PRELIMINARY EVALUATION OF THE FIRE-RELATED DEBRIS FLOWS ON STORM KING MOUNTAIN, GLENWOOD SPRINGS, COLORADO

Large fire-related sedimentation events are generally initiated by storm precipitation. The connection between forest fires and major sedimentation events has been recognized for some time, particularly in Southern California where the concept of "fire-flood sequences" was first defined (e.g. Kotok and Kraebel, 1935; Rowe et al., 1949, 1954).

• normal or dilute streamflow,
• hyperconcentrated flow,
• slurry, or debris flow,
• granular flow.

Dilute Streamflow. Flows in which the sediment load has no effect on flow behavior, or imparts no yield strength to the flow, are considered as normal streamflow (Pierson and Costa, 1987). Turbulence is the primary mechanism for sediment support in such flows (Smith, 1986). The conditions of sediment transport and deposition are controlled by a complex set of variables, including flow velocity and depth, and channel configuration. Streamflow results in deposits generally associated with flooding and water transport.

Hyperconcentrated flow. Sediment-water flows in which the concentration, size distribution, and/or composition of the entrained sediment lead to a measurable yield strength have been defined as hyperconcentrated flow (Pierson and Costa, 1987). Intermediate ranges of sediment concentration and low to moderate clay contents result in generally low yield strengths. Smith and Lowe (1991) recommend that hyperconcentrated flow be used to refer to non-Newtonian flows with little or no strength that produce deposits that are intermediate in nature between streamflow and debris-flow deposits. Deposition occurs by particles dropping out of the flow as individual grains, and the remaining fluid continues to move; hyperconcentrated flow thus results in deposits with the particles in contact with each other (i.e. clast supported). Hyperconcentrated-flow deposits also show some sorting and gradation, depending on the velocity and depth of flow at the time of deposition.

Slurry or debris flow. Slurry flow is characterized by a substantial yield strength and plastic behavior, yet the fluid retains at least partially liquid properties (i.e. it will spontaneously assume the shape of its container) (Meyer, 1993). The onset of slurry flow in sediment-water mixtures is defined by Pierson and Costa (1987) to occur at the point where yield strength increases rapidly with increasing sediment concentration, probably due to internal friction that arises from interlocking of grains. In hyperconcentrated flows, particles are deposited as individual grains from suspension, and the remaining fluid continues to move; in debris flow, the sediment-water mixture flows as a single phase, and only the very largest particles may fall out of suspension. Deposition occurs essentially as a freezing in place of this single-phase mixture, and results in sharp, well-defined flow boundaries with significant relief. Levees lining the flow path and lobes of material at the path terminus are characteristic of this flow process. Deposits may contain gravel-sized, or larger, particles which are supported in a fine-grained matrix. A large number of terms have been used for the processes and deposits of slurry flow, including debris flow, mudflow, and debris torrent. In this paper, we'll refer to the results of such a process as debris flow.

Granular flow. Granular flow occurs at high ranges of sediment concentration, where the mass loses it ability to liquify, and frictional and collisional particle interactions dominate the flow behavior (Pierson and Costa, 1987). At the lower end of this range, granular flows may have similar field characteristics to high-strength slurry flows; the upper end of the range extends to flows with no water, requiring steep slopes or high inertia for movement. In the upper end of the range, dry ravel, the particle-by-particle transport of material downslope due to gravity, can occur. This process has been described on steep slopes following a fire, where loose, noncohesive material was formerly anchored by vegetation. Dry ravel has been described as an important post-fire process in southern California, where the channels are loaded with sediment, increasing available sediment for large events (e.g. Florsheim, et al., 1991; Scott and Williams, 1978; Wells, 1981, 1987). Debris avalanches are very rapid or extremely rapid inertial granular flows, most often generated by the en masse failure of sizeable masses of material (Pierson and Costa, 1987). This type of mass movement has been linked to fires in the humid conifer forests of the Cascade Range, where failures commonly involve all or part of a thick, commonly water-saturated colluvial and soil mantle overlying bedrock (Swanson and Dyrness, 1975). These failures probably occur due to decay of tree roots and resulting loss of anchoring effects (Meyer, 1993).

• Interception of rainfall, which extends the time for water to reach the ground surface and absorbs raindrop impact energy.
• Mulching of the ground surface to provide temporary water storage and slow release, slope roughness, and energy absorption.
• Structural support of loose, surficial material.
• Reinforcement of the deeper soil by roots, which increases the natural slope stability.
• Maintains conditions necessary for soil micro-organisms that provide soil structure.

On unburned slopes, live vegetation and vegetative litter intercept and slowly transmit precipitation to the soil. During normal rainfall events, the volume of rainfall that infiltrates into the soil is generally high, so that little surface flow occurs. Water from the surface soil percolates to the ground water table and migrates down gradient in slope-forming materials. Rainfall may ultimately emerge as surface flow from streams and stream channels. Water flows slowly through the soil, often traveling a few meters or less per day. This reduces the size of flood peaks and allows streams to continue to run significantly beyond individual storms. When a watershed is burned a number of possible hydrologic and geomorphic changes occur, including (modified from Swanson, 1981):

• Enhanced wind erosion.
• Increased rates of dry ravel of loose material.
• Decreased rainfall interception and infiltration, resulting in increased surface runoff.
• Increased rainsplash, rill, and sheetwash erosion.

All of these processes result in destabilization and accelerated erosion of slopes. Enhanced wind erosion and increased rates of dry ravel of loose material occurred on Storm King Mountain in the time following the fire and up to the September rain storm. In the days following the fire, residents of Glenwood Springs reported seeing huge clouds of dust flying above the mountain; presumably the loose, friable and virtually unprotected burned mineral soil and ash were being transported by wind and dry ravel and deposited on the hillsides and in the side drainages. Further, decreased rainfall interception and infiltration and increased rainsplash, rill, and sheetwash erosion occurred during the September storm. Following a fire, instead of the volume of rain water being routed through the hydrologic system over a period of days to months as soil water and ground water, it may force its way through the system over a period of several hours as surface flow or runoff. Greatly increased surface runoff is expected from intensely burned slopes primarily due to the lack of rainfall interception by vegetation (Spittler, in press). This causes the infiltration rate of the soil to be rapidly exceeded in brief, intense storms. In addition, the volume and velocity of the surface runoff can increase rapidly due to the lack of impedance by vegetation and litter (Wells, 1981, 1987); such high-discharge flows will result in the formation of gullies or rills on hillsides. The erodibility of a soil can be considered to be a function of the dispersivity of the clays present. Dispersivity is defined as the degree to which clay particles repel each other, and results in the disintegration of the clay structure, and thus erosion. The presence of ash in a soil, in some cases, may increase its dispersivity (Durgin, 1985). Wells (1987) describes a process where the excess water that cannot penetrate into the soil saturates only the most surficial material which may then fail as very small-scale debris flows or grain flows. This then leads to the formation of gullies and rills on a hillside. In addition, the gullies and rills provide an efficient means for transporting surface runoff and sediment to the stream channels. Peak flows in the channel may occur with less of a lag time than those observed in unburned watersheds, and flood peaks are often much higher and more capable of eroding sediment stored in the channel. Any loose, permeable material in the channel can easily be entrained by the increased surface flow, and channel incision can also occur. Finally, the size of storms necessary to surpass the critical stream power required to mobilize sediment stored in the channel is reduced. This is a consequence of the increased peak flow of streams in burned watersheds caused by rapid runoff (Spittler, in press).

Table 1. Physical properties and classification of materials.

The watershed can be divided into seven major drainages, labeled as A through G on Plate 1, all of which are direct tributaries of the Colorado River. These drainages typically have short, steep stream channels and precipitous side slopes. This topographic configuration is conducive to a rapid concentration of runoff and, when combined with intense rains, is a primary cause of high peak discharge and erosion rates (Rowe et al, 1954). In addition, two watershed front areas (H and I) are delineated on Plate 1. The areas of each drainage and watershed front, and the percentage of the drainage or front burned, are tabulated in Table 2.

Table 2. Areas of major drainages, burned area, and percentage of drainage burned, within the Storm King Mountain Watershed.

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