USFS Burned Area Emergency Response (BAER) Team Hydrologists Examine Potential Water Run-off from Delta and Hirz Fires
US Forest Service BAER Team hydrologists Christine Mai and Jesse Merrifield are examining the areas burned by the Delta and Hirz Fires. They are not only looking at the damage done to the vegetation, soils, and drainage structures affected by the fire, but they are also determining what values at risk exist within and below the fire area. Concentrating on areas with high and moderate burn severities, they are calculating the flow and sedimentation increases that can be expected after the fire.
Winter storms that occur up to two years after a severe burn typically result in increases in surface flow, erosion and stream sedimentation. Post-fire run-off increases occur as a result of the loss of ground cover and physical and chemical changes to soil properties. Fires consume the duff layer, composed of rotting leaves, twigs, and other organic matter on the surface of the ground. In some areas, fires may burn hot enough to consume plant roots and other organic matter that are normally incorporated in the upper layer of soils, leaving behind the mineral components.
The duff and organic matter are critical to regulating the rate water runs off of a slope. These materials serve as sponges, helping absorb rainfall. After a fire, less rain might be absorbed, leading to higher rates of run-off from a slope. A chemical reaction from the burning organic matter may create a waxy layer in or on the soil, blocking water absorption and further increasing the rate of run-off.
On Wednesday, Christine and Jesse traveled with geologist Johnathan Schwartz to the west side of Delta Fire to evaluate some of the drainages flowing into the East Fork Trinity River. Much of this area is mapped as having moderate to high burn severity and is located above numerous homes and Alpen Vineyards, some of the identified values at risk. The hydrologists and geologists checked the extent of burn severity, steepness of slopes, and overall stability of the stream beds and banks. They looked specifically at the size and quantity of rocks and logs upstream from the roads. Road drainage structures were evaluated to see if they were likely to handle an increase in water run-off, floating debris, and bedload (rocks and sediment) from the burned areas.
In most areas, general surface run-off and water in streams is supposed to flow through culverts under the roads. Many of the culverts examined on Wednesday are unlikely to handle an increase in water flow, especially if the flow is bulked up by woody debris, sediment, rocks and boulders. Fire killed vegetation, including branches and entire trees, will be falling, breaking apart, and rolling downhill into the streams. Materials entering a stream channel are likely to be mobilized by increased flows. A few of the culverts are already partially blocked by rocks or chunks of wood, some from before the fire. Large areas with high and moderate soil burn severity and steeper slopes are the highest concerns for increased stream flow and sedimentation rates.
Back in the office, Christine and Jesse calculate pre-fire stream flows using stream gage data, where available, and stream flow models. They delineate the specific drainages where special resource values may be at risk. To estimate post fire flow effects, they utilize data that comes from the soil scientists (soil burn severity, erosion and sediment transport) and the geologists (debris flow potential).
They will be identifying the watersheds with the greatest potential for increased run-off and damage. This information is utilized to develop plans for treatments on federal lands. Treatments developed for roads typically provide the greatest water quality protection by reducing the potential for road erosion and failing stream crossings. Areas with the highest potential for impacts to life, property, or other values found to be at risk will be prioritized for work under the BAER program.