Success Story: Mitigating Hazardous Debris Flows
By Kelly Hanson, Process Improvement & Change Management Intern
May 5, 2021
Learn about post fire debris flow risks and see how Golder Associates Inc. developed a preliminary debris flow model to help the Colorado Department of Transportation (CDOT) mitigate these hazards by estimating post-fire maintenance locations, road closure durations, and preliminary mitigation designs and costs.
As many Coloradans know, last summer and fall had an unprecedented wildfire season. Most of the front range was engulfed in smoke for months, which led to many residents having to evacuate from their homes. Multiple wildfires existed across the state. One of the fires in Grizzly Creek, caused interstate 70 (I-70) to be closed for a portion of the summer due to a fire starting in the median of I-70 several miles east of Glenwood Springs, Colorado.
The Grizzly Creek fire burned nearly all the tributary watersheds to the Colorado River within the Glenwood Canyon. Burned watersheds have a higher likelihood of causing a debris event. According to the U.S. Geological Survey (USGS), debris flows are fast-moving landslides consisting of mud and rock. Due to debris flows having high speeds and destructive force, they are capable of; destroying homes, washing out roads and bridges, sweeping away vehicles, knocking down trees, and obstructing streams and roadways with mud and rock deposits.
So, why are debris flows such a concern after wildfires? This is because the area that has been affected by the wildfire is left with a burn scar, which causes the natural environment to be temporarily altered. After a wildfire, the soil is not as permeable, so rainfall does not soak into the soil matrix. Severely burned forests areas are incapable of holding this rainfall after undergrowth is removed from the landscape, so rainwater moves through the watershed at a faster rate and higher volume. This one-two punch from the burn scar results in major runoff during minor rainstorms and results in flash floods and debris flows. Over time the soil and vegetation will return to their natural state and will intercept rainwater and reduce flood and debris potential; in the meantime, extra precautions have to be taken to prepare for post-wildfire hazards.
To locate high-risk areas, Golder Associates Inc. (Golder) has conducted preliminary debris flow modeling. Finding a modeling program for debris flows can be challenging since debris flows are considered a non-Newtonian fluid and are very complex. Luckily, the two-dimensional hydraulic modeling software FLO-2D can handle these calculations. Golder used FLO-2D to develop a complex preliminary debris flow model to help locate areas of concern in prioritized basins. To run the model, the following input parameters were required: the hydrologic event, the basin volume, the solids concentration of debris flow event, the duration of the mudflow event, the runoff curve number (CN), the duration of the simulation, the sediment specific gravity, the resistance parameter for the laminar flow, the manning’s n of burned area, the manning’s n of road surfaces, and the mudflow properties. Much of this information was provided by federal agencies and quickly incorporated into Golder’s detailed analyses.
The preliminary model simulates a debris flow event triggered by a hypothetical precipitation event. Golder used the preliminary model results to estimate the runout extents, volumes, and depths that may impact I-70. The debris volume includes the matrix of soil, sediment, and water. Likely, the debris flow will also transport cobbles, boulders, and vegetation/trees.
An example of the preliminary model results is shown below in Figure 1. Figure 1 shows the watershed that intersects I-70 at MP 120. The model results show the debris flow location across I-70 in the westbound (WB) and eastbound (EB) lanes. It also shows the potential depth of material deposited on the Interstate after the debris flow event, and the approximate volume of debris left on the road.
The preliminary model results will help estimate post-fire maintenance locations, road closure durations, and/or preliminary mitigation designs and costs. CDOT is grateful for Golder’s help in keeping the residents of Colorado safe.
