Embedded Files
Red-legged Fron Tunnel, Sea-to-Sky Highway, British Columbia. Source: Toronto Globe and Mail, October 6, 2010
case studies
case studies
To better understand the potential of existing culverts to serve as wildlife connectors, and how such connectors can be engineered, we provide an overview of relevant examples designed to facilitate animal mobility and ecosystems connectivity.
Washington State's King County Fish Passage Restoration Program
The program inspected 3,000 locations to identify where salmon habitats are potentially blocked by roads and trails. They ranked more than 700 of the barriers identified and determined that completing 50 restoration projects would restore access to at least half of the habitat that is currently blocked by infrastructure that blocks salmon from swimming upstream. The projects reopen connectivity between salmon and amphibian habitat patches by removing barriers and creating water-flowing underpasses with box culverts.
Sea-to-Sky Highway Frog Tunnels, British Columbia
When British Columbia hosted the 2010 Winter Olympics, $1 billion was invested in the reconstruction of the Sea-to-Sky Highway to Whistler Mountain. The highway passed through wetland habitat of at-risk Red-Legged Frogs. Underpasses were built for the frogs to allow them to move between their habitat on either side of the roadway. According to a 2012 study by Josh Malt (Ministry of Forests, Lands, and Natural Resource Operations South Coast Region), mortality remained high, however; later addition of fencing to guide frogs into the tunnels reduced mortality 50%.
Design feature:
Multiple underpass tunnels below major highway through wetlands
Monkton Wildlife Crossing, Monkton, Vermont
The Monkton Wildlife Crossing aims to help endangered salamanders, lizards and amphibians to migrate. It consists of an underpasses network under specific highways that are barriers to connectivity between habitat patches. These crossing tunnels have allowed thousands of salamanders and frogs to safely make their annual journey to and from their breeding pools.
Design features:
Large underpass tunnels
Fences to guide species through the tunnel and prevent them from going on the road
San Diego Zoo Alliance, in San Diego, California
The Alliance encouraged a plan for four wildlife culverts or underpasses to prevent animals from crossing roads. Results included a significant decline in animal-car collisions, as animals used the underpass. Moreover, although these structures were designed for the endangered San Bernardino kangaroo rat, they were used frequently by other animals, including racoons and coyotes.
Design features:
Underpasses with guiding fences
Grates to prevent big animals to go through the tunnel
Amphibian Tunnel, Waterton National Park, Alberta, Canada
Amphibian Tunnel, Waterton National Park, Alberta, Canada
In Waterton National Park, crossing have been designed for amphibians, and other small species. Amphibian walls or drift fences are required to guide amphibians and reptiles to the location of the crossing structure.
Design features:
Walls or fences are needed as reptiles and amphibians cannot orient their movement to tunnel entrances without these structures.
Large tunnels provide greater airflow and natural light conditions, but smaller tunnels with grated slots can provide ambient light and moisture.
Amphibian and Reptile Tunnels in New England
In a 2003 study of salamander and reptile tunnels in Massachusetts, Scott Jackson (Department of Natural Resources Conservation, University of Massachusetts, Amherst) established typologies for amphibians and reptiles to facilitate underpass implementation.
Design features:
A 2’x 2’ tunnel with very small openness ratio (cross-sectional area divided by length).
Small low-cost. allowing installation of multiple structures creating roadway perforations rather than one large passage.
Open at the top with an iron grate to allow ample rain, light and air circulation into the culvert
Sandy soil as a natural substrate.
Likely made of concrete
Wing walls should angle out from each end of the tunnel at approximately 45 degrees to guide amphibians into culvert
Vertical retaining walls at least 18” high should angle out away from wing walls at a broad angle for 100-200 feet, flush with the ground surface on the side closest to the road
Tunnels placed no more than 200 feet apart
Design strategies
Design strategies
The goal is to connect the pond networks and wetlands to have better habitat connectivity. However, human infrastructures present barriers, necessitating infrastructure that assures safe passage for amphibians.
Culverts exist under Highway 101, presenting an opportunity for habitat connectivity, especially between our target area, Irwin Creek and San Rafael Creek. The goal is to connect the wetlands and other wild areas to other habitats, offering a migration route without risks associated with crossing a roadway. Culverts redesigned for amphibians could perforate Highway 101, while retaining the water management function of the culverts. The redesign involves adding features such as fences, grates, and new soil textures.
Strategies
Revegetation along the highway and along the creeks to be connected
Fences to protect roadside habitat and guide species into the culvert
New sandy soil in the culvert bed
Sloping wing walls
Replacement of some culverts with 2’x2’ tunnels
Grates and other features to enable light, ventilation and humidity in the tunnel
Plan for new corridors opportunies with existing water infrastructures
Opening of the culvert (plan and cut-section)
The margins and bed of existing culverts would have new substrate, revegetation, and openings for light and humidity.
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