Miles and miles of streams flow through northeastern forests, serving as habitat for fish, freshwater mussels, invertebrates, and other aquatic organisms. These waterways feed our rivers, bringing nutrients from the upper watersheds to sustain natural systems downriver, including estuaries and ocean bays. Just like our larger rivers, streams have sustained our communities for millennia, supplying food, power, and recreational opportunities. For more than a century, road and energy infrastructure has fragmented naturally occurring habitats in many stream systems.
Where roads and railroads cross woodland streams, their builders often installed culverts to allow continued water flow. Historically, these structures were not built with knowledge of or care for ecosystem processes, such as the passage of fish and other organisms. And they typically were not built in anticipation of the full range of variable weather conditions and how the culvert size and shape might influence, or be influenced by, the flow of floodwaters.
The increased unpredictability and intensity of storms due to climate change compounds the impacts of improperly sized and constructed culverts. During Tropical Storm Irene, for example, culvert failure contributed to catastrophic damage in many communities across the Northeast, and subsequent heavy rainfalls have also significantly damaged community infrastructure.
In the illustrated example below, a bridge designed to increase flood resiliency and to mimic a natural system for fish and other wildlife replaced a poorly designed culvert.
Poorly Designed Stream Crossings
Create barriers for the movement of fish, invertebrates, and other aquatic organisms
- Stream flow is constricted, increasing water velocities and turbulent conditions, which make it difficult or impossible for organisms to move upstream. The streambed downstream of the culvert is eroded, and dislocated sediment can create additional downstream barriers, further obstructing passage.
- A perched culvert is one where downstream scour has eroded the streambed to create a significant drop or where an installed pipe is set too high, creating a barrier to upstream movement as well as the potential for fish to be swept back out of the culvert due to higher velocities.
- Improperly designed or installed culverts can also create low-flow conditions where water depth is inadequate for fish to swim through.
- Culverts will often eliminate natural resting pools inside the structure, further restricting upstream fish migration.
Alter hydrology of the stream system to such an extent that they exacerbate impacts of high water flow during and after rain events and storms
- Undersized culverts frequently cause a ponding effect and streambank erosion upstream, further reducing its ability to handle high flows; if the culvert and road wash out during a flooding event, this can send a wall of water downstream leading to catastrophic damage.
- They can also increase water velocity and create the “firehose effect,” contributing to streambed erosion and bank destabilization downstream.
- Undersized crossings can reduce hydraulic capacity (how much water can safely go through the crossing) over time mainly due to sediment deposition at the culvert inlet.
- As a stream crossing becomes clogged over time with sediment, wood, and other debris (exacerbated during large storm events), the upstream channel widens, further contributing to streambank erosion, which in turn further changes how the system responds to heavy rain events.
Properly Designed Stream Crossings
Facilitate passage of aquatic organisms
- The width of the culvert or bridge abutment exceeds the width of the natural stream channel.
- Stream and streambed conditions look similar both upstream and downstream of the crossing.
- Slope of the streambed through a properly sized structure matches the natural slope of the larger stream reach.
- Low velocity occurs under normal conditions; a restored streambed provides enough diversity and instream features to provide sufficient seasonal water depths.
- Naturally occurring substrate – sediments, cobble, boulders, and so on – can move along with the natural flow of the stream to form small pools or eddies and riffles.
- Channel banks are often necessary for the movement of wildlife such as turtles and mammals.
Make roads and communities safer
- Channel dimensions and characteristics are similar to the natural stream channel design.
- A properly designed and installed stream crossing allows for minimal interference of stream and floodplain processes.
- Stream crossing size allows for passage of floodwaters, sediment, wood, and debris in a wide range of conditions.
- Design of the stream crossing takes into account changing weather patterns and the unpredictability and increased frequency of high-intensity rain and storm events.
This infographic and the accompanying article, Floodplain Forests: Nature's Flood Relief, were made possible by a grant from the Davis Conservation Foundation.