Downstream Dilution of a Lahar: Transition From Debris Flow to Hyperconcentrated Streamflow

Abstract

Nearly instantaneous melting of snow and ice by the March 19, 1982, eruption of Mount St. Helens released a 4 x 10 6 m 3 flood of water from the crater that was converted to a lahar (volcanic debris flow) through erosion and incorporation of sediment by the time it reached the base of the volcano. Over the next 81 km that it traveled down the Toutle River, the flood wave was progressively diluted through several mechanisms. A transformation from debris flow to hyperconcentrated streamflow began to occur about 27 km downstream from the crater, when the total sediment concentration had decreased to about 78% by weight (57% by volume). The hyperconcentrated lahar-runout flood wave, transporting immense quantities of sand in suspension, continued to experience progressive downstream dilution. Although turbulence was significantly dampened by the extremely high suspended load, very large standing waves and antidune waves were observed. The hyperconcentra. ted lahar-runout flow deposited an unusual, faintly stratified, coarse sand which locally contained small, isolated gravel lenses. Very similar deposits in .the Quaternary stratigraphy of Moun•t St. Helens and other Cascades volcanoes suggest that lahars may be more frequent than previously recognized. INTRODUCTION Lahar Generation and Transformation On the evening of March 19, 1982, an explosive eruption of Mount St. Helens, Washington [Waitt et al., 1983] rapidly melted about 107 m 3 of snow and ice in the confined southern end of the crater. Subse.quently, 4 x 10 6 m 3 of nearly clear water flowed out of the crater breach as two flood surges and entered a steep and unstable gully system on the north flank of the volcano. Both surges eroded and incorporated enough volcanic debris to emerge 2 km downstream as lahars (volcanic debris flows). The lahars then flowed 27 km down the North Fork Toutle River (Figure 1), overtaking the streamflow in the channel and becoming progressively more dilute with distance downstream. Flo TM transformations occurred within each surge between 27 and43 km downstream from the crater; the evidence for this was a couplet of coarse sedimentary facies deposited by each surge within this reach. The deposits were not typical of either debris flows or normal floods. Downstream of this transition reach the two surges merged and continued as a single flood wav e of hyperconcentrated streamflow, leaving behind an unusual type of fluvial deposit. Sediment concentration continued to decrease with distance downstream. The flood wave reached the Cowlitz River confluence (Figure 1), 83 km downstream from the crater, in approximately 5 hours. Flood wave celerity averaged 4.6 m/s and slowed with distance downstream. Mixing with the Cowlitz River at the confluence reduced peak sediment concentration to Well below hyperconcentrated levels. Rheological Definitions of the Flood "Fluids" The rheological behavior of sediment-water mixtures can be either Newtonian or non-Newtonian, depending on Sediment concentration, sediment type, and particle size distribut...

Evaluation of ASTER and SRTM DEM data for lahar modeling: A case study on lahars from Popocatépetl Volcano, Mexico

“…Due to strong glacier retreat in recent years (Huggel and Delgado, 2000;Julio Miranda and Delgado Granados, 2003), the water equivalent of ice has significantly decreased to about 2.8 × 10 6 m 3 at present. In accordance with studies on sedimentological characteristics of recent lahars in the Huiloac gorge (Capra et al, 2004;Julio Miranda et al, 2005) and more general flow-type considerations (e.g., Pierson and Scott, 1985;Pierson and Costa, 1987;Hungr et al, 2001), a sediment concentration of 25 to 65% is assumed for potential lahars and hyperconcentrated flows. Hence, given the maximum water volume available from melting processes, a maximum flow volume of 3.7 × 10 6 m 3 to 8 × 10 6 m 3 results.…”

“…However, for lahars the definition of a minimum H/L ratio is considerably complicated by processes such as flow transformation. Volcanic debris avalanches may transform into debris flows and eventually into hyperconcentrated streamflows (Pierson and Scott, 1985;Scott et al, 2001).…”

Acute sedimentation response to rainfall following the explosive phase of the 2008–2009 eruption of Chaitén volcano, Chile

“…The Chaitén River was not gauged, and peak-flow discharge during aggradation was not measured. Peak-flow stage just before the permanent avulsion occurred (obtained from high-water marks along the abandoned channel) was about 1.5 m above the aggraded channel bed near the airport, where channel width was about 100 m. If peakflow velocity had been 2-3 m/s, a reasonable range for high-concentration floods about 1 m deep in similar channels (Pierson and Scott 1985), then peak discharge immediately prior to channel avulsion can be estimated to have been in the range of 350-550 m 3 /s. For comparison, summer low flow in 2010 at this location was 5-10 m 3 /s (A. Iroumé, Universidad Austral de Chile, 2010, written communication).…”

Development and routing of mudflow resulting from hypothetical failure of Spirit Lake debris dam, Washington

“…In this report, sediment concentrations are expressed by volume, as the ratio of the volume of sediment solids to the volume of the mixture; or by weight, as the ratio of the weight of the sediment to the weight of the mixture; or by mass per unit volume, as the number of milligrams of sediment per liter of mixture. A flowing water-sediment mixture typically will begin exhibiting non-Newtonian flow characteristics when the sediment concentration by weight of the mixture exceeds about 80 percent (Pierson and Scott, 1985;and Beverage and Culbertson, 1964). The sediment concentration by volume of a mudflow with a concentration of 80 percent by weight would be approximately 60 percent, assuming a specific gravity of 2.65 for the entrained sediment.…”