Post-Fire Erosion and Debris Flows: Causes, Impacts, and Mitigation - Prof. Chad M. Hoffma, Lab Reports of Forestry

Download Post-Fire Erosion and Debris Flows: Causes, Impacts, and Mitigation - Prof. Chad M. Hoffma and more Lab Reports Forestry in PDF only on Docsity! 1 FOR 434 Measuring Fire Effects Post-fire Erosion and Debris Flows Zack Holden and Chad Hoffman My name is Zack Holden. Today I’ll be talking about one of the most serious consequences of severe wildfires, post-fire erosion and debris flows. 2 FOR 434 Measuring Post-Fire Effects Severe fires can result in accelerated erosion by: 1.) removing the forest canopy and litter layer, exposing mineral soil to the direct impact of rainfall 2.) heating and combusting soil organic matter 3.) burning logs that trap soil on steep slopes or store sediment in stream channels 4.) reducing the root strength of the soil Severe stand replacing wildfires can dramatically accelerate post-fire erosion. Removal of overstory vegetation increases the impact of rain events. Once surface litter and fuels have been removed, rain can quickly strip away upper soil layers. Soil heating and combustion of organic matter can weaken the structure of soils. Finally, a severe fire can significantly alter the above and below ground structure of a site, by burning up downed dead logs that trap soil on steep slopes, and weakening roots give the soil structure and strength. 5 FOR 434 Measuring Post-Fire Effects Rainfall Infiltration Subsurface flow Overland flow The fate of precipitation… •Interception •Evaporation •Interception •Overland flow - precipitation intensity exceeds infiltration Several things can happen to preciptation on it’s way to the ground. As we’ve already noted, canopy interception by overstory vegetation can significantly reduce the amount of rain the falls to the ground. Evaporation can also reduce the amount of water that enters the soil. Once on the ground, one of two things can happen to precipitation. Water either infiltrates the soil surface and then travels downward via subsurface flow, or travels across the soil surface through what is called Overland flow. 6 FOR 434 Measuring Post-Fire Effects Infiltration Rates • Infiltration = the process of water entering the soil • Infiltration capacity= the maximum rate at which water can enter the soil • Influenced by • Soil properties (texture, porosity) • Ambient moisture • Soil OM Infiltration is the process of water entering the soil, and infiltration capacity in the maximum rate at which water can enter the soil. Characteristics of the soil, for example it’s mineral content, porosity, particle size, texture and organic matter content all influence its infiltration capacity. 7 FOR 434 Measuring Post-Fire Effects Time In fil tra tio n R at e (m m /m in ) clay sand Clay soils have smaller average particle sizes with less interstitial space. This increases the surface area to volume ratio within the soil matrix, increasing the attraction forces between water molecules. This results in decreased porosity and decreased infiltration rates in clay soils compared to sandy soils. 10 FOR 434 Measuring Post-Fire Effects 1996 Rattlesnake fire, AZ Photo courtesy of Tree ring lab, U.Arizona 1996 White springs fire, AZ Photo courtesy of Dan Neary Here, we see two deep channels that formed following wildfires in Arizona in 1996. As we’ve learned, the post-fire environment is an important determinant of long- term fire effects on ecosystems. Fires in the Southwest generally occur during dry spring and early summer months. These fires are often followed by monsoon rain storms. These storms are often localized and can be extremely intense, and an cause severe post-fire erosion. 11 FOR 434 Measuring Post-Fire Effects Hydrophobic Soils • A water-repellent layer of soil that prevents infiltration below that layer. • Derived from plant material burned during a hot fire. The hydrophobic compounds (hydrocarbons) penetrate the soil surface as a gas and solidify after cooling, forming a waxy coating. • Sandy soils with large pore spaces and areas with thick litter accumulations that experience very hot fires are especially susceptible. Let’s briefly revisit water repellency in soils, as this can become an important contributor to debris flow initiation. Hydrophobic from the root HYDRO meaning water and phobic meaning fear, is another word for water repellent soil. Compounds that are volatilized during a fire can penetrate the soil surface when heated, and then cool to form a waxy coating around soil particles. Susceptibility of soils to water repellency formation varies by soil type, by sandy soils with large pores are especially prone. 12 FOR 434 Measuring Post-Fire Effects Hydrophobic Soils (con’t) • The thickness and continuity of hydrophobic layers varies, as does their persistence. • Recovery: plant roots, soil microorganisms, and soil fauna help break up the hydrophobic layer. • Negative feedback: reduced infiltration will decrease the amount of water available for plant growth and biological activity in the soil. The thickness of these layers and their duration in the soil varies. Eventually, recovery of vegetation and soil organisms will break up the hydrophobic layer. However, for several years after a fire, a water repellent soil layer will reduce water infiltration, increasing overland flow and soil erosion. This has a negative feedback effect, reducing the amount of water available to plants and soil microorganisms re- establishing after a wildfire. 15 FOR 434 Measuring Post-Fire Effects Rill and channel erosion (1988 fire, 1989 storm, Yellowstone) Loss of root strength, saturation-failure of colluvium (1989 fire, 1997 storm, Idaho) Initiation of events through runoff and sediment bulking, early post-fire In the left photgraph, we see an example of surface run off with rill and gully formation following a storm that occurred 1 year after the 1989 yellowstone fires. To the ride, we see the result of landslide initiated debris flows that created this gully incision. 16 FOR 434 Measuring Post-Fire Effects Copyright © Tom Black 2002 This is another example of a psot-fire debris flow from the Payette River drainage of Idaho, USA. Debris flows are a common occurrence following fire on the granitic soils that characterize this part of the country. 17 FOR 434 Measuring Post-Fire Effects 1989 debris flow-dominated event, NE Yellowstone This is another example of debris flow-dominated post-fire erosion in Yellowstone National Park following the 1988 fires. 20 FOR 434 Measuring Post-Fire Effects Time since fire Li ke lih oo d of la rg e- sc al e er os io n Runoff-Dominated Surface Erosion Saturation-Induced Slope Failures Two-Phase Erosional Response 0 – 5 yrs 5 – 15 yrs It is useful to think of post-fire erosion occurring in two phases; Most surface erosion occurs within the first 5 years of a fire and decreases quickly after the first few years. Debris flows can occur within the first 15 years of a wildifire. The probability of debris flow occurrence peaks several years after the fire because the root strength of fire-killed vegetation continues to decrease and hydrophobic soil layers can persist in the soil for many years after a fire. 21 FOR 434 Measuring Post-Fire Effects Years after forest removal / fire R el at iv e ro ot re in fo rc em en t From Ziemer 1981 Decay of dead roots Live roots 5 – 15 yrs The resilience of a site to post-wildfire debris flow events depends on the decay of roots from vegetation that was killed by fire and on the contribution of roots from new plants that colonize a site after a fire. 22 FOR 434 Measuring Post-Fire Effects Probability of Occurrence • Depends upon: • Soil type • Slope steepness (> 45%) • Contributing area of steep slopes to basin • Size and distribution of severely burned patches • post-fire storm characteristics (intensity and timing of post-fire rain events) Landform characteristics strongly influence the probability of post-fire debris flow events. Topography, particularly slope steepness and the amount of area steep slopes contribute to a basin or watershed is the most important factor. Soil type and the extent of the severely burned area will also influence probability of debris flow occurrence. Again, the timing and intensity of post-fire rain events will ultimately determine whether debris flows occur. 25 FOR 434 Measuring Post-Fire Effects Rehabilitation Measures • On-site: revegetation • grass seeding • straw mulch • hydromulch • Off-site: sediment retention devices • straw bale check dams • directional log felling • sediment retention ponds • silt fences Several techniques are commonly applied by BAER teams after wildfires. A common method for preventing post-fire erosion is to plant seed on site in areas that are severely burned. Revegetation is common, and both native and non-native grasses are often spread out over severely burned areas. Straw mulch and hydromulch are two types of materials that are often applied in conjunction with seeding. These treatments are intended to reduce the impact of rain drops, improve infiltration and protect seeds from being washed away by rain. A variety of sediment retention devices are now applied in and around severely burned areas to mitigate post-fire erosion and retain sediment. These include straw bale dams, straw wattles and contour log felling. 26 FOR 434 Measuring Post-Fire Effects Post-wildfire Erosion Mitigation The photograph to the left is of a straw wattle, nylem mesh netting filled with packed straw. These are used in small drainages and are used to retain small amounts of sediments in small, first order drainages or side slopes. Contour log felling is another method of sediment retention. The effectiveness of this treatment depends largely on the size and design of the log structure. 27 FOR 434 Measuring Post-Fire Effects Aerial Straw Mulching This photograph is taken from a short video of a helicopter dropping straw mulch on a burned area.