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Road-Stream Crossings

Wildcards for Alaska’s transportation infrastructure and fish


Stream simulation culvert in Tyonek.

Ash Adams


Putting a road across a stream in Alaska is a lot like going on an adventure. Survival of road and traveler alike hinges on careful planning and weighing risks.


Christy Cincotta | Tyonek Tribal Conservation District

This site in Tyonek had frequent washouts on an almost yearly basis. After this 3-foot diameter culvert washed out in spring of 2012, it was finally replaced with an 8-foot embedded culvert and hasn’t washed out since.

Plan for the 1 Percent

Rivers and streams are powerful and ever-changing. They carry rocks and logs and heavy loads of silt and debris. They flood. They carve out canyons. And when they encounter a road crossing not custom-built to their unique characteristics, they find a way around it. To avoid floods that overwhelm roads, it’s necessary to design crossings that give rivers and streams room to be their dynamic selves — from their variable flows down to their moving streambeds.

Case in point: in September 2012 a week-long storm generated severe flooding that overwhelmed the Matanuska-Susitna Borough’s road system. Willow Creek rose three feet and swelled to roughly ten times the volume it carried the previous week. Montana Creek ate away at Yoder Road and commandeered its roadbed. Oil Well Road, West Kenny Boulevard, Burrows Road, and others washed out. People were stranded. The governor declared a federal disaster and FEMA stepped in.

Each year, there is a 1 percent chance that a flood of this magnitude (the so-called “100-year” flood) will occur. It’s a common mistake to think a 100-year flood will only happen once every century. Although statistically unlikely, it is possible that it will occur many years in a row or many times in a decade. In fact, Southcentral Alaska has seen multiple flood events of this magnitude in the past twenty years that caused localized and widespread road failures on the Kenai Peninsula, Kodiak Island, and the Mat-Su.

Planning for the 1 percent should be a major consideration for those businesses that need to reliably transport goods over land or for those operating or traveling through flood-prone areas.


US Fish and Wildlife Service

A stream’s natural bed can include boulders, cobble, gravel, sand, or silt. It moves. It creates roughness that breaks up the flow. Water moves much slower here than just a few inches higher in the stream’s main flow and this is where small fish (like these trout and salmon) like to be. Ideally these same conditions exist within the culvert itself.

Pay Now or Pay Later

Many have seen it, especially in the harder-to-access areas of remote Alaska: the culvert graveyard. They tend to be similar, often including some mangled 12-inch diameter pipes, a couple 18-inchers, maybe a 4-footer. Sometimes they’re still buried, filled with debris, unearthed only when the excavator comes in to make way for a new pipe.

Damaged or blown-out culverts often result from a failure to provide sufficient space for a stream to carry flood flows, sediment (from silt to large boulders), and debris. An undersized culvert can quickly become overwhelmed as the volume of water increases and cannot physically drain through it fast enough. The situation worsens when the culvert inlet becomes clogged with trees or other debris that have become dislodged and carried downstream. As the water backs up, a reservoir forms upstream and puts pressure on the road. Roads are not designed to be dams and can be overtopped and washed away under the strain of floodwaters. Even if the road or culverts are not washed away, they are often damaged and can pose a hazard to drivers.

Let’s consider the other end of the culvert spectrum where structures placed at road-stream intersections are designed to retain the characteristics of the natural channel under the road. This can be achieved using a technique called “stream simulation” (first developed by the US Forest Service). Unlike traditional designs that focus solely on moving water under a road, stream simulation design considers the movement of water, sediment, and debris and also maintains natural conditions for normal fish movement.

US Fish and Wildlife Service

Stream simulation culverts benefit all fish species, including weaker swimmers like juvenile Arctic Grayling.

Bill Rice, a fish passage engineer with the US Fish and Wildlife Service, remembers one project at North Sitze Road at Colter Creek in Mat-Su: “A year after construction I noticed a twenty foot log in the middle of the ten foot wide pipe sitting on the streambed. That fall it was gone, moved downstream. If the previous five foot pipe had still been there, Borough maintenance would have likely had to come out and remove it."

So where on this spectrum of upfront investment versus back-end cost is best for business? Looking at the performance of different culvert designs during flood events provides insight. Prior to the 2012 floods, the Mat-Su Borough had already been working with partners to install wider, embedded stream simulation culverts (at eighty-one road-stream crossings, to be exact) for another reason: salmon. All stream simulation design culverts held fast during the 2012 flood event, while culverts previously installed under lesser design standards did not fare so well.

Vermont had a similar outcome in 2011 following Tropical Storm Irene. Where nearly 1,000 traditional culverts were damaged or destroyed, stream simulation culverts in the Green Mountain National Forest that had been upgraded to improve fish passage for Brook Trout and reduce risk for debris plugging and failure during large storm events incurred no maintenance or replacement costs.

It turns out that designing culverts to accommodate natural stream processes is good for the bottom line, and as of the fall of 2017, 108 crossings in Alaska featured stream simulation culverts. Evolving design standards and engineering techniques are demonstrating that it is possible to improve flood resilience, decrease maintenance costs, and extend the lifespan of a culvert, all while concurrently providing increased benefits to fish, people, and industries that depend on stable infrastructure. Communities in Alaska are beginning to recognize the multiple benefits of upfront investments in flood/fish-friendly designs. This is corroborated by the Mat-Su and Kenai Peninsula Boroughs and Municipality of Anchorage adopting proactive, preventative ordinances that include fish-friendly standards and designing new crossings to accommodate at least the 100-year flood.

US Fish and Wildlife Service

The US Fish and Wildlife Service’s stream simulation design approach for Alaska. The goal of this type of crossing is to recreate the natural stream channel (top) through the road crossing (bottom). An engineered streambed mimics what’s in the reference reach and the crossing accommodates natural movement of floods, fish, and debris. 

Jim Jenson, the operations and maintenance manager for the Mat-Su Borough, has been responsible for the maintenance of roads, facilities, utilities, and vehicles since 2009 and has experienced the cost saving benefits of stream simulation design culverts first hand. “The 2012 flood cost us millions of dollars in road damages where we still had low capacity culverts. Now when we put crossings in we make them bigger and put rocks in them for the fish. A big benefit is they pass the floods.” Per Borough code, culverts on “anadromous” streams where migratory species like salmon are present must meet fish passage criteria and pass a 100-year flood event.

The Native Village of Tyonek is another Southcentral community that’s had enough with their roads blowing out and impeding access to their lands for both people and fish. It shares in common with Mat-Su and the western Kenai Peninsula a large land base with extensive intersections of salmon streams and roads but is only accessible by air or barge, so infrastructure installation and repair costs are higher, making each decision more important. A number of Tyonek’s road-stream crossings are located on “flashy” river systems that rise and fall quickly during or after storms and have a history of being replaced every year; also, more than thirty of these crossings are impassible to juvenile fish. Tyonek has worked with partners to install five stream simulation culverts on their road system since 2012 and removed several undersized traditional culverts where roads have been decommissioned. And more upgrades are planned.

“The reason we really like these projects is they’re a win-win. Developers need to be able to get places on the roads out there, and if they’re washed out, projects can get delayed. In 2012 and 2013 the only roads that didn’t wash out had been replaced with fish-friendly culverts. Everyone’s on board,” says Christy Cincotta, Tyonek Tribal Conservation District executive director.

Type of Barrier Conditions
Jump excessive distance between the culvert outlet and water surface or excessive jump height to pool depth ratios
Velocity constricted and/or high flows or lack of roughness through the crossing
Depth insufficient water depth in the crossing or subsurface flows in the vicinity of the crossing
Turbulence excessive aeration at the culvert outlet
Behavioral inadequate/excessive attraction flows at the culvert outlet/inlet or lack of light

Despite costing more upfront, the stream simulation approach is proving to increase infrastructure durability and lower (and sometimes even eliminate) costly maintenance. What’s more, the return on higher upfront investments in stream simulation crossings may become even more pronounced in the future. Published in 2016, “Climate Change Damages to Alaska Public Infrastructure and the Economics of Proactive Adaptation” by Melvin et al. predicts that climate change-related expenses to Alaska’s public road, building, airport, rail, and pipeline infrastructure will increase through the end of this century. The largest total damages they predicted involved roads with 75 percent of road damages caused by flooding. Proactive measures such as planning and building higher capacity road drainage systems could reduce predicted flood damage by an order of magnitude and result in a significant reduction in the cost to maintain Alaska’s road network.


Alaska Department of Fish and Game

Examples of inadequate culverts in Alaska.

Life-time Costs

Let’s break down these costs over the life of a crossing.

  • Design: The engineering design cost for all stream crossings depends on project complexity. Added nominal costs for stream simulation designs can include more extensive stream assessments and streambed stability analyses, contrasted to the conventional hydraulically-based designs.

    One area in which Alaska is lacking design information compared to other states is in its long-term and widespread hydrological data that can help predict the magnitude and frequency of flows. Specifically, the state needs more and better stream gage data. Gages are instruments placed on a river or stream that measure the volume of water flowing past a given point over a period of time (discharge). A stream’s width and depth varies day-to-day with how much water it’s carrying, so gages provide great insight into the stream’s range of flows. Gage data that captures extreme flood events is useful to properly design culverts. Without local gage data, hydrologists must rely on regression equations that use regional weather statistics and the type of land cover to predict flows in a stream. These estimates have wide margins of error and have profound effects on the stability of culverts and their ability to pass water and fish.

    Stream simulation provides the antidote to this unpredictability by looking to the form and function of streams and their interaction with the adjacent landscape. By using fluvial geomorphology principles, it’s possible to use channel shape and substrate characteristics to add a higher level of confidence to the flow predictions. In Alaska, engineers designing fish passage culverts are doing just that and ending up with culverts that will pass the 100-year flood flows without failing.

  • Installation/Construction: Stream-simulation culvert installation in Alaska requires several extra steps including a deeper excavation compared to traditional culverts and adding a nature-like streambed and streambanks within the culvert to make a seamless transition between the simulated and natural channels. The highest cost tends to be the larger capacity culvert itself with additional costs for the deeper excavation and equipment/labor to move the streambed and streambank materials to site and into the crossing structure. To do it right the first time in Alaska, it typically costs around 5 percent to 20 percent more to install a stream simulation versus a traditional round culvert, but the long-term savings can far exceed these costs.

    In a published study highlighting culvert designs in Vermont titled “Flood Effects on Road-Stream Crossing Infrastructure: Economic and Ecological Benefits of Stream Simulation Designs” by Gillespie et al., similar cost comparisons for traditional and stream simulation designs found the latter increased construction costs between 9 percent and 22 percent but were shown to be more economical when compared to costs exceeding more than four times that for repairs after Tropical Storm Irene.

  • Operation, Maintenance, Flood Damages: Road-stream crossings are a long-term infrastructure liability. Mobilizing heavy equipment and materials to conduct routine or emergency repairs is costly and can directly impact drivers (delays and safety). Other maintenance factors to consider include beavers, which tend to be attracted to constrictions at too-small crossings, and debris clogs. This is where stream simulation culverts outperform other designs; operation and maintenance needs are often minimal, resulting in long-term cost savings, according to Perrin and Jhaveri’s “The Economic Costs of Culvert Failures” report published in 2004.


Other Key Considerations

Given their expense and long-term liability, it’s worthwhile to consider if crossings can be avoided completely, and if not where along the continuum of the stream or river they’re least likely to fail. Developers in Mat-Su are now finding ways to plan their developments to minimize the number of crossings or avoid them altogether to save costs. If a crossing is unavoidable, crossing a stream higher in its watershed is generally better because it’s usually more entrenched, meaning less floodplain and less volume of flowing water.


Costs to Alaska’s Fisheries

Externalities that aren’t reflected in the initial price tag are potential impacts to subsistence ways of living that depend on healthy fish populations and the cost to Alaska’s multi-billion dollar commercial and sport fishing industries.

Since 2001, the Alaska Department of Fish and Game (ADF&G) has assessed 3,099 crossings on state and local roads and roughly 40 percent are considered partial or full barriers to weak-swimming fish such as juvenile salmon and Arctic Grayling. On federal roads maintained by the US Forest Service Tongass National Forest, there are similar patterns with 2,019 fish stream crossings, of which 34 percent have documented passage concerns.

According to Gillian O’Doherty, a habitat biologist with ADF&G, “We’ve assessed over 90 percent of culverts in the state for impacts to fish passage. We also look at the condition and age of structures and collect data to help develop initial cost estimates for replacement. That data is publicly available in ADF&G’s interactive online mapper. The initial investment in assessment and ongoing prioritization efforts means we can be strategic and work over large areas and time frames to restore fish passage in an organized and cost-effective manner.”

Alaska can be a pretty harsh place for fish to eke out a living. Fish need to constantly move to access feeding areas, ice-free areas for overwintering, and cooler areas during the summer. They also need to be able to quickly find refuge during seasonally high and low flows and temperatures. Some species move short distances, but many Alaska species are migratory and need to swim long distances to complete their life cycle. It’s generally known that salmon return from the sea to spawn, a behavior called anadromy, but few people know that there are twenty anadromous species of fish in Alaska ranging from lamprey to whitefish. Even species that spend their whole lives in freshwater may complete long migrations. Migration maximizes growth and reproductive potential, but it can quickly become a disadvantage when barriers, such as culverts, delay or prevent movement and migration among key habitats. This has happened in the Northeast United States and Pacific Northwest where roads rank second only to main stem dams as the most significant impediment to salmon recovery, according to Perrin and Jhaveri.

Decades-long research by the University of Washington in the Bristol Bay region has shown that the stability of salmon fisheries hinges on the availability of a wide array of intact freshwater habitats, as published in “Population Diversity and the Portfolio Effect in an Exploited Species” by Schindler et al. Analogous to asset diversity on the stability of financial portfolios, access to diverse habitats throughout the year, and from year to year, gives fish populations the flexibility and resiliency they need to thrive over the long term.

With roads and fish, Alaska can have its cake and eat it too. Design approaches that maximize the life and safety of transportation networks (new and existing) and the sustainability of Alaska’s fisheries are being adopted and are establishing a track record of success.


Learn More

The ADF&G Fish Passage Improvement Program and USFWS Fish Passage Program jointly offer free training workshops to practitioners including engineers, resource managers, road owners, and permitting staff. For specific information on topics including fisheries biology, regulatory questions, design methods, Alaska hydrology, and construction techniques, contact the nearest agency office in your area.



Katrina Liebich is Fisheries Outreach Coordinator for the US Fish and Wildlife Service in Alaska.


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