Temporary Sream and Wetland Crossing

This contribution was supported by the Great Lakes Protection Fundand the University of Minnesota, College of Natural Resources, ExtensionService, and Agricultural Experiment Station under Project MN 42-42.

TABLE OF CONTENTS

Page

Introduction ....................................................................................... 1Stream and Wetland Crossing Considerations..................................... 4 Abutments and Geotextiles .............................................................. 4 Erosion Control Measures ............................................................... 6Temporary Stream Crossing Options .................................................. 9 Fords .............................................................................................. 9 Permanently constructed fords ................................................... 10 Temporarily constructed fords .................................................... 11 Culverts ........................................................................................ 12 PVC and HDPE Pipe Bundles ......................................................... 13 Bridges .......................................................................................... 15 Ice bridges .................................................................................. 16 Timber bridges ........................................................................... 17 Used railroad cars and flatbed truck trailers ............................... 20 Steel bridges............................................................................... 20 Pre-stressed concrete bridges ..................................................... 21Temporary Wetland Crossing Options ............................................... 21 Wood Mats .................................................................................... 22 Wood Panels .................................................................................. 23 Wood Pallets .................................................................................. 24 Bridge Decking .............................................................................. 24 Expanded Metal Grating ................................................................ 24 PVC and HDPE Pipe Mats or Plastic Road ...................................... 25 Tire Mats....................................................................................... 26 Corduroy ....................................................................................... 27 Pole Rails ...................................................................................... 27 Wood Aggregate ............................................................................. 28 Low Ground Pressure Equipment .................................................. 29 Equipment with wide tires, duals, bogies, and/or tire tracks ...... 30 Equipment with tracks ............................................................... 31 Lightweight equipment ............................................................... 32 Equipment with central tire inflation .......................................... 32Economic Considerations for Crossing Options ................................. 32Environmental Impacts Associated with Crossings ........................... 33 Background................................................................................... 33 Studies of Stream Crossings .......................................................... 35 Studies of Wetland Crossings......................................................... 39Summary of Some Stream and Wetland Crossing Statutes ................ 40 Michigan ....................................................................................... 40 Minnesota ..................................................................................... 40 New York ....................................................................................... 42 Pennsylvania ................................................................................. 42 Wisconsin ...................................................................................... 43 Ontario.......................................................................................... 44 Quebec .......................................................................................... 44Research and Education Needs......................................................... 45Summary ......................................................................................... 46Selected Pertinent Literature ............................................................ 46

Appendices ....................................................................................... 56 Appendix 1.Partial List of Commercial Vendors for Temporary Crossing Options ..................................................................... 56 Appendix 2.Specifications for Some Temporary Stream Crossing Options ..................................................................... 62 Appendix 3.Specifications for Some Temporary Wetland Crossing Options ..................................................................... 74 Appendix 4.Informational Summaries for Temporary Stream Crossing Options ..................................................................... 79 Appendix 5.Informational Summaries for Temporary Wetland Crossing Options ................................................................... 101

List of TablesTable 1.Criteria to consider when selecting a stream or wetland crossing option ......................................................................... 3

Table 2.Informational categories summarized in Appendices 4 and 5 for each stream and wetland crossing option ......................... 4

Table 3.Properties of some geotextiles used in stabilization and separation applications ............................................................ 6

Table 4.Minimum ice thickness required to support a given load above a flowing river or stream or on a lake ............................. 18

Table 5.Central office contact information for water regulatory authorities in Michigan, Minnesota, New York, Pennsylvania, Wisconsin, Ontario, and Quebec.............................................. 41

List of IllustrationsFigure 1. Stream that may need to be crossed by an access road or trail .......................................................................................... 2Figure 2.Wetland area that may need to be crossed by an access road or trail .............................................................................. 2Figure 3.Rutting that can occur when crossing soft, wet soils ......... 2Figure 4.Anchoring of a bridge to a nearby tree .............................. 5Figure 5.Abutment logs below a log stringer bridge (approach yet to be constructed) ..................................................................... 5Figure 6.Non-woven geotextile ......................................................... 5Figure 7.Woven geotextile ............................................................... 5Figure 8.Installation of hay or straw bale barriers ........................... 7Figure 9.Installation of silt fencing.................................................. 7Figure 10.Mulched surface ............................................................. 8Figure 11.Erosion blanket samples ................................................. 8Figure 12.Riprap slope ................................................................... 8Figure 13.Seeding an area with a hand seeder ................................ 9Figure 14.Ford crossing .................................................................. 9Figure 15.A geotextile used in a ford .............................................. 11Figure 16.A tire mat used to provide support in a temporary ford .. 11Figure 17.Culvert .......................................................................... 12Figure 18.Packing earth around a culvert during installation......... 13Figure 19.PVC or HDPE pipe bundle.............................................. 13Figure 20.Installation of a geotextile under a pipe bundle .............. 14Figure 21.Pipe bundle crossing layered to the desired height ......... 14Figure 22.Temporary bridge spanning a stream ............................. 15Figure 23.Ice bridge ....................................................................... 17Figure 24.Log stringer bridge ......................................................... 18

Figure 25.Cabled logs in a log stringer bridge ................................ 18Figure 26.Solid sawn stringer bridge ............................................. 18Figure 27.Stress-laminated timber bridge panel ............................ 19Figure 28.Bridge made from a used railroad car ............................ 20Figure 29.Steel bridge ................................................................... 20Figure 30.Pre-stressed concrete bridge .......................................... 21Figure 31.Wet area in a haul road.................................................. 21Figure 32.Applying a geotextile under a temporary wetland crossing .................................................................................. 22Figure 33.Wood mat ...................................................................... 22Figure 34.Cable loop at the end of a wood mat............................... 23Figure 35.Wood panel .................................................................... 23Figure 36.Wood pallet .................................................................... 24Figure 37.Expanded metal grating over a geotextile ....................... 25Figure 38.PVC or HDPE pipe mat in a road.................................... 25Figure 39.PVC or HDPE pipe mat .................................................. 25Figure 40.Plastic road mat ............................................................ 26Figure 41.Tire mat ........................................................................ 26Figure 42.Another tire mat design ................................................. 27Figure 43.Corduroy ....................................................................... 27Figure 44.Pole rails ....................................................................... 28Figure 45.Wood aggregate .............................................................. 28Figure 46.Use of a geotextile under wood aggregate ....................... 29Figure 47.Wide tires ...................................................................... 30Figure 48.Dual tires ...................................................................... 30Figure 49.Tire tracks ..................................................................... 30Figure 50.Bogie system ................................................................. 31Figure 51.Tire tracks on a bogie .................................................... 31Figure 52.Tracked machine ........................................................... 31Figure 53.Central Tire Inflation (CTI) system ................................. 32Figure 54.Tire footprint at two pressures ....................................... 32

Charles Blinn is a Professor/Extension Spe-cialist, Department of Forest Resources,University of Minnesota, St. Paul, MN.

Rick Dahlman is a Utilization and MarketingSpecialist, Minnesota Department of NaturalResources, Division of Forestry, St. Paul, MN.

Lola Hislop is a General Research Engineer,with the U.S. Department of Agriculture,Forest Service, Madison, WI.

Michael Thompson is a General Engineer,U.S. Department of Agriculture, Forest Ser-vice, Houghton, MI.

Temporary Stream and Wetland Crossing Optionsfor Forest Management

Charles R. Blinn, Rick Dahlman, Lola Hislop,and Michael A. Thompson

INTRODUCTION

Expansion of economies and populationsworldwide has increased the demand for forestproducts and other uses of forests. Removingwood products from the forest requires accesssystems, such as truck roads and skid trails.Roads and trails must often cross streams (fig.1) and wetlands1 (fig. 2). The construction anduse of these access roads and trails has thepotential to negatively impact streams andwetlands directly by soil compaction, rutting,or the placement of fill (fig. 3). Streams andwetlands can also be impacted indirectly byfunneling the movement of sediment, debris,and nutrients into the water body or by caus-ing changes in hydrologic flows across thearea.

The best way to protect streams and wetlandsis to avoid crossing them. If this is not fea-sible, it is important to minimize and mitigateimpacts while using the crossing. For anyparticular application, selecting a crossingoption that is cost-effective for the contractorand/or landowner, that adequately addressesthe environmental concerns of society, andthat satisfies the wide range of regulatoryconstraints is becoming increasingly difficult.

In many areas, fords, culverts, and ice bridgesare the types of stream crossings most com-monly used, although use of portable bridgesis rapidly increasing. Corduroy, permanentfill, and frozen soil are the most commonchoices for crossing wetlands. While there arenumerous other options, lack of informationhas been an obstacle to expanding the range ofoptions that landowners, land managers,contractors, and regulatory agencies are willingto consider.

The purpose of this paper is to help reduce thisobstacle by providing detailed informationabout a broad range of reusable temporary2

stream and wetland crossing options. Toaccomplish this, we have:

Summarized information about many of thetemporary stream and wetland crossingoptions,

Reviewed some of the reported environmen-tal impacts associated with using theseoptions,

Highlighted some of the key points fromstatutes that regulate stream and wetlandcrossings in Michigan, Minnesota, New York,Pennsylvania, Wisconsin, Ontario, andQuebec, and

Identified some research and educationneeds.

1 For the purpose of this report, wetlands are areasthat contain soil with poor load-bearing capacity andhigh moisture content or standing water that areoften affected by seasonal fluctuations in water level.Examples include peat, muck, wet mineral soils, orunstable sections of access roads and trails.

2 For the purpose of this report, a temporary cross-ing is one that is used for a maximum of 3 yearsbefore it is removed or rendered unusable.

Information was compiled through contactswith individual loggers, staff in several forestproducts companies and land managementand regulatory agencies, and through a reviewof published literature.

The options discussed in this report includeboth commercial and homemade devices thatare either transported to the site or built on-site. Increased use of these options can helpminimize the cost of protecting water re-sources. While the initial price of a reusabletemporary option may exceed the cost of acurrently used alternative (e.g., a culvert for astream crossing or fill for a wetland crossing),the fact that it is reusable and that it may beeasier to install may make it the lowest costoption in the long-term. Also, some of thetemporary options reduce environmental

Figure 1.Stream that may need to be crossedby an access road or trail.

Figure 2.Wetland area that may need to becrossed by an access road or trail.

Figure 3.Rutting that can occur when crossingsoft, wet soils.

impacts and can be installed and removedmore rapidly than the options most frequentlyused today.

It is likely there are additional excellent cross-ing options not identified in this paper. It wasnot possible to identify all available options,and new ideas for crossings and new applica-tions of existing technologies are constantlybeing developed. We recommend that youcontact vendors, contractors, and othersources for more detailed information on localexperience regarding costs, installation andremoval, the availability of specific options inyour area, and additional options not coveredin this report.

We also recommend that you compare eachvendors recommendations and specificationsto your long-term needs and applications.Seek the advice of a licensed engineer whenpurchasing a stream crossing product thatlacks engineering specifications, when using aproduct in an application for which it was not

2

designed, or when constructing a crossingyourself. Inspect all products before eachinstallation. Some criteria to consider whenevaluating which option(s) to select for yourparticular application are noted in table 1.Before deciding to install a crossing, be sure toweigh the potential value(s) of the resourcewithin the accessed area against the costs andpotential negative environmental impacts.

Additional details and quick reference tablesare provided in the Appendices. Appendix 1contains a list of commercial vendors. Tablessummarizing specifications, approximate cost,installation and removal time, and equipmentneeds are provided in Appendices 2 (stream

crossings) and 3 (wetland crossings). All costsand prices presented in this report are in $USunless specified. Arnold (1994) and Mason(1990) also provide excellent overviews of manyproducts, including design specifications,drawings, and photographs. Copstead et al.(1997) have also prepared an extensive anno-tated bibliography of published literature onwater/road interaction technologies andassociated environmental effects. Appendices4 (stream crossings) and 5 (wetland crossings)contain additional information for each optionaccording to the categories listed in table 2.Although we recognize that there is someredundancy built into this report, we chose todo this to make it more useful for a number ofdifferent applications and users.

Effectiveness for reducing potential impacts

Relative safety of the option

Compliance with safety regulations

Compliance with other applicable non-safetyregulations, including permitting requirements

Potential conflicts with insurance coverage

Season(s) of use

Length of time the crossing will be needed

Ability to handle anticipated traffic loads and speeds

Purchase price or construction cost

Maximum distance of crossing

Bridge abutment requirements (stream crossingoptions only)

Availability of engineering specifications, especiallyfor bridging options (stream crossing options only)

Ease of transport, installation, and removal withavailable labor and equipment

Local availability

Installation and removal time and costs

Number of reuses possible

Anticipated future need for a particular option

Maintenance requirements

Driving surface traction under anticipated operatingconditions

Potential costs avoided, such as the purchase andplacement of fill or the need to divert stream flowduring a culvert installation or removal

Anticipated fluctuation in water level during use

Site conditions (e.g., soil type, hydrology, amount ofrock present, stream width and depth, volume ofwater in the stream, types of aquatic life in thestream, permitting requirements, etc.)

Potential value(s) of the resource within the area tobe accessed

Landowners management objectives

Rehabilitation requirements following removal

Table 1.Criteria to consider when selecting a stream or wetland crossing option. Unless otherwisenoted, all criteria are appropriate for both stream and wetland crossing options.

3

STREAM AND WETLAND CROSSINGCONSIDERATIONS

There are several considerations that apply tonearly all stream and wetland crossings. Donot cross unless absolutely necessary. If it isnecessary to cross a stream or wetland, thenumber of crossings should be limited to asfew as possible and the location(s) should becarefully selected. Existing crossings shouldbe used whenever possible, unless theirrehabilitation and use would be more damag-ing than establishing a new one.

The crossing should be as short as possible.Stream crossings should be perpendicular tothe direction of water flow to the degree practi-cal. Wetland crossings should be parallel tothe direction of water flow to the degree practi-cal. Approaches to crossings should be directand have a low grade. Water diversion struc-tures, such as a broad-based dip or waterbars, should be constructed to direct waterflowing down the road or skid trail into avegetated area before it reaches the crossing.This will help minimize the movement ofsediment into the water body.

Stream crossings should be located on astraight segment of the stream channel thathas low banks (except for bridge crossingswhere higher banks are preferred to supportthe abutments). This will minimize the need todisturb the bank or to alter the natural shapeof the channel. It will also reduce the impactof turbulent water action against the crossingstructure itself or against any portions of thebank that need to be disturbed to permit

installation of the structure. Where there is arisk of flooding, structures should be anchoredat one end to allow them to swing out of themain channel without washing downstream orobstructing water flow (fig. 4).

Proper installation, maintenance, and siterehabilitation are essential for any crossingoption to be fully effective. All necessarypermits should be obtained in advance andterms communicated clearly to the employeesor contractors working on the crossing. Thecrossing structure should be carefully in-spected before each installation and appropri-ate repairs made. If the crossing structurebecomes damaged, a licensed engineer shouldbe consulted to certify that it is still safe forthe anticipated loads. Structures that areinstalled either in or over open water shouldbe cleaned (away from the water body) beforeeach installation to remove accumulateddebris, such as mud and branches. Streamcrossing options that are placed in the water(e.g., culverts, pipe bundles) need to be fre-quently checked for blockage to avoid dam-ming the stream.

Abutments and Geotextiles

Many of the structures are most effective whena proper foundation is provided. Bridges needa log, railroad tie, or similar abutment to reston to help level the structure, to minimizedisturbance to the stream bank, and to makeremoval easier (fig. 5).

Some stream crossing options and mostwetland crossing options function best with ageotextile under them. Geotextile, also called

Table 2.Informational categories summarized in Appendices 4 and 5 for each stream and wetlandcrossing option.

Description of option

Area of application

Advantages

Disadvantages

Source(s)

Recommended supplemental material

Approximate price of materials (March 1998)

Construction directions and/or diagram and timerequired

Equipment needed for construction, installation,and/or removal

Installation approach and time required

Patent protection of product design

4

Figure 4.Anchoring of a bridge to a nearbytree.

Figure 5.Abutment logs below a log stringerbridge (approach yet to be constructed).

filter fabric, is a fabric mat used to prevent anew layer of material from mixing with thematerial below (usually native soil when usedwith crossing structures). Geotextiles allowwater to drain through them, provide addi-tional support for a crossing, and make re-moval of the crossing easier. A non-wovenfabric is recommended for use with temporaryinstallations because it is less slippery thanwoven fabrics, reducing movement of thestructure during use (fig. 6). Also, non-wovengeotextiles exclude fine particles while allowingwater to flow through from above and below. A

needle-punched non-woven polypropylene orhigh-density polyethylene (HDPE) geotextile oflow (3 oz/yd2 [100 g/m2]) to medium (6 oz/yd2

[200 g/m2]) weight has been used in mosttrials of temporary crossings.

Another type of geotextile is a woven geotextile(fig. 7). It is mainly used in applications thatrequire high tensile strength and low elonga-tion of the fabric, such as permanent roadbuilding. The woven fabrics tend to allow morefine particles to flow through. Some propertiesof a few representative non-woven and wovengeotextiles are compared in table 3.

A non-woven fabric will not continue to tear asa woven fabric may if it becomes punctured.Despite this, it is important to limit the num-ber of high spots (e.g., rocks, stumps) toreduce the potential of punctures during use ofthe crossing. At the same time, care should betaken to avoid damaging the root or slash mat

Figure 6.Non-woven geotextile.

Figure 7.Woven geotextile.

5

to the degree possible. This mat will provideadditional support during use, lessen themovement of sediment, and can speed reveg-etation of the site after the structure is re-moved.

The geotextile should be removed at the sametime as the crossing option. Sometimes, thegeotextile may be too heavy with soil and waterto be easily recovered in a reusable condition.For this reason, it may be beneficial to useshorter lengths with the ends overlapped (e.g.,25-ft [7.6-m] lengths with 2 ft [0.6 m] of over-lap) or sewn (e.g., leave about 3 in. [7.5 cm] ofoverlap in the sewn seam). On very soft soilwhere the geotextile sinks when it is steppedon, it is best to sew the overlapped area to-gether. If it is not possible to remove thegeotextile, a biodegradable fabric should beused. Fiber options for biobased geotextilesinclude coir, jute, kenaf, flax, sisal, hemp,cotton, and wood fiber (English 1994).

Paper machine felt may be a low-cost alterna-tive to geotextile (Bridge 1989). Used carpetand other materials may also be viable alterna-tives to geotextile. However, some materials

may contain toxic substances that could leachinto the water. Therefore, the appropriateregulatory agencies should be contacted beforeany substitute materials are used.

Erosion Control Measures

The installation, use, and removal of streamand wetland crossing structures frequentlyresults in movement and exposure of soil. Useof temporary erosion control measures isstrongly recommended to prevent movement ofsediment into water bodies while these soilsare being adequately stabilized by vegetation orarmoring. Adamson and Harris (1992) recom-mend development of a sediment control planto reduce sediment concerns at water cross-ings. The four broad categories they recom-mend that need to be considered in formulat-ing a plan for a particular water crossing arenoted below. Not all measures will apply for allcrossings.

Administrative measures (e.g., purchase ofadequate pipe length, training and instruc-tion of workers, timing of the construction toavoid fish spawning periods, inspectionfrequency, contingency plan to follow in caseof change).

Protection (e.g., protecting the existingground cover, limiting the area of distur-bance to reduce the area requiring stabiliza-tion).

Table 3.Properties of some geotextiles used in stabilization and separation applicationsa (metricunits are reported in brackets).

Property Non-woven geotextiles Typical woven4546 4551 4553 geotextile 2002

PhysicalWeight (oz/yd2) [g/m2] 4.6 [156] 7.0 [237] 9.2 [312] 5.0 [170]Grab tensile strength (lbs) [kN] 100 [0.44] 150 [0.67] 203 [0.90] 200 [0.89]Grab tensile elongation (%) 50 50 50 15Packaging and priceRoll width(s) (ft) [m] 15 [4.6] 15 [4.6] 15 [4.6] 12.5 [3.8] or 18 [5.5]Roll length(s) (ft) [m] 300 [91] 300 [91] 240 [73] 504 [154] or 351 [107]Gross weight/roll (lbs) [kg] 145 [66] 220 [98] 229 [104] 220 [100]Area/roll (yd2) [m2] 500 [418] 500 [418] 400 [334] 700 [585]Priceb ($/yd2) [$/m2] 0.56 [0.47] 0.84 [0.70] 0.95 [0.82] 0.60 [0.50]aThe geotextiles shown here are produced by Amoco Fabrics and Fibers Company3.bThe reported 1998 price (FOB) assumes the purchase of one roll of geotextile. Prices may vary betweenvendors and according to the amount of fabric purchased.

3 The use of trade, firm, or corporation names in thispublication is for the information and convenience ofthe reader. It does not constitute an official endorse-ment or approval of any product or service by theUniversity of Minnesota, the Minnesota Departmentof Natural Resources, or the USDA Forest Service tothe exclusion of others that may be suitable.

6

Strategic planning of the sequence of opera-tions to manage the factors affecting sedi-ment entering the water body (e.g., stormwater flowing toward the water body, flowagewithin the water body during construction).

Structural controls that will be used indi-vidually or in combination for reducingsediment (e.g., silt fencing, hay or strawbales, mulch, erosion blankets, diversionditches, water bars).

Options that may be used for reducing sedi-ment include silt fencing, hay or straw bales,mulch, and erosion blankets. Silt fencing andhay bales are installed as barriers across theslope of exposed soils to intercept and slow themovement of water down the slope. They trapthe sediment the water is carrying and releasethe water slowly. To be most effective, theyshould be placed at spacings similar to thatrecommended for water bars on long slopes(refer to the Best Management Practices [BMP]guidelines in your State or Province). Thebarrier closest to the water or wetland is mostcritical. Proper installation and maintenanceis essential for these barriers to work effec-tively.

To properly install hay or straw bale barriers(fig. 8):

Set the bales in a shallow trench to preventwater from flowing under the barrier. Ifdigging is impractical due to frost, packsnow against the uphill side of the bales.

Overlap the bales to avoid leaving gapsbetween them.

Drive stakes or lath through the bales sothat the stakes are buried 6 to 10 in. (15 to25 cm) into the soil to firmly anchor them inplace.

To properly install silt fencing (fig. 9):

Drive wooden stakes or lath spaced 4 to 6 ft(1.2 to 1.8 m) apart into the ground to holdthe geotextile or other permeable fabric siltfencing in place.

Cut the fabric in long strips that are 2 to 3 ft(0.6 to 0.9 m) wide.

Attach a continuous length of the fabric tothe uphill side of the stakes so that pressurefrom water and sediment will not pull thefabric loose. The lower 6 in. (15 cm) of thefabric should form an L facing uphill. Thisshould be buried, preferably in a shallowtrench, or covered with soil to prevent waterfrom running under the barrier. Pack thesoil firmly over this part of the fabric.

Silt fence and hay bale barriers should beinspected periodically and maintained asnecessary to make sure they remain func-tional. They may fill with sediment or becomedamaged, rendering them ineffective. Once thesite is effectively stabilized by revegetation, thebarriers should be removed.

In some situations, additional erosion protec-tion may be needed. The force of raindrops onexposed surfaces can loosen and wash awaysubstantial amounts of soil. In most cases, a

Figure 8.Installation of hay or straw balebarriers.

Figure 9.Installation of silt fencing.

7

layer of mulch or an erosion blanket over theexposed soil will provide good protection.Mulch or erosion blankets shield the soil andhelp disperse and slow the surface flow ofwater. Mulching with loose straw or hay (fig.10) on level or moderate slopes can work well.On steeper slopes, it may be necessary tolightly disk the mulch into the soil to keep it inplace. Where the risk of erosion is high orwhere concentrated flows of water are antici-pated, such as the bottom of a ditch, an ero-sion blanket may be needed. These blankets(fig. 11) are mats made of shredded wood orother fibers. They are commercially marketedby several manufacturers and should beavailable through suppliers that sell culvertsand geotextiles.

Steep excavated cuts, particularly on the bankof a stream, may require permanent protec-tion, such as heavy rock riprap (fig. 12). Infor-mation on this and several other possibletreatments for slope failure prone areas can befound in Moll (1996) and Mohoney (1994).Moll (1996) also discusses techniques used toclose and obliterate access routes after use.The level of restoration required for a particu-lar access corridor will depend on many site,management, and political factors.

Figure 10.Mulched surface.

Figure 11.Erosion blanket samples.

Figure 12.Riprap slope.

In any case, speeding the revegetation ofdisturbed soil surfaces is always a good idea.Vegetation provides a root mass that holds soilin place and soaks up water, reducing runoff.Fertilizing and seeding disturbed soil facilitatesand speeds revegetation, minimizing erosionpotential (fig. 13). Care should be taken to usenative seed sources to avoid introducing non-native species to the ecosystem unless theyhave been proven to be innocuous. Fertilizerand seed mixture recommendations for ex-posed soil are generally available from mostlocal land management organizations.

8

Figure 14.Ford crossing.

Figure 13.Seeding an area with a hand seeder.

TEMPORARY STREAM CROSSING OPTIONS

Crossing streams is often perceived to be oneof the most controversial, time consuming,and expensive parts of any forest managementoperation. Properly designed, installed, andmaintained temporary stream crossing struc-tures can greatly reduce costs and help meetthe concerns of regulatory agencies. Thetypes of temporary stream crossing optionsdiscussed below include fords, culverts,polyvinyl chloride (PVC) and high-densitypolyethylene (HDPE) pipe bundles, and por-table or on-site constructed bridges.

It is important to follow the proper permittingprocess when installing and using a streamcrossing. A local hydrologist should be con-tacted to determine whether a permit isrequired. The permit may specify what type ofcrossing as well as when it can be installedand/or used.

As noted previously, engineering input from alicensed engineer into the design of manystream crossing options is recommended.However, engineering design information forsome of the alternatives is limited, and theadditional costs for an engineered design maybe considered exorbitant for a temporarycrossing. Caution is necessary when usingany crossing that has not been engineeredand/or that has not been inspected duringand between uses.

The condition of any crossing should bemonitored as long as it exists. Regular main-tenance may be needed to keep the crossing

functional. Maintenance is especially impor-tant for culvert and pipe bundle crossings tomake sure that they are clear and free ofdebris. Weak soils on the approach to acrossing can be stabilized by placing one ormore of the temporary wetland crossingoptions (e.g., corduroy, wood mats, woodpanels, expanded metal grating, tire mats,etc.) on top of a non-woven geotextile.

Appendix 1 contains a list of commercialvendors for several stream crossing options.Appendix 2 presents a summary of productspecifications and approximate costs for someof the options discussed. Additional informa-tion can be found on the Internet. The Log-ging and Sawmilling Journal has a World WideWeb page (http://www.forestnet.com/log&saw/stream/introstr.htm) that includesinformation on a variety of stream crossingoptions, planning tips, and suggestions forprotecting aquatic resources and rehabilitat-ing the site.

Fords

A ford or low-water crossing uses the streambed as part of the road or access trail (fig. 14).They are best suited for short-term, limitedtraffic. Use should be limited to periods of lowflow when the water is less than 2 ft (0.6 m)deep. Because the spawning beds of manyfish species occur within the same areas thatmake a good ford crossing, fords should notbe constructed or used during periods of fishspawning. Also, construction should notoccur during fish migration periods. Fordsshould maintain the natural shape andelevation of the stream channel to avoidcreating obstructions to the movement of fishand other aquatic organisms.

9

Because equipment will be directly in thestream channel when using a ford, it is espe-cially important to keep vehicles clean andwell-maintained. Mud and debris dragged inon skid loads or on tracks and tires, as well asoil and fuel leaks, contribute to polluting thewater. Protecting the approaches to a fordwith clean gravel, corduroy, wood mats, woodpanels, expanded metal grating, or othertemporary surfacing material is recommendedfor this reason. (See the section on wetlandcrossing options for a description of corduroy,wood mats, wood panels, expanded metalgrating, tire mats, etc.)

Existing crossings should be used wheneverpractical unless rehabilitating and using thecrossing would be more damaging than estab-lishing a new one. New fords should be lo-cated where the banks are low, less than 4 ft(1.2 m) high, and gently sloping; where thegrade of the approach to the stream does notexceed 5:1 (horizontal to vertical); and wherethe stream bed is firm rock or gravel. Suchlocations require little or no modification toaccommodate traffic. More often some grad-ing, excavating, and placement of road basematerial is needed.

To facilitate construction of a ford where thestream bed is not dry, it may be desirable totemporarily divert the main flow of the streamaround the work site using ditches, berms,dikes, piping, high capacity pumps, or anexisting alternate channel crossing. Thismakes placement and compaction of fill mucheasier and minimizes sediment movement inthe stream. It may also be desirable to placelarge rocks or logs or to construct a sedimenttrap below a ford to catch sediment introducedby use of the ford. However, excavation ofmaterial within the stream bed should only bedone with the advice of a qualified hydrologistor licensed engineer and approval of the appro-priate regulatory authorities.

A mucky or weak stream bed is not acceptableas a base for a ford unless it is frozen. Whenthe soil within the stream bed is weak, it willbe necessary to cover or to replace it withmaterials that will support traffic. There areseveral alternative materials for creating a firmbase for a ford. Those applicable to normalforestry operations include:

Permanently constructed fords using cleangravel or rock with or without geotextile or a

plastic cell webbing known as a cellularconfinement system.4 A ford constructedwith the cellular confinement system issometimes known as a plastic ford.

Temporarily constructed fords using matsmade of wood, tires, expanded metal grating,logs or poles, or a floating rubber mat.

Use of a geotextile below any material (e.g.,clean gravel, rock, wood, or tire mats) in eithera permanently or temporarily constructed fordcan provide additional support to the crossingwhile separating the material from weak nativesoil. However, use of the geotextile should beapproved by the appropriate regulatory au-thorities because of concerns about impacts ifany downstream movement of the geotextileoccurs.

Permanently constructed fords

Constructing a firm crossing using gravel orrock is common when the existing bed of anintermittent or perennial stream is too weak tosupport traffic. When using clean gravel orrock fill to construct a ford, the existing weakmaterial in the stream bed should be exca-vated first. This allows the natural shape ofthe stream channel to be maintained andreduces the problem of the gravel mixing withthe mud, making the fill ineffective. A mini-mum of 6 in. of gravel or rock fill should beused unless it is not possible to excavate deepenough to accommodate this much fill. Thegravel or rock fill should not raise the crossinghigher than the original stream bed level.

We recommend that a geotextile be laid downbefore placing gravel or rock fill (fig. 15), aslong as it is approved by the appropriateregulatory authorities. This keeps the fillseparated from the weak native materials andprovides added support to the crossing. Al-though fords that are constructed with gravelor rock fill are intended as temporary cross-ings, the gravel or rock fill is not removed whenthe crossing becomes inactive.

4 A cellular confinement system is an expandablehoneycomb plastic panel into which different types offill material can be added. The panels confine the fillinto small compartments, holding them in place.

10

Pence (1987) and Tufts et al. (1994) describeplastic fords that were constructed to helpkeep the gravel or rock fill in place. Withoutthe cellular confinement system, the fill can bepushed aside by heavy vehicles or frequenttraffic over a crossing. Also, the cellularconfinement system reduces the chance thatfast currents will wash the gravel downstream.

A vented ford or emergency spillway can beformed by providing culverts to handle theday-to-day flow and by partially lowering theroad grades for passing floods (Adamson andRacey 1989). This type of ford is suitablewhere the normal flow may exceed a fordabledepth either seasonally or following stormevents. A wall must be provided along thedownstream edge of the spillway to keep thegravel fill from eroding under the action of theflowing water. Various wall types includegabions and precast concrete sections. Ripraperosion protection is required downstream ofthe wall. Coarse granular fill is required tomake the vented ford erosion resistant.

Temporarily constructed fords

A firm base for a ford can be temporarilyestablished by using materials such as matsmade of wood or tires, or by using expandedmetal grating, logs, or floating rubber mats.(See the section on wetland crossing optionsfor a description of wood mats, tire mats, andexpanded metal grating.) Wood or tire mats orexpanded metal grating, in combination with anon-woven geotextile, can be placed directlyacross a stream to create a firm base for theford (fig. 16). Installation is quick and easy.Removal is also simple, but may require re-turning at a later date if the site is frozen whenthe operation is finished. In some instances,

Figure 15.A geotextile used in a ford.

Figure 16.A tire mat used to provide supportin a temporary ford.

the stream bed or bank may be too weak forthe geotextile and mats or expanded metalgrating to provide sufficient support for traffic.Supplemental corduroy, gravel, or rock fill maybe needed in the weakest portions of thecrossing to create a firm base for the ford.

In some jurisdictions, a pole or log ford may beestablished for crossing small streams. Forthis type of ford, the stream channel is filledwith logs laid parallel to the direction of streamflow. This works well for crossings that will beused only a few times and where the logs canbe easily removed as soon as the work iscompleted. It is best suited for dry ditches andintermittent stream channels because of therisk of blocking stream flow, particularlyduring spring breakup or following a heavyrain. Placement of two or more steel cableslaid bank-to-bank below the pole ford willfacilitate removal of the logs. To minimize therisk of blocking stream flow, polyvinyl chloride(PVC) pipe bundles can be used in place oflogs. (See the section on PVC pipe bundlesbelow for a description of this option.)

11

A dam bridge or rubber mat bridge can alsobe used to provide a temporary ford (Arnold1994, Looney 1981). It is constructed fromstrips of rubber conveyor belting joined side-by-side. Looney (1981) describes a dam bridgeconstructed of 0.5-in.- (1.3-cm)-thick strips ofrubber conveyor belt that was cemented andbolted together. Support cables hold the sidesupright so the mat floats on the water when novehicles are in the crossing. When a vehicleenters the dam bridge the mat is pushed downto the stream bottom, momentarily dammingthe stream. Once the vehicle is across thestream, the mat floats up again, allowing thestream to flow normally. Looney (1981) recom-mends placing rock on the approaches to thedam bridge to assist vehicles in climbing out,particularly for skidders with chains.

Culverts

A culvert is a structure that conveys waterunder a road or access trail (fig. 17). It is oneof the most common methods of crossingintermittent and perennial streams. Manufac-tured culverts come in many shapes (round,oblong, or arched), lengths, and diameters,and may be made of corrugated steel, concrete,or polyethylene. An arch culvert is an open-bottom arch with appropriate footings intowhich the arch is fitted. New culverts areavailable from a variety of suppliers. Usedculverts suitable for temporary installationsmay be available from state, provincial, or localroad authorities or from construction, pipeline,or drilling companies.

Although corrugated steel culverts are usedmost frequently, the use of polyethylene pipesis increasing for temporary installations and

Figure 17.Culvert.

low standard roads. Polyethylene pipes haveseveral limitations that should be considered(Stjernberg 1987); however, proper installationcan overcome most problems. Other materialsare often used as substitutes for manufacturedculverts on temporary forest roads and skidtrails. Some examples are hollow steel piling,well casings, gas pipeline, wooden box culverts,and hollow logs. Small culverts can be in-stalled and removed with a bulldozer or back-hoe. An excavator may be needed to installand remove larger culverts. Low-cost culverttransportation systems have been developed(Ewing 1992).

Proper sizing and installation of culverts arecritical to constructing a successful crossing.Both culvert diameter and length need to beconsidered when determining culvert sizerequirements. The recommended minimumdiameter for any culvert is generally 12 in. (30cm). Smaller sizes are difficult or impossible toclean out if they become clogged. Temporaryinstallations that are removed seasonally mayonly need to accommodate estimated peak flowfor the season of use. Year-round installationsneed to consider peak flows of longer termevents (e.g., 10-, 25-, 50-, or 100-year floods).In all cases, it is important to confirm sizingrequirements with a hydrologist or licensedengineer and the appropriate permittingauthority.

A single large-diameter culvert is better thantwo or more smaller culverts. A commonmistake is to assume that two 12-in. (30-cm)culverts equal the capacity of a single 24-in.(60-cm) culvert. However, it takes at leastthree 12-in. (30-cm) culverts to equal thecapacity of one 24-in. (60-cm) culvert. Thegreater surface area of the three smallerculverts will also cause more turbulence,reducing the flow rate through each culvert.The pipe must also be long enough to extend toat least the toe of the fill slope. Placement ofthe culvert should follow the same grade as theexisting stream channel and should be placeddeep enough to avoid washing out from below.If the upstream end of the culvert becomes tooelevated, the water may move through theculvert too fast and impede the migration ofaquatic organisms. Culverts should not beinstalled during fish spawning and migration.

The pipe should be placed in line with thedirection of stream flow. Attempting to use the

12

culvert to redirect the stream may violate theterms of a permit and can result in the culvertbeing washed out. The side slope of the fillshould be no steeper than 2 to 1 (horizontal tovertical) to ensure it is stable. The culvertshould be covered with fill to a depth of 12 in.(30 cm), or one-half the diameter of the pipe,whichever is greater. This protects the pipefrom being crushed and avoids damage to thepipe during grading of the road surface. Whenearth fill is used, it should be compacted inlayers (fig. 18). This is especially important forthe bottom half of the culvert, which may bewashed out if the earth fill is not properlycompacted. Compaction of the road surfaceover the culvert will help ensure that thedriving surface sheds water rather thanturning to mud.

Figure 18.Packing earth around a culvertduring installation.

Some jurisdictions may permit logs or brush tobe used as fill around a temporary culvert toavoid placement of soil that would need to bedisturbed when the pipe is removed. A hy-drologist or the appropriate permitting author-ity should be consulted before using logs orbrush. There may be concerns about the logsor brush washing away during use of thecrossing, or that the fill material will not beremoved from the stream when the culvert isremoved. In either case, downstream move-ment of the logs and/or brush may dam thestream.

To facilitate installation and/or removal of aculvert where the stream bed is not dry, it maybe desirable to temporarily divert the main flowof the stream around the work site usingditches, berms, dikes, piping, high capacity

pumps, or an existing alternate channel cross-ing. This makes placement, compaction, and/or removal of fill much easier and minimizessediment movement in the stream. It may alsobe desirable to place large rocks or logs or toconstruct a sediment trap below a culvert tocatch sediment introduced by use of thecrossing. However, this should only be donewith the advice of a qualified hydrologist orlicensed engineer and approval of the appropri-ate regulatory authorities.

Periodic maintenance of culverts is importantto ensure proper function. Both ends of theculvert should be checked and cleared ofdebris to allow an unimpeded flow of water.This is especially important in areas subject tobeaver activity. Problems with unnaturalerosion around the culvert should be correctedto minimize the chance of washout of theculvert and sedimentation of the stream.

PVC and HDPE Pipe Bundles

A pipe bundle crossing is constructed using 4-in.- (10-cm)-diameter Schedule 40 PVC orSDR11 HDPE pipes that are cabled togetherforming loose mats that can be formed intobundles. The bundle provides mechanicalsupport for the vehicle while allowing water topass through the pipes unimpeded (fig. 19).Streams with a U-shaped channel to containthe pipes are most appropriate for this option.

Because standard PVC pipe is light-sensitiveand will lose strength when exposed to sun-light, using PVC pipe that has been exposed tothe sun should be avoided. Strength of thecrossing can be maintained by covering or

Figure 19.PVC or HDPE pipe bundle.

13

painting PVC pipe or by using an ultraviolet-resistant type of pipe, such as HDPE. HDPEpipe also tolerates temperature extremes of-40 F (-40 C) better than PVC without becom-ing brittle or losing shock resistance and willreturn to its original shape after being de-formed (Lgre 1997). No published studieshave evaluated the use of PVC or HDPE pipebundles for stream crossings during winter inan environment where temperatures areconsistently below freezing.

HDPE pipes may be more expensive thanstandard PVC and may need to be purchasedthrough a vendor that specializes in plasticpipe. The thickness of many alternative plasticpipes is often specified using the term stan-dard dimension ratio (SDR), which is calcu-lated by dividing the average outside diameterof the pipe by its minimum wall thickness. Forany given outside diameter, SDR will increaseas wall thickness decreases.

It is important to make the crossing wideenough for the widest vehicle that will use it.Plastic pipe is generally sold in multiples of 10-ft (3-m) lengths; therefore, it may be necessaryto purchase 20-ft (6.1-m) sections that can becut to the desired length. Mason andGreenfield (1995) proposed using shorter pipesections, end-to-end, between full-length pipesmaking a 14-ft- (4.2-m)-wide crossing.

Constructing pipe bundles consists of drilling1/4-in.- (6.4-mm)-diameter holes at 1 ft (0.3m) and 4 ft (1.2 m) from each end of each pipe.Four 3/16-in.- (4.8-mm)-diameter galvanizedsteel cables are threaded through the holes toconnect the individual pieces. It is importantto drill round holes to avoid creating potentialstress points that could later facilitate pipeshattering. It may be necessary to controlcable end fray before stringing the cablethrough the sections. Loops should be madeat the end of each cable, extending beyond thelast pipe, and then secured using 3/16-in.-(4.8-mm)-diameter cable clamps to preventindividual pipes from rolling or moving in otherdirections.

Our experience has shown that it takes about1 hr for two people using two hand drills andone saw to cut, drill, and cable 4-in.- (10-cm)-diameter and 20-ft- (6.1-m)-long PVC into a10-ft x 12-ft (3-m x 3.7-m) section. The initialconstruction cost of this size section, including

materials and labor, is about $500. Two 10-ftx 12-ft (3-m x 3.7-m) sections are necessary tobuild a crossing for a 6-ft (1.8-m) wide x 2-ft(0.6-m) deep channel. Each cabled sectionshould be loose so the pipes can conform tothe stream channel. A tight connection isused for a single layer crossing.

The crossing is created by first laying down ageotextile fabric (fig. 20) to ensure separationfrom the stream bottom and to facilitateremoval (Mason and Greenfield 1995). A layerof connected pipes is then placed on top of thegeotextile along the stream bottom, parallel tothe stream flow. If necessary, loose or con-nected pipes are then layered to the desiredheight (fig. 21). A layer of connected pipesshould be placed as the top mat. Loops at theend of connecting cables should be covered sothat they dont hook onto the underside of apassing vehicle. The top and bottom layers ofpipe should be long enough to go beyond thestream edge to protect the stream banks.

Figure 20.Installation of a geotextile under apipe bundle.

Figure 21.Pipe bundle crossing layered to thedesired height.

14

Typically, a stiff surface such as expandedmetal grating, tire mats, or wood panels arelaid over the top mat to limit pipe movement(wave action) and to provide traction. Thecrossing surface needs to be sufficiently con-nected to the pipe to avoid flipping up under acrossing vehicle (Mason and Greenfield 1995).Wood mats or expanded metal grating can beplaced on top of a non-woven geotextile tostabilize the approaches to the crossing if theyare weak.

The time required to install the crossing de-pends on the crossing length, stream depth,water volume, equipment available, and theamount of room needed for the equipment tomaneuver. It took about 1.5 hr to place a PVCpipe bundle crossing with a wood pallet run-ning surface within a stream channel that was35 ft (11 m) wide (the bundles covered 25 ft[7.5 m] of that span). It took about 20 minutesto remove a PVC pipe bundle crossing consist-ing of a non-woven geotextile placed below two10-ft x 12-ft (3-m x 3.7-m) bundle sections, awood plank running surface, and one 10-ft x12-ft (3-m x 3.7-m) wood mat that was used oneach side of the stream to protect the banks.

Transportation equipment needed to bring thepipe or bundles to the crossing site depends onthe length and amount of pipe (Mason andGreenfield 1995). In some cases, the drilledindividual pipes may be transported by pickuptruck and constructed on-site. Preconstructedmats may be too heavy for a pickup truck,requiring a dump truck or lowboy. Front-endloaders or skidders are typically used to placethe mats.

Bridges

By spanning the stream, bridges keep fill andequipment out of the water body to the greatestdegree of any stream crossing option (fig. 22).For this reason, they have the least impact onstreams. Designs are available for a widerange of span lengths and load capacities (e.g.,pickup trucks, skidders and forwarders, orloaded semi-tractor trailers). Temporarybridges can be made from ice, timber, usedrailroad cars (flatcars and boxcars), usedflatbed truck trailers, steel, or pre-stressedconcrete.

Little site preparation is normally requiredwhen installing a temporary bridge. To provide

stable, level support for the structure, it isrecommended that bridges be installed onabutments on both sides of a stream. This willalso facilitate removal if the ground is frozen.Logs or large wooden beams are often used forabutments. Crossing permit requirements,local statutes, or specific site conditions mayrequire more extensive abutments. Also,crossing permit requirements or local statutesmay specify the minimum clearance betweenthe bridge and the stream to accommodatepeak flows and/or recreational use.

Ramps from the approaches up to the bridgetraffic surface often need to be created on-site.Soil, snow, and corduroy are usually adequate,but permit requirements and/or site condi-tions, such as weak soil, may require use ofother materials. Also, if the soils within theapproaches to the bridge are weak, or if theywill be significantly disturbed during installa-tion and use of the bridge, we recommendusing corduroy, wood mats, wood panels,expanded metal grating, or other temporarysurfacing material to strengthen the ap-proaches. (See the section on wetland crossingoptions for a description of corduroy, woodmats, wood panels, and expanded metalgrating.) This will reduce rutting or otherdamage that may inhibit use of the crossing. Itwill also minimize the potential for sedimentreaching the stream.

Bridges are generally designed to be a fixedlength. Unfortunately, a fixed-length bridge isnot applicable to all streams. Engineeredbridges have been evaluated and a load(weight) rating established for a maximumspan distance. If that maximum span distance

Figure 22.Temporary bridge spanning astream.

15

is lengthened and/or the maximum load isexceeded, the bridge may fail. It may befeasible to span longer distances; however, themaximum load will need to be decreased andshould be determined only by a licensedengineer.

Some bridge designs are open in the middle, orhave holes or gaps in the traffic surface. Thesedesigns may be less expensive or easier toinstall, but also allow dirt and debris to fallinto the stream. As a result, some jurisdic-tions may not permit use of these designs.Also, movement of individual bridge panelsmay occur during use if they are not ad-equately connected. For structures with a gapin the traffic surface, it is recommended that adecking material (e.g., plywood, lumber) beadded to close this space. Installation of curbsor guardrails is an important safety consider-ation for bridges designed for truck traffic.They help the driver position the vehicle safelyon the structure. The curbs or guardrails mayneed to be removed for skidder traffic if theskid loads will damage them.

Most temporary bridges can be installed andremoved with a knuckleboom loader, front-endloader, bulldozer, or skidder. Long spans forwide streams may require special equipment,such as a crane or excavator. Keliher et al.(1995) recommend that individual bridgesections weigh less than 5,000 lb (2,300 kg)and be under 30 ft (10 m) long if a grappleskidder is to lift the structure. Sections couldweigh as much as 11,000 lb (5,000 kg) if theskidder drags, winches, or pushes them. Thesize and weight of the bridge or its individualcomponents is also an important considerationfor loading, unloading, and transport from onelocation to another. In some circumstances,the machine installing the bridge may need tocross through or work in the stream channel toget the structure in place. This should beavoided whenever possible and may requireprior approval from the local permitting au-thority.

Prefabricated, portable bridges made of steel,treated or untreated lumber, and pre-stressedconcrete can be purchased from commercialmanufacturers. When they are retrofitted,used railroad cars and flatbed truck trailerscan also serve as a prefabricated crossing.Although they are generally more expensivethan a structure built of locally available

materials, prefabricated options offer twoimportant advantages. First, commerciallymanufactured bridges generally incorporateengineering input into their design. This willhelp reduce concerns about safety and liabil-ity. Second, they can be used many times overseveral years, considerably reducing the costper crossing. Prefabricated designs includesingle, rigid structures, and hinged or modularpanels. The hinged or modular panels facili-tate transport, installation, removal, andhandling. Prefabricated bridges can usually beinstalled or removed in less than 2 hr.

Bridges fabricated from locally available mate-rials also come in a wide variety of shapes andsizes. Some are built on-site for a single use;others are portable and can be used severaltimes. Examples of on-site constructedbridges include ice and log stringer bridges.Portable bridges may be constructed from solidsawn timbers and planking, flatbed trucktrailers, steel beams, pre-stressed concretepanels, or other materials. Because the initialcost of a bridge fabricated from locally avail-able materials is often much lower than that ofcommercially manufactured bridges, they are aviable alternative for many stream crossings.The installation and removal time may belonger for a bridge fabricated from locallyavailable materials than for a prefabricatedbridge.

We recommend that a licensed engineer reviewthe design of any bridge that is fabricated fromlocally available materials to ensure that thestructure will be safe and is adequate for theintended use. However, this review may bedifficult to obtain and the cost may be consid-ered too exorbitant in some cases. Construc-tion specifications have not been establishedfor some materials (i.e., hardwood lumber),and others may be converted to a use forwhich they were not originally designed (e.g.,flatbed truck trailers). Many materials orstructures have had substantial wear and tearbefore their use as a bridge structure (i.e.,railroad flat cars, flatbed truck trailers, con-crete panels), which may have significantlyreduced their strength or limited their remain-ing service life.

Ice bridges

Ice bridges are a common type of winter cross-ing over streams, lakes, and rivers in areas

16

where there are extended periods of tempera-tures below freezing (fig. 23). In some areas,the ice may be thick enough that no construc-tion is needed. Where construction is neces-sary, ice bridges are made by packing snowand/or pumping water onto the existing ice.Some jurisdictions permit slash or brush to beplaced in the stream channel when there is noice to build on, but this is generally undesir-able because it is often difficult to remove thebrush. Because an ice bridge will take longerto melt than the stream, a new channel maybe cut around the blockage on streams withlarge or high velocity spring flows. Therefore,ice bridges may not be appropriate on thosestreams.

Figure 23.Ice bridge.

On lakes, ice bridges are sometimes created byplowing snow off the path chosen for the road.Removing the snow enables the ice to growthicker faster, thereby increasing the loadbearing capacity. Because the plowed snow-banks represent concentrated loads on the ice,they should be spread over as large an area aspossible. When a vehicle crosses floating ice, adeflection bowl moves with it, generating wavesin the water (Haynes and Carey 1996). If thespeed of these waves is the same as the vehiclespeed, the deflection of the ice sheet is in-creased and will likely lead to failure of the ice.The problem is more serious for thin ice andshallow water depths. When in doubt, opera-tors should drive less than 15 miles (24 km)per hr.

The following formula was developed to esti-mate the minimum ice thickness required tosupport a given load above a flowing river orstream or on a lake (Haynes and Carey 1996).

h=4(P)1/2

Where: h = ice thickness in inches P = the load or gross weight of the vehicle plus its contents, in tons.

This equation can also be expressed in tabularformat, where the vehicle class equals the totalweight of the vehicle plus its load (table 4).

Timber bridges

Donnelly (1997) provides a good overview oftimber bridges. Engineering design guidelinesare available for many different types of timberbridges. Two popular designs for temporarystructures are log stringer bridges and solidsawn stringer bridges with or without a plankdeck. Log stringer bridges (fig. 24) are built bycabling logs together from trees felled in thearea of construction. It is especially importantto bundle stringers together with cable (fig. 25)when building a log stringer bridge to improveperformance. A narrow stream can sometimesbe crossed by bundling stringers together withchains or cable and placing them only in thewheel path. Several log stringer bridge optionsare discussed in Peterson (1987). Solid sawnstringer bridges (fig. 26) are built with newlumber, railroad ties, or demolition materials.Kittredge and Woodall (1997) describe a solidsawn stringer bridge made with 6-in. x 6-in.(15-cm x 15-cm) and 6-in. x 8-in. (15-cm x 20-cm) cants. This design provides an unevendriving surface for better traction.

Creating an ice bridge requires cold weather.Night temperatures below 0 F (-18 C) are best,with several days required to build up ice thickenough to safely support traffic. Once an icebridge is in use, the thickness and condition ofthe ice should be checked frequently to be sureit remains adequate. This may need to be doneas often as once or twice a day, or more, on fastmoving streams or large rivers and lakes and astemperatures warm up.

17

Table 4.Minimum ice thickness required to support a given load above a flowing river or stream oron a lake (Haynes and Carey 1996)a

Vehicle classb Minimum ice thickness Minimum distance between vehiclesc(tons) [tonnes] (in.) [cm] (ft) [m]

0.1 [0.1] 2 [5.1] 17 [5.2] 1 [1.1] 4 [10] 34 [10] 2 [2.2] 6 [15] 48 [15] 3 [3.3] 7 [18] 58 [18] 4 [4.4] 8 [20] 67 [20] 5 [5.5] 9 [23] 75 [23] 10 [11] 13 [33] 106 [32] 20 [22] 18 [46] 149 [45] 30 [33] 22 [56] 183 [56] 40 [44] 26 [66] 211 [64]

a Information in this table assumes clear, sound ice. If white, bubble-filled ice makes up part of the icethickness, count it only half as much as clear ice. If the air temperature has been above freezing for at least6 of the previous 24 hr, multiply the vehicle class by 1.3 to obtain a larger minimum thickness. If the airtemperature stays above freezing for 24 hr or more, the ice begins to lose strength and the table no longerrepresents safe conditions. Maximum recommended speed is 15 mi (24 km) per hr.b Vehicle class equals the gross weight of the vehicle plus the weight of its contents in tons.c At ice thicknesses greater than the minimum, the spacing between vehicles can be reduced on sound ice.

Figure 24.Log stringer bridge. Figure 25.Cabled logs in a log stringer bridge.

Figure 26.Solid sawnstringer bridge.

18

Plank decks provide a smooth running surface,decrease lateral movement of the structureduring use, and reduce the amount of dirt anddebris that may fall into the stream. If a plankdeck is to be installed on either of these bridgetypes, annularly threaded (ring-shank) orhelically threaded (spiral) spikes should beused to attach the surface deck. The spikesshould be at least 9 in. (23 cm) long to reducewithdrawal during use of the crossing. If careis used during installation, removal, andtransport, stringer bridges can be reusedseveral times.

The main concern with log stringer and solidsawn stringer designs is that there is limitedinformation on the engineering properties oflogs, railroad ties, and demolition materials.Thus, designs using these materials typicallyinclude little, if any, engineering input. Careshould be exercised when using these materi-als if they have not been evaluated to deter-mine their structural strength and engineersare not involved in the design or in accuratelyestimating their load ratings. Rot, decay,knots, and grain can greatly affect theirstrength properties. Some of these factorsbecome more important the longer a bridge isin use, especially if the species does not have ahigh decay resistance.

Muchmore (1976) presents basic design crite-ria to determine whether single lane logstringer bridges are designed and constructedto safely support specific loadings and to meetminimum safety criteria. Bradley and Krag(1990) present span designs for seven treespecies (white spruce [Picea glauca], easternwhite pine [Pinus strobus], jack pine [Pinusbanksiana], red pine [Pinus resinosa], easternhemlock [Tsuga canadensis], quaking aspen[Populus tremuloides], and balsam poplar[Populus balsamifera]) and two types of bridges(with and without needle beams5). Guidelinesare also available for evaluating the capacity ofsawn timber and log stringers in existingbridges (Ontario Ministry of Natural Resources1989). The USDA Forest Service is currentlydeveloping guidelines for different types ofportable timber bridges.

Other timber bridges have been constructedusing treated panels of stress-laminated,

glued-laminated (glulam), dowel-laminated, ornail-laminated materials. Stress-laminatedpanels consist of lumber placed edgewise andheld together with high-strength steel rods (fig.27) that are stressed in tension, up to 100 lb/in.2 [PSI] (7,000 g/cm2). There is a need toretension the steel rods periodically. Russell(1997) presents a review of stress-laminatedtechnology. Glulam panels consist of dimen-sion lumber glued together on the wide face.Dowel- and nail-laminated panels are similarto stress-laminated panels, except that dowelsor nails are used to connect each successivepiece of lumber. In all cases, the panels areplaced side-by-side across a stream.

One advantage of these designs, as well as asolid sawn stringer design using new lumber,is that the lumber is a known species andgrade. Thus, structural properties are knownand an engineer can properly design a safestructure. Also, because they are panelized,transportation to the site may be easier thanfor some other bridging options. Field perfor-mance for some of these engineered bridges

5 A needle beam distributes a live load to all string-ers and is positioned across the stringers at mid-span.

Figure 27.Stress-laminated timber bridgepanel.

19

installed for permanent use indicates that theirperformance is generally satisfactory (Ritter etal. 1995).

Behr et al. (1990) compared the initial costs oftimber, steel/concrete, and prestressed con-crete bridges. Five New England generalcontractors performed cost estimates on bridgedesigns spanning 20, 40, and 60 ft. Also,three timber bridge suppliers provided costestimates for nine bridge designs, three at eachspan length. Results from general contractorsindicated that timber was cost competitive withsteel/concrete and was less expensive thanprestressed concrete. Results from timberbridge suppliers showed a distinct initial costadvantage for timber over both steel/concreteand prestressed concrete.

The USDA Forest Service Wood in Transporta-tion Program has established the Wood inTransport National Information Center. TheCenter maintains a website at http://wit.fsl.wvnet.edu that describes the programand provides an on-line order form to obtainfree publications, design plans, cost informa-tion, etc. The Center can also be contacteddirectly at 304-285-1591 (voice) or 304-285-1505 (FAX). The Forest Products Lab also hasa website at http://www.fpl.fs.fed.us/wit/containing some of their publications on wood-in-transportation technology. The ForestService has also published an excellent refer-ence on timber bridge design, construction,inspection, and maintenance (Ritter 1992).

Used railroad cars and flatbed truck trailers

Railroad cars (flatcars and boxcars) and flatbedtruck trailers can be retrofitted for use asstream crossings (fig. 28). The retrofittingprovides additional reinforcing to the mainsupport beams so they will support vehicletraffic. Bradley and Pronker (1994) present astandard design for using railcars as tempo-rary bridges. Carraway (1997) indicated thatrailroad boxcars are much lighter than flat-cars, allowing a 50-ft- (15-m)-long x 10-ft- (3-m)-wide bridge to be transported on a lowboytrailer. A railroad boxcar bridge can be un-loaded with a knuckleboom loader and placedacross the stream with a skidder. A crane maybe needed to lift and install a railroad flatcarbridge. Individual railroad cars and flatbedtruck trailers tend to be narrow which may bea limitation for hauling applications.

Figure 28.Bridge made from a used railroadcar.

Used railroad cars may be purchased fromthird-party vendors who purchase old railroadcars from railroad companies. Contact yourlocal railroad company or railroad car repairfacility to find out how to obtain one. Usedflatbed truck trailers are lighter and are morereadily available locally through trailer repairand salvage operations. While it is possible topurchase retrofitted used railroad cars for useas bridges, most used flatbed truck trailervendors do not provide the additional rein-forcement needed.

Steel bridges

Steel bridges include hinged portable bridgesand modular bridges (fig. 29). Where permit-ted, two or more bridge spans or bridge panels(multi-spans) may be connected across a pierto span wide crossings. Hinged bridges fold upfor transport. Modular steel bridges are de-signed as a series of individual panels thatinterlock, forming a bridge of variable length.

Figure 29.Steel bridge.20

We are aware of a locally fabricated, non-engineered, steel bridge that is about 50 ft (15m) long and was constructed using I-beams.Also, catwalks with wood decking have beenused.

Pre-stressed concrete bridges

Precast, pre-stressed concrete panels can belocally fabricated. Generally, two or morepanels are placed side-by-side to form thebridge (fig. 30). Although the initial cost of thisbridge may be low, they are usually heavy andrequire larger equipment to install and remove.It is important to make sure that the panelsare engineered to handle the anticipated loads.Highway departments or local road authoritiesmay be a source for used panels.

Figure 30.Pre-stressed concrete bridge.

TEMPORARY WETLANDCROSSING OPTIONS

This overview of temporary wetland crossingoptions focuses on alternatives that can beapplied to the surface of a wetland soil, includ-ing a wet spot on a haul road, to stabilize it forshort crossing distances (fig. 31). While wedefine short as being less than 200 ft (61 m),the distance may depend on the initial cost topurchase or construct the selected option, thevalue of whatever is to be accessed, and thecosts associated with other travel routes.Although a very long distance could be crossedwhen the option is matched to site needs, thecost may be prohibitive. The ability to reuseoptions makes them more viable, especiallythose with a higher initial cost.

Temporary wetland crossing options includewood mats, wood panels, wood pallets, bridgedecking, expanded metal grating, PVC andHDPE pipe mats or plastic road, tire mats,

Figure 31.Wet area in a haul road.

corduroy, pole rails, wood aggregate, and lowground pressure equipment. Low groundpressure equipment includes machines withwide tires, duals, tire tracks, bogies, tracks,light weight, and/or central tire inflation (CTI).We have chosen not to discuss road construc-tion activities or wetland dredging and fillingoperations that are associated with constructinga new road or crossing over long distances. Wehave also not included cable yarding systems,helicopters, or balloons. Although use of frozenground may be the most viable crossing optionin many areas, that option is also not dis-cussed.

Many of the options should not be placed onareas that have firm high spots (e.g., stumps,large rocks) to reduce bending stress andbreakage during use. Hislop and Moll (1996)recommend blading the surface as flat aspossible before installation. For sites with grassmounds or other uneven vegetation, bladingshould not disturb the root mat associated withthe vegetation. The performance of any wetlandcrossing option is enhanced if there is a root orslash mat to provide additional support to theequipment.

Maintaining the root mat can also speed reveg-etation of the site following removal of thecrossing. The performance of the crossing isalso enhanced by use of a geotextile (fig. 32),which helps segregate the crossing from theunderlying soil and provides additional flota-tion. Most of the options are best suited to beused in conjunction with hauling and forward-ing, but not during skidding. If used duringskidding operations, the options will wear fasterand may move out of position when trees aredragged over them. Also, if a geotextile is used,it may become torn and displaced by skidding.

21

The length and width of a crossing optionneeded to achieve a particular wetland cross-ing will vary according to the site characteris-tics, soil strength, anticipated loads, andinstallation equipment available. The crossingoption used should cover the entire length ofarea to be crossed so that ruts do not developbeyond the end of the option. Ruts may causedrivers to steer the vehicle out of a wheel pathon the crossing option and to rut outside theedge of the crossing. On very weak soils thathave a low bearing strength (e.g., muck, peat),the options may need to be wider than what isrequired on other soils. The additional widthis needed to spread the weight over a largerarea. Additional width may also be needed atroad intersections and curves to providenecessary maneuvering room for vehicles.

Most of the options are best applied on roadsections with straight alignments, grades up to4 percent, and no cross slope. Steeper grades,cross slope, or curves may result in loss oftraction or lateral movement of the optionoutside of the planned travel area. Tractionloss may occur between the tires and thesurface of the option, especially when thesurface is wet. Slippage can occur between thecrossing option and the geotextile below it.Because most wetland crossing options have arough surface, they require a reduction ofvehicle speed. They should be placed in areaswhere the speed is low or where there is goodvisibility and plenty of distance to slow thevehicle.

For the options constructed from wood (e.g.,wood mats, wood panels, wood pallets), theground surface should be fairly level before

Figure 32.Applying a geotextile under atemporary wetland crossing.

installation to reduce breakage of the woodmembers. Use of a dense hardwood speciesand treated timbers may extend the life of thecrossing. However, use of treated timbers maynot be cost-effective if the anticipated life anduse of the panels do not require that applica-tion (e.g., only a short-term use is anticipated,skidding material over the option). Also, bestmanagement practices for the use of treatedwood in aquatic environments should be used(WWPI 1996).

Each temporary wetland crossing option isbriefly described below. Rummer and Stokes(1994) also discuss some of these options. Aswith stream crossings, there are several waysto accomplish each of the various options.Appendix 3 includes further information aboutmany of the temporary wetland crossingoptions, including information about dimen-sions, product weight, and approximate pur-chase price. A list of some vendors of tempo-rary wetland crossing options is presented inAppendix 1.

Wood Mats

Wood mats are individual cants or logs cabledtogether to make a single-layer crossing (fig.33). A 10-ft- (3-m)-long, 4-in. x 4-in. (10-cm x10-cm) cant or log is the recommended mini-mum size. Longer cants or logs may be neededto distribute the weight better on very weaksoils or under heavy loads. Mason andGreenfield (1995) tested both 4-in.- (10-cm)-and 6-in.- (15-cm)-square cants on a silty sandsoil within the Osceola National Forest inFlorida. They found that while both matsworked well, the smaller mats cost less andwere lighter weight, facilitating on-site installa-tion.

Figure 33.Wood mat.22

Constructing wood mats consists of drillingholes 1/4-in. (6.4-mm) in diameter througheach cant or log about 1 to 2 ft (0.3 to 0.6 m)from each end. Two 3/16-in. (4.8-mm) galva-nized steel cables are then threaded throughthese holes to form the mat. Loops should bemade at the end of each cable, extendingbeyond the last cant, and then secured with3/16-in.- (4.8-mm)-diameter cable clamps (fig.34). These loops are used to handle the matduring installation and removal. The connec-tion between the cants should be tight toreduce any rippling or wave movement thatmight occur when vehicles pass. Improperlytightened cable clamps can lead to slip andloss of connectivity within the mat. We notedthat a 10-ft x 12-ft (3-m x 3.7-m) mat con-structed from 4-in. x 4-in. (10-cm x 10-cm)material over deep, organic, muck soil devel-oped a rippling motion in front of the tires of amoving flatbed truck. This was caused by themat sinking into the soil in the area immedi-ately below the tire. We speculated that awider mat with the cants more tightly con-nected may have avoided this problem.

Figure 34.Cable loop at the end of a wood mat.

Hislop (1996a) reported that it took threepeople up to 3 hr to cut, drill, and cable to-gether a 20-ft- (6.1-m)-long x 10-ft- (3-m)-widemat. To reduce drilling time and errors inmarking, she recommends making one set ofdrilling marks on the ground and ensuringthat several hand drills are available. A weld-ing torch or some other means of controllingcable end fraying will increase threading speed.The cost to initially construct a 10-ft x 12-ft (3-m x 3.7-m) mat using 4-in. x 4-in. (10.2-cm x10.2-cm) cants is about $200 (including about$40 for labor) and the mat can be expected tolast several years under normal use.

Individual mats can be connected to oneanother on-site to form the complete crossing.Limiting mat length to about 10 ft (3 m) re-duces weight and facilitates installation.Shorter lengths may be needed if the mats arewider than about 12 ft (3.7 m). During instal-lation, it is important to tuck the ends of allcable loops under the mats to avoid their beingcaught by a passing vehicle. If the surface ofthe crossing becomes slick during use, ex-panded metal grating (as described later in thisreport) can be added as a running surface toprovide traction.

Wood Panels

Two-layer wood panels can be constructed bynailing parallel wood planks to several perpen-dicular wood planks where the vehicles tireswill pass (fig. 35). The actual running surfacemay be on either side of the panel, unless thenails have gone all the way through it. Theindividual panels can be either preconstructedor constructed on-site. We constructed panelsusing 3-in.- (7.6-cm)-thick x 8-in.- (20-cm)-wide planks. A gap of about 1 in. (2.5 cm) wasleft between each plank during assembly.Each finished panel was 8 ft x 12 ft (2.4 m x3.7 m).

Figure 35.Wood panel.

Annularly threaded (ring-shank) or helicallythreaded (spiral) spikes can be used to attachthe planks. For ease of construction, starterholes should be pre-drilled into the top board.To reduce withdrawal, spikes should be placedat slight angles from vertical with one spikeangled toward the traffic and one away from it.To facilitate picking up the panels duringinstallation and removal, loops can be createdby attaching 3/16-in.- (4.8-mm)-diameter

23

galvanized steel cables to each section using3/16-in.- (9.5-mm)-diameter cable clamps.The initial construction cost of an 8-ft x 12-ft(2.4-m x 3.7-m) wood panel is about $150(including about $40 for labor). Connectorsand non-woven geotextile are extra.

Interconnecting adjacent panels in a crossingwill help minimize the rocking that occurswhen vehicles drive over the panels and willimprove the overall flotation provided by thecrossing. However, interconnecting panels willalso increase the time required for installationand removal of the crossing. Adjacent panelscan be interconnected using eye hooks screwedinto the end of each panel with quick links orother heavy duty connectors through thehooks. If the panels wont be interconnectedwhen installed, about 6 in. (15 cm) should beleft between the individual panels to facilitateinstallation and removal.

Wood Pallets

Wood pallets for crossings are sturdy, three-layered pallets similar to those used for ship-ping and storage but specifically designed tosupport traffic (fig. 36). They are a commer-cially available product generally made fromdense hardwood planks that are nailed to-gether. They are specially designed so thatthey can interconnect, so that they are revers-ible, so that broken planks can be easilyreplaced, and so that nail points wont surface.Some pallets are designed so that the top andbottom pieces are already interconnectedsimilar to a traditional pallet, while others aredesigned so that the top and bottom pieces areseparate and interlock during installation toprevent longitudinal movement.

Figure 36.Wood pallet.

His

Temporary Sream and Wetland Crossing

Documents

Transcript of Temporary Sream and Wetland Crossing