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LOCATING BRIDGES FOR SUSTAINABILITY J.S. Groenier; R.A. Gubernick

ABSTRACT Selecting the proper location for a bridge is as important as the characteristics of the bridge itself. This paper will discuss a common sense approach, combined with science, to help select sustainable locations for bridges. Problems associated with bridge location and construction can be alleviated by conducting a proper site investigation, paying attention to geomorphic indicators, knowing road template design needs, and understanding how and watersheds function. classification provides a simple framework to help understand the potential problems that may exist at bridge locations and to help with route locations. An interdisciplinary approach is required to incorporate all the considerations involved in choosing the best bridge location for sustainability.

INTRODUCTION A poor location or the wrong size structure can make a bridge more susceptible to failure. Bridges can be the most expensive item on a road, so it’s important to get them right the first time. Good bridge siting involves many disciplines and includes preliminary engineering, hydrology and hydraulics, geomorphologic concerns, roadway alignment, and environmental and geological concerns. All of these topics must be addressed to make sure that the structure is appropriate for the site. This paper will focus on construction of new bridges for sustainability, but the same considerations can be used for bridge relocation or reconstruction.

PRELIMINARY ENGINEERING Preparations for site investigations include collecting topographical maps, infrared photography, remote sensing images, GIS coverages, and aerial photographs. Topographic maps can help when you are locating a bridge. Infrared maps (figure 1) may show areas that are prone to being wet and other problem areas (springs or wetlands).

FIGURE 1 Infrared photograph of proposed bridge locations in Alaska.

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Reviewing multiple years of aerial photographs is helpful when determining the stability of streams. Stable streams will show up in the same location year after year, while unstable streams may change locations or widths in photographs taken during different years. Site work includes site investigation, site surveys, and geotechnical investigations. Some sites require simple site investigations, because the abutment locations and sites are controlled by the highway, railroads, lined ditches, or . Complex bridge sites require a thorough investigation due to problems associated with s, dynamics, wildlife concerns, etc. The more complex the site, the more important it is to form an interdisciplinary team, which may include Bridge and Transportation Engineers, Geologists/Geotechnical Engineers, Fisheries and Wildlife Biologists, Hydrologists, Botanists, Archeologists, and Soil Scientists. Site investigation includes conducting a site reconnaissance by walking the upstream and downstream reaches and talking to long-time residents of the area about flooding and debris jams. Some of the questions that should be addressed are: • What time of year have the occurred? • How high does the water get? Does the stream over its banks? • Does the stream have ice flows or debris damming problems? During field reconnaissance, the stream should be reviewed for dynamic sections and problem areas that should be avoided, such as deltas, alluvial fans, actively aggrading/degrading sections, sharp curves, multithreaded channels, sloughs, wetlands, and . Numerous photos (figure 2) should be taken of the site, banks, stream corridor, and other important features.

FIGURE 2 Example of a photograph of a proposed bridge location.

A stream bankfull determination should be made in the field [Stream System Technology Center 2004] to provide verification for structure length and future modeling

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efforts. The bankfull flow value will be compared to the Q2 flow value to determine whether modeling outputs are similar to the known Q2/Qbankfull relationship. Also, it is valuable to get a field estimate of the elevation that corresponds to large floods. This elevation can be checked with estimates of the Q100 flood to verify model projections and assure that modeling is as accurate as possible. A rule of thumb used when estimating the Q100 is to determine the approximate Q2 in the field and double the maximum bankfull depth in a representative channel section to estimate Q100 depth. The stream should be investigated for at least 500 meters upstream and downstream of the proposed bridge. This reconnaissance will help identify factors affecting the structure. For example, a bedrock control stream will have less chance of scour and the abutments will normally be perched high enough above the water to allow enough clearance for floating debris. Additional items that require investigation include: • Structures upstream and downstream • Channel control structures, such as or • Natural control points, such as wood and rock steps and bedrock channels • Bankfull indicators and high-water marks • Ice damage, scars, or marks • and stream stability • Springs and groundwater flow • Flood plains and deltas • Visual geotechnical investigation of soil types and streambed strata • Navigational clearance requirements

All features not normally included in a survey map should be flagged to ensure they won’t be missed by the survey crew. A topographic map should be prepared after site surveys have been conducted. The amount of geotechnical investigation required varies depending on the site. This investigation should be completed by a geotechnical engineer. The site should be probed for soil and bedrock conditions. An easy probing method used by the U.S. Forest Service is the Williamson Probe [Hall and others 2004]. The method works best when used in gravel or sand, giving the operator an idea of relative density and when soft zones are encountered. Borings are desirable for sites with unacceptable and complex soils or highly fractured shear bedrock faces. Bedrock should be assessed for the degree of fracturing, gaps between the fractured surfaces, the material’s hardness, and the degree to which it has weathered. Wet and unstable sites and sites with clay and silt soils should be avoided, if at all possible. Unsuitable foundation material can cause structures to settle and fail. All major bridge sites should have a geotechnical study completed with at least one boring drilled for each abutment or pier. The type of bridge substructure is site specific and should be designed in conjunction with a geotechnical engineer [Michigan Department of Transportation 2004, Davis 2001].

HYDROLOGY AND HYDRAULICS Hydrology calculations should be completed by a hydrologist familiar with the local conditions and streamflows. These calculations should include at least the Q2 and Q100 flows. Streamflow in the United States is usually calculated using a model or equations, such as the Hydrologic Modeling System (HEC-HMS) or U.S. Geological Survey Regression Equations for Streamflows. The results should be compared to arrive at a logical solution because calculations are not an exact science. Another good method compares the watershed being crossed to an adjacent watershed with similar physical characteristics that already has hydrologic data. A nearby gauged stream

4 can be used to compare your results and calibrate the modeled stream flow. Discharge measurements are a great way to calibrate your flow model for your site [Harrelson and others 1994]. In addition, a hydrologist should make a pebble count and gather substrate information to allow the channel roughness value and scour potential to be estimated. The channel roughness values, as well as substrate and streamflow information, will be used to calculate the hydraulics for the site. Hydraulic calculations can be performed using many different computer programs. Two of the most common in the United States are Hydrologic Engineering Center— Analysis System (HEC–RAS) and WSPRO, a computer model for Water-Surface PROfile computations. After calculations are completed, make sure to verify results with site investigation field observations, such as bankfull indicators, high-water marks, streambed strata, stream velocity, and information from local residents. A scour analysis should be completed for every stream-crossing project. The timings of peak flows vary from region to region around the world. For instance, the peak flows may be caused by runoff from mountain snowpack or from hurricanes or monsoons. Season and cause of the peak flows should be taken into account in bridge design. Navigational clearance is required in many streams and must be provided at high water. Minimum clearance for navigation will vary, depending on the type of boat traffic. Floating trees or debris present another problem during floods. The minimum clearance for floating trees can be estimated as half of the root wad’s longest dimension, plus 1 meter added for safety.

GEOMORPHIC CONCERNS The geomorphology of the watershed and channel play key roles in the siting of bridges. Basic geomorphic principles allow designers to understand the geomorphic processes and difficulties presented when bridges cross various positions in the watershed. These processes change with location in the watershed and along the reach where the crossing will be located. Channels are extremely dynamic, responding to changes in the watershed by propagating changes downstream to upstream and vice-versa depending on the channel position in the watershed, the type of disturbance, and the channel types along the stream. To choose the best location for a bridge in this dynamic environment, the designer should address the following questions: 1. Where is the crossing location in the watershed and how does the stream transport water, , and wood at that location? 2. How is the channel configured? a. What is the degree of channel containment? b. Is there conveyance? If so, how much, and are there side channels or flood swales? c. Can the stream move laterally and affect the crossing during the structure’s design life? Are the stream’s banks erodible or not? d. What is the range of vertical fluctuation of the streambed during the structure’s design life? 3. How well does the road and bridge alignment mate with the stream alignment? 4. Is the channel stable? Is the channel adjusting to recent large-scale disturbances?

The location of a stream reach in its watershed determines its channel morphology and responsiveness to natural or manmade disturbances [Gubernick and others 2003]. Slope, discharge, sediment, and vegetation are the main controlling factors, which also vary with topography and position in the watershed. The way a channel is configured provides

5 information that can help you decide whether a crossing is a good, safe location or an expensive, complex location that will require extensive analysis and design. Channel classification has been an excellent tool for describing stream configurations and for interdisciplinary communication. The two main channel classification schemes used are the Rosgen system [Rosgen 1994] and the Montgomery and Buffington system [Montgomery and Buffington 1993]. The Montgomery and Buffington system is based principally on watershed position, slope, and the geomorphic description of bed characteristics. The Rosgen system (figures 3 and 4) is based on slope, entrenchment ratio (figure 5), bankfull-width to bankfull- depth ratio, sinuosity, and bed material. Both have utility, but this paper will focus on the Rosgen system for bridge siting [Rosgen 1994, 1996].

FIGURE 3 Rosgen system showing examples of broad level delineation of stream types. Courtesy of Wildland Hydrology [Rosgen 1998].

FIGURE 4 Rosgen system showing broad level stream classification delineation of major stream types. Courtesy of Wildland Hydrology [Rosgen 1998].

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At an ideal bridge crossing, all floodwater and watershed byproducts would stay within the confines of the existing channel. Such crossings would have high banks with a narrow floodplain or none at all. Rosgen’s channel classification system illustrates that certain channel types are more vertically contained than others. In channels with low entrenchment ratios (channel types A, B, F, and G), the majority of the discharge remains within the confines of the bankfull or active channel area, even during flood events. When a bridge crosses such channels, it is relatively easy to provide good vertical clearance between the stream and the bottom of the bridge’s girder. Channels with high entrenchment ratios (channel types C, D, DA, and E) tend to have active flood plains with low banks. They will require deep fills for acceptable vertical clearance. Streams with high entrenchment ratios often require additional drainage structures on the flood plain and wider crossings. Bridges built on such streams may pose problems for animals that need to cross the area. Identifying how much water flows and its flow width over a flood plain is a major consideration when crossings have a high entrenchment ratio. Flood plain conveyance is a balance between open span width, vertical clearance, and scour. Even when a bridge spans the active channel bed, the bankfull width, or even more of the flood plain, if the flood plain has high conveyance, the bridge opening will constrict the flow during floods, causing higher shear in the bed and deeper scour depths than normal. If crossings must be located in these channels, riprap or other materials are recommended to prevent excessive scour. If flood channels or swales exist, additional culverts or slab structures can help reduce the constriction caused by roadway approaches and may also help preserve flood plain processes at the crossing.

FIGURE 5 Illustration of different entrenchment ratios (ER). Wfp is width of flood plain. Wbf is width of Bankfull width.

All stream channels migrate laterally over time. Because of bank resistance and increased bed shear, confined channels usually migrate more slowly than unconfined

7 channels. If the banks are composed of highly erodible materials (non-cohesive, finer grained sands, gravels, and cobbles), the banks adjust more easily than if they are composed of non- erodible materials (boulders, bedrock, and cohesive materials). Vegetation can also be a major factor in the susceptibility of bank . If deeply rooted native species are present, banks are less erodible. Material alone is not always the sole indicator of the likelihood of lateral movement. Streams with low entrenchment ratios and lower width-to-depth ratios (channel types A, B, F, and G) tend to have lower migration potential than those with high entrenchment and high width-to-depth ratios (channel types C, D, and E). Type E channels and channels with dense, deep-rooted woody vegetation can be very stable and migrate laterally very slowly. Transport reaches usually have heavily armored streambeds (channel types A, B, and G) and response reaches have fine-grained, non-cohesive streambeds (channel types C, D, DA, E, and F). The armored streambeds in the transport reaches indicate that scour is usually localized, and streambeds tend to be more rigid, limiting large vertical changes to the bed. Streambeds in the response reaches tend to aggrade or degrade with changes in sediment supply and discharge. In situations where response reaches are composed of cohesive material (clay), these channels tend to be very stable and can have good crossing sites. Establishing solid foundations at such crossings can be very expensive and floodplain issues and stream sinuosity will need to be addressed. Headcuts may be encountered in a streambed that migrates upstream. Depending on their size (depth), headcuts can undermine bridge foundations or materials intended to prevent scour. Characterize the bed materials and use a long longitudinal profile to determine headcut locations. The range of potential vertical changes in bed elevations will help determine the risks and costs associated with constructing crossings in response reaches. When a wide stream flows into a narrow bridge opening, back eddies can form, constricting the area available for unrestricted flow area in the channel. This condition causes observable changes in and increases localized scour. Field evidence of this condition includes above the structure, usually seen in the longitudinal profile as a flat sediment wedge with little streambed structure or as gravel bars. Bank scour can occur above or below the site because the changes in cross-section area create back eddies, increasing the boundary shear stresses and directing flow into the banks instead of parallel to them. Bed scour commonly occurs downstream, caused by increased outlet velocities and increased water surface slope. Understanding how an unstable landform can behave over time can be helpful when planning for future maintenance needs and design considerations. For example, active alluvial fans are sediment zones. Their channels change location frequently, sometimes rapidly when sediment and debris deposits cause the channel to seek a lower level. If a crossing is located on an active fan, the channel can be abandoned after an occurs upstream, or the crossing may fail catastrophically because of sediment or debris deposition. The best crossings in such areas would be below the fan or near its apex. If the crossing must be on an , large channel changes should be anticipated and the design should minimize the downstream consequences of failure by minimizing diversion potential at the crossing [Grant 1998]. The channel needs to be assessed for stability at both the watershed and reach scales. It is particularly important to identify system-wide instability such as , because the design will have to account for predicted changes in the channel if the structure is to be stable over the long term. It is best to avoid crossings in unstable channels because it can be difficult to predict the amount of change in width and depth that might occur. System-wide instability usually can be seen in series of aerial photos as noticeable changes in channel width, rapid growth and movement of depositional bars, alluvial fans at mouths, and

8 so forth. Frequently, large-scale channel changes are associated with observable land-use changes such as mining, agriculture, subdivision and road development, or logging. As a rule of thumb, the heavily armored transport reaches (channel types A, B, and G with cobble and larger substrates) tend to be more stable and less affected by watershed changes than the response reaches (channel types C, D, E and F).

ROAD ALIGNMENT A good horizontal road alignment should provide adequate sight distance with required horizontal curves and/or straight approaches for the design road speed. An ideal bridge approach would allow vehicles approaching the bridge to see oncoming traffic. Bridges with horizontal alignments constructed perpendicular to the stream are the shortest structures and usually cost less, but may cause safety problems on the approaches because of inadequate stopping sight distances and turning radius. If the bridge is not located in a relatively straight section of road, the bridge may have to be widened to allow large trucks to make the turns without damaging the bridge railings. Vertical road alignments are also important. Bridges with a slight grade will shed water. Water will pond and debris will collect on bridges in the bottom of a sag in the alignment. Gravel and debris on the bridge deck or frozen ponded water will cause maintenance (increased rust or decay) and safety problems. Less efficient alignments are acceptable when conditions are controlled by large trees, banks, wildlife habitat, or high stream sinuosity. Straightening stream channels or modifying channel alignments is not recommended and requires complex hydraulic and geomorphic investigations.

ENVIRONMENTAL CONCERNS Wildlife and fisheries concerns, including those involving threatened or endangered species, should be taken into account when locating a bridge. For example, seasonal construction closures may be needed near salmon spawning habitat in Alaska or Indiana bat roosting trees in the Midwest.

SUMMARY Investigating the site properly, paying attention to geomorphic indicators, and understanding how streams and watersheds function can help alleviate problems associated with bridge location and construction. Channel classification provides a simple framework to help understand the potential problems that may exist at any bridge location and will help designers locate travel routes. Proper bridge siting requires an interdisciplinary approach and common sense to assure selection of the best bridge site location for sustainability.

REFERENCES [1] Davis, T. Geotechnical Testing, Observation, and Documentation. ASCE Press. Reston, VA. 2001. [2] Grant, Gordon E. The RAPID Technique: A New Method for Evaluating Downstream Effect of Forest Practices on Riparian Zones. Portland, OR: U.S. Department of Agriculture Forest Service General Technical Report PNW-GTR 220. 1988. [3] Gubernick. R; Clarkin. K; and Furniss. M. Site Assessment and Geomorphic Considerations in Stream Simulation Culvert Design. International Conference of Ecology and Transportations Proceedings, Lake Placid, NY. 2003. [4] Hall, D., Long, M, Remboldt, M. Slope Stability Reference Guide for National Forests in the United States, Volume 1. Washington, D.C. U.S. Department of Agriculture Forest Service. 2004.

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[5] Harrelson, C; Rawlins, C.L; Potyondy, J. Stream Channel Reference Sites: An Illustrated Guide to Field Technique. USDA Forest Service General Technical Report RM-245. 1994. [6] Michigan Department of Transportation. Geotechnical Investigation and Analysis Requirements for Structures. Michigan Department of Transportation. 2004. [7] Montgomery, D.R.; Buffington J.M. Channel Classification, Prediction of Channel Response, and Assessment of Channel Condition. Report TFW-SH10-93-002. Washington State Timber/Fish/Wildlife Agreement. 1993, p 54. [8] Rosgen, D. A Classification of Natural . Catena, vol. 22 Elsevier Science, B.V. Amsterdam. 1994. [9] Rosgen, D. Applied River Morphology. Wildland Hydrology, Pagosa Springs, CO. 1996. [10] Rosgen, D. Field Guide for Stream Classification. Wildland Hydrology, Pagosa Springs, CO. 1998. [11] Stream System Technology Center. Identifying Bankfull Stage in the Eastern and Western United States. DVD-ROM. Fort Collins, CO: U.S. Department of Agriculture Forest Service. 2004.