Maintain habitat connectivity: Retrofit or replace culverts

Priority crossings for possible culvert replacement or retrofit are represented by green dots and triangles. If these are not visible, use the plus sign to zoom in. You can layer in locations for the top 5%, 10% and 15% of crossings with the highest restoration potential as estimated by landscape modeling; darker green represents more potential for restoring aquatic connectivity. Green dots are crossings that have been assessed in the field and the potential for restoring aquatic passability is reliable. Green triangles are crossings that have not yet been assessed and the restoration potential is hypothetical. Data are from the Critical Linkages Phase I (2016). Top 5% and 10% Coldwater Stream Crossings (blue dots and triangles) are from a specialized run of Critical Linkages for coldwater streams (summer mean temperature < 16C) conducted in 2017.

Priority crossings for possible culvert replacement or retrofit are represented by green dots and triangles. If these are not visible, use the plus sign to zoom in. You can layer in locations for...

Maintain habitat connectivity: Retrofit or replace culverts

Adaptation type:

Removal of dams and other barriers to aquatic connectivity

Roadway infrastructure, crossings, and dams

Strategy:

Restore and maintain terrestrial and aquatic connectivity sufficient to maintain healthy ecosystems and wildlife populations

Animal movements (of individuals or their offspring) across the landscape are important for maintaining healthy wildlife populations. Climate change is likely to result in changes to habitat conditions (temperature, rainfall, vegetation) that will require adjustments in the areas occupied by many species. Restoring and maintaining landscape connectivity sufficient to allow wildlife populations to adjust their distribution over time is a critically important strategy for adapting to climate change.

Action

Replace or retrofit deficient culverts at strategic locations
It has been recognized that dams are significant barriers to upstream movement of fish and other aquatic organisms. Road-stream crossings, especially culverts, can also constitute barriers to aquatic organism passage. Although the impacts of dams may be more severe, road-stream crossings are more numerous. Some culverts are severe barriers to upstream movement; others represent little or no barrier at all, and there are many in between.

Tools are now available to assess the passability of road-stream crossings and model their effects on aquatic connectivity. Volunteers and technicians working with state agencies and environmental organizations are using protocols from the North Atlantic Aquatic Connectivity Collaborative (NAACC) to assess the passability of bridges and culverts in the field. The University of Massachusetts Amherst uses Critical Linkages software to model crossings where upgrades would result in the largest benefits in terms of aquatic connectivity.

Field assessments and passability scores along with Critical Linkages analyses can be used to target specific culverts for replacement or retrofit. Culvert replacement involves the complete replacement of a sub-standard crossing with another, usually larger, structure. Retrofits are halfway measures such as the use of rock weirs to back up water just below the structure and eliminate outlet drops. In some cases, retrofits can improve aquatic passability at a road-stream crossing until such time that a full replacement is possible.

Mitchell Brook in Whately, MA, before and after culvert replacement. The PVC piping in the photos are antennae to monitor the passage of fish through the crossing. Researchers report that not only are more brook trout moving from the West Brook up into Mitchell Brook but dace are now being found in the stream for the first time. Photo credit: Scott Jackson.

Culvert replacements are tricky business and replacement structures must be carefully designed, permitted and constructed. Attention must be paid to the potential for downstream flooding (larger structures will pass water more quickly) as well as the potential for stream adjustments such as erosion and head cutting (progressive erosion up the stream channel above the culvert). The stream channel and bed characteristics either need to be retained (bridged) or created within the crossing structure. Appropriate expertise and experience is required to design a stream bed that both simulates characteristics of the natural stream and is able to withstand velocities that are often higher in the structure than in the natural stream during periods of high flow.

Older crossing structures are often single or multiple culvert crossings designed with the sole objective of getting water from one side of the road to the other. Replacement structures that meet the Massachusetts River and Stream Crossing Standards are generally bridges, open bottom arches or large culverts designed according to the principles of Stream Simulation. Stream Simulation design seeks to create crossings that simulate the channel and bottom characteristics and replicate the water depth and velocity conditions found in the natural stream. The U.S. Forest Service has an in-depth manual on Stream Simulation Design that is the best current resource available for culvert replacements.

The Massachusetts Division of Ecological (DER), Stream Continuity Program works closely with municipalities and other project proponents to help them and their culvert replacement projects meet the Massachusetts River and Stream Crossing Standards. DER is a division of the Massachusetts Department of Fish and Game, and is a state leader in culvert replacement and improving stream continuity. The Stream Continuity Program has completed several culvert replacements across Massachusetts to improve river continuity and health. Municipal highway departments and local officials are encouraged to contact Tim Chorey, Stream Continuity Specialist with questions about replacement or retrofit projects. Tim Chorey has extensive experience with culvert replacement, Stream Simulation design, and construction. He is available to provide technical assistance and can be reached at: email: timothy.chorey at state.ma.us or phone: (617) 626-1541.