Each of the Divisions at Hoyle Tanner has a work item in common: stream crossings. Any form of transportation – pedestrian or bike pathway, road, railroad, airport runway or taxiway – that crosses over a waterway includes consideration of stream crossings. Although we often have projects that are specific to addressing a bridge or culvert, other transportation projects and even private development projects include stream crossings when there is a waterway in the project footprint that needs to be crossed. Design and permitting of single or multiple stream crossings can be simple or complex, no matter what the form – a bridge, culvert or pipe.
Every one of the New England states in which we work, as well as Florida, has state-issued stream crossing standards or guidelines. Our staff tracks all of these varying requirements in order to best present the design, restoration or replacement stream crossing plan that is the most effective to meet the specific project goals, schedule and budget.
The following common factors must typically be considered while designing stream crossings:
Sizing
Designing the crossing, where feasible, to allow for free passage of the river through an opening that is similar or wider than the upstream and downstream conditions. A parameter called “bankfull width,” which is the width of a stream channel at the water level when it’s full and about to spill into the floodplain, is used to determine the necessary crossing size. This is usually measured by someone in the field, but in some cases, we can use existing survey or LiDAR data to estimate this value. To address most states’ size requirements during permitting, structures typically need to be larger than the bankfull width to account for the floodplain and variances in flow balanced between size/cost and requirements. This is important ecologically so that the stream is given the opportunity to flow as needed along its path without blockages, but this is also one of the most important and sometimes challenging permitting requirements that must be addressed because the result of this analysis often means building larger structures than what’s currently there.
Connectivity
Also tied to sizing is a crossing’s allowance for Aquatic Organism Passage (AOP) and wildlife crossing. Optimal consideration for connectivity is size and project goal dependent. A bridge carrying a four-lane highway would require different accommodations for AOP and wildlife crossing considerations than a culvert under a driveway. Design options can include a wildlife shelf through a pipe/culvert/bridge if it is large enough. Sideslope design can also affect connectivity. Covering a structure’s sideslopes with soil and seeding to cover stone or riprap creates a more inviting surface for large wildlife and cover for smaller organisms. Restoring AOP can also include smoothing the transition between the streambed and the pipe/culvert to remove “perched” culvert situations. This can happen over time where a steep drop has been created at the outlet where the stream exits the crossing. Perched culverts impede upstream movement for fish, turtles, salamanders/newts, and invertebrates.
Hydraulic analysis
Designing stream crossings involves studying how water flows and making sure the bridge or culvert is the right size and shape to handle it without causing problems. There are multiple parameters that need to be assessed, either in the field or using online data, to determine how the stream currently flows and how the proposed condition should be designed to meet the state’s requirements to the maximum extent practical. Not only bankfull width but also bankfull depth (under varying flow scenarios), width to depth ratio, channel slope and sinuosity (how curvy or winding the stream channel is) all play into the determination of what kind of structure is needed for each specific location. Our permitting team works hand-in-hand with the engineers who are trained in hydraulic analysis software and modeling to provide this type of analysis.
Crossing type
For crossings requiring a culvert, there are many different styles of pipes, which can be round or elliptical to fit the site parameters. A box culvert can be larger than a pipe crossing due to the stronger side stability. For the largest crossings, a bridge that spans the stream and allows for an open bottom with a free-flowing stream and maintains the existing streambed material is ideal; typically in cases like these, where we are able to fully meet a state’s stream crossing requirements, the path to achieving a permit is much smoother. For non-bridge crossings, a simulated streambed through the crossing to match the upstream and downstream conditions is essential to stream health, water quality, wildlife and AOP.
Your Permitting Experts
The factors above can be fully addressed under ideal circumstances, but sometimes site conditions or project budget make full compliance impossible. In these situations, Hoyle Tanner is always willing to work with our clients to create a stream crossing design that achieves a substantial improvement over the existing crossing. We can assist in providing the appropriate justification to the permit reviewers as to why the fully compliant structure isn’t feasible so that the project is able to move forward through permitting while keeping the project within the necessary budget.
In conclusion, the design and implementation of stream crossings are integral components of numerous transportation and development projects, necessitating careful consideration of local regulations, hydraulic analysis, and ecological impact. Hoyle Tanner has worked with clients across New England and Florida in collaboration with state and federal permitting agencies to design successful stream crossings. Achieving the balance of maintaining resilient transportation infrastructure and improving connectivity while meeting budgetary and site constraints is an important part of what we can do for clients. Whether addressing a simple culvert or a complex bridge structure, the goal remains to ensure effective waterway passage while supporting aquatic and wildlife connectivity. At Hoyle Tanner, we are dedicated to providing innovative and compliant solutions tailored to meet the specific needs of each project. If you have any questions regarding stream crossings, our experienced team is ready to help you – reach out to me!
