A USGS Tool That Drafts Culvert Designs for Massachusetts Stream Crossings

Massachusetts has about 25,000 structures where roads cross rivers and streams, and as of 2026 many of them are undersized. A culvert or bridge that is too small can block fish and wildlife, break up habitat and harm aquatic life. It may also lack the resilience to survive large floods, which climate change could make bigger and more frequent.

Better designs can help on every count: easier passage for aquatic organisms, a continuous stream, and less flooding upstream and over the road, less erosion and less damage to habitat.

Two crossings in Ashby, Massachusetts, on October 1, 2021. At Pearl Hill Brook and Mill Creek Drive (A), the drop at the culvert's outflow may block aquatic organisms; on an unnamed tributary to Trapfall Brook at Harris Road (B), shallow water in the culvert and the drop at its outflow may do the same. Photos: Brian Nelson, U.S. Geological Survey.

In July 2019 the U.S. Geological Survey (USGS), the Massachusetts Department of Environmental Protection (MassDEP) and the University of Massachusetts Amherst began a series of joint studies to build an automated geographic information system (GIS) hydraulic modeling tool for preliminary culvert designs. The USGS plans to offer its designs in StreamStats, a web application, so municipalities and engineers can see possible designs for a crossing they plan to replace. The application is meant to:

• give information on hydrology, hydraulics and ecological conditions at each crossing;
• offer culvert designs that improve aquatic organism passage and flood resilience;
• help MassDEP apply the Wetlands Protection Act regulations to stream crossing projects.

How the state's standards came about

The handbook sets out six standards for a well-designed crossing:

1. Type: a bridge, a three-sided box culvert, or an open-bottom arch culvert.
2. Embedment: a closed-bottom culvert sits at least 2 feet or 25 percent of its height or diameter into the streambed, whichever is greater.
3. Span: 1.2 times the stream channel's bankfull width.
4. Openness ratio: cross-sectional area divided by length of at least 0.82, so the culvert is wide and tall for its length.
5. Substrate: a natural bottom that matches the streambed above and below.
6. Water depth and velocity: comparable to those in the natural channel upstream and downstream.

Under the regulations, a replacement crossing must meet the standards to the maximum extent practicable (310 CMR 10.24(10) and 10.53(8)). A project has to show that the crossing's harmful effects are prevented or minimized and mitigated, and must weigh site constraints, undesirable effects or risk, and environmental benefit against cost.

In the early 2010s a USGS and MassDEP study built hydraulic models of seven crossings around the state. None of the existing culverts met the standards or could carry the modeled floods, which overtopped the road and backed water up upstream. Replacement designs that met the standards mostly carried the floods without backwater.

The grant program funds culvert designs that are structurally and environmentally better, more flood-resilient, and meet the standards, and the tool's designs are built to fit its goals. Meanwhile the North Atlantic Aquatic Connectivity Collaborative (NAACC) continues the crossing assessments UMass Amherst began in 2000, across Massachusetts and other northeastern states; StreamStats carries each crossing's NAACC code so users can look them up.

How the tool works

The tool is built with ArcGIS, the Python programming language, and the U.S. Army Corps of Engineers' HEC–RAS (Hydrologic Engineering Center's River Analysis System). It drafts box, arch and pipe culverts that:

• carry the 10-, 4-, 2- and 1-percent annual exceedance probability (AEP) floods, the same as the 10-, 25-, 50- and 100-year floods. These come from the design flows the Massachusetts Department of Transportation (MassDOT) sets by a road's functional classification;
• meet the stream crossing standards.

The aim is that an undersized crossing is never replaced in kind, with the same design, and that every replacement gives some flood resilience and passage for aquatic life. A range of designs is offered, matched to how good the habitat is and how much stream connectivity could be restored, while meeting the standards as far as practicable.

Step by step:

1. Flood flows come from the Massachusetts flood flow equations.
2. Cross sections — the approach, the culvert's inlet and outlet faces, and the exit — are cut from high-resolution lidar elevation models, with channel depth estimated from the state's bankfull channel geometry equations.
3. HEC–RAS runs start from a span equal to the estimated bankfull width, which keeps velocities in the culvert close to those upstream.
4. The culvert grows through standard precast concrete box and pipe sizes until it carries each flood.
5. Freeboard — the gap from the water surface to the culvert's top, its low chord — is set to at least 1.0 or 2.0 feet, to satisfy the review of municipal bridge projects under chapter 85, section 35 of the Massachusetts General Laws.
6. A standards design is also produced, with a span of 1.2 times bankfull width and an openness ratio of at least 0.82.

What a StreamStats user sees

A crossing's culvert replacement report gives:

• the delineated drainage area and location: latitude, longitude, stream, road, and town or city;
• the NAACC assessment summary, with a link through the crossing code;
• the basin characteristics used in the flood and channel geometry equations;
• a habitat quality score from five factors: BioMap, cold-water fisheries resources, areas of critical environmental concern, wild and scenic rivers, and diadromous fish runs;
• a stream connectivity restoration potential score from two UMass Amherst factors, critical linkages and cold-water critical linkages;
• the road's functional classification, its hydraulic design flow, the AEP flood flows and the bankfull channel geometry.

MassDEP and UMass Amherst are writing guidance, as of 2026, on how the two scores could help decide a final design.

An example of the tool's preliminary designs for one crossing, as shown in StreamStats. Figure: U.S. Geological Survey.

The report then lists the tool's designs for three-sided box, three-sided arch and pipe culverts: dimensions, length, cross-sectional area, selected elevations and how each measures against the standards. Users compare them alongside the two scores, choose the design or designs that best serve their community's flood resilience and aquatic passage, and can download the HEC–RAS input and output files.

Sources

• Bent GC, McCarthy BA, Sturtevant LP, McCallister MA, Tudor AL, Armstrong IP, Poe MW, Graziano AP, Carlson CS. "An automated geographic information system-based hydraulic modeling tool for developing preliminary culvert designs for stream crossings in Massachusetts." U.S. Geological Survey Fact Sheet. https://pubs.usgs.gov/publication/fs20263065/full
• StreamStats: https://streamstats.usgs.gov/ss/ · NAACC crossing search: https://naacc.org/naacc_search_crossing.cfm
• The fact sheet's figures 2 (UMass Amherst photographs), 3 (a photograph used with permission) and 4 (a diagram modified from a UMass Amherst original) are not reproduced here.