Stream Continuity Designs for Culvert and Bridge Crossings

Continuity simply means: “Uninterrupted Connection”

Stream continuity means that the stream is very similar in character upstream, downstream, and through the road crossing. This includes features such as slope, streambed material, channel width and shape, streambed features (i.e. pools, steps, riffles), and connectivity to its floodplain.

Crossings can be undersized due to lack of design, changing climactic and watershed conditions, or by using the traditional hydraulic design approach which focuses on designing to accommodate just the right amount of hydraulic conditions and can include provisions for overtopping. The issues highlighted below can be prevented or minimized utilizing the stream continuity design approach.

Backwatering and Gravel Bars: At high flows, undersized pipes and bridges often back up water like a dam, causing material that streams naturally carry (such as gravel) to drop out above the pipe inlet. This “gravel bar” is a constant source of maintenance for road owners as it requires regular cleanout after storms.

Clogging: Undersized pipes and bridges are prone to clogging during large flow events due to debris jams and gravel deposition.

Channel & Road Erosion: Undersized bridges and culverts flowing full can act like a fire hose to erode banks and scour streambed for hundreds of feet downstream. This also increases maintenance and repair work and costs.

Structure Failure: With increased storm frequency and severity, more undersized crossings eventually experience a total washout or failure due to the concerns outlined here.

Aquatic Barriers: Undersized bridges and culverts can fragment habitat by creating barriers that prevent organisms, ranging from fish to frogs and invertebrates, from passing through the structure.

The traditional hydraulic design approach focus on passing water through the road during storm events, and stream continuity approach focuses on passing the stream through the road.

A different approach: Let the stream be a stream, even through the road. Designing the crossing around the stream provides stable stream continuity and hydraulic design can be met through it.

Wider Structures: Structures must be wide enough not only to accommodate water flow, but to maintain channel shape and width while passing debris and maintaining channel stability through the road.

Consistent Slope: Maintaining a natural channel slope is critical, as an increase in slope can lead to erosion, and a decrease in slope can lead to gravel deposition. Establishing slope continuity often requires doing work further upstream and/or downstream to correct legacy stream impacts from the undersized structure.

Grade Control: As the name implies, grade controls are essentially immobile objects that set the elevation of the streambed. In most PA streams, grade control consists of large rock clusters. Reestablishing the spacing, type, and sizing of natural grade controls will help to promote long term channel stability.

Streambed: Maintaining streambed through the structure is key for long term stability. Bottomless structures are encouraged, especially on steeper channels. Structures with inverts (bottoms) should be buried sufficiently to maintain streambed. The streambed mix composition should mimic natural conditions.

Channel Shape: Typical streams have a low-flow channel, bank margins, and a floodplain. This includes maintaining a appropriate bank-full channel width. Stream continuity mimics these features as best as possible through the structure as shown in the picture above.

Aquatic Organism Passage: Aquatic Organism Passage (AOP) means that aquatic can move naturally up and down the stream through the road corridor. Creating a stream with continuous slope and streambed features through the road will ensure AOP is also achieved.

Reduced Maintenance and Storm Damage: the design approach by facilitating all the design features mentioned above results in less erosive and damaging storm flows through the structure and less clogging with debris. This reduces storm damage the amount and cost of regular structure maintenance.

Climate Change Resiliency: By better passing larger storm flows which are increasing with climate change, these structures offer strong climate change resiliency.

The below examples show the construction of a stream continuity culvert replacement in Union County. The structure was installed in July 2024, just before the rains of hurricane Debby came through. The photographs and video show the Debby rain event and how the structure looked immediately after the water levels receded.

Yes, larger structures and additional in-channel work can mean more up-front costs. However, it is important to consider the 75–100- year lifetime costs of structures, especially when severe storms are becoming more frequent. A study by the U.S. Forest Serviceshowed long-term cost savings from these larger, more resilient structures when accounting for reduced erosion, maintenance, and the potential for failure.

Stream Continuity Long-Term Cost Savings Research

• Flood Effects on Road-Stream Crossing Infrastructure: Economic and Ecological Benefits of Stream Simulation Design
• Long-Term Construction and Maintenance Cost Comparison for Road Stream Crossings: Traditional Hydraulic Design Vs. Aquatic organism Passage Design (see Page 23-24 for findings and cost savings) – Prepared for AASHTO Standing Committee on Environment
• An Economic Analysis of Improved Road Stream Crossings – The Nature Conservancy

Designing stream crossings for continuity is catching on in PA. A variety of organizations such as the PA State Conservation Commission (SCC), PA DEP, PA Fish and Boat Commission, PA Game Commission, PA Department of Conservation and Natural Resources, Trout Unlimited, Western PA Conservancy, and others are trying to expand the knowledge and use of stream continuity. 

In July of 2022, the SCC, working in collaboration with the Penn State Center for Dirt and Gravel Road Studies and others, approved a comprehensive set of stream crossing document for use under this program. These documents and policy require the use of a design standard for stream continuity.

The design standard and technical manual for stream continuity are the first PA specific stream continuity standards and design manual specific to Pennsylvania. These are great resources for all potential bridge and culvert owners, not just ones participating in the program.

Stream Continuity Beyond PA: Efforts to design stream crossings for continuity extend well beyond PA and have been implemented for a longer time in other states. The U.S. Forest Service is a national leader in this design approach, and has been actively working on education, research, and implementation nationally. Several states in the Northwest and New England provide standards for stream continuity. The U.S. Forest Service’s “Stream Simulation” is the most comprehensive guidance. See the education and design resources below for more information.

There can be misconceptions that this design approach will bring about additional questions, scrutiny, or require more complex permitting processes. While its true that the design approach has many components of the design which should be included in a Chapter 105 water obstruction and encroachment permit application or registration, many of these design approaches for replacements are likely eligible for authorization using General Permit 11 (GP-11).

The methodology in the design approach seeks to meet the terms and conditions of both GP-11 and the Chapter 105 regulations through ensuring the design restores stream continuity which improves its overall function physically and biologically. The inherent major goal of the approach is to maintain natural flows, form, and function of the stream, where a hydraulic design approach typically alters these aspects of a stream to obtain the hydraulic design at the crossing.