-inch threaded stainless steel rod,

 18 inches long 2 —

Wooden form 1 Made from 2- by 4-inch boards; used in pouring concrete pad

Felt paper 1 Piece slightly larger than form dimensions

Hog wire 1 Piece slightly smaller than form dimensions

Quickrete concrete, 50-pound bag 3–4 —

Sandbag, 50 pound 30–50 —

Washer and nut 6–8 For securing flume to concrete pad and 2- by 8-inch boards

to flume

Bolt 4 For securing 2- by 8-inch boards to flume

2- by 8-inch board, 12 foot long 2 Use treated wood

Black plastic sheet, 6 mil 3 One approximately 3- by 6-foot piece, plus two pieces for use in

covering sandbags

Chainsaw 1 —

Wheelbarrow 1 For use in mixing concrete

Shovel Several Recommend at least one “sharp-shooter” shovel, as well as a flatbladed and normal blade shovel

Portable drill (with bits) 1 —

Level 1 —

Pencils/markers Several —

Carpet knife 1 —

Socket wrenches (and drivers) Set —

Grass seed — Planted on soil/plastic covering sides of flume
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length of the pad. We set one post approximately 1 inch

from one edge of the form, the other about 2 inches from the

opposite edge. We allowed the pad to cure for 24 to 48

hours before transporting it to the study site.

At the site, we selected an appropriate location in the ditch.

Selection criteria included (a) a relatively stable, straight

section of ditch; (b) a location sufficiently downstream to

enable data collection; and (c) ease of access.

We used sandbags to temporarily block stream flow and

excavated an area in the center of the ditch, including the

thalweg, to allow for level placement of the concrete pad

and installation of a stilling well. The stilling well is built

onto the side of a flume to allow accurate measurement of

water volume and provide a place to attach other sampling

devices. We laid the pad and checked for level (fig. 2).

Placing the flume directly over the pad, we marked it and

drilled holes through which the threaded rods would pass.

We fastened the flume to the pad with washers and nuts,

checking for level several times in the process. After

ensuring that the hole for the stilling well was sufficiently

deep (so the flume rested level on the concrete pad), we

secured the structure with another washer and nut.

2. Bolting entrenched wooden barriers to the flume’s inlet

and outlet.

We used sharp-shooter shovels to dig trenches directly in

front of and behind the flume, extending approximately 1 to

1.5 feet into the ditch banks and 3 to 4 inches below the

ditch bottom. We cut 12-foot lengths of 2- by 8-inch boards

and set them into the trenches. We then sketched onto the

boards the inflow and outflow openings and used a

chainsaw to cut the openings. Anchoring the boards into the

ditch banks added considerable stability to the structure.

3. Placing a plastic apron beneath the flume’s outflow

opening.

Before fitting boards into the trenches, we placed a 3- by 6-

foot sheet of 6-mil black plastic under the outflow opening

and across the adjacent trench to reduce chances that

turbulence would undercut the rear of the flume during

periods of high flow. We then positioned the sections of

board (the rear board holding the apron in place) and drilled

holes through both board and flume. After bolting the rear

board to the flume, we trimmed excess plastic from the

outflow opening and packed excess soil around board ends.

4. Surrounding the flume with sandbags.

We used 50-pound fiberglass sandbags to fill in around and

stabilize the flume. We positioned one bag directly under the

flume outflow—on top of the plastic apron—and arranged

other bags at about a 45o angle to direct and confine water

flow in front and back of the flume. We placed additional

sandbags on both sides, level with the flume’s upper surface.

We packed excavated soil between the sandbags to further

stabilize the installation. The appearance of the flume after

this stage is shown in figure 3.

Flume installation was complete. Additional equipment,

such as stilling-well towers, water-level recorders, and

automated water samplers, could now be attached or

installed as needed.

Discussion

Flumes installed using the modified procedure were

significantly more stable and more resistant to undercut and

bank erosion than those we installed earlier. Although the

modified method costs more initially and is more laborintensive, it has withstood the high-intensity, short-duration

rain events common to the study site.

Figure 2—The concrete pad (showing embedded stainless steel rods) fitted

into an excavated hole in the ditch bed. Incipient trenches for the 2- by 8-

inch boards and a cavity for the stilling well of the flume are also visible.

Figure 3—View of completed installation. The notched 2- by 8-inch board

bolted to the outflow opening, the plastic apron, and the sandbag emplacements are visible.
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Approximately 8 months after installation, minor

maintenance was necessary—mainly replacing torn or

damaged sandbags. As a result of exposure to ultraviolet

radiation, some of the fiberglass bags had deteriorated and

were replaced. To reduce deterioration, we covered the bags

with black plastic, covered them with soil, and planted a

variety of hardy grasses. Although planting significantly

reduced bag deterioration, concrete-filled sandbags may

provide more stability and require less maintenance. After 4

years, installed flumes have sustained no significant

damage, and it will be possible to remove them once the

study is complete. They will be suitable for re-use at other

locations.

Summary

By making simple modifications to basic installation

methods, we were able to use small trapezoidal flumes in

deep ditches that experience brief but intense rainfall events.

Increased stability and durability of the flumes has reduced

necessary maintenance, as well as potential data loss, thus

making the additional cost and effort well worthwhile.

Acknowledgments

This work was supported in part by a grant from Oak Ridge

National Laboratory (DE–AI05–96OR22560). Todd

Tompkins, Matthew Eldridge, Jim Aaron, Bir Thapa, Mark

Farina, and David Ayer provided assistance in the field;

Cindy Bunton contributed to the preparation of figures. Carl

Trettin supervised the project and provided comments on the

manuscript. John Parsons and Masato Miwa also reviewed

the manuscript and offered constructive suggestions. The

authors extend heartfelt thanks to all.

Literature Cited

Brakensiek, D.K.; Osborn, H.B.; Rawls, W.J. 1979. Field manual for

research in agricultural hydrology. Agric. Handb. 224. Washington, DC:

U.S. Department of Agriculture, Science and Education Administration:

77–103.

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