Culverts under pressure: North Cascades roads and trails in a wetter, warmer climate

Roads and trails in the North Cascades were built for floods, avalanches and rockfall. A warmer climate changes which of those they must survive. Climate models project average warming in the Pacific Northwest of 2.1 °C by the 2040s and 3.8 °C by the 2080s, and that will shift many northern Cascades watersheds from snow-dominated to rain-dominated systems. The result: more autumn and winter floods, higher peak flows and lower summer flows.

What it means for park transportation

Park roads and trails are designed to withstand local conditions. In the North Cascades those include periodic high water, avalanches and debris flows, landslides and rock fall. They should also protect aquatic organisms, vegetation and wildlife corridors.

Watersheds are classed by the share of precipitation that falls as rain rather than snow. A rain-dominated watershed receives less than 10% of its winter precipitation as snow; a snow-dominated one receives more than 40%; mixed watersheds fall between. Rain-dominated watersheds usually peak in the fall, snow-dominated ones in spring snowmelt.

Share of roads by watershed type in North Cascades, historical (current), 2040s and 2080s. Chart from the National Park Service's page.

Today the park complex's roads are spread evenly across the three watershed types. With warmer temperatures and less snowpack, climate scientists expect most roads to sit in rain-dominated watersheds. Streams are projected to warm, flooding and flow timing to change, and summer minimum flows to fall. Higher flows will stress culverts, bridges, roads and trails, and both low and high flows threaten some fish spawning, fish passage and the habitat of aquatic organisms.

Culverts, which carry streams under roads and trails, face a higher risk of inundation, scour, clogging and failure as flood extremes grow. How they perform matters for stream function and aquatic organism passage as well as for visitors and staff travelling the park.

Cascades River Road washout in August 2013 following thunderstorms. NPS photo.

What the park is doing

• Park scientists, facilities staff and data managers are working with the University of Washington to build projections of future streamflow into transportation planning and management.
• The park is updating its culvert inventories with a Culvert Inventory App, built by the University of Washington for Survey123 for ArcGIS, which records each culvert's location and data.
• Current culvert capacities are being compared with future flows, to find the culverts most at risk from higher flows and those that may hold up.
• Culvert databases are being combined with known locations of fish and amphibians.
• The aim is to protect ecosystems, recreation and natural processes by adapting the transportation system to a changing climate.

The project was run through the Pacific Northwest Cooperative Ecosystem Studies Unit, funded by North Cascades National Park Service Complex and the National Science Foundation. The park complex's summary is dated June 2016.

Sources

• National Park Service, North Cascades National Park Service Complex, “Climate Adaptation: Adapting Road and Trail Infrastructure to Changing Hydrologic Flows” (https://www.nps.gov/articles/adapting-road-and-trail-infrastructure.htm); rewritten in hubnx's own words.