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Joseph A. Burns CWB
National Threatened and Endgangered Species Program Leader
Watershed, Fish, Wildlife, Air, Rare Plants
1400 Independance Ave SW - MS1121
Washington, D.C. 20250-1121
(202) 205-0919


You are here: HOME » Resources » Retrofitting Existing Structures for Wildlife Passage: Assessment Tools

Retrofitting Existing Structures for Wildlife Passage: Assessment Tools

Retrofitting existing structures to enable animals to use them for passage across highways is possibly the most cost-efficient method of increasing the number and type of passage opportunities across the country. Literally millions of culverts and hundreds of thousands of bridges are present on our roads. Some of these already function to allow wildlife or aquatic organisms to use them, but others might be useful with minor tweaks.

Several mechanisms such as fish ladders have been devised to retrofit existing culverts so that they provide effective fish passage, but relatively few retrofitting systems have been suggested or tested for terrestrial passage. An example of a retrofit for terrestrial animals is the construction of a dry concrete shelf  inside a culvert that is usually partially inundated.

This section on retrofitting existing structures results from a grant from the Coordinated Technology Improvement Program. It was enhanced by cooperation with several other entities, including Portland State University (Portland, OR), Washington Department of Transportation and consultants Julia Kintsch and Dr. Patricia Cramer.

Along with our partners, we are actively improving and modifying the tools for this section, based on several field units that have used the Passage Assessment System (PAS). Currently, tools upcoming by the end of FY15 include a peer-reviewed journal article on Wildlife Crossing Guilds (in review; and a presentation at ICOET15), a smart phone app for the PAS, and updates to the system based on user critiques.

This section contains assessment tools to help you inventory existing structures and decide if they are appropriate for retrofitting. A more complete process can be found in the PAS report from the Washington State DOT (pdf, 8.9 MB).

Determining if Retrofitting is Appropriate

In partnership with Washington DOT, and Portland State University, the Forest Service developed a standardized system for evaluating existing structures for their ability to pass terrestrial wildlife. Foundational to this system is the classification of wildlife species based on their responses to roads and crossing structures – behavior that is largely influenced by predator detection and avoidance strategies, as well as their capacity for locomotion.

Practitioners are guided through an evaluation process to assess the characteristics of a given passage relative to the needs of the species movement guilds of interest to determine if the structure can be retrofit to accommodate those needs. The evaluation process can be accomplished with either a GPS-data recorder or a paper form. The GPS data recorders obtained for the field trials of the system use software readily available to the public, so the format can be used by anyone with a programmable GPS unit.


Process Outline

The process to determine if retrofitting is appropriate is outlined below. Three outcomes are possible when assessing retrofitting potential: the structure has a long enough remaining design life that it is cost-effective to retrofit but not replace it; the structure is unsafe, too expensive, or otherwise unsuitable for retrofitting; or the structure can not be made suitable for wildlife passage and must be replaced to ensure functionality.

The process is briefly as follows:

  1. Determine Species at Risk of population-level impacts in your area of concern.
  2. Assign any species at risk in your area of concern to Species Movement Guilds, which determines the types of structures they will accept.
  3. Inventory structures with a Field Inventory in the project area for retrofitting attributes.
  4. Identify structures suitable or unsuitable for retrofitting using a Decision Tree. The Decision Tree also suggests retrofitting options available.


Species at Risk

The potential for retrofitting is species-specific. Consider two very different species such as deer and salamanders. A crossing structure suitable for salamanders, such as a small, moist culvert, will never be suitable for a deer regardless of retrofitting. Therefore, each structure must be investigated based on its potential for the species of management concern.

In order to determine what species are of management concern, the Table here indicates the attributes of species that tend to put them at risk of population-level impacts.

Species Movement Guilds

Each species approaches highways with a different set of behavioral attributes, generally strongly determined by the type of anti-predator adaptations it possesses. For example, many small animals readily accept small, enclosed culverts as passageways because the darkness and cover simulates conditins they seek in other situations to avoid detection by predators. Contrarily, pronghorn seek open vistas and plenty of manuevering room because of their adaptation to seeing their predators from afar and running away; thus any hint of enclosure is percieved as dangerous to them.

This table characterizes the types of structures that animals tend to prefer. Once the species at risk is identified, and the type of structure that it can accept is determined, then the structures in a highway inventory segment can be investigated.

When interpreting the Species Movement Guilds, it may be helpful to refer to the table introduced on the Glossary page that discusses structure types by how they function as wildlife passages. Animals see structures differently than people do, and this table helps to categorize structures in animal behavioral terms.

Field Inventory

This field inventory process can be used either as a paper spreadsheet, or loaded onto a GPS data recorder.

The GPS data recorder process uses ArcPad software.

Decision Tree

The decision tree below is to be used after data is gathered on structures in an inventory segment. The bulleted items are further discussed with options in links. The Case Histories>Retrofitting section of the Wildlife Crossings Toolkit provides some real life examples of how these concepts have been applied.

  1. Existing Structure Suitable for Wildlife Passage
    1. No Modification Necessary
    2. Supporting Infrastructure or Vicinity Needs Retrofit
      • Add or modify diversion or barrier fencing
      • Add escape structure
      • Modify approach situation (vegetation; visibility; etc.)
      • Non-structural solution (modify human use; manage maintenance; etc
  2. Existing Structure Unsuitable for Wildlife Passage
    1. Not Feasible for Retrofit
      • Structure is too old or unsafe
      • Culvert is too long relative to other dimensions for target species
      • Culvert gradient is too steep
      • Culvert is curved with no through-visibility
      • Location is not suitable relative to cost of retrofit
      • Benefits of retrofit would not be cost-effective before replacement schedule
      • Location is not suitable because of land use or ownership
      • Approach is not capable of being managed for suitable habitat
    2. Feasible for Retrofit
      • Remove obstruction, barrier or physical impedimentBiologist looking at obstruction at opening of medium culvert
      • Add pathway or facilitate movementdesert tortoise pathway through cobble
      • Enhance or encourage approach
      • Reduce intimidation (increase visibility; increase hiding cover; reduce sound)
  3. [TOP]

Conceptual Retrofits

Retrofitting has not recieved as much attention from designers as new construction techniques, so there are relatively few highly innovative designs. This is probably due to funding opportunities. New construction projects carry funding with them, and mitigation measures can be applied in many ways especially if designed into the process. However, retrofits may be standalone projects, where a problem is recognized but no project is planned that can piggyback a solution.

Few conceptual retrofitting ideas were uncovered in the retrofitting project. We'd like to hear from you, if you have a great idea, however. Please send it in and we will post it in this section in the future.


Page Last Modified: April 07, 2017

Additional Information

Rocks in culvert used as a dry shelf to allow some wildlife to stay dry

Image: Randall Reeve. Rocky shelf that terrestrial wildlife can use to travel through inundated culvert.


Dry shelves are a type of retrofit commonly used in Europe to modify inundated culverts so that terrestrial wildlife can better use them.

Retrofits are cheaper than new construction but have fewer funding options.