1 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 streams and watersheds function. Channel 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. 2 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 canals. Complex bridge sites require a thorough investigation due to problems associated with s, stream 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 floods occurred? • How high does the water get? Does the stream flood 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 floodplains. 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 3 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 dams or weirs • Natural control points, such as wood and rock steps and bedrock channels • Bankfull indicators and high-water marks • Ice damage, scars, or marks • Bank 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 discharge 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—River 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 spring 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