Road-Stream Crossings: Wildcards for Alaska's transportation infrastructure and fish.

• Author: Liebich, Katrina
• Position: Transportation

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

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. The) 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 PF.M A stepped in.

Each year, there is a I 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, South-central 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.

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 win 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.

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...