United States

Department

of Agriculture

Forest Service

Rocky Mountain

Research Station

General Technical

Report RMRS-GTR-64

September 2000

Constructing

Temporary Sampling

Platforms for

Hydrologic Studies

Manuel H. Martinez

Sandra E. Ryan
Rocky Mountain Research Station

324 25th Street

Ogden, UT 84401

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Abstract __________________________________________

Martinez, Manuel H.; Ryan, Sandra E. 2000. Constructing temporary sampling platforms for hydrologic studies. Gen. Tech. Rep. RMRS-GTR-64. Ogden, UT: U.S. Department of Agriculture, Forest

Service, Rocky Mountain Research Station. 10 p.

This paper presents instructions for constructing platforms that span the width of stream channels

to accommodate the measurement of hydrologic parameters over a wide range of discharges. The

platforms provide a stable, safe, noninvasive, easily constructed, and relatively inexpensive means

for permitting data collection without wading in the flow. We have used the basic techniques described

herein for building platforms on channels of small to medium size, up to 70 feet in width. Lightweight

joists, commonly used in housing construction, form the primary structural support for the platform. The

structures can be constructed by persons having only rudimentary knowledge of building practices and

using simple tools. Cost estimates for materials range from $200 to $1,500 depending on the required

length of structure. The platforms have been used successfully in measuring flow and sediment

discharge in subalpine channels in Colorado and Wyoming during runoff events with 2 to 5 year return

frequencies. They are quite stable and are safer than wading in channels with fast moving flow.

Keywords: hydrologic measurements, sediment measurements, mountain streams, safety

Acknowledgments _________________________________

Many useful comments concerning organization and clarification of earlier versions of this

technical report were provided by John Buffington, Mark Dixon, Sue Hilton, Dan Marion, and

Mike Solomon. We also acknowledge financial and logistical support provided by the Stream

Systems Technology Center, Fort Collins, CO.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-64. 2000 1

Introduction ____________________

Land managers often need information on flow and

sediment transport to evaluate the impacts of changing regime on channel processes, morphology, and

aquatic habitat. A typical sampling scheme involves

collecting sediment or measuring flow characteristics

(depth, velocity, turbidity, and so forth) from many

points within a cross-section, which may require wading in strong, deep currents during high flows—a

particularly onerous task prone to error and safety

problems. More specifically, wading may alter the

dynamics of water movement in the area of collection,

thereby affecting measured values and conclusions

drawn from them. Sediment may be kicked up or

disturbed by wading, altering the relationship between measured sediment and flow. From a safety

standpoint, rivers undergoing high flow and intense

transport are dangerous and often impossible to

traverse safely. It is therefore advisable to collect

samples without wading to eliminate the potential for

sampling error and to assure the safety of the person

collecting samples.

Collecting samples from vehicular bridges is one

way to reduce problems introduced by wading. However, bridges are not always in desired locations,

particularly in pristine mountain environments where

there are few roads. Additionally, flow conditions in

the vicinity of bridges are often altered by reduced

channel width or through abutment placement during

construction. Smaller footbridges may be used if in

suitable locations, though high handrails on these

structures may make it difficult to collect samples.

Construction of new footbridges may disturb the riparian area, alter the shape of the channel and banks,

and may be more permanent than necessary. What is

needed is a temporary, stable, safe, noninvasive, easily constructed, and inexpensive means of spanning

the channel in order to take measurements without

wading in the flow.

We describe in this paper the design and construction of platforms that span the width of streams for the

purposes of making stream measurements during

high runoff. We have constructed about 20 of these

structures on streams ranging from 15 to 70 feet in

width. Most sites are within the subalpine environment

of Colorado and Wyoming where total annual flow is

dominated by the influence of snowmelt, and so the

flow rarely exceeds the banks by more than a few

inches in most years. The platforms have been used

successfully to aid in measuring flow and sediment

during runoff events with 2 to 5 year return frequencies. The structures were fabricated using steel bar

joists or TJI® joists1 (as used in housing construction),

plywood, and 2 x 4s. Either two or three joists are used

to form the primary structural component of the platform; three joists are used to construct wider platforms that are needed when a structure is longer than

about 40 feet. The plywood deck and 2 x 4 cross-members provide lateral stability and a surface to walk on.

The structures are relatively inexpensive and can be

assembled onsite in less than a day by those with only

basic knowledge of construction practices. No heavy

equipment is needed for assembly, thus minimizing

disturbance to the riparian area. They may be used for

a number of years on a seasonal basis and require

minimal maintenance.

Platforms Constructed From Bar

Joists _________________________

The earliest versions of our sampling platforms were

constructed from steel bar joists obtained from construction salvage. Six platforms on St. Louis Creek on

the Fraser Experimental Forest, near Fraser, CO,

were constructed using these heavy supports (fig. 1).

They were assembled by four persons and have been in

place since 1993. The structures are still usable, having withstood particularly heavy snowloads in 1995,

1996, and 1997. The main difficulty with platforms

constructed from bar joists is that the supports are

heavy, requiring many persons to haul to the site; the

size of the platform is, therefore, limited to that which

may be packed in. Once in place, they are not readily

removed and so are more permanent than what may

be desired.

Constructing Temporary Sampling

Platforms for Hydrologic Studies

Manuel H. Martinez

Sandra E. Ryan

1 These joists are used in construction for floors and roofs. Use of the joists for

sampling platforms may not be an approved use of the supports by the

manufacturer.
2 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-64. 2000

TJI® DF Joist Platforms

We began constructing platforms from TJI® joists

instead of bar joists in 1996. The TJI® joists are sturdy,

lighter in weight and more easily handled by workers;

a 40-foot section of joist requires only three to four

people to move into place. The platforms are removed

at the end of the season or before winter because there

is potential for breakage due to heavy snowloads on

these lighter structures.

There are several manufacturers of TJI® joists. We

used TJI®/35 DF Joist and TJI®/55 DF Joist manufactured by Trus Joist MacMillan (fig. 2 shows the ones

we used plus other sizes available). Regardless of

manufacturer, there are building guides and installation information on the manufacturer’s specification

sheets that can be consulted to determine the appropriate size of joist to use. Joists can be ordered by the

foot in a variety of depths; we used 16 inch deep joists

on our platforms. There was, however, a 40 foot limit

on the length of joist that could be transported in one

piece to our remote sites. While it is generally recommended that one avoid splicing the ends of the joist, we

have used splices to create structures up to 75 feet in

length; we describe this in more detail later. Construction of the sampling platforms over substantially wider

channels has not been attempted and will likely require structural engineering support. We suggest using cableways to deploy samplers in swift flow in

streams wider than 70 feet (Paradiso 2000).

Site Selection___________________

The primary consideration in selecting a site is to

establish the cross-section in a location suitable for the

intended measurement. For example, the platform and

cross-section should be located in a relatively straight

stretch of channel for measuring bedload samples. The

banks in the area should be stable, as identified by

vegetative growth (grasses, herbs, and sedges) and

absence of cracks or raveling faces. Banks are rarely

overtopped in our snowmelt streams and so they provide a suitable base for the platform. To determine the

length of joists required, measure the distance between

the two edges of bank where the break in slope occurs

(the height of the banks across the channel do not need

to be even). This is the maximum width of the channel

to be sampled (fig. 3). A minimum of 4 feet is then added

to this value to obtain the length of the joists needed. In

areas of where there are sloping or less-well defined

banks, a minimum of 6 feet should be added to the

measured width of the channel. This permits greater

flexibility in the placement of the platform in channels

with uneven banks. The joists may be trimmed should

the length be excessive.

Supplies _______________________

The supplies needed for a typical platform constructed from two TJI® Joists and 2 x 4 lumber (shown

in fig. 4) are listed below; this list is for a platform that

Figure 1—Platform constructed using steel bar joists at a site on St. Louis

Creek, near Fraser, CO.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-64. 2000 3

Minimum length of TJI® joists

24” Width at top of bank 24”

Minimum Minimum

1 “ 1 2 12 3 “ “

“ “ “ “

“ 3 5

3 3 3 7

1 4 16

8 8 8 16

2

1 “ 1 2 1 “ 1 2 1 “ 1 2 1 “ 1 2

TJI /Pro , TM

9 and 11 depths 1 7 2” ”8 9 and 11 and 16 depths 1 7 2” ”8 ” 11 ,14 and16 depths 7 8” ” ” 11 ,14 and16 depths 7 8”” ”

and TJI /15 DF JOISTS TJI /25 DF JOIST

chord

vertical

“ ” shoulder “ ” shoulder

TJI /35 DF JOIST TJI /55 DF JOIST ®® ® ® ®

Figure 2—Some types of TJI® joists that are available; the drawing is based on specification

sheets from Trus Joist MacMillan. Chord refers to the pressed wood support along the top and

bottom of the joist. Shoulder refers to the interior portion of the chord where pressed particle

board (the vertical) connects to the chord.

Figure 3—Field measurements needed to determine length of platform

required.
4 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-64. 2000

is 2 feet wide and less than 40 feet long. Material

amounts will vary depending on the desired size of the

platform. Hence, the total cost of a platform depends

on the size of the structure plus available tools on

hand. We determined that the costs for materials and

hardware ran typically between $500 (two joist supports) and $1,500 (three joist supports).

Materials:

2 joists of same length and series, minimum of

16 inches deep

3⁄4 inch CDX grade plywood in 2 x 8 foot pieces—

enough to equal total length of structure

2 x 4 lumber, cut to lengths so that the joists are

spaced for a 2 foot wide platform (length will

vary depending on joist dimensions selected)

Several pieces of lumber of varying thickness

(such as 4 x 4 and 6 x 6 inches) but at least 2.5

feet in length to construct sills

Paint (preferably brown or green to blend with

environment)

Hardware:

Screws 2 to 3 inches long (high twist, coated deck

screws work well)

1⁄2 inch bolts (6 inches long), washers, and nuts

3⁄4 or 1 inch rebar, 18 to 24 inches in length to

stabilize sills

Tools:

2 or more hand-held drills with re-chargeable

batteries

Drill bits–1⁄2 and 3⁄4 inch plus other sizes as

needed

Carpenter’s level

Hammer

Wrenches

Sledge hammer

Portable generator (needed if cutting plywood

and spacers in field)

Circular saw (needed if cutting plywood and

spacers in field)

Other:

Waders for assemblers

Eye protection

For platforms longer than 40 feet we use three joists

to construct a structure 3 feet wide. The spacers are

cut to fit between the three joists so that the total

width at the top of the platform equals 3 feet. Plywood

is cut into 3 x 8 foot sections. We have also used

rectangles cut from plywood as joist spacers (fig. 5)

Figure 4—An example of platform frames constructed using 2 x 4 lumber as

spacers. Most of the construction for smaller platforms (15 and 24 feet shown

here) can be done off-site because the frames are light enough to be carried to

the site by two to three persons.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-64. 2000 5

Figure 5—Plywood rectangles are used as

spacers on this 3 x 60 foot platform on South Fork

Cache la Poudre, near Fort Collins, CO.

instead of 2 x 4s (fig. 4). The plywood spacers add less

weight to the structure, which is important when

constructing longer platforms.

General Strategy ________________

Construction material may be cut, painted, and

predrilled at an off-site work station and hauled to the

site. Alternatively, some companies will deliver supplies to remote locations for moderate cost, which is

worth investigating if one doesn’t have access to a

truck and trailer. In the latter case, construction

materials must be prepared in the field and so a

generator and circular saws are needed for cutting

plywood and spacers onsite. Wood pieces should be

primed and painted, preferably a neutral color, to help

preserve and maintain the condition of the platform,

as well as reduce attention to the structure from forest

users. Platforms are assembled and disassembled

during low flow because the channel must be waded

during construction. The structure is assembled over

the channel, so all of the assembly work must be done

using battery-powered or hand tools. The sequential

order of construction onsite is:

• Prepare the site

• Splice the joists (if necessary)

• Move the joists into position over the channel

• Assemble the frame of the platform using spacers

• Cover the top of the structure with plywood

• Stabilize structure

Site Preparation

Site preparation involves clearing vegetation from

the area near the stream cross-section and bank excavation for sills, if needed. Sills are the base upon which

the structure rests and are formed from 4 x 4 inch or

6 x 6 inch pieces of treated lumber (fig. 6 and 7). The

platform feels wobbly without the sills, due mainly to

the instability of the bank under the joists. Different

sized pieces of lumber may be needed where there are

differences in bank heights; the elevation at the top of

the sills on either side of the channel should be the

same so that the platform is level. Excavating and

placing sills into sloping banks helps stabilize their

position and prevents them from slipping. Steel reinforcement bars (or rebar), 3⁄4 or 1 inch in diameter

and 18 to 24 inches in length, are driven through holes

drilled into the sill and into the ground using a sledge

hammer to prevent the sills from shifting.

Splices

The splice support is created from two 8 foot lengths

of 3⁄4 inch plywood cut to the depth of the joist (16

inches); one section of plywood is placed on both the

inside and outside of the splice. The plywood overlaps

the ends of the two joists by 4 feet and is bolted through

the joist vertical using 6 inch carriage bolts and 11⁄2 x

10 inch steel plating (fig. 7 and 8). The ends of the bolts

should face the interior of the platform to prevent

catching on clothing or interference with instruments.

The splices are staggered at different locations on the

joists over the length of the structure so that there is

no point that is less stable due to presence of two or

more splices (fig. 7).

Joist Placement

A crew of three to four persons is needed to move the

joists into position across the channel; the ends of the

joist rest on or overlap the sills. Joists should be

positioned so that they are roughly 24 inches apart

and 18 inches downstream from desired sampling

cross-section.
6 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-64. 2000

20 feet

20 feet

40 feet

40 feet

30 feet

30 feet

Sill

4x4 or 6x6

Figure 6—Platform (3 x 24 feet) resting on sill at Silver Creek, near

Wolf Creek Pass, CO.

Figure 7—An example of lengths of joists and splices used in constructing

a platform 60 feet in length. Note that the positions of the splices are

offset across the frame of the platform.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-64. 2000 7

Assembly Instructions

The shell of the platform is formed using spacers

between the top and bottom chord of the joists (fig. 9).

Plywood rectangles, cut to fit the inner dimensions of

the platform, or 2 x 4 pieces of lumber are used as

spacers. The spacers rest on the inner side of the

shoulders or are fitted into the space between the top

and bottom chord. They are attached using two to

three wood screws drilled into the edge of the spacer

through the vertical portion of the joist. Spacers are

placed 18 inches apart (on center). When using 2 x 4s

(fig. 4), the second piece of lumber is placed under the

shoulder of the top chord and attached; alternating

between the top and bottom chords adds lateral stability and prevents the joists from twisting. The frame of

the structure is complete and stable once the spacers

are in place.

Plywood Top

The entire surface of the structure is covered by

plywood laid end to end so that the plywood edges are

flush with the edges of the joists and there are no gaps

(fig. 10). Plywood is attached using wood screws drilled

from the top of the structure into the top of the joist; the

screw head is countersunk slightly to avoid catching

clothing or tripping while sampling.

Stabilizing Structure

While the weight of the structure is sufficient to

prevent the completed platform from moving during

typical use, there is potential for the structure to slip

off the sill. To prevent separation at the sill-to-platform contact, the platform is directly attached to the

sill using several 6 inch carriage bolts (and washers)

drilled through the bottom shoulder of the joist into

the top of the sill. The entire structure may be cabled

to a nearby tree to ensure its retention if flows are

unexpectedly high.

Safety Precautions and

Limitations _____________________

Our experience with these platforms has generally

been limited to smaller channels in the subalpine

environment of Colorado and Wyoming where flow

rarely exceeds channel banks by more than a few

inches during peak discharge. These channels are

easily waded during low flows, facilitating straightforward construction of the platforms. Furthermore, channel banks are generally stable and distinct and the

area of inundation is well defined. While platforms

such as those described herein are useful under these

environmental conditions, they may be unsafe in channels with less predictable flow or unstable ground.

Figure 8—One of the 8 foot splices on a platform (2 x 48 feet) at Little

Granite Creek near Jackson, WY. Note the metal plate (above the bent

knee of the worker) used to add strength to the chord at the point where

the two joists are joined.
8 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-64. 2000

Deck

3/ ply 4” wood

Figure 9—Joists, spacers (either 2 x 4s or plywood rectangles), and plywood deck (cut

away) used in constructing platform.

Figure 10—Plywood placed on a platform (3 x 40 feet) at Halfmoon Creek, near Leadville,

CO. Note that the edges of the plywood are flush against the edges of the platform frame.
USDA Forest Service Gen. Tech. Rep. RMRS-GTR-64. 2000 9

We recommend that the platform be built using

estimates of the highest measurable flows for the

environment in which they are constructed. This is

because the platforms are unsuitable for use when

flow is higher than the bottoms of the joists. If it

appears that flows may exceed this level, platforms

must be raised using additional lumber placed under

the joists at the sill contact before it is used at higher

flows.

As a cautionary note, floating pieces of woody debris

may catch under platform during periods of sustained

high flow (though this has not been encountered at any

of the sites where these platforms have been constructed to date). Woody pieces must be removed

before platform is used because of interference with

instruments and samplers and potential safety issues.

Unfortunately, the platforms often draw the attention of forest users and recreationists; they may therefore be subject to vandalism and should be inspected

prior to each use. Study sites and platforms should be

located at inconspicuous locations, if possible. Educational signs may be attached to the structures explaining the nature of the hydrologic project, emphasizing

the structure’s impermanence. Posting warning signs

in the area provides cautionary notice to forest users

that helps cover the land manager’s liability. As an

additional safety precaution, fencing may be placed

Figure 11—Wire fencing used to discourage use of sampling platform (3 x 75 feet) at East Fork

San Juan River, near Pagosa Springs, CO.

around the ends of the platforms, particularly in areas

frequented by families whose children are unable to

read warning signs. We have constructed fences using

wire and metal t-posts (fig. 11); locks and chains may

be used around an opening in the fence to permit

operator access. If the stream is large enough for

recreational boating, signs should be placed upstream

warning of the presence of an obstacle. The platform

should also be flagged and marked on the upstream

side, increasing its visibility to boaters.

Handrails were not used on our platforms mainly

because they interfered with movement and placement of samplers with long handles. Our employees

were comfortable without them. If handrails are preferred, we suggest consulting with an engineer in their

design because improperly designed handrails are

subject to failure and may give a false sense of security.

We recommend placing handrails only on the downstream side of platform; the upstream side should be

kept clear to prevent interference with instruments

and samplers.

The platforms are quite stable and much safer than

the alternative of wading in channels with fast moving

flow. The platform will support several persons at one

time, though we generally limit its use to two persons;

this is mainly because the structure is narrow and

caution is needed when passing by other users. In
10 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-64. 2000

6 years of use, no person has fallen from any of the

platforms we have used in research projects. Still,

crews should be outfitted with ropes and throw bags to

use in the event that a person accidentally falls from

one of the structures. Adhesion tape, such as that used

around swimming pools, may be attached to the surface of the platform to help prevent slippage when

surfaces are wet.

Summary ______________________

The primary benefit of sampling platforms is that

they ensure safety in hydrologic research by elevating

persons and equipment above the water surface,

thereby limiting bodily contact with swift currents

while also eliminating disturbance to the flow and

streambed from wading. In addition, they are relatively inexpensive and easily constructed and disassembled within a few hours using simple hand tools.

The primary difficulty in platform construction is in

hauling long sections of joists to remote sites on unpaved roads. There are, however, some companies

that will deliver to remote locations for moderate cost,

which is worth looking into.

Literature Cited _________________

Paradiso, J. J. 2000. A bank-operated traveling-block cableway for

stream discharge and sediment measurement. Gen. Tech. Rep.

RMRS-GTR-44. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 35 p.

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