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Chapter 7
Bridge Maintenance
7.3. Enhancements to Bridges and Stream Access

In the course of bridge maintenance and improvements, a number of DOTs are exploring ways to improve the environment for people and animals. While the Design stage is key to building in improvements such as bridge extensions, for wider floodplain and stream channel protection, as Florida DOT considers as a matter of policy, much can be done to enhance the environment in the course of maintenance.


7.3.1 Fishing Access
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DOT maintenance staff usually know where the public is trying to access fishing sites and where such access is presenting a safety problem on the roadside and/or improvements could easily be made that would benefit the traveling public, with minimal investment.

NYSDOT Niagara County maintenance staff took the lead in forming a partnership with local business, the NYS Office of Parks Recreation and Historic Preservation and the New York State Department of Environment Control, to provide a public fishing access site and picnic area at a popular salmon and trout stream - Keg Creek. Prior to the improvement, anglers parked along the state highway and made their way down a very steep, slippery and dangerous ravine to fish for Lake Ontario's world famous migrating trout and salmon. NYSDOT maintenance crews designed and constructed a paved parking area, a series of wooden stairs and a picnic area with lumber donated by a local company and tables donated by the State Parks Department. [N]


7.3.2 Bird and Bat Roosts in Bridges
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Nests in and on bridges are often regulated under the Migratory Bird Treaty Act. Migratory bird concerns often require DOTs to time projects for "off- season" work so as not to disturb active nests. In some cases, large unoccupied nests may be relocated during the off-season. Maintenance forces have used deterrents for nesting swallows on bridges such as netting or discouraging coatings. [N]

Virginia Department of Transportation Program Key in Comeback of Peregrine Falcons

Peregrine falcons were classified nationwide as an endangered species from 1970-1999. In the 1980s, the falcons began showing a preference for VDOT's coastal bridges. At that time, VDOT began working with the Virginia Department of Game and Inland Fisheries and the College of William and Mary to aid bridge-nesting efforts. VDOT installed nesting boxes on ten bridges and video cable on one. From 1978 through 1985, the Department of Game and Inland Fisheries led a recovery effort that released 115 young birds on the Eastern Shore. Rather than moving to western Virginia as anticipated, VDOT bridges became one of the most popular nesting sites. Native to Virginia's Allegheny and Blue Ridge mountains, peregrine falcons were nearly wiped out by unintended side effects of pesticide use by the early 1960s. All known breeding pairs east of the Mississippi had disappeared and their numbers were drastically reduced worldwide by the mid-90s.

The Department reports falcon activity to wildlife experts, and limits maintenance work to avoid disturbing nesting pairs or their young. In 1998 VDOT's Environmental Division earned a Federal Highway Administration Award for Excellence in Highway Design for Environmental Protection and Enhancement for the effort. VDOT employees have been recognized by the Board of Directors of Virginia Game and Inland Fisheries.

VDOT has continued its habitat enhancement and sensitive maintenance practices even after the species was delisted. In the spring of 2001, VDOT and eight other public and private agencies began FalconTrak, a three-year program to protect eggs and hatchlings, track young falcons via satellite, and monitor nests with video cameras for researchers and the public to view on VDOT's web site, Pairs of peregrine falcons are currently nesting on eight VDOT bridges and offspring are thriving. [N]

Protecting and Increasing Bat Roost Habitat in Bridges

Bats are primary predators of vast numbers of insect pests that are extremely costly to farmers and foresters. One bat can easily eat 20 female corn earworm moths in a night, and each moth can lay as many as 500 eggs, potentially producing 10,000 crop-damaging caterpillars. Yet as few as eight caterpillars per 100 plants can force a farmer to apply pesticides. [N]

A number of bat species nationwide are listed as threatened or endangered under the federal Endangered Species Act. Bats are especially susceptible to extinction because most species form large colonies in vulnerable locations, such as caves that are sometimes inadvertently sealed. In addition, bats usually produce only one pup per year. As a consequence of losing natural roosts in caves and old growth forest snags, bridges and culverts have become havens of last resort. Bridges from Canada to Florida are being used by at least 24 of the 46 North American bat species; it is estimated that within the southern United States alone, 3,600 highway structures are being used by approximately 33 million bats. [N]State DOTs can contribute to bat recovery at little or no cost, through proactive measures. Most bat species that will roost in bridges choose concrete crevices that are sealed at the top, at least six to 12 inches deep, .5 to 1.25 inches wide, and ten feet or more above the ground, typically not located over busy roadways. Day roosts are places that protect bats from predators and buffer weather conditions while resting or rearing their young. Such roosts are usually in expansion joints or other crevices. In contrast, night roosts, where bats gather to digest their food between nightly feeding bouts, are often found in open areas between bridge support beams that are protected from the wind. Retrofitting existing bridges and culverts proved highly successful in attracting bats, especially where bats were already using them at night.

Figure 8 : TxDOT Bats in Bridge Retrofit Partnership
Figure 8: TxDOT Bats in Bridge Retrofit Partnership

Citizens have gotten involved as well. When 33,000 Mexican free-tailed bats became a nuisance in the school attic at Canadian Middle School in Canadian, Texas, teachers and students purchased materials, constructed, and installed alternate roosts for up to 50,000 bats in a nearby highway bridge by collaborating with Bat Conservation International (BCI), the TxDOT and local businesses.

Bats have the largest surface area to body mass of any mammal, and this requires greater energy to maintain body temperatures. Sun-warmed bridges help adult bats to conserve energy and foster development of their young. During the summer months, sun-exposed bridges act as thermal sinks, often achieving and holding temperatures above the ambient average for most of the 24-hour cycle.

Bat Conservation International cooperated with 20 state DOTs in a national study of bats in bridges and found 217 highway structures used as day roosts and 714 highway structures used as night roosts. Information from this study is summarized in this section and those following, pertaining to construction and retrofit recommendations. [N]

The study found that the higher, more consistent bridge temperatures are especially important in mountainous or desert regions where ambient temperatures fluctuate dramatically within a 24-hour cycle. An Oregon study found that bats prefer bridges with greatest sun exposures. Bridges receiving no sun had little or no bat use. This preference was especially obvious within partially shaded bridges, where roosting activities occurred only in the sun-exposed halves of bridges [N] The northernmost day roost discovered in this study was occupied by a maternity colony of roughly 300 little brown myotis in an Idaho bridge at 44 north latitude. However, the number of day roosts appears to drop rapidly above 42 north latitude. [N]

Bats use parallel box beam bridges as day roosts more than any other kind. The next most preferred bridges are cast in place or made of prestressed concrete girder spans. These designs are the most likely to contain spaces suitable for bats. Although parallel box beam bridges were rarely encountered during the survey, they can provide numerous crevices of suitable width. Metal and small concrete culverts are the most frequently encountered highway structures and are the least preferred as roosts. Even ideal structures were rarely used by bats in areas dominated by open plains, perhaps due to a lack of appropriate habitat. Creation of day-roost habitat for bats in new or existing highway structures is easy, often at little or no extra cost to the taxpayer. For new structures, the minimum needs for day-roosting bats can be met by specifying the proper dimensions for crevices such as expansion joints.

Night use of highway structures is even more common; 29 percent of all structures surveyed had signs of night-roost activity. In some regions of the southwest, all suitable structures were used by night-roosting bats. Night-roosting bats are believed to be attracted to bridges that provide protected roosts and have a large thermal mass that remains warm at night. Bridges constructed of prestressed concrete girder spans, cast-in-place spans, or steel I-beams are preferred. Vertical concrete surfaces located between beams provide ideal protection from wind and are especially used when they are heated by full sun exposure. Bats typically do not use bridges with flat bottomed surfaces that lack inter-beam spaces. They will avoid small culverts but will roost at night in the long concrete box culverts that often pass under divided highways, if the culverts are at least 5 feet (1.5 m) tall. Night roosts appear to play important roles in body temperature regulation and social behavior.

Figure 9 : Night Roosts Located in Open Spaces between Bridge Beams. Credit: Bat Conservation International
Figure 9: Night Roosts Located in Open Spaces between Bridge Beams

TxDOT Study and Bridge Modifications to Support Bat Usage

TxDOT and Bridge Engineer, Mark Bloschock, have received an award of excellence from Bats Conservation International, the first given to a person outside the field of wildlife conservation. Bloschock began a study with BCI in 1994 after a large colony of Mexican Free-Tailed bats settled under the Congress Avenue bridge in Austin and each wanted to determine why the bats settled there, whether bats might damage the bridge, and if there might be potential effects on human health. The study determined that slot-shaped crevices under the bridge were similar in size to spaces found in bat caves and uncovered bat roosting preferences in both bridges and culverts throughout the state. The study indicates that minor modifications to highway structures can maximize or minimize the potential for use by bats, and that less than 0.01 percent of Texas highway structures currently meet the day-roosting requirements for bats. In central, southern, and western Texas, there is a 62 percent chance that structures with suitable characteristics will be used by bats. The Mexican free-tailed bat (Tadarida brasiliensis) was found to be the most frequent bat species day roosting in highway structures. Bridge characteristics preferred by day-roosting bats were defined by a paired comparison study where bat-occupied and unoccupied bridge characteristics were statistically compared.

Today, 1.5 million Mexican Free-Tailed bats migrate from Mexico every year and stay from March to October under the popular Austin bridge. It's estimated the bats eat ten to 15 tons of insects on their nightly flights. The sight of the bats taking flight at dusk from beneath the Congress Avenue bridge had another unexpected benefit as thousands of tourists visit the bridge to see the nightly flights. The bridge is listed as a top tourist attraction on the City of Austin web page. Each year, the bridge attracts tens of thousands of tourists from all over the world, and has been estimated to generate more than $8 million for the local economy. [N]

Based on the success of the bat habitat in Austin, Bloschock established the Bats and Bridges program, which has spread to 24 states and 17 countries. Overall, TxDOT has 218 structures currently used as roosts, almost three times as many habitats for bats as any other state taking part in the program. [N]

Practices to Incorporate in Design, Construction, and Retrofitting of Bridges for Bats

TxDOT performed a statewide evaluation used to identify the distribution of highway structures used by bats. Day-roosting bats prefer concrete bridges and culverts with secluded locations such as crevices that are 0.5 to 1.25 inches-wide(1.2 to 3.2 cm), especially those that are 12 inches deep (30 cm), have covered tops, and are located in central, southern, and western Texas. Additional experiments further supported the results of the paired comparison and statewide evaluations. Bat colonies, even large ones, do not damage highway structures and water sources under roosts are not negatively impacted. Human health risks are minimized by educating people not to handle bats. [N] In sum, the study found that:

  1. Highway structures that incorporate crevices between 0.5 and 1.25 inches wide (1.2 and 3.2 cm) can provide ideal roosting habitat for several of the most rapidly declining and valuable bat species in Texas, especially if these crevices are 12 inches (30 cm) or more in depth and covered at the top.
  2. Bats typically use only concrete or wooden roosting surfaces, preferring the highest, darkest locations.
  3. Structures can be retrofitted with Bat-Abodes or concrete panels to create bat habitat.
  4. Large concrete culverts under divided highways can provide excellent roosts for threatened and endangered bats, though most would require provision of roughened ceiling cavities during construction.
  5. No structural damage, aquatic pollution, or disease transmission to humans has been associated with even the largest bat colonies living in Texas bridges and culverts, but warnings not to handle downed individuals or inhale dust associated with bird or bats droppings are recommended.
  6. Where bats are unwanted, simple elimination of preferred crevice widths can prevent potential nuisance problems.

Incorporating characteristics into new structures specifically for bats can be relatively inexpensive and easy to do. TxDOT has developed a bat-friendly domed culvert, for which customization costs are minimal; modifications can even be implemented during construction. [N]

As part of their national study, Bat Conservation International determined in consultation with state DOTs that bat-friendly habitat can be provided in either new or existing bridges or culverts, at little or no extra cost to taxpayers. During construction planning, there are no costs for an engineer to specify the appropriate crevice widths of 3/4 to 1-inches (1.9 to 2.5 cm) for expansion joints or other crevices. Existing structures can be retrofitted with bat-friendly habitats using the designs described in the following sections. Signs of bat use in nearby bridges and culverts increase the chances of success for habitat enhancement projects.

Ideal day roost characteristics for crevice-dwelling bat species that use highway structures, include (in descending priority): [N]

  • Location in relatively warm areas, primarily in southern half of the country
  • Construction material: concrete
  • Vertical crevices: 0.5 to 1.25 inches (0.25 to 3 cm) wide
  • Vertical crevices 12 inches (30 cm) or greater in depth
  • Roost height: ten feet (three meters) or more above the ground
  • Rainwater-sealed at the top
  • Full sun exposure of the structure
  • Not situated over busy roadways


  • Location in relatively warm areas
  • Concrete box culverts
  • Between five and ten feet (1.5 and 3 meters) tall and 300 feet (100 meters) or more long
  • Openings protected from high winds
  • Not susceptible to flooding
  • Inner areas relatively dark with roughened walls or ceilings
  • Crevices, imperfections, or swallow nests

The Texas Bat-Abode, Big-eared Bat-Abode, and the Oregon Bridge Wedge bat roosts are designed for day-roosting bats in bridges and culverts. In the protected environment of a bridge or culvert, a properly constructed and installed bat habitat made of quality materials should last as long as the highway structure. BCI would appreciate photographs of the installation and especially of bats using any bat-friendly modifications. For more information on adapting the designs to specific bridges, or to report occupied units, contact Bat Conservation International (BCI), Inc., at 512/327-9721. BCI maintains a list of bats, documented bridge/culvert use, potential use, roost type (crevices or open beams), preference, nationwide distribution, and status at:

Texas Bat-Abode
The Texas Bat-Abode is designed to retrofit bridges with bat habitat for crevice-dwelling species. It has an external panel on either side and 1x2-inch (2.5 to 5.1 cm) wooden spacers sandwiched between 0.5 to 0.75 inch (1.2 to 1.9 cm) plywood partitions (Figure 29). Recycled highway signs are ideal construction materials. Note that only the external panels need to be cut to fit the bridges' inter-beam spaces. The internal partitions should provide crevices 0.75 inch (1.9 cm) wide and at least 12 inches (31 cm) deep.

Smooth roost surfaces need to be textured to provide footholds for bats on at least one side of each plywood partition (preferably both), creating irregularities at least every 1/8 inch (0.3 cm). Many methods have been tested to create footholds, such as

  • Using rough-sided paneling
  • Coating the panel with a thick layer of exterior polyurethane or epoxy paint sprinkled with rough grit
  • Attaching plastic mesh with silicone caulk or rust-resistant staples
  • Mechanically scarifying the wood with a sharp object such as a utility knife
  • Lightly grooving the wood with a saw (do not penetrate to the first plywood glue layer)
  • Lightly sandblasting the wood with rough-grit

The use of rough-sided paneling or polyurethane sprinkled with grit have provided the longest lasting results. Rust resistant wood screws should be used to assemble the spacers and partitions.

Figure 10 : Texas Bat-Abode for crevice-dwelling species.
Figure 10: Texas Bat-Abode for crevice-dwelling species
Credit: Bat Conservation International

The Texas Bat-Abode should be installed in bridges that are at least ten feet above ground, free of vegetation, and not susceptible to flooding or easy vandalism. Measurements of the exact location where the Bat-Abode is to be placed will ensure a proper fit. The number of partitions is arbitrary and limited only by availability of materials and support for the weight of the Abodes. Because of the weight, it may be easiest to assemble the cut pieces in the bridge. In wooden bridges, the unit should be anchored to the structure with heavy-duty rust-resistant lag-bolts.

Big-eared Bat-Abode
Big-eared bats are frequent bridge users in both the eastern and western United States. They prefer open roost areas such as cave entry rooms, large hollow trees, darkened undisturbed rooms in abandoned houses, or between the darkened beams of quiet bridges over streams. The Big-eared Bat-Abode creates these conditions.

The Big-eared Bat-Abode has two external panels with 1x2-inch spacers that are used as braces to hold the panels together with a plastic mesh lining to provide footholds for bats. The netting should be attached using rust-resistant staples (Figure 30). The other methods of creating footholds mentioned above would also be effective.

Figure 11 : Big-eared Bat-Abode. Credit: Bat Conservation International.
Figure 11: Big-eared Bat-Abode

Modified Bat-Abode

Several designs of the Texas Bat-Abode, such as this one modified for a steel I-beam bridge, have been used to attract thousands of bats. Complements of TxDOT and BCI.

It may be easier to partially assemble the structure on the ground leaving one end panel off until it is placed in its chosen location. Units installed in wooden bridges can be anchored using heavy-duty rust-resistance lag bolts. Because big-eared bats are very sensitive to disturbance, units should be placed in areas of low activity and painted a color that does not attract attention.

Big-eared bats are often found in low bridges darkened by thick vegetation growing along the sides. The Big-eared Bat-Abode should be placed at least six to ten feet (two to three meters) above the ground in a secluded portion of the bridge. However, access to the fly-way entrance should not be blocked. Other bat species are likely to use this structure.

The Oregon Wedge
The Oregon Wedge (Figure 12) is an inexpensive method of retrofitting bridges or culverts with day-roost habitat for bats. The Wedge is made from an 0.5 to 0.75 inch (1.2 to 2 cm) exterior grade plywood panel that is at least 18 inches high and 24 inches wide (46x61 cm) with three 1 x 2 inch (2.5x5 cm) wood strips attached along the top and sides, leaving an opening along the bottom. If larger panel sizes are used, vertical wooden pieces should be placed every 24 inches (61 cm) to support the plywood and prevent warping. The pieces should not run from the top to the bottom so that bats can move about within the panel.

Figure 12 : Oregon Wedge
Figure 12: Oregon Wedge
Designs courtesy of David Clayton and Dr. Steve Cross and Bat Conservation Int'l

The Wedge can be attached to a vertical concrete portion of a bridge or culvert using concrete anchor-bolts or a fast-drying environmentally safe epoxy cement (such as 3M Scotch coat 3-12). The transportation department should install the panels if anchor bolts are used. If the panel is to be attached to wood, then use appropriate rust resistant wood screws. Before applying the epoxy, check the preferred installation site to make sure the support strips fit flat against the concrete surface.

Wedge placement is possible on any adequately sized, flat concrete or wood surface. However, we recommend that the panels be placed near the sun-warmed road slab (preferably as high as possible between heat-trapping bridge beams). They should be at least ten feet (three meters) above ground, with a clear flyway (at least ten feet), and be out of view or reach of vandals. The Wedge can be installed in the middle sections of culverts higher than five feet (1.5 cm). A Wedge should not be placed in structures that flood. As a precaution against flooding, a 1.5 inch (3.8 cm) gap can be left at each corner where the support strips join to act as an escape route in the event of fast-rising water.

Bat-domed Culverts

Figure 13 : Bat-domed culvert. Graphics courtesy of TxDOT and BCI.
Figure 13: Bat-domed culvert

The Bat-domed culvert is a modified concrete box culvert designed to accommodate large colonies of bats. The dome has several bat-friendly characteristics:

  • The height is increased
  • Warm air is trapped
  • Light intensity is reduced
  • Air movement is reduced

Bat-domed culverts should be at least 5 feet (1.5 m) in height with an additional 1 to 2-foot (0.6 meter) raised portion centered in the culvert. The raised area can be any length from 2 to 50 feet, depending on the colony size preferred. The walls and ceilings of the raised area should be roughened to provide footholds for bats. The following method was used to produce suitable wall and ceiling textures. Using a crowbar, thin strips were removed from the surface of recycled plywood. The resulting roughened wood was then used as the form for pouring the concrete, which produced the desired textured surface within the domed area of the culvert. In addition, a method of attaching panels or partitions, such as female threaded inserts, can be incorporated into the raised walls and ceiling to create more surface area once the culvert is completed.

Bat-domed culverts should not be placed in areas susceptible to flooding. However, in the event of rising water, the dome may serve as a temporary air-trap. Almost any cave-dwelling species may use these, including several that are endangered.

Success in Retrofitting Bridges to Accommodate Bats

Retrofitting habitat into existing highway structures has become a popular and successful method of accommodating bats. Pre-surveys to look for bat signs in nearby bridges are useful to predict the success of proposed enhancement projects. Four bridges in Oregon and five bridges and two culverts in Texas with signs of night roosting were retrofitted with ideal crevices, and all were occupied by bats within the first year. All retrofit designs tested in bridges and culverts so far have successfully attracted bats, and at least six states are already using retrofitting projects to accommodate bats. Retrofits are adaptable to almost any structure and can be placed where they will have a high potential for success and that will minimize disturbance from maintenance or vandalism. Retrofits are generally inexpensive and can be sized to accommodate small or large colonies, with potential expansion by adding more units if initial efforts are successful. Retrofits are usually highly beneficial to local agricultural and can be easily moved if necessary.

Figure 14 : TxDOT's Concrete Version of the Oregon Wedge
Figure 14: TxDOT's Concrete Version of the Oregon Wedge

Two basic designs can be used to retrofit almost any bridge or culvert. Texas Bat-Abodes can accommodate thousands of bats each, and have been modified to fit three different bridge designs. Four of the five tested were fully occupied, one within the first month.

The Oregon Wedge can house several hundred bats and has been accepted for day roosting by 12 species, including a maternity colony of Yuma Myotis (Myotis yumanensis). This design has been successful in both bridges and culverts in Oregon, Arizona, and Texas. The Texas Department of Transportation developed a concrete version that attracted bats within a year.

Preserving Portions of Old Bridges as Habitat

When old bridges must be replaced, some of those occupied by bats have been retained as wildlife sanctuaries. The Santa Barbara Public Works Department and Caltrans are collaborating to preserve a colony of 10,000 Mexican free-tailed bats and 200 pallid bats (Antrozous pallidus) by retaining a portion of an old bridge that is surrounded by agricultural fields (Storrer, 1994). It is calculated that these bats consume roughly 10,000 pounds (4,540 kg) of insects each summer, many of which are pests. Bridge habitat enhancement techniques are being developed in other countries. In Australia, the roost portion of an old wooden bridge was retained and incorporated into the underbelly of a new replacement bridge.

Further DOT Efforts to Identify Characteristics and Design Features of Roost Bridges and Conservation Efforts

FDOT is in the process of undertaking a survey of bridges with bats to help FDOT predict and control where bats will roost. At least five species of bats in Florida use concrete highway bridges as roosting sites. Because many natural roosts such as caves and large hollow trees are rare, bridges serve as the most common or primary roosting sites for bats in some areas. The objectives of this project are to: 1) identify FDOT maintained bridges in Florida that are occupied by roosting bats; 2) summarize characteristics and design features of roost bridges, and correlate bridge features with presence, number, and species of roosting bats; 3) prepare guidelines for FDOT employees to record the presence of bats during routine activities; and 4) identify all bridges that support bat roosts and are planned for replacement by 2020, and to identify ways to conserve the roosts when these bridges are replaced. [N]

Likewise Georgia DOT will 1) identify the highway bridges and select culverts in Georgia that are occupied by roosting bats; 2) evaluate the characteristics and features of bridges being used as roosts, including an assessment of surrounding habitat features; 3) recommend bridge design elements that provide the roost features preferred by bats; and 4) prepare standard procedures for assessing the presence of bats, minimizing disturbance to the bats, and preserving the existing or potential roosting opportunities during management, repair, and demolition of highway bridges by the Georgia Department of Transportation (GDOT). Those research results will be available in mid-2005. [N]


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Table of Contents
Chapter 7
Bridge Maintenance
7.1 Preventative Bridge Maintenance Practices
7.2 Avoiding and Minimizing Impacts to Fish and Wildlife
7.3 Enhancements to Bridges and Stream Access
7.4 Bridge Painting/Coating/Sealing and Containment Stewardship Practices
Lists: Examples | Tables | Figures
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