A few months ago, we wrote about the widerange of practices used to protect water qualityon logging jobs. We gave only a brief mention

to bridges, since it is a complex subject.Readers have asked for more, so here it is.

1 • Sawmill & Woodlot

Streams can be crossed by a number of creative meth-ods, and, as always, each state will have its ownrequirements, based on water flow, permanent vs.

temporary crossings, and other parameters. So, check withyour county service forester or private consulting forester tosee which methods are allowed and what permits orapprovals might be required. Most states have a specificbooklet with “Best Management Practices” laid out insome detail. Forestry operations may be less restricted thanbridges for development or other use. This is based on thetemporary nature of forestry crossings and lower overallenvironmental impact of forest management compared toother land use. Forestry water crossings come in several categories.

First, there are truck roads, and next, skidder or forwardertrails. Another category is whether they are permanentstructures or temporary crossings that might be used forone or two logging seasons. The size of the drainage area isanother category, with different states delineating minorand major streams and drainages. Preventing erosion and protecting water quality are

important aspects of forest management. We also need effi-cient and cost-effective approaches to moving wood prod-ucts and influencing forest conditions for desired growth.In most states, this comes down to commonsense applica-tion of practices that allow us to move wood from thestump to the road, taking into account the various cate-

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B R I D G E SWOODLOT MANAGEMENT • by Robbo Holleran

Example of a map showing planning and layout on a large lot. (from the Maine BMP manual page 14)

gories of water crossings. “Plan ahead,”my father always says. The best crossingis the one you can avoid.You probably know your own land,

including the streams, wet soils, steepareas, and other features to avoid for roadand trail construction. A good map withthese features is helpful. Before any log-ging project gets started, it is important toplan for truck access and the main skidtrails, with particular emphasis on streamcrossings. A truck needs to get to a land-ing area to pick up logs. The area needsto be firm and well drained and as closeto the logging action as feasible. Decidingwhether to build a longer truck road ordrag the logs further is part art and partscience. But the size of the trucks and thetype of logging equipment are part of thatdecision. Perennial streams, intermittent streams, andeven dry drainages are often protected. Planningshould include the location of main skid trails, avoidingwetlands and stream buffers as much as possible.Truck roads and skid trails should be planned foracceptable grades and so that crossings can be avoidedor minimized. Stream crossings are the most likelylocations to cause sedimentation and need to be care-fully planned and executed. Existing trails and cross-ings might be the best spots or they might not. Justbecause the last person did it that way does not meanyou should. In some states, it is required to map outand locate stream crossings, buffers, or wetlands beforeany logging starts for review by state foresters. Theplanning stage involves anticipating site conditions andthe risks to water quality, such as heavy summer rain or

a thaw in midwinter.Each stream crossing should be selected carefully. It

has to fit the overall access layout, but should be asclose to perpendicular to the stream as possible, and onfirm ground with stable approaches. Sometimes ourterrain limits us to less-than-perfect locations, whereextra care is needed. Once we have a location, we needto determine if this is permanent or temporary, inter-mittent or perennial, and if it will be used by heavy logtrucks or skidders. This will determine the appropriatestructure, and what approvals are needed. What is a stream? Here in Vermont, an intermittent

stream is defined as having a well-defined channel, butsurface flow is not consistent. Perennial streamsexceed 320 acres of drainage area and have higher lev-els of protection, which may include allowing for fish

Precast blocks make a sturdy abut-

ment, especially if they are properly set on a

good base, below the streamlevel and protected by rip-rap.

The flow has been kept in a culvert to keep the water clean.

The abutments are now protected fromflood flow by rip-rap and the stream is flow-

ing pure. Another row of blocks will beadded to raise the bridge further.

Precast blocks make a sturdy abut-

ment, especially if they are properly set on a

good base, below the streamlevel and protected by rip-rap.

The flow has been kept in a culvert to keep the water clean.

passage. Generally, intermittent streams can be crossedwith a culvert, ford, or even logs and brush for tempo-rary skidder use. Bridges are generally required forperennial streams and may be permanent or temporary.Temporary bridges, especially if they will be left in

for the spring runoff or rainy season, need to haveenough room to allow the typical annual high-waterflow, or the three-year high in some states. The annualflow is usually apparent by the scouring and lack ofvegetation along the banks. Permanent crossings needto be able to handle the average “25-year storm” eventand may require an engineer and special permits.Some states offer a “flow chart” to determine theopening size based on the number of acres of water-shed drainage. Or, the average flow in square feet canbe estimated by the width and depth of the annualflow. For example, a small stream that is 6 feet acrossthe top of the banks, and averages 1.5 feet in depthwould have 9 square feet of area for normal annualflow. This is normally multiplied by 2.5 for the 10-yearhigh and 3.5 for the 25-year high level. So, 9 squarefeet times 3.5 is 31.5 square feet, the cross section thatneeds to be maintained to handle the 25-year floodevent. A 6-foot round culvert has 28 square feet ofcross section, so this would not be big enough for apermanent structure. A 12-foot span with 3 feet ofaverage depth (12 X 3 = 36, exceeding the 31.5 squarefeet) underneath should be adequate for a permanentstructure. The size of a watershed can be determined with a

topographic map and area measuring tool like basic

mapping software. The divide is the height of land,which delineates where water will flow into one oranother watershed. The Continental Divide in theRocky Mountains distinguishes where water will flowinto the Atlantic or Pacific oceans. The term “water-shed” is often used improperly in normal English,where the word “divide” would be better. “TheGettysburg battle was a watershed moment in the CivilWar.” No, it was a divide. If you can map the height ofland (divide) all around the point at which your crossingwill be, you can see which acres will flow toward yourbridge. This area, along with the soil type, terrain,slopes, and rainfall events will determine the correctsize of a bridge. Most states have permit thresholdsbased on this area, where large drainages require addi-tional levels of approval and possibly a consulting engi-neer.There are many kinds of temporary bridges that can

be easily installed for either trucks or skidders. Woodenbridge mats are in common use, and for short spans of10–14 feet, they are great! These heavy wood panels,usually 8 or 10 inches thick, are 16–20 feet long and bolt-ed together into panels 4-feet wide. They can be han-dled with a log truck, skidder, or excavator, and three orfour can be set into place in less than an hour. These aregenerally set on “sill or header” logs, placed parallel tothe stream above the bank to make a higher, stable plat-form for the panels. Then bumper logs are laid along theedges to create an edge for machines or dragged stems.Each panel sells for about $500 new, and used ones areavailable. In many states these are available “on loan” or

for a small rental fee. Since they are easyto install and remove, minimize siltationand damage to the streambed, and arereusable, they are better than culverts inmany cases. Our larger logging crewsgenerally own a stack of them and bringthem from job to job and move themaround as needed to different smallcrossings as the job progresses. Thesewooden panels have a relatively shortlife span of a couple years of heavy useand can rot if left in wet locations oreven if stacked up with no spacebetween them. There are commercial metal

portable bridges of all sizes for tempo-rary use. Twenty- to 40-foot bridgescan be rented and may be worthwhilefor a temporary crossing. One contrac-tor I work with has a steel bridge witha 20-foot span that costs about $10,000.It folds up to fit on a log truck, is easyto install, and holds up a tractor-trailer.He has had it at least 10 years, and it isstill in good shape. I have seen a pair ofold flatbed trailers used to make a 40-

3 • Sawmill & Woodlot

Completed bridge with steel I-beams, pressure treateddeck and railings. All disturbed areas are stabilized andvegetated. While the stream looks small, a permanent

bridge has to plan for major flood events.

Completed bridge with steel I-beams, pressure treateddeck and railings. All disturbed areas are stabilized andvegetated. While the stream looks small, a permanent

bridge has to plan for major flood events.

foot span “on the cheap.” And if you are logging, thereare always logs. Many temporary bridges have beenbuilt with logs from nearby trees for spans up to 20feet. These are subject to decay and should be built tominimize soil contact with the structural logs, but theycan last 10 to 20 years under good conditions. Eacharea will have preferred species for rot resistance, suchas cedar, hemlock, oak, or locust. While log bridgeshave a low cash cost, they can take considerable laborand machine time to construct. Permanent bridges for log trucks over perennial

streams can be a major project. Once you select thebest location and determine the drainage area andneeded flow space, you should check to see what per-mits are required. This is best started with your con-sulting or state service forester since the rules may bedifferent for a forestry bridge compared to other uses.There may be fisheries or wetlands review by thestate, and the federal Army Corps of Engineers mayhave some jurisdiction. This is a case where it is betterto ask permission than forgiveness. The needed flow space will determine the height

and span of the bridge. This has to be designed withthe existing contour and flood flow in mind for the 25-year flood event in most cases. Once you have the spanand expected load, you can determine the structureneeded. For most small timber-access bridges, experi-enced contractors can create a suitable structure. Onceyou get beyond a 24-foot span for a 50-ton log truck,you probably need a structural engineer to design theactual bridge. Basic options include abutments made with logs,

precast concrete blocks (refrigerator-sized), or pouredconcrete. The span can be covered by logs, but steel I-beams are typical. There are also precast structuresthat can be set in place, and, while expensive, theysave a lot of on-site labor/equipment costs. The sur-face decking is best done with pressure-treated lum-ber. You can save a few bucks with local “rot resistant”wood, but for a permanent bridge, the pressure-treatedwood is worth it.

Abutments are the structures upon which the beamsrest. These should make a firm foundation when setwell below stream level to withstand frost and floodflow. Setting these involves working in or directlyalong the stream, so special care needs to be taken toprotect flowing water from siltation. If the flow can berestricted to a culvert for construction, that works well.A poured concrete base is expensive, but probablybest. I have built several bridges that use precastblocks that usually weigh about a ton. Setting a firmflat base with crushed stone is a good way to start, andthen the blocks are stacked in place and backfilledwith good material like crushed stone. They can alsobe set back along the edges as a retaining wall, and toprotect against flood flow, and this goes quickly if youstart with a good base. Many bridge abutments havebeen built with local stone and have lasted a century ormore, but the concrete blocks are easier. Using logs isanother option with lower “cash cost,” but there is a lotmore labor involved in cutting and stacking logs like alog cabin for a solid base. Since they are in contactwith soil and water, they have a limited life span. I-beams are the typical way to span the abutments

and hold the weight. They are reliable, relatively inex-pensive, and easy to use. Logs, large wooden timbers,laminated wood, and precast concrete panels are otheroptions. Used steel beams are often available for abouthalf the cost of new ones and are adequate for the proj-ect. Decking is normally lumber unless you are usingprecast concrete products. Thick, roughsawn planksmight be fine, but even our rot-resistant species willeventually decay. I prefer pressure-treated wood, as Imentioned, and 2 x 6 lumber set on edge and nailedtogether makes a firm and flat surface. With any bridge or water crossing, there is mainte-

nance involved. Obstructions such as trees or rocksmoved by the current can block the flow and become ahazard. These can also build up other materials to forma dam and cause more trouble downstream. Any woodshould be checked for decay periodically, along withchecking steel beams for rust. So, it is important tocheck these crossings at least once per year.Minor stream crossings are a normal part of land

management, and major stream crossings can be daunt-ing and expensive, but can be worthwhile as a perma-nent improvement to the land and can reduce loggingand trucking costs. And, they can be fun! n

Robbo Holleran is a private consulting forester helpinglandowners meet their goals in Vermont and adjacentareas. His work has him outdoors about 150 days eachyear, plus play time. He is one of the authors of the mostrecent Silvicultural Guide for Northern Hardwoods inthe Northeast.

April 2018 • 4

Rented bridge panels being moved into place.

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