CHARACTERISTICS AND PROPERTIES OF DEBRIS FLOWS
The characteristics of debris flows on Mount Shasta, which change dramatically in a downstream direction, are indicated by the various travel distances, sizes, location, thickness, cross-sectional shapes, and physical properties of the deposits (tables 3 and 4, figs. 13 and 14 below). A comparison of stream profiles for different channels on Mount Shasta (fig. 13 below) indicates that the slope of most channels near the summit is about 35°. On Whitney Creek, the upper slope is about 25°.
Pattern of Travel and Deposition
The paths of debris flows are dependent on the size of the flow, capacity, sinuosity and slope of the channel, presence of overflow areas, and patterns of deposition of previous flows. Overbank flow caused by debris flows generally occurs in reaches below altitudes of 2,130 m (slopes flatter than 15°). An example of the effects of previous flows on existing channels can be seen from data obtained for Ash Creek. On the upper road crossing of Ash Creek (fig. 14 above), previous flows had partly filled the channel, and the debris flow of 1962 was so large that it could not be contained in the channel. Where the existing channel curved, a part of the debris flow continued straight downslope, with the leading edge coming to rest more than 700 m from the original channel.
Debris-flow deposition is affected also by the capacity of the existing channel and the height of channel banks. If banks are low, large debris flows may overtop them, and the new flow path may diverge from the main channel. During the series of debris flows on Mud Creek between 1924 and 1931, one or more flows overtopped the right bank near the Mud Creek Dam and traveled 6,000 m outside the main channel before coming to rest (fig. 15 below). Near the terminus of the overbank flow, this deposit (fig. 15 below) lies more t.han 3.2 km from the Mud Creek (main) channel.
An additional example of a debris flow occupying both existing and new channels occurred on Whitney Creek July 6, 1985. The path of this debris flow and the areas subject to inundation were not confined to the existing channel in the lower reaches even though parts of the channel were diked. Inundation of the flood plain began below an altitude of about 1,070 m and extended about 27 km from the summit. Areas of overflow were documented by aerial photographs taken during July 1985. The inundated areas of the fan, known as Juniper Flat in Shasta Valley (fig. 16 below), included the existing main (east) channel, an old left bank overflow (west) channel, and the adjacent fan. Deposits from this debris flow tended to feather out and were less than 0.3 m thick near the terminus, rather than ending with an abrupt snout as noted for earlier debris-flow deposits at higher altitudes on Whitney Creek.
Thickness of Debris-Flow Deposits
Stratigraphic surveys of historical debris-flow deposits were made by inspecting cut banks and excavating pits at selected locations (fig. 17 below) in the channels and on fans of several streams on the flanks of Mount Shasta. These data (table 3), collected during 1983 and 1984, indicate that the average thickness of debris-flow deposits on fans is about 0.98 m, with an approximate range between 0.4 and 2.5 m. The thickness varies laterally across the channel or fan. The data in figure 18 below indicate that the thickness of deposits tends to decrease in a downstream direction. Most debris-flow deposits occur on fans between 10 and 20 km from the summit.
The greatest thickness of historical deposits are found in stream channels that confined the flow. Deposits with depths of more than 1 m commonly represent a terminal lobe or inundation in a former channel. Debris-flow deposits from different flows were difficult to separate because of the similarity of the transported material and lack of distinct bedding planes. Thus, one deposit might, in some cases, represent several flows that occurred within a few days or a few years of each other. Figure 18 above represents average thickness of debris flow deposits at several sites on five streams.
Cross-Sectional Shape of Deposits
Debris-flow deposits on Mount Shasta have a convex-shaped transverse profile. Cross sections of flows at seven selected locations on Mud, Inconstance, Gravel, Whitney, Bolam, and Ash Creeks (fig. 17 above) illustrate the convex surface shape typical of the larger debris deposits (fig. 19 below) on Mount Shasta. The cause of the convex shape of the deposits is unknown, but is possibly related to the physical composition of the debris-flow slurries. cross section of Mud Creek (M35.5, fig. 19 below) shows remnants of various multiple debris-flow deposits that are incised within terraces formed by more current flows. Generally, the centers of the deposits are as much as 5 m higher than margins near the canyon walls.
Existing debris flow deposits on Mount Shasta indicate that as new flows occur, older incised channels are filled and obliterated. Subsequent streamflow then cuts a new channel, which may not occupy the lowest part of the valley cross section. For some debris flows, large boulders and rubble (debris-flow levees) line the outer perimeter of the deposit (fig. 20 below).
Sediment and Water Ratio of Debris-Flow Slurries
Using analytical procedures described by Pierson (1985), the approximate original water-sediment ratios of debris-flow slurry deposits at 19 sites on Mount Shasta (table 4) were determined by reconstituting dry samples in the laboratory. The results indicated that, on the average, original debris-flow mixtures consisted of a ratio of solids to the total mass by volume of 0.68 (table 4). Slurry consistency was sensitive to very small changes in water content. For example, a decrease of only 1 to 2 volume percent from optimum rendered the slurry too viscous to flow; an increase of 1 to 2 volume percent diluted the slurry to the point at which it could not hold coarse particles in suspension. All debris flows were characterized by a fairly small range of sediment concentrations (table 4), with the ratio of solids to water by volume ranging from 0.62 to 0.77, which is equivalent to a minimum water concentration of 32 percent by volume. A cumulative size distribution curve of debris-flow deposits for the Mount Shasta volcano (fig. 21 below) shows that the median grain diameter of the smaller material in the deposits was larger than 0.01 mm, which is very fine sand.
