Initiation and Flow Behavior of the 1980 Pine Creek and Muddy River Lahars, Mount St

Initiation and flow behavior of the 1980 Pine Creek and Muddy River lahars, Mount St. Helens, Washington

THOMAS C. PIERSON U.S. Geological Survey, Water Resources Division, David A. Johnston Cascades Volcano Observatory, 5400 MacArthur Boulevard, Vancouver, Washington 98661

ABSTRACT curacy by comparing computed lahar arrival ing a finite yield strength. Although a number of times with recorded arrival times at Swift well-defined hydrodynamic equations can be Two large, high-velocity lahars (volcanic Reservoir. The computed velocities appear to used to accurately compute how water flows debris flows) were triggered by a pyroclastic be -15% slower than the recorded velocities, under specific sets of idealized conditions, very surge during the first few minutes of the May which is consistent with the restriction that little is known, quantitatively, about how large 18, 1980, eruption of Mount St. Helens. The the velocity formulas produce only minimum lahars behave. initial surge cloud evolved progressively by values. The objectives of this paper are to describe gravity segregation from a gas-mobilized, the initiating mechanism(s) of these lahars, to highly inflated density flow to a dense, water- INTRODUCTION reconstruct flow dynamics and fluid properties, mobilized, basal debris flow (lahar) and ac- and to describe downstream and interchannel companying ash cloud as it flowed down the Within the first few minutes of the May 18, variation in flow. No attempt has been made to east flank of the volcano. The main source of 1980, eruption, numerous lahars were generated derive flow equations for these lahars because the water for the lahars was probably from on the upper western, southern, and eastern (a) too many variables are unconstrained and eroded snow and ice incorporated into the flanks of Mount St. Helens. Two of the largest of (b) the size and accuracy of the data set is flow by turbulent mixing, but ground water, these lahars were debris flows that originated on limited. expelled together with the rock debris by the the eastern side of the volcano (Figs. 1 and 2) initiating volcanic explosions, also may have and flowed down two parallel channels to Swift METHODS contributed. Reservoir. The Pine Creek lahar traveled down Peak lahar discharge from the Pine-Muddy across the Pine-Muddy fan and then down the Indirect Velocity Computation for Lahars fan, upper Smith Creek, and Ape Canyon east fork of Pine Creek (Figs. 1, 2, 3). Lahars probably exceeded 250,000 m3/s initially but entered the Muddy River drainage at four differ- From hydraulics, the velocity-head equation decreased exponentially in the downstream ent points, coalesced, and continued down the and the simplified superelevation equation direction. Total volume of the lahars was in lower Muddy River channel as the Muddy (Chow, 1959; Apmann, 1973) can be rewritten excess of 1.4 x 107 m3. Initial peak-flow ve- River lahar. Together, these east-flank lahars to solve for mean velocity, U. The accuracy of locities in excess of 30 m/s also decreased destroyed 16 bridges, buried several kilometres these equations has not been tested for lahars, markedly downstream. Where flow was not of road, and raised the level of the 14-km-long but they have, nevertheless, been previously ap- 3 impeded, velocity was strongly related to the reservoir 0.8 m by depositing 13.4 million m of plied to mudflows and debris flows (for depth-slope term (R2/3S1/2) from the Man- slurry in the water (Cassidy and others, 1980; example, Nakamura, 1926; Johnson, 1970; ning uniform-flow equation as a power-law Schuster, 1981). Additional deposits on the Guy, 1971; Dietrich and Dunne, 1978; Fink and function. During much of the route traveled, newly formed delta and in the channels would others, 1981; Janda and others, 1981). The first lahar flow appears to have been supercritical. bring the total volume to well in excess of 14 equation makes use of the effect of velocity head Deposits left in channels were generally thin million m3. The sequence of events, as well as relative to flow depth (0 to 2.5 m). Particles general descriptions of these flows and their up to small boulder size were randomly dis- deposits, are given in Janda and others (1981) tributed in the poorly sorted, nonstratified and Cummans (1981). Figure 1. Slopes and channels impacted by matrix, indicating complete suspension in a Lahars (volcanic mudflows or debris flows) major lahars triggered by the May 18, 1980, fully developed debris-flow slurry; however, are dense, viscous slurries of poorly sorted eruption of Mount St. Helens. Circled num- much larger clasts were transported as "bed- gravel, sand, mud, and water. The coarse frag- bers are designated cross sections where load." Computed sediment concentrations of ments are suspended in a pore fluid of silt- and equation 3 was used for velocity determina- matrix slurry samples ranged from 84% to clay-sized particles plus water and are held in tion; numbers in squares are sites where 91% solids by weight and were similar for the suspension by varying combinations of fric- equation 2 was used. Values from Cassidy two lahars. tional, buoyant, dispersive, turbulent, and cohe- and others (1980), Fink and others (1981), Two indirect methods for computing peak- sive forces (Bagnold, 1954; Hampton, 1975, and Janda and others (1981) are included. flow velocity, previously only tentatively ap- 1979; Pierson, 1981). Such slurries are non- Letters in triangles are localities mentioned in plied to debris flows, were tested for ac- Newtonian fluids, differing from water by hav- text.

Geological Society of America Bulletin, v. 96, p. 1056-1069, 17 figs., 2 tables, August 1985.

1056

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EXPLANATION Area devasted by direct blast •I Areas swept by surge and

lahars (not studied) 0123 miles I 1—S r-h 1—1 Assessed for east flank only: 012345 kilometers H Zone of gas-inflated surge • Transitional zone CH Lahar zone 11 Lahar overflow into standing forest Boundary between net erosion and net deposition J_

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Crater rim

Pine-Muddy fan

Figure 2. Composite oblique aerial view of east flank of Mount St. Helens taken July 1,1980. Ape Canyon is in the lower right; the upper Pine-Muddy fan is in left center. Small dark flows on fan are small lahars triggered after May 18 (photo: Stewart Lowther, for U.S. Geol. Survey).

in causing runup of flow on hills, ridges, or valley sides oriented perpendicular to the flow path

1 -T - -' 1 1 1 (Fig. 4A). It assumes that all kinetic energy is converted to potential energy: \\ PINE CREEK - H\\ MUDDY RIVER mil2 = mgh , (1)

which reduces to

\ \ \ \ U = [2ghf- (2) - V "V where m = mass, g = gravitational acceleration, and h = height of runup. The energy coefficient, a, that is often applied to open-channel flow to a. , , X correct for nonuniform velocity distribution, is assumed to be 1 and is not used in the equation.

DISTANCE DOWNSTREAM FROM CRATER, IN KILOMETERS Because frictionless flow is assumed, energy losses are not taken into account. Figure 3. Longitudinal profiles of the Pine Creek and Muddy River (via Ape Canyon) The superelevation equation is based on the channels. Vertical exaggeration is times 5.3. tilting of the free surface in a bend due to the

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r h I T L

0 10 20

METERS

Figure 4. Flow conditions useful for indirect velocity computation: A. Schematic longitudinal section of a lahar running up on a hillslope perpendicular to the flow path. The distance "h" is used in equation 2 to calculate velocity. B. Transverse section of a lahar showing superelevation in a bend. The distance "Ah" is used in equation 3 to calculate velocity. This is an unexaggerated scale drawing of a surveyed cross section at the head of Pine Creek (PI): Ah = 20 m.

action of centrifugal force (Fig. 4B). The surface when maximum cross-wave height occurs on of discharge values. If migrating cross waves oc- rises on the outside and lowers on the inside of the inner bank, it cancels the effect of superele- curred, average depth and, therefore, discharge the bend in proportion to U2. The difference in vation and the resulting water surface is flat would have been somewhat overestimated. water-surface elevations (mudlines) is defined as (Chow, 1959, p. 450). Except for apparent the superelevation, Ah. If it is assumed that "cross-valley sloshing" of the lahar in upper Ape Sediment Samples (1) all filamental velocities in the bend are equal Canyon, no multiple mudline maxima were seen to the mean velocity, (2) all streamlines have the in bends of the channels studied, even though Samples (1-2 kg) of lahar deposits were col- same radius of curvature, and (3) flow is subcrit- the bends were long enough. If cross waves did lected at different points along the channels from ical, application of Newton's second law of mo- develop, they migrated because of unsteady flow as high as possible on channel sides or terraces in tion to the centrifugal action in the curve yields conditions, leaving relatively smooth mudlines. order to determine composition of the lahars a simple equation for velocity (Chow, 1959, Superelevation equations remain valid for veloc- during peak flow. Sample sites were selected to p. 448): ity computation even if cross waves occur, be- avoid clasts larger than ~ 100-mm diameter; this cause the "effective superelevation" measured procedure provides data only on the matrix would be the difference between the outside and slurry and biases the results against the coarse inside cross-wave maxima, which is the same fraction. If larger clasts were included, however, elevation difference caused by superelevation it would be extremely difficult to get representa- where rc = centerline radius of curvature and b = channel width. This same equation has been de- alone (Chow, 1959). tive samples. Samples were analyzed by stan- rived from the principle of radial acceleration of The superelevation equation does not con- dard dry sieving and pipette techniques, and a unit mass, assuming that the free surface is sider the effects of channel roughness, resistance envelopes of cumulative particle-size distribu- perpendicular to the vector difference between of the bend, increased density of the fluid, or tion curves were drawn. the body force and the radial acceleration internal resistance to flow; therefore, this equa- (Johnson, 1970). Equation 3 can also be derived tion and equation 2 should both provide only LAHAR INITIATION from the parabolic equation for the free surface minimum velocity values. Coal slurries with the of a rotating fluid, given the assumptions that the same sediment concentration as typical lahars Lahars may originate in two general ways. material is a perfect fluid, rc is constant, and can flow very efficiently, however—with less One involves the sudden release of large vol- there is no relative motion between fluid than half the friction loss of pure water (Hughes umes of water on a slope; in the turbulent particles—conditions that may be approached in and Brighton, 1967). With properties apparently downhill rush of the flood, sufficient loose vol- "plug flow" of viscous slurries, as defined by providing compensating effects, the accuracy of canic debris may be incorporated to form a Johnson (1970). these equations remains to be verified. lahar. Lahar-generating events in this category Interpretation of mudlines in channel bends is include (1) the breakout of water from debris- more ambiguous for supercritical flows. When Discharge Determination dammed lakes or from crater lakes; (2) the sud- streamflow becomes supercritical, cross waves den release of subglacial reservoirs (jokulhlaups) may be generated in bends and in other localities When an average peak-flow velocity has been which may or may not coincide with eruptions; of irregular channel alignment and shape. A dis- determined at a cross section, peak discharges (3) the action of heavy rain during an eruption, turbance pattern from intersecting wave fronts can be estimated by multiplying mean velocity often generated by the eruption itself; and (4) the can form and cause multiple maximum and by the surveyed cross-sectional area of the lahar sudden melting of snow and ice during an erup- minimum water-surface elevations at regular in- at the cross section. If channels were incised or tion by hot gases, radiant heat, or hot debris on tervals on both channel banks (Chow, 1959). aggraded slightly between the time of lahar flow snow and ice (Neall, 1976; Waitt and others, When maximum cross-wave height occurs on and the time the channels were surveyed, some 1983). the outer bank, it can double the superelevation; error would have been introduced into estimates The other mechanism involves large volumes

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/8/1056/3445037/i0016-7606-96-8-1056.pdf by guest on 29 September 2021 Figure 5. Photo sequence of May 18,1980, eruption taken by Ken Seibert from Calamity Point, 32 km to the south-southwest. A. East-west line of volcanic explosions (light puffs) across summit (from release of landslide block?) as surge cloud starts down outer flanks. Directed blast cloud already far to the north. B. Low, ground-hugging pyroclastic surge accelerating down slope. C. Surge has reached head of Pine-Muddy fan. D. Surge is partway down upper fan.

of volcanic debris that are set in motion by ex- northwest, east, and southeast sides (Janda and plosive and/or gravitational forces and evolve others, 1981; Cummans, 1981; Fairchild and into lahars during, or shortly after, transport. Wigmosta, 1983); smaller lahars filled the gaps The debris must either be wet to start with or in between (namely, Major, 1984). The surge is able to pick up sufficient water during transport known to be the triggering event for the lahars to liquefy. Specific events in this category in- because no other events took place on the con- clude (5) hot pyroclastic flows moving over and tinuously observed and photographed outer eroding snow or ice or flowing into streams, flanks for the next several hours, and lahars (6) wet debris ejected by volcanic explosions, reached Swift Reservoir (Fig. 1) within one-half and (7) large landslides of water-saturated debris hour of the beginning of the eruption. (Neall, 1976; Janda and others, 1981). Field and photographic evidence suggests that the surge started out as an inflated mixture of Initiation of the East-Flank Lahars rock debris, gas, and possibly liquid water—a turbulent and, presumably, well-mixed gaseous Photographs of the early moments of the May suspension. It collapsed to a high-density, satu- 18 eruption1 show a collar of dense, turbulent, rated basal flow (lahar) with an overriding low- ground-hugging cloud descending the east, density, highly gas-expanded ash cloud by the south, and west flanks of Mount St. Helens time it had traveled 4 km from the crater rim (Fig. 5). The ring of outward-expanding cloud (Fig. 6). It is hypothesized that this transforma- appears to be a lateral component of the vol- tion occurred through settling out of coarser canic explosions released along the failure scarp clasts (gravity segregation), in a manner similar of one of the landslide blocks (probably block to that described for dry block-and-ash flows III) during failure of the catastrophic rockslide (Fisher and Heiken, 1982). The transformation avalanche (Moore and Rice, 1984; compare probably also involved the incorporation of ad- with Voight and others, 1981). This interpreta- ditional water and sediment from the flanks of tion is supported by the appearance of a linear Figure 6. Schematic representation of vol- the volcano. ridge of uniformly growing, light colored explo- canic explosion, laterally directed pyroclastic Three "impact zones" can be designated on sion clouds (Fig. 5A) in the approximate posi- surge, and evolution of surge into a laher on the east flank on the basis of erosional and depo- tion of a headscarp just before the darker cloud the east flank of Mount St. Helens, beginning sitional characteristics (Fig. 1). They appear to "boiled over" the crater rim and descended. The at -08:34 PDT, May 18,1980. Top: volcanic reflect the type and severity of impact from the appearance of the cloud in the photographs fits explosions, outward movement. Middle: collapsing pyroclastic surge. very well with the general definition of a vol- gravity takes over as driving force, heavier The upper zone, extending from the crater canic "base surge." This phenomenon, a type of particles segregate out, flow is turbulent and rim to —1-2 km downslope, is characterized by pyroclastic surge (Sheridan, 1979), is defined as erosive. Bottom: deflation to water-mobilized relatively evenly spaced furrows cut into snow a turbulent, high-velocity density flow of gas, basal flow (lahar); accompanying ash cloud and ice on the upper flanks of the volcano rock debris, and sometimes liquid water, which lifts, decelerates, and stalls. (Fig. 7). Aerial photography shows that these is driven first by the lateral component of a volcanic explosion and then by gravity (Moore, 1967; Williams and McBirney, 1979). Because the cross-bedded, dune-like pyroclastic deposits commonly attributed to base surges (Fisher and Waters, 1970; Waters and Fisher, 1971) were not observed in the field, the more general term "pyroclastic surge" will be used to describe the turbulent cloud that initiated the east-flank lahars. Inferring from descriptions of other volcanic eruptions and their deposits, lahars may have been triggered by pyroclastic surges during eruptions of Bandai-san in 1888 (Sekiya and Kikuchi, 1890), Mount Lassen in 1915 (Day and Allen, 1925), Hekla in 1947 (Kjartansson, 1951), and Bezymianny in 1956 (Gorshkov, 1959). The pyroclastic surge photographed at Mount St. Helens (Fig. 5) evolved into lahars over the entire outer flank of the volcano (Fig. 1). Large, particularly mobile lahars came down the

'Sequences of photographs were taken from the east by John V. Christiansen, from the south-southeast by Ken Seibert, from the south by Harold Fosterman, and from the west by Ty Kearney (Foxworthy and Hill, Figure 7. Furrowing in snow on southeast flank of Mount St. Helens directly below crater 1982, p. 50-51). rim, June 19,1980 (photo: Austin Post, U.S. Geol. Survey).

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features were in place before the onset of pumi- compact, poorly sorted, muddy, gravelly sand at started descending the south and east flanks at ceous pyroclastic flows in the afternoon. Such the same stratigraphic position as the loose about the same time and was a well-developed an erosion pattern can be created by base surges gravelly sand farther upslope. This deposit, which surge by 08:33.8. It reached the upper Pine- above the main zone of deposition (Moore, commonly contains abundant vesicles, is inter- Muddy fan by 08:35.0 and the base of the cone 1967), indicating an inflated condition. Above preted as being a saturated lahar deposit. Flow on the south side by 08:35.1. At 08:35.5, the ~l,800-m elevation (slope 3=28°), generally lit- at the head of the lahar zone was still erosional, surge cloud still appeared to be moving on the tle deposition occurred. Where examined on the but when it reached the dotted line, as shown in fan, but by 08:36.5, forward motion had been southwest flank (Jon Major, 1985, written Figure 1, net erosion changed to net deposition. arrested and the ash cloud was rising over the commun.), deposits form thin (<10 cm), discon- Trees just outside the flow path at the head of middle of the fan. Bracketed between 08:33.7 tinuous beds of subangular to angular, fine lithic this zone (location B, Fig. 1) were blasted by a and 08:35.0, the surge had an average velocity sand mixed with small amounts of silt- and clay- fine-grained ash cloud which left a gray, sand- down the east flank of -50 m/s. The average sized grains and occasional pebbles. Some bub- sized ash deposit and which was apparently able velocity of the visibly slower part of the surge on ble vesicles were present, suggesting that some of to shear off tops of trees at progressively greater the south flank was -30 m/s (Moore and Rice, the material was wet when deposited. heights above the ground with distance down- 1984). The middle zone extends from — 2 to 4 km slope. This damage to the forest may correspond North of Ape Canyon (Fig. 1), the upper (in- from the crater rim (Fig. 8). It lacks the furrow- to the expanding ash cloud lifting off the ground flated surge) zone appears to merge with the ing, yet erosion in this zone was very severe, as it lost horizontal momentum. Needles remain- area devastated by the directed blast. The di- removing all vegetation and some upper soil ing on fir and pine trees were killed but not rected blast was a pyroclastic surge with greater layers. The lowermost May 18 deposit in this charred by the ash cloud, suggesting a tempera- energy, higher velocities, and higher tempera- zone is a loose, poorly sorted gravelly sand ture >50 °C but <250 °C (Winner and Casade- tures than the one affecting the outer flanks composed of predominantly lithic ash and la- vall, 1981). (Kieffer, 1981; Moore and Sisson, 1981). pilli. Deposition is highly variable; the deposit is Timing of the east-flank pyroclastic surge was commonly absent on topographic highs but is up accomplished by comparing Seibert's photo- Source of Water in the Lahars to 0.5 m thick in swales. At location A (Fig. 1) graphs with the Christiansen photographs that at the flow margin, gravel-sized clasts are im- were timed by Moore and Rice (1984). Moore Approximately 14 million m3 of lahar slurry bedded in tree trunks at orientations diverging as and Rice were able to time that photo sequence reached Swift Reservoir from the Pine and much as 90° from the main flow direction, indi- by correlating a number of different photo sets, a Muddy drainages (Cassidy and others, 1980), cating significant turbulence. These features, to- videotape, and a key radio transmission with and the slurry is assumed to have had a water gether with the absence of characteristic debris- accurately timed satellite imagery. Correlation content of -32% by vol (see section on charac- flow deposits, suggest that the surge had not yet was based on cloud morphology and front teristics of the lahar slurries). About 2 km2 of fully collapsed into a saturated basal flow and position. the upper slopes on Mount St. Helens' east that this is a zone of transition. The outer-flank pyroclastic surge emerged flank showed evidence, on the aerial photo- The lower zone starts at ~4 km from the from the crater at 08:33.7 PDT, following a graphs, of significant scour which was accom- crater and extends downstream. It is first charac- major collapse of the south crater rim (slide plished by the pyroclastic surge and by later, hot terized by the occurrence of a thin (2-5 cm), block III) at 08:33.4 (Moore and Rice, 1984). It pyroclastic flows. Brugman and Meier (1981) reported that an average of 6 m of snow and ice was eroded from east-flank glacier surfaces on May 18, and that figure may actually be closer to 10 m (M. Brugman, 1983, written commun.). Head of The amount of water in the lahars reaching Ape Canyon Swift Reservoir could be accounted for by a 4.6-m layer of snow (assumed density, 0.5) which was removed from this area and incorporated into a dry surge cloud. Flow would have to be turbulent for this mixing to occur, but the process is not well understood. If the surge cloud already contained some moisture, a lesser thickness of snow would be required to account for the water in the lahars. The debris in the surge cloud may have been already wet when it was ejected, but the evidence supporting this possibility is only circum- stantial. Voight and others (1981) inferred that extremely high ground-water pore pressures were present in the volcanic cone immediately prior to the eruption, and a number of eyewit- nesses reported "mud balls" falling close to the mountain very early in the eruption (Rosen- Figure 8. Lower east flank of Mount St. Helens, July 7, 1980, that was swept by the baum and Waitt, 1981). On the other hand, wet collapsing pyroclastic surge (photo: Robert M. Krimmel, U.S. Geol. Survey). base surges typically emplace their moist, cohe-

Figure 9. Swift Reservoir stage record and infilling rate on May 18, 1980. Approximate configuration of waves caused by lahar entry shown by dashed lines. Infilling rate is equivalent to lahar discharge into the reservoir. Total volume deposited under water in the morning was 13,431,000 m3; volume from afternoon event was 520,000 m3. Stage record courtesy of Pacific Power and Light Company. Computations by John Cummans (1980, written commun.).

flow velocity. This is a reasonable assumption because, for debris flows set in motion by a relatively instantaneous "pulse" of debris, such as by a landslide, peak flow occurs immediately behind the steep flow front and because front velocity is approximately equal to peak-flow mean fluid velocity (Pierson, 1985). As a debris flow slows with distance downstream, the peak can lag significantly behind the front. It is doubtful, however, that a separation of any significance occurred in the Pine Creek lahar because of the shape of the inflow hydrograph and because it did not occur with the similar South Fork Tou- 1000 1200 1400 1600 tle River (SF) lahar. The peak of the SF lahar, TIME (PACIFIC DAYLIGHT TIME) triggered in the same early moments of the May 18 eruption on the west flank, lagged only 7 min behind the front 43 km downstream (Cummans, sive sediments in cross-bedded dunes and as rise that was observed (Fig. 9) in the reservoir 1981), and total flow duration was ~ 1 hr (Fair- plastered mud coatings on upright obstructions level. child and Wigmosta, 1983). The Pine Creek such as tree trunks (Moore, 1967; Fisher and Entry of the first lahar into the standing water lahar, flowing down a steeper channel, entered Waters, 1970; Waters and Fisher, 1971). Such of the reservoir generated a wave 0.4 m high at Swift Reservoir within half that distance, so an features have not been found in the path of the the dam (Cassidy and others, 1980). An incre- even closer proximity of flow front and flow east-flank surge. mently averaged hydrograph of the rate of infill- peak would be expected, and the velocities The most likely source of water for the lahars ing was constructed from the Swift Dam would be in close agreement. was snow and ice eroded by the pyroclastic stage-height record (John Cummans, 1980, surge (and developing basal flow) and incorpo- written commun.). This hydrograph, equivalent To estimate traveltime for the Pine Creek rated through turbulent mixing; however, the to lahar discharge, has steep rising and falling lahar from velocity data, peak-flow velocities possibility of water being explosively ejected limbs and a very approximate peak discharge of (Table 1) were computed at the cross sections with the pyroclastic debris and gas cannot be 7,500 m3/s (Fig. 9). More than half of the flow shown in Figure 1 by equations 2 and 3. The discounted. volume was deposited within the first 30 min, Pine Creek channel was then divided into and total flow duration was only -2 hr. The reaches centered on cross-section locations, and LAHAR FLOW BEHAVIOR stage-height recorder at Swift Dam, operated by velocity values were assigned, in turn, to the Pacific Power and Light Company, recorded the reaches. Reach length was multiplied by velocity Verification of Velocity-Computation arrival of the first wave at between 09:04 ±3 to obtain time of passage through the reach; Methods min (a time span attributed to error in resolution these times were then summed for all reaches to of the record and establishment of the time obtain computed arrival time at Swift Reservoir. /2 Virtually all of the —14 million m3 of lahar base). Wave celerity (c = (gD)' , where average The computed traveltime was 23 min. This is slurry that reached Swift Reservoir on the morn- reservoir depth = 61 m) would have been 24.5 -15% longer than the recorded traveltime, ing of May 18 arrived via Pine Creek and m/s, and it would have taken ~10 min for a which is consistent with the fact that the com- Muddy River.2 The Pine Creek lahar was prob- wave to travel the length of the reservoir. The puted velocities resulted from equations that ably the first of the two lahars to enter the reser- Pine Creek lahar therefore must have entered yield only minimum velocity values. voir, to which it had the shorter and steeper the lake at 08:54 ±3 min, having traveled 22.5 route. A small lahar also entered the reservoir km in 20 ±3 min. Unchannelized Lahar Flow on the Pine- from Swift Creek (Fig. 1), but its volume was An approximate verification of the velocity Muddy Fan not great enough to effect the sustained rapid equations can be made by comparing the recorded arrival time for the Pine Creek lahar with At the head of the lahar zone (Fig. 1), the the computed arrival time obtained by routing lahar flowed as a broad sheet 1.3 km wide and 2A volume of 13.4 million m3 was deposited under the lahar down the channel. It is first necessary, water in the reservoir, causing the water level to rise. A at least 5 m deep in places, as estimated from large, but unknown, volume was deposited above the however, to assume that velocity of the flow abraded trees on the flow margin. Its momen- water surface on the newly formed delta. front is approximately equal to computed peak- tum carried it 47 m up and over a ridge 3.8 km

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from the present crater rim at location A (Fig. 1). If a mean depth of 5 m is assumed, equation 2 yields a minimum velocity at 29 m/s. Fink and others (1981) reported 31 m/s at the head of West Fork Pine Creek, just under 4 km from the crater rim, and Janda and others (1981) computed 40 m/s at the head of Ape Canyon, 3.0 km from the crater rim. On the west side, 33 m/s was calculated for the South Fork Toutle River lahar (Fairchild and Wig- mosta, 1983), and 36 and 41 m/s were computed for small lahars on the southwest flank (Major, 1984). Peak velocities for these outer-flank lahars are among the highest known velocities reported for lahars. A mean velocity of 42 m/s was computed using eyewitness accounts for a lahar triggered by an explosive eruption of Mt. Tokati (Japan) on a mean slope of 16°, ~2 km from the crater (Nakamura, 1926). This lahar was probably still evolving from a pyroclastic surge because it was described as a dark, avalanching cloud by those who viewed from a distance, but it was also described as a "mud-flow" by a person who narrowly escaped being carried away. Farther downstream, velocities of 26 to 34 m/s Figure 10. Boulder-strewn surface of lower Pine-Muddy fan. May 30, 1980. Battered re- on slopes 3° to 8° were computed from high mains of small trees in middle distance were too flexible to break. (Photo: Terry Leighley, for mudlines using equation 3. The next highest U.S. Geol. Survey.)

TABLE 1. HYDRAULIC AND PHYSICAL PROPERTIES OF EAST-FLANK LAHARS AT MOUNT ST. HELENS, 18 MAY 1980

Sites Peak Peak Hydraulic Channel Channel Downstream Froude Estimated Estimated Estimated (in downstream velocity discharge depth width slope distance number F solids solids bulk order) UCra/s) Q(m-Vs) D(m) b(m) Sim. m j from crater J~ I concentration concentration density 3 L(km) (gDr by weight by volume Db(g/cm )

Cw Cv

Pine-Muddy fan F1 29+ 190,000+ 1,300 0.121 4.1 + F2 28 190,000 1.350 0.083 4.0 F3 20 65,000 1.060 0.059 3.7 Muddy River system AO 40 0.144 3.0 Al 28.0 66,800 20.6 116 0.028 6.8 2.0 A2 26.8 63,900 4.9 488 0.019 7.5 3.9 M0 23 20,000 0.110 9.8 MI 14.4 22,000 4.7 295 0.020 13.9 M2 6.8 7,150 3.5 305 0.011 18.1 1.2 0.90"f 0.78+ 2.28+ M3 3.7 2,440 5.3 124 0.003 21.4 0.5 M4 4.4 2,690 2.2 276 0.007 22.7 0.9 M5 6.4 3,460 5.1 106 0.003 24.5 0.9 0.86 0.69 2.14 M6 3.2 4.860 8.7 174 0.005 28.1 0.4 0.83 0.64 2.06 LI 6 0.006 31.0

Pine Creek system WP 31 0.123 4.4 PI 23.5 17,100 9.8 74 0.154 9.1 2.4 P2 17.7 28.600 15.2 106 0.092 9.9 1.4 P2.I 20.8 25.900 12.6 99 0.065 10.1 1.9 P3 13.1 28,200 14.5 148 0.041 10.8 1.1 P4 12.4 21,700 14.9 117 0.042 11.5 1.0 P5 10.9 19,900 14.8 123 0.043 12.2 0.9 0.86 0.70 2.16 P6 14.2 21,000 13.9 106 0.036 14.1 1.2 2.10 P7 21.1 19,200 10.7 85 0.031 14.8 2.1 0.84 0.66 P8 15.3 16,600 9.4 116 0.026 16.8 1.6 P9 9.3 6,250 9.3 72 0.027 19.5 1.0 PIO 11.0 8,930 9.0 90 0.019 19.6 1.2 Pll 12.0 7,320 6.0 100 0.030 20.9 1.6 L2 9 0.009 22.5

*Basal deposit on fan. 'l ower depositional unit.

As in Muddy River, the lahar left coatings of mud, sand, and fine gravel on tree trunks near the edge of the flow (Fig. 11). It deposited a thin (0-0.5 m) veneer of slurry in the channel and on terrace surfaces, and it left tangles of organic debris along the flow margin. Boulder accumulations were noted on the insides of tight bends. Comparison of a pre-lahar bridge survey with a post-lahar cross section just above the mouth of Pine Creek shows no net change in bed elevation; erosion on one side of the channel (-1 m deep) was compensated for by an equal amount of deposition on the other side (H. Martinson, 1983, written commun.). Relatively undisturbed forest soil beneath lahar deposits at a number of locations in lower Pine Creek indicates that the lahar tended not to be erosive there. Even in the steeper upper part of the channel, erosion by peak lahar flow appears to have been minor.

Flow in Tributaries to Muddy River

Another lahar evolving from the east-flank Figure 11. Coatings of mud, sand, and fine gravel left by the lahar on tree trunks, lower pyroclastic surge entered Ape Canyon (Fig. 1) at Muddy River. Note person on terrace (right side) for scale. (Photo: Lyn Topinka, U.S. Geol. a velocity of 40 m/s (Janda and others, 1981), Survey). scouring most of the upper canyon to bedrock. Wavy, uneven trimlines on the walls of the middle part of the canyon suggest either cross-valley known lahar velocity is 25 m/s, reported by Iida boulders and piles of battered logs mark the "sloshing" or the formation of standing cross (1938) from Mount Bandai. Such extreme ve- boundaries of the main flow, although along waves during supercritical peak flow. In the locities are inferred as being the result of energy much of the southern flow boundary (Fig. 1), lower canyon, flow depth exceeded 20 m, aver- imparted by the explosive ejection of debris and the slurry spilled over into the forest and moved age velocity was 28 m/s, and peak discharge 3 of momentum accumulated during the downhill passively among the trees for several kilometres. was -67,000 m /s. There, the lahar scoured sweep of the inflated surge cloud, which would most of the soil from the canyon walls but left a Pine Creek Lahar have experienced much less frictional resistance dark gray, poorly sorted, unstratified deposit as to flow than would a fully developed lahar. The unchannelized lahar split on the lower much as 1 m thick on the canyon floor. At the Peak discharge on the upper fan at the cross Pine-Muddy fan, the larger portion funneling canyon mouth, it spread out, surged across the section through location A was estimated to be into East Fork Pine Creek at a velocity of 21 to Smith Creek valley, and ran up the opposite at least 190,000 m3/s. Two kilometres down- 24 m/s (Fig. 1). As soon as it was channelized, valley side at 27 m/s. fan, velocity had decreased to only 28 m/s. On superelevation of the debris slurry around bends Preliminary examination of sediments depos- the lower fan, the lahar continued to flow as a was dramatic (Fig. 4). As the lahar entered the ited on May 18 in upper Smith Creek (location broad sheet ~3 m thick, decelerating to -20 East Fork channel, peak discharge was -29,000 C) suggests that the east-flank pyroclastic surge, m/s before entering the channels of Muddy m3/s, and flow depths were on the order of 10 to coupled with, or replaced by, the energetic River and Pine Creek, 8.2 and 9.4 km, respec- 15 m. In the upper channel where width was "blast" pyroclastic surge, was still in the transi- tively, from the crater rim. constricted, flow depth remained at -13 m for tional stage. Rather unusual deposits at location Along this flow path, almost all trees were several kilometres and average velocity ranged C suggest that a dense, high-velocity, and fairly sheared off at ground level by the lahar, leaving between 11 and 20 m/s. Sharp bends in the complicated flow event came down upper Smith splintered stumps only a few tens of centimetres channel appear to have played an important role Creek. It appears to have had a partly devel- high. Those trees remaining near the flow mar- in providing hydraulic resistance which slowed oped, dense basal flow with a dense, but gins were severely abraded on parts that were the lahar. The lower half of the Pine Creek somewhat more inflated, flow above. submerged. Lahar deposits on the Pine-Muddy channel was wider than that above; here, aver- A lahar phase from upper Smith Creek fan ranged from vesicle-rich mud coatings, just a age flow depth decreased to 8 m, and average reached the mouth of Ape Canyon after the Ape few centimetres thick and only locally preserved velocity ranged from 9 to 12 m/s. At the mouth Canyon lahar, and it was partly dammed by on the upper fan, to deposits on the lower fan as of Pine Creek, peak discharge had decreased to Ape Canyon deposits. Neither the velocity nor 3 much as 1 m thick. Over large areas, a veneer of -7,300 m /s. From the mouth, it ran up the discharge of this lahar is known. poorly sorted, unstratified muddy, sandy gravel opposite side of the Lewis River valley before Another minor lahar descended the unnamed 0.3 to 0.5 m thick and scattered boulders as turning the corner and flowing the last kilometre canyon immediately south of the Muddy River much as 2 m in diameter was all that was de- to Swift Reservoir at an average velocity of -9 Canyon (Fig. 1). Its velocity and discharge were posited by this massive lahar (Fig. 10). Levees of m/s (Cassidy and others, 1980). not determined.

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Muddy River Lahar Reservoir at -17:10 PDT (timing from another transport by suspension or saltation in the main sudden rise in reservoir level). It deposited current of the lahar but with a tendency to settle Two lahar units deposited in Muddy River -520,000 m3 of slurry in the reservoir, and its out of suspension in slack-current locations. The below the Muddy-Smith confluence indicate solid component was made up almost exclu- largest known boulder transported was found that the lahar arrived there in two major pulses. sively of white rounded pumice clasts, ash, and resting on a terrace on May 18 deposits near The underlying first unit is sedimentologically uncharred fragments of wood. This composition location M2 (Fig. 1). It measured 10.4 x 6.0 x similar to the Ape Canyon flow. The finer, sec- suggests derivation from a hot pumiceous pyro- 4.6 m and was probably rolled there as ond unit probably came down Muddy River clastic flow. Peak pyroclastic-flow production "bedload" by the lahar. Flow depth at that site Gorge, where peak velocity was 23 m/s (Cas- occurred from -15:30 to 16:10 (Criswell, was only -4 m. sidy and others, 1980) and peak discharge was 1983). If the initiating pyroclastic flow occurred Particle-size distributions (Fig. 12) are ap- 3 -20,000 m /s. Peak flow immediately down- within this time frame, traveltime down the proximately similar for the two lahars, but there stream from location Ml (Fig. 1) was -22,000 Muddy River would be between 1 and 1.7 hr, are significant variations within each flow. On 3 m /s, and average velocity was 14 m/s. The which is reasonable. The denser morning lahar the basis of Varnes' (1978) classification, the 3 peak attenuated rapidly to 7,200 m /s only took -1.1 hr to cover the same distance. samples of matrix slurry from both lahars are 4 km downstream, and it further decreased to close to the boundary between "mudflow" and 3 2,400 m /s with a velocity of 7 m/s in only PHYSICAL CHARACTERISTICS OF "debris flow"; however, if all unsampled coarse another 3.3 km. One eyewitness account (tes- THE LAHAR SLURRIES clasts could be included, both lahars would be timony of Paul and Kathy Hickson) states that classified as debris flows. the highway bridge on the lower Muddy (im- Deposits left by the Pine Creek and Muddy Statistical measures of particle-size distribu- mediately upstream of location M3) was gone River lahars had a number of characteristics tion (Folk, 1965) were calculated and are sum- and the river was still "in flood" at -10:00 PDT; quite different from normal fluvial deposits. The marized in Table 2, where data from the North they reported that by -10:45 or 11:00, the sediments were poorly sorted, nonstratified, and Fork and South Fork Toutle River lahars are "flood" had receded considerably but was still generally ungraded mixtures of clay-sized to shown for comparison. Texturally, the sample flowing with "thick mud." Below the bridge, boulder-sized particles which were deposited in deposits can be described as either muddy, peak velocities fluctuated between 3 and 6 m/s Pine Creek as a single flow unit and in Muddy sandy gravels or muddy, gravelly sands (Folk, and peak discharge varied between 2,700 and 3 River as two successive units. When examined 1965). The gravel fraction was predominantly 4,900 m /s, apparently increasing downstream the following year, the deposits resembled dried porphyritic dacite; pebble-sized clasts were very because of hydraulic damming in very narrow concrete, forming a layer of variable thickness angular, but larger clasts were more rounded. A reaches. Deposits here comprise a single flow on the Pine-Muddy fan surface, channel beds, small amount of juvenile "blast dacite" was unit. Either they were deposited by the finer, and inundated terraces. Deposits were, in places, found in these deposits, slightly more in Muddy second pulse or they resulted from a mixture of as thick as 2.5 m, but they were generally less River deposits than in Pine Creek (R. Janda, the two pulses. After entering the Lewis River, than 0.5 m and thin relative to the depth of 1983, oral commun.). the lahar was still flowing at ~6 m/s (Cassidy flows that deposited them. Thin coatings of The Muddy River samples were, on the aver- and others, 1980) when it flowed into Swift Res- muddy sand also adhered in places to steep age, coarser than those from Pine Creek, but the ervoir. Routing this lahar down the channel channel banks and tree trunks (Fig. 11). Pebble- samples of slurry matrix of both east-side lahars (via Ape Canyon), using calculated velocities to small boulder-sized particles were randomly were considerably coarser than samples of the and the same starting time as the Pine Creek distributed throughout the deposits; this was the North Fork and South Fork Toutle River lahars lahar, yields an arrival time at the head of Swift result of their complete suspension in a structur- (Table 2). All were very poorly sorted, and most Reservoir of 09:38 PDT. The leading edge ally coherent slurry. Larger boulder-sized parti- had weakly fine-skewed distributions (with the should have reached the bridge across the cles (>-50 cm) were commonly found both exception of Pine Creek). The distributions were Muddy River by -08:56. (1) as accumulations behind obstructions and on mesokurtic to platykurtic. This means that the Later in the day, another large lahar of very the insides of sharp channel bends and (2) on tails were better sorted than were the central different composition came down Smith Creek terrace surfaces many metres above the channel portions of the distributions and that the distri- and Muddy River, arriving at the head of Swift bed. These contrasting depositional sites indicate butions were slightly deficiently peaked com- pared to perfectly normal distributions. No significant downstream changes in mean grain TABLE 2. SUMMARY OF STATISTICAL GRAIN-SIZE CHARACTERISTICS FOR SAMPLES OF PEAK-FLOW DEPOSITS FROM THE PINE CREEK AND MUDDY RIVER LAHARS size or sorting were evident for the matrix slurry of either lahar (Fig. 13). The Muddy River lahar Mean grain Sorting Skewness Kurtosis did show a distinct difference, however, between diameter coefficient Sk| KG the coarse lower unit and the finer upper unit. ( units) (fit units) Sediment concentration by weight for the

Pine Creek lahar 0.19 t 0.68 3.19 ± 0.28 -0.15 ± 0.08 0.86 i 0.07 lahar matrix slurries (weight of solids divided by n = 13 total weight) (Table 1) was estimated by recon- Muddy River lahar -0.36 ± 1.50 3.03 ± 0.35 0.20 ± 0.27 0.87 ±0.13 stituting dry samples with water to optimum n = 11 slurry consistency in the laboratory; the conver- North Fork Toutle 1.22 ± 1.13 3.58 ± 0.51 0.13 ± 0 18 1.04 ±0.11 sion to volume concentration (volume of solids River lahar n = 13 divided by total volume) was made by assuming

South Fork Toutle 1.20 t 0.94 3.06 ± 0.62 0.04 ± 0.15 1.06 ± 0.15 a particle density of 2.65. Although reconstitu- River lahar tion is a subjective technique, it is relatively ac- n = 7 curate because slurry consistency is extremely Note: data in this table as defined by Folk (1965). Characteristics for North Fork and South Fork Toutle River lahars (K. Scott. 1982, written commun.) are shown sensitive to very small changes in water content. for comparison. A decrease in water content of only 2 to 3 wt %

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/8/1056/3445037/i0016-7606-96-8-1056.pdf by guest on 29 September 2021 GRAIN DIAMETER, IN MILLIMETERS -r- i T 1 1 — o PINE CREEK LAHAR MUDDY RIVER LAHAR 1st & 2nd UNITS 100 10 1 0.1 0.01 3 • a _ A LAHAR ON PINE-MUDDY FAN - • • • O o o 3 o • - o a - N °o (0 - o

er h-" z LU O 3.0

O O

S 2

'•" 0 10 20 30 Figure 12. Envelopes of cumulative grain-size curves for the Pine DISTANCE DOWNSTREAM FROM CRATER RIM, IN KILOMETERS Creek and Muddy River lahars. Sampling was limited to peak-flow deposits and sites where clasts were finer than 100 mm. Figure 13. Downstream variation in mean grain size (Graphic Mean) and degree of sorting (Inclusive Graphic Standard Deviation) in peak-flow lahar deposits. After Folk (1965). 1 1 T A LAHAR ON PINE MUDDY FAN

O PINE CREEK LAHAR

• MUDDY RIVER LAHAR

Ii go

o o UJ CO _1_ cc jy LU 0 10 20 30 CL '8 DISTANCE DOWNSTREAM FROM CRATER (L), IN KILOMETERS CO a LU Figure 14. Downstream variation in calculated mean lahar velocity b- LU at peak flow in Pine Creek and Muddy River. 5

O PINE CREEK LAHAR

• MUDDY RIVER LAHAR

Figure 15. Change in mean lahar velocity with respect to R2/3S1/2, where R is hydraulic radius of the channel and S is channel slope. The regression equation is U = 20.15 (R2/3S1/2)0-73, with r = 0.93. Values plotted are only for relatively unimpeded open-channel flow; values at 2/3 1/2 severe channel constrictions and very tight bends were omitted. R S , IN METERS

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-i—l i i i | 1 1 1 1—i—r Figure 16. Downstream variation in calculated lahar peak discharge A LAHAR ON PINE-MUDDY FAN in Pine Creek and Muddy River.

\ O PINE CREEK LAHAR ,A \ • MUDDY RIVER LAHAR \

\ 1 1 OP/v 0 PINE CREEK LAHAR • MUDDY RIVER LAHAR • A LAHAR ON PINE-MUDDY FAN -

0\ - ' A Q A

O 0 o ° 0 o O 0 o 0 o n o SUPERCRITICAL FLOW p — -® H — SUBCRITICAL FLOW ^O D • • 0 10 20 30

DISTANCE DOWNSTREAM FROM CRATER , IN KILOMETERS

Figure 17. Downstream variation in Froude number of lahars at DISTANCE DOWNSTREAM FROM CRATER (L), IN KILOMETERS peak flow in Pine Creek and Muddy River.

from optimum can render a slurry too viscous to rapidly with distance away from the crater significantly with distance downstream (Fig. flow; an increase of 3 to 4 wt % can dilute the (Fig. 14). Flow velocity decreased steadily 16). Flow magnitude was attenuated by more slurry to the point where it cannot hold gravel- down Pine Creek, but in the low-gradient than two orders of magnitude in the 21 and 30 sized particles in suspension. Because the lahars Muddy River, velocity became approximately km traveled by the two lahars. The trend was moved with unambiguous fluidity and held even constant (in the range, 3 to 6 m/s) beyond 22 interrupted, however, apparently by channel boulders in suspension, sediment concentrations km downstream. The velocity decrease is due to constrictions that caused hydraulic damming must have been within a few percent of the es- the combined effect of decreasing channel gra- and a build-up of flow. The linear trend of the timated values. One error not accounted for, dient, decreasing flow depth, and progressive data points on a log-log plot indicates that the however, is the unknown amount of fines lost deposition along the flow path. A nonlinear decrease is approximately described by a power- from the deposit during dewatering of the slurry (power curve) dependence of velocity (U) on law function. deposits. Samples from both lahars had a range depth and slope can be shown for sites of unob- Froude number (F) was calculated for each of sediment concentrations (Table 1) typical for structed flow by plotting U versus R2/3S1/2 surveyed cross section (Table 1). Assuming that debris flows (Pierson, 1980), and there appears (where R = hydraulic radius and S = channel the concept of Froude numbers is valid for vis- to be no significant difference between Pine gradient), the combined depth-slope term in the cous slurries, the results suggest that flow in Pine Creek and Muddy River samples. well-known Manning uniform-flow equation Creek remained near, or above, critical veloci- (Fig. 15). The correlation coefficient, r, for the ties along the entire flow path, whereas the VARIATION IN FLOW BEHAVIOR linear regression of log-transformed values is Muddy River lahar flowed subcritically through 0.93. It might be tempting to use such a relation- the lower channel reaches (Fig. 17).3 No inde- The magnitude and nature of lahar flow in ship to predict velocities of other lahars; how- pendent evidence for hydraulic jumps or drops Pine Creek and Muddy River changed conspic- ever, data from smaller debris flows (Pierson, was observed in the field. uously in the downstream direction. Similar 1985) indicate that flow velocity is also highly changes have been observed or inferred for other dependent on slurry sediment concentration, a lahars at Mount St. Helens (Fairchild and Wig- factor that may vary significantly between dif- 3 ! mosta, 1983; Pierson and Scott, 1985). ferent flows. The reason for the nonlinearity of In subcritical flow [U <(gD)' ], gravitational forces the plot is not known, suggesting that other fac- are dominant, and flow depth is greater than critical Peak-flow velocities between 30 and 40 m/s l/! tors may also be important. depth; in supercritical flow [U >(gD) ], inertial forces occurred near the base of the volcano on the are dominant, and flow depth is less than critical open slopes (Table 1), but velocities dropped off Like velocity, peak discharge also decreases depth.

CONCLUSIONS REFERENCES CITED Mount St. Helens, Washington: U.S. Geological Survey Professional Paper 1250, p. 379-400. Apmann, R. P., 1973, Estimating discharge from superelevation in bends: Kjartansson, G., 1951, Water flood and mudflows, in Einarsson, T., Kjanans- Proceedings of the American Society of Civil Engineers, Hydraulics son, G., and Thorarinsson, S., eds., The eruption of Hekla, 1947-1948: Eyewitness accounts and photographs, dam- Division Journal, v. 99, p. 65-79. Visindafelag Islendinga, Reykjavik, p. 1-51. age to vegetation, and erosion and deposition in Bagnold, R. A., 1954, Experiments on gravity dispersion of large solid spheres Major, J. J., 1984, Geologic and rhéologie characteristics of the May 18,1980, in a Newtonian fluid under shear: Royal Society of London Proceed- southwest flank lahars at Mount St. Helens, Washington [M.S. thesis): the flow path indicate that the Pine Creek and ings. ser. A, v. 225, p. 49-63. State College, Pennsylvania, Pennsylvania State University, 225 p. Brugman, N. M., and Meier, M. F., 1981, Response of glaciers to the eruptions Moore, J. G., 1967, Base surge in recent volcanic eruptions: Bulletin Volcano- Muddy River lahars, totaling in excess of 1.4 x of Mount St. Helens, in Lipman, P. W., and Mullineaux, D. R., eds.. logique, v. 30, p. 337-363. 7 3 The 1980 eruptions of Mount Si. Helens, Washington: U.S. Geological Moore, J. G., and Rice, C. J., 1984, Chronology and character of the May 18, 10 m of rock debris and water, evolved pro- Survey Professional Paper 1250, p. 733-741. 1980, explosive eruptions of Mount St. Helens, in Explosive gressively from a single turbulent, gas-mobilized Cassidy, J. J., Coombs, H. A., and Shannon, W. L„ 1980, Study of effects of volcanism—inception, evolution, and hazards: Studies in geophysics, potential volcanic activity on Lewis River projects: Unpublished report Washington, D.C., National Academy Press, p. 133-142. to the Pacific Power and Light Co., Portland, Oregon, 91 p. pyroclastic surge during descent down the east- Moore, J. G., and Sisson, T. W„ 1981, Deposits and effects of the May 18 Chow, V. T„ 1959, Open-channel hydraulics: New York, McGraw-Hill, 680 p. ern flank of Mount St. Helens. The main source pyroclastic surge, in Lipman, P. W., and Mullineaux, D. R., eds.. The Criswell, C. W., 1983, Chronology, morphology, and stratigraphy of pumi- 1980 eruptions of Mount St. Helens, Washington: U.S. Geological Sur- of water for the lahars was probably eroded ceous pyroclastic-flow deposits (ignimbrite) from Mount St. Helens on vey Professional Paper 1250, p. 421-438. May 18,1980 [abs.]: EOS (American Geophysical Union Transactions), Nakamura, S., 1926, On the velocity of reoent mud-flows in Japan, in Proceed- v. 64, no. 45, p. 893. snow and ice incorporated into the flow by tur- ings of the Third Pan-Pacific Science Congress: Tokyo, p. 788-800. Cummans, John, 1981, Chronology of mudflows in the South Fork and North Neall, V. E., 1976, Lahars as major geologic hazards: International Associa- bulent mixing, but ground water, expelled to- Fork Toutle River following the May 18 eruption, in Lipman, P. W.. tion of Engineering Geologists Bulletin, v. 14, p. 233-240. and Mullineaux, D. R., eds.. The 1980 eruptions of Mount St. Helens, gether with the rock debris by the initiating Pierson, T. C., 1980, Erosion and deposition by debris flow at Ml. Thomas, Washington: U.S. Geological Survey - Professional Paper 1250, North Canterbury, New Zealand: Earth Surface Processes, v. 5, p. 479-486. explosions from the volcanic vent, may have p. 227-247. Day, A. L., and Allen, E. T„ 1925, The volcanic activity and hot springs of 1981, Dominant particle support mechanism in debris flows at Mount contributed also. Computed initial flow veloci- Lassen Peak: Carnegie Institution of Washington Publication no. 360, Thomas, New Zealand, and implications for flow mobility: Sedi- 190 p. ties of these large lahars are extremely high mentology, v. 28, p. 49-60. Dietrich, W. E., and Dunne, T., 1978, Sediment budget for a small catchment 1985, Flow behavior of channelized debris flows, Mount St. Helens, in mountainous terrain: Zeitschrift fur Geomorphologie, Supplement (>30 m/s); however, both velocity and peak Washington, in Hillslope processes: Proceedings of the 1985 Geo- Band 29, p. 191-206. morphology Symposium, Buffalo, New York (in press). discharge decreased markedly with distance Fairchild, L. H., and Wigmosta, M., 1983, Dynamic and volumetric character- Pierson, T. C., and Scott, K. M., 1985, Downstream dilution of a lahar: Transi- istics of the 18 May 1980 lahars on the Toutle River, Washington, in tion from debris flow to hyperconcentrated streamflow: Water Re- downstream. Where flow was not impeded, Proceedings of Symposium on Erosion Control in Volcanic Areas: Pub- sources Research (in press). lic Works Research Institute (Japan), Technical Memorandum mean peak-flow velocity was strongly related to Rosenbaum, J. G., and Waitt, R. B., Jr., 1981, Summary of eyewilness ac- no. 1908, p. 131-153. counts of the May 18 eruption, in Lipman, P. W., and Mullineaux, a combined term containing hydraulic radius Fink, J„ Malin, M„ D'Alli, R. E., and Greeley, R„ 1981, Rheological proper- D. R., eds., The 1980 eruptions of Mount St. Helens, Washington: U.S. ties of mudflows associated with the spring 1980 eruptions of Mount St. and channel gradient. Two simple velocity equa- Geological Survey Professional Paper 1250, p. 53-67. Helens volcano, Washington: Geophysical Research Letters, v. 8, no. 1, Schuster, R. L., 1981, Effect of the eruptions on civil works and operations in p. 43-46. tions, the runup and the superelevation formu- the Pacific Northwest, in Lipman, P. W., and Mullineaux, D. R., eds,, Fisher, R. V., and Heiken, G„ 1982, Mount Pelee, Martinique: May 8 and 20, The 1980 eruptions of Mount St. Helens, Washington: U.S. Geological las, appear to provide estimates of mean velocity 1902, pyroclastic flows and surges: Journal of Volcanology and Geo- Survey Professional Paper 1250, p. 701-718. thermal Research, v. 13, p. 339-371. at peak flow that are -15% slower than actual Sekiya, S., and Kikuchi, Y., 1890, The eruption of Bandai-san: Imperial Uni- Fisher, R. V., and Waters, A.C., 1970, Base surge bed forms in maar volcanoes: versity of Japan, Tokyo, College of Science Journal, v. 3, pt. 2, American Journal of Science, v. 268, p. 157-180. velocities. p. 91-171. Folk, R. L., 1965, Petrology of sedimentary rocks: Austin, Texas, Hemphill's, Sheridan, M. F., 1979, Emplacement of pyroclastic flows: A review: Geological 159 p. Society of America Special Paper 180, p. 125-136. Foxworthy, B. L„ and Hill, Mary, 1982, Volcanic eruptions of 1980 at Mount Varnes, D. J., 1978, Slope movement types and processes, in Schuster, R. L., St. Helens—The first 100 days: U.S. Geological Survey Professional and Krizek, R. J., eds., Landslides, analysis and control: Transportation Paper 1249,125 p. Research Board Special Report 176, Washington, D.C., National Gorshkov, G. S., 1959, Gigantic eruption of the volcano Bezymianny (Kam- Academy of Sciences, p. 12-33. chatka): Bulletin Volcanologique, v. 20, p. 77-109. Voight, B., Glicken, H., Janda, R. J., and Douglass, P. M., 1981, Catastrophic Guy, H. P., 1971, Flood flow downstream from a slide: Proceedings of the rockslide avalanche of May 18, in Lipman, P. W., and Mullineaux, American Society of Civil Engineers, Journal of the Hydraulics Div- D. R., eds., The 1980 eruptions of Mount St. Helens, Washington: U.S. ACKNOWLEDGMENTS ision, v. 97, p. 643-646. Geological Survey Professional Paper 1250, p. 347-377. Hampton, M. A., 1975, Competence in fine-grained debris flows: Journal of Waitt, R. B., Jr., Pierson, T. C„ MacLeod, N. S., Janda, R. J., Voight, B„ and Sedimentary Petrology, v. 45, p. 834-844. Hotcomb, R. T., 1983, Eruption-triggered avalanche, flood, and lahar at Cheng-lung Chen, Richard Janda, and Kevin 1979, Buoyancy in debris flows: Journal of Sedimentary Petrology, Mount St. Helens: Effects of winter snowpack: Science, v. 221, v. 49, p. 753-758. Scott provided helpful critical reviews of an ear- p. 1394-1397. Hughes, W. F., and Brighton, J. A., 1967, Theory and problems of fluid Waters, A. C., and Fisher, R. V., 1971, Base surges and their deposits: Capelin- lier version of this paper, and editorial critiques dynamics: New York, McGraw-Hill, 265 p. hos and Taal volcanoes; Journal of Geophysical Research, v. 76, lida, K., 1938, The mudflow that occurred near the explosion crater of Mount p. 5596-5614. of the later version were made by John Costa, Bandai on May 9 and 15, 1938, and some physical properties of vol- Williams, Howel, and McBirney, A. R., 1979, Volcanology: San Francisco, canic mud (English summary): Tokyo University Earthquake Research Richard Janda, David Meyer, and Christopher Freeman, Cooper, and Co., 397 p. Bulletin, v. 16, pt. 3, p. 658-681. Winner, W. E., and Casadevall, T. J., 1981, Fir leaves as thermometers during Newhall. Special thanks are due to the Pacific Janda, R. J., Scott, K. M., Nolan, K. M., and Martinson, H. A., 1981, Lahar the May 18 eruption, in Lipman, P. W.. and Mullineaux, D. R., eds., movement, effects, and deposits, in Lipman, P. W„ and Mullineaux, Power and Light Company for providing gage D. R., eds., The 1980 eruptions of Mount St. Helens, Washington: U.S. The 1980 eruptions of Mount St. Helens, Washington: U.S. Geological height data from Swift Reservoir and to Ken Geological Survey Professional Paper 1250, p. 461-478. Survey Professional Paper 1250, p. 315-320. Johnson, A. M., 1970, Physical processes in geology. San Francisco, Freeman, Seibert for providing the eruption photo se- Cooper, and Co., 577 p. MANUSCRIPT RECEIVED BY THE SOCIETY JANUARY 18,1984 quence in Figure 5. Kieffer, S. W., 1981, Fluid dynamics of the May 18 blast at Mount St. Helens, REVISED MANUSCRIPT RECEIVED JANUARY 31,1985 in Lipman, P. W„ and Mullineaux, D. R„ eds., The 1980 eruptions of MANUSCRIPT ACCEPTED FEBRUARY 4, 1985

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