Effects of Catastrophic Floods and Debris Flows on 

the Sediment Retention Structure, North Fork Toutle 

River, Washington

By Roger P. Denlinger

 

Open-File Report 2011–1317

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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Suggested citation:

Denlinger, R.P., 2012, Effects of catastrophic floods and debris flows on the sediment retention 

structure, North Fork Toutle River, Washington: U.S. Geological Survey Open-File Report 2011–1317, 

25 p. 

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endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report.
iii

Contents

Abstract ......................................................................................................................................................... 1

Introduction.................................................................................................................................................... 1

Methods......................................................................................................................................................... 5

Numerical Model............................................................................................................................................ 7

Initial Conditions ............................................................................................................................................ 7

Results and Analysis ................................................................................................................................... 10

Conclusions................................................................................................................................................. 12

References Cited......................................................................................................................................... 12

Figures

1. Photograph of Mount. St. Helens and Castle Lake upstream of the Sediment Retention Structure on 

the North Fork Toutle River, Washington ................................................................................................ 4

2A. Shaded relief map of numerical domain used for flow simulations.......................................................... 4

2B. Close-up image of Sediment Retention Structure (SRS) as it is discretized in the numerical domain, 

with the elevation of each point shown with flooded contours................................................................. 5

3. Initial condition for Castle Lake and blockage............................................................................................ 9

4. Initial conditions assumed for a debris flow from the crater of Mount St. Helens..................................... 10

5. Initial flooding conditions associated with breaching of the blockage forming Castle Lake and 

spillway ................................................................................................................................................. 14

6. Initial flooding conditions associated with breaching of the blockage forming Castle Lake and 

spillway ................................................................................................................................................. 15

7. Water depth at peak stage at the dam, as steady flow is achieved through the Sediment Retention 

Structure ............................................................................................................................................... 16

8. Peak water surface elevation (stage), as steady flow is achieved through the Sediment Retention 

Structure for a partial dambreak flood from Castle Lake and a total flow volume of 29 Mm3 ................ 17

9. Water depth at peak stage at the dam, as steady flow is achieved through the Sediment Retention 

Structure for a dambreak flood from complete failure of the blockage of Castle Lake .......................... 18

10. Peak water surface elevation (stage), as steady flow is achieved through the Sediment Retention 

Structure for a full dambreak flood from Castle Lake and a total flow volume of 56 Mm3...................... 19

11. Hydrographs for both partial and complete failure of the blockage of Castle Lake, producing a 

dambreak flood that impacts the Sediment Retention Structure ........................................................... 20

12. Initial conditions for a debris flow formed from the blockage and lake volume, where outlet is 

blocked and lake volume is assumed to be fluidized debris to a depth of 41 meters, giving a total 

volume of 56 Mm3 ................................................................................................................................. 21

13. Final deposit for a debris flow with the initial configuration in figure 12.................................................. 22

14 A. Snapshot of flow depths of hypothetical crater debris flow 1,500 seconds after release.................... 23

14 B. Snapshot of flow depth of hypothetical crater debris flow 1,500 seconds after release ..................... 24

15. Deposit created by a debris flow originating in the crater of Mount St. Helens nearly 

3 hours after release. ............................................................................................................................ 25

Tables

1. Material Properties and Dimensionless Parameters for Model Debris Flow Predictions...........................7

2. Stage at Sediment Retention Structure for Different Dambreak Volumes. .............................................. 12
iv

Conversion Factors and Datum

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

meter (m) 3.281 foot (ft) 

meter (m) 1.094 yard (yd) 

Area

square meter (m2) 0.0002471 acre 

square meter (m2) 10.76 square foot (ft2) 

Volume

cubic meter (m3) 6.290 barrel (petroleum, 1 barrel = 42 gal)

cubic meter (m3) 264.2 gallon (gal) 

cubic meter (m3) 0.0002642 million gallons (Mgal) 

million cubic meters (Mm3) 0.8107 thousand acre-feet (taf) 

cubic meter (m3) 35.31 cubic foot (ft3)

cubic meter (m3) 1.308 cubic yard (yd3) 

cubic meter (m3) 0.0008107 acre-foot (acre-ft) 

Density

kilogram per cubic meter (kg/m3) 0.06242 pound per cubic foot (lb/ft3)

Viscosity

square meters/sec (m2/s) 10.8 square foot per second (ft2/s)

Flow rate

cubic meter per second (m3/s) 70.07 acre-foot per day (acre-ft/d) 

cubic meter per second (m3/s) 35.31 cubic foot per second (ft3/s)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32. 

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8. 

Datum

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83). 

Elevation as used in this report, refers to distance above the vertical datum.
Effects of Catastrophic Floods and Debris Flows on 

the Sediment Retention Structure, North Fork Toutle 

River, Washington

By Roger P. Denlinger

Abstract

The eruption of Mount St. Helens in 1980 produced a debris avalanche that flowed down 

the upper reaches of the North Fork Toutle River in southwestern Washington, clogging this 

drainage with sediment. In response to continuous anomalously high sediment flux into the 

Toutle and Cowlitz Rivers resulting from this avalanche and associated debris flows, the U.S. 

Army Corps of Engineers completed a Sediment Retention Structure (SRS) on the North Fork 

Toutle River in May 1989. For one decade, the SRS effectively blocked most of the sediment 

transport down the Toutle River. In 1999, the sediment level behind the SRS reached the 

elevation of the spillway base. Since then, a higher percentage of sediment has been passing the 

SRS and increasing the flood risk in the Cowlitz River. 

Currently (2012), the dam is filling with sediment at a rate that cannot be sustained for its

original design life, and the U.S. Army Corps of Engineers is concerned with the current ability 

of the SRS to manage floods. This report presents an assessment of the ability of the dam to pass 

large flows from three types of scenarios (it is assumed that no damage to the spillway will 

occur). These scenarios are (1) a failure of the debris-avalanche blockage forming Castle Lake

that produces a dambreak flood, (2) a debris flow from failure of that blockage, or (3) a debris 

flow originating in the crater of Mount St. Helens. In each case, the flows are routed down the 

Toutle River and through the SRS using numerical models on a gridded domain produced from a 

digital elevation model constructed with existing topography and dam infrastructure. The results 

of these simulations show that a structurally sound spillway is capable of passing large floods 

without risk of overtopping the crest of the dam. In addition, large debris flows originating from 

Castle Lake or the crater of Mount St. Helens never reach the SRS. Instead, debris flows fill the 

braided channels upstream of the dam and reduce its storage capacity. 

Introduction

The Sediment Retention Structure (SRS) on the North Fork Toutle River was completed

in 1989 by the U.S. Army Corps of Engineers (Willingham, 2005). The purpose of the SRS is to 

block downstream transport of the deposits and sediment left by the catastrophic eruptions of 

Mount St. Helens in the summer of 1980, thereby reducing flood risk created by sediment 

deposition in the Cowlitz River. A photograph of the drainage basin upstream of the SRS is 

shown in figure 1, and a map of the SRS and its drainage basin are shown in figure 2a. The SRS
2 

is 56 m high and 575.5 m wide, with a crest elevation of 304.8 m and an original capacity of 

197.3 million m3, and was designed to contain sediment until the year 2035. Since construction 

of the SRS, however, higher than anticipated sediment flux into SRS by occasional debris flows 

from the crater of Mount St. Helens as well as by common bank failures into the North Fork 

Toutle River during large storms (Major and Mark, 2006) has reduced the capacity of the SRS 

such that it is nearly full and passing substantial sediment (U.S. Geological Survey, 2012). 

In addition to chronic sedimentation, potential catastrophic events could radically shorten 

the useful life of the SRS by causing rapid filling the valley behind the SRS dam with sediment. 

One such event could be a failure of the blockage forming Castle Lake, which was impounded 

when a southern tributary of the North Fork Toutle River was blocked by the debris avalanche on 

May 18, 1980 (Glicken, 1998, figs. 1 and 2). The lake presently has a volume of about 23 million 

m3. The U.S. Army Corps of Engineers constructed a spillway across this blockage in 1981 to 

stabilize the maximum lake level and prevent overtopping of the blockage. However, seepage 

failure of the blockage still poses flooding and debris flow hazards to the SRS (Roeloffs, 1994). 

Another plausible cause of rapid sediment accumulation behind the SRS is a debris flow from 

the crater of Mount St. Helens (Wolfe and Pierson, 1995). 

To evaluate the likely outcomes from these events, numerical simulations were conducted

to model large dambreak floods and large debris flows flowing into the drainage of the upper 

North Fork Toutle River and towards the SRS. The initial condition for the simulations consisted

of specifying source conditions at Castle Lake and in the crater of Mount St. Helens, and 

specifying flow composition on the basis of local geologic and geomorphic conditions. 

Numerical flow modeling allowed computation of flow and deposition conditions for the three 

dimensional terrain down to and through the SRS. These calculations rely on algorithms 

developed either for dambreak floods (Denlinger and O’Connell, 2008) or for debris flows 

(Denlinger and Iverson, 2001) depending on whether the source volume consists of water or 

saturated debris. No assessment of structural integrity of the SRS is made in this work. 
3 

Figure 1. Photograph of Mount St. Helens and Castle Lake upstream of the Sediment Retention Structure 

on the North Fork Toutle River, Washington. The braided channel of the Toutle River, which originates in

the crater of Mount St. Helens, is in the lower middle part of the photograph. The blockage forming Castle 

Lake was emplaced during the catastrophic debris avalanche on May 18, 1980. The blockage is now 

vegetated but is permeable. The Castle Lake outlet now feeds a tributary of the North Fork Toutle River.
4 

UTM = 10NNorth American Datum of 1983 with x = 527800 and

y=5102800 substracted from distance east and distance north.

Figure 2A. Shaded-relief map of numerical domain used for flow simulations. Elevation data are from 

LiDAR mapping of this region (Watershed Sciences, 2009, unpub. data), and have been generalized to a 

50-meter cell size for the flow calculation. The impact of a 50-meter cell size generalized on grid 

discretization near the spillway is shown in figure 2B. 
5 

Figure 2B. Close-up image of Sediment Retention Structure (SRS) as it is discretized in the numerical 

domain, with the elevation of each point shown with flooded contours. Each black square is the center of a 

computational cell on the numerical grid, which has a spacing of 50 meters. The spillway of the SRS is 

adequately sampled. 

Methods

Two specific conditions were modeled, reflecting the impact of inundation, either from 

debris flows or from sediment-laden floods. The distinct physical properties and behaviors 

associated with these flow types require different modeling approaches. 

Shallow water flooding: For numerical modeling of the sediment-laden water floods, 

analysis was based on a depth-averaged model for flow of shallow water over three-dimensional 

terrain with slopes up to 32 degrees (Denlinger and O’Connell, 2008). This model has been 

tested on flows in the U.S. Geological Survey (USGS) flume near Blue River, Oregon, and on 

catastrophic dambreak floods in France and the United States. This model accounts for non-
6 

hydrostatic forces resulting from flow over rugged terrain, making the algorithm appropriate for 

dambreak analyses of flows in the rugged topography of the upper Toutle River drainage. 

Debris flow routing: Debris flows are mixtures of sediment and water with a variable 

amount of fine (silt and clay) material. At Mount St. Helens, most debris flows are composed of 

gravel, sand, and silt (Iverson, 1997). Numerical modeling of these flows over three-dimensional 

terrain is described in Denlinger and Iverson (2001), who did simulations of such flows in the 

USGS debris flow flume. The material properties assumed for the debris flows simulated here 

(table 1) are based on the bulk composition of the 1980 debris avalanche at Mount St. Helens. 

The composition of this avalanche is representative of debris flow material for flows affecting 

the upper North Fork Toutle River basin (Iverson, 1997; Glicken, 1998). 

The equations used to model debris flows are similar to the shallow water equations used 

for flood routing (Denlinger and Iverson, 2001), but account for the substantially greater internal 

and boundary stresses than those achieved in water floods, even in sediment-laden floods. The 

numerical model is partly based on the condition that for flowing debris, intergranular fluid 

pressure, or ‘pore pressure’ typically increases to 90 percent of the overburden load at a few

percent bulk shear strain (Iverson, 1997). Continued straining during flow regenerates internal 

pore pressures and allows saturated debris to continue to flow, but pressure in slow moving 

zones dissipates, inhibiting motion and promoting deposition (through cessation of flow). A 

kinematic condition given in Denlinger and Iverson (2001) defines patterns of pore pressure 

diffusion based on surface slope and flow movement, and typically enhances diffusion by an 

order of magnitude along advancing margins of the flow. With these conditions and a higher 

bulk density, observed and simulated debris flows accelerate and decelerate much more rapidly 

than water floods, particularly when flowing over steep terrain. Simulated debris flows mimic 

observed debris flows in that debris is stranded as levees along flow margins, often filling

embayments along the channels they occupy. 

A debris flow also can transition to sediment-laden water flow if it incorporates sufficient 

in-channel river water. Progressive dilution results in fewer enduring particle contacts among the 

granular debris composing the flow, and eventually the debris flow transforms into a water flow 

carrying fine-grained suspended load and a coarse bed load. This typically occurs at a fluid/solid 

ratio of 50–60 percent by volume. Although dilution is gradual, the transition from a debris flow 

to a flood can be quite abrupt as the ratio of water to sediment increases (Denlinger and others, 

2001). Because this transformation is not yet predictable and quantifiable, this study assumes 

that flows are either sediment-laden water floods or debris flows that do not transform within the 

study area.
7 

Table 1. Material properties and dimensionless parameters for model debris flow predictions. 

[kg/m3, kilogram per cubic meter; m2, square meter; m2/s, square meter per second]

Property or parameter Value

Basal friction angle 30 degrees

Internal friction angle 30 degrees

Solid volume fraction 60 percent

Fluid volume fraction 40 percent

Fluid viscosity 0.1 poise

Solid density 2,600 kg/m3

Fluid density (muddy water) 1,200 kg/m3

Hydraulic permeability 10-11 m2

Hydraulic diffusivity 10-4 m2/s

Initial pore pressure/weight 1,200/,2600

Maximum pore 

pressure/weight

0.9

Numerical Model

To apply either the shallow water flow model or the debris flow model to the flow 

simulations, a 2 m regular grid from LiDAR elevation data was constructed for the Mount St. 

Helens region (Watershed Sciences, 2009, unpub. data). The numerical domain, constructed 

from LiDAR data (from Watershed Sciences, 2009, unpub. data), is in UTM zone 10N (NAD83) 

and has x=527800 and y=5102800 (in meters) subtracted from the distance east and distance 

north, respectively, in all figures in this report. This is done in the computation to improve 

resolution, and in all figures to make them easier to read. To accommodate computer memory 

limitations, the grid was generalized to a 50-m cell size on a square mesh grid for the numerical 

modeling (fig. 2A). A closeup of the meshed domain in the vicinity of the SRS and its spillway is 

shown in figure 2b. Flows on this three-dimensional surface grid are routed by specifying an 

initial volume of water or saturated debris, either at Castle Lake or in the crater of Mount St. 

Helens, and then applying the applicable numerical modeling scheme to let the flow translate 

downstream. The terrain controls the path of the flow such that as the stage rises the flow will 

take a straighter path downstream, often cutting off point bars and meanders (Denlinger and 

others, 2001). Because flow direction depends on stage, the thalweg varies considerably with 

stage along the flood wave, particularly when flow occurs over rugged terrain. For debris flows 

and water floods, the terrain routes the flow in local downhill directions regardless of number of 

multiple channels or channel width. The boundary conditions for simulated floods allow them to 

run off the grid downstream of the SRS without affecting flow upstream. 

Four distinct scenarios were modeled: Two different-sized water floods from different 

scenarios of breaching of Castle Lake; another considered a debris flow from Castle Lake; and 

the fourth was a debris flow originating from the crater area of Mount St. Helens. 

Initial Conditions

Dambreak from Castle Lake at controlled elevation: This scenario entails breaching of 

the impounded lake associated with the current, controlled, lake elevation but resulting in 

complete emptying of the lake. The total volume of water and sediment are derived from U.S. 

Army Corps of Engineers (1990) and assumes a limited 100-m wide breach channel on the 

southeastern side of the blockage with the channel bottom even with the lake bottom (fig. 3). 
8 

This scenario produces a total flood volume of 28×106 m3, and includes 4.6×106 m3 of sediment 

incorporated from failure of the breach. 

Full Dambreak from Castle Lake: This worst-case scenario assumes that the controlled 

outlet is somehow blocked, resulting in an elevated lake of 804 m that overtops the blockage 

prior to breaching. Instantaneous failure was assumed to accompany water overtopping the 

blockage, allowing release of the entire volume of blockage, down to the current elevation of the 

lake bottom. This scenario results in a total flow volume of 56×106 m3,which includes material 

entrained from the blockage. This worst case scenario tests the spillway capacity of the SRS to 

pass an anomalously large flood in which the flow does not damage the spillway. 

Debris Flow from Castle Lake blockage: Given the amount of water in Castle Lake, it is 

implausible that failure of the blockage will produce a large debris flow rather than a flood. 

Nonetheless, it is useful to look at the difference between a large debris flow originating at 

Castle Lake, and a large flood resulting from failure of the blockage. Here, I defined how much 

volume comprises the blockage at Castle Lake using the results from U.S. Army Corps of 

Engineer (1990). The base of the blockage was even with the level of the lake bottom (as with 

the full dambreakscenario). Computationally, I did not include water and debris in a single 

simulation, so I assumed that the lake and blockage formed 28×106

 m3 of saturated debris with 

the geometry shown in figure 3 and the properties in table 1. 

Debris Flow from Mount St. Helens crater: This scenario assumes mobilization of an 

arbitrary brick with 25.4×106 m3 of saturated debris sitting on the floor of the crater (fig. 4). The 

volume of this simulated flow is a worst-case scenario as the volume used is much larger than 

the crater is likely to produce at this time (2012). This volume lies between the volumes of the 

two largest debris flows originating from Mount St. Helens since 1980 (65 million m3 on May 

18, 1980, and 4 million m3

 on March 19, 1982 (Tilling and others, 1997). The initial cubic 

geometry of the debris flow volume in the crater at Mount St. Helens is not critical, because any 

debris volume flows and changes shape within a few hundred meters in response to valley and 

channel geometry as well as in response to internal pore pressure fluctuations. For this scenario, 

the initial volume (fig. 4) was fully saturated but not pressurized, and pore pressure was 

calculated to develop completely at 1 percent bulk shear strain as debris flowed downhill. 
9 

Figure 3. Initial condition for Castle Lake and blockage. The blockage was formed by the debris avalanche 

of May 18, 1980, and produced the lake by damming a tributary of the North Fork Toutle River. The terrain 

model is based upon LiDAR mapping of the surrounding region (Watershed Sciences, 2009, unpub. data), 

and the thickness of the blockage is determined from U.S. Army Corps of Engineers (1990) studies of 

Castle Lake. For a partial dambreak, the normal lake depth of 21 meters is used, giving a total flow volume 

of 28×106 cubic meters, which includes 5×106 cubic meters of blockage material. For a larger dambreak 

assuming blockage of the controlled outlet, the maximum initial depth is 41 meters and the total volume is

56×106 cubic meters. See text for explanation of UTM datum.
10

Figure 4. Initial conditions assumed for a debris flow from the crater of Mount St. Helens. The initial 

unrealistic prism shape for the debris flow mass does not affect results because the flow will quickly evolve 

in accordance to downstream topography. The volume of the flow is close to the maximum volume 

observed for debris flows originating in the crater of Mount St. Helens since 1980. See text for explanation 

of UTM datum.

Results and Analysis

The model simulations indicate that all scenarios result in substantial flow volumes and 

discharges into the drainage system of the North Toutle River upstream of the SRS. The potential 

effects of these simulated flows on the SRS depend on whether the flow reaches the dam and 

whether the stage at the dam exceeds spill capacity and overtops the dam crest. Additionally, 

long-term decreases in storage capacity may result from deposition behind the SRS. The various 

scenarios have different outcomes with respect to these issues. 
11

Dambreak floods from Castle Lake: The differences between the controlled elevation 

and full dambreak floods from Castle Lake owe to the rate at which water is released from the 

lake and the total volume of the flood. The differences in the opening in the blockage between 

these two scenarios control the rate of release, and the differences are apparent in the resulting 

flow soon after failure of the blockage (figs. 5 and 6). A large opening (full dambreak) has a high 

rate of discharge and a much greater effect on the outlet channel at early in the simulation. As the 

dambreak flood wave lengthens, the differences between the two scenarios result primarily from 

total volume released (figs. 7–10). A high discharge into the Toutle River valley results from the 

enhanced volume of 56 ×106

 m3 of water and sediment produced by the full dambreak scenario. 

In this case, the model predicts a peak discharge of 40,000 m3/s in the Toutle River Valley 

upstream of the SRS (fig. 11); however, this peak discharge is attenuated within the wide Toutle 

River valley and by existing storage capacity of the SRS, such that the peak discharge at the SRS 

is only 6,000 m3/s and is predicted to pass through the spillway without overtopping the crest of 

the SRS dam. That the dam does not overtop with such a huge input discharge upstream is 

testimony to the huge conveyance in the Toutle River valley. As the flow at the spillway crests at 

6,000 m3

/s, the water surface is still 3 m below the crest of the dam. 

Debris flows from failure of Castle Lake blockage: An alternative scenario is given by 

the unlikely formation of a debris flow from failure of the blockage of Castle Lake. To 

demonstrate the impact of a large debris flow rather than a large flood, the model assumes the 

lake and blockage volume (fig. 12) combine to form saturated debris that flows downhill. The 

final condition is shown in figure 13. The debris flow stops before it reaches the SRS, but affects

SRS storage capacity because debris fills in the braided channels upstream in the Toutle River 

valley. Filling and modification of the braided channels in this way is expected for all debris 

flows, whether the flows initiate at Castle Lake or in the crater of Mount St. Helens.

Debris flows from Mount St. Helens crater: Debris flows that initiate in the crater of 

Mount St. Helens are far more likely than a debris flow that initiates from failure of the Castle 

Lake blockage, but the results are similar. The impact and results are sensitive to the volume of 

the initial debris (fig. 4), but not the initial geometry (shown in figure 4 as a block). As the crater

debris flow progresses downslope, material at the margins of the flow becomes stranded in 

embayments along the channels. Additionally, some of the initial debris is left behind as some 

flow is forced to completely pressurize (liquefy) the flow (Denlinger and Iverson, 2001). Farther 

downslope, the debris flow from the crater divides at the base of Mount St. Helens; about onehalf of the flow spreads east, entering Spirit Lake and forming a delta and about one-half

proceeds down the Toutle River toward the SRS (figs. 14a and 14b). For these modeled 

conditions of a debris flow from the crater of Mount St. Helens, a total volume of 25×106

 m3/s

with specified physical properties (table 1) never reaches the SRS. The debris flow does partially 

fill the channel of the North Fork Toutle River upstream of the SRS and does leave deposits at 

embayments and bends in the channel (fig. 14). The full volume of the crater debris flow remains 

upstream of the SRS (fig. 15), with a large part of the flow coming to rest closer to the dam than 

did the simulated Castle Lake debris flow. Given the simulated excessively large, and unlikely, 

volume of debris flows from the crater, and the natural limit to the volume of debris flows from 

the blockage of Castle Lake; the simulations show that there is insufficient volume and regional 

slope for debris flows from either site to flow to or over the SRS structure without additional 

entrainment of water sufficient for these flows to transition to a water flood. In the long term, 

however, the deposits left by debris flows upstream of the SRS accumulate, increasing sediment 
12

loads in the channels feeding into the SRS. This reduces channel conveyance and increases flood 

hazards downstream. 

Table 2. Table 2. Stage at Sediment Retention Structure (SRS) for different dambreak volumes. 

[m, meters; Mm3, million cubic meters]

Initial conditions and flood volume Maximum stage at SRS Maximum depth at SRS

Partial breach of Castle Lake, 28 Mm3 297.0 m 9.9 m (near spillway)

Full breach of Castle Lake, 56 Mm3 299.8 m 11.7 m (near spillway)

Conclusions

Extreme scenarios of debris flows and floods in the upper North Fork Toutle River 

drainage show that the Sediment Retention Structure (SRS) operated by the U.S. Army Corps of 

Engineers on the North Fork Toutle River in southwestern Washington, is still viable in its 

present form. Flows were simulated from either a partial or a complete failure of the blockage 

forming Castle Lake, as well as a large debris flow emanating from the crater of Mount St. 

Helens. The results show that current SRS spillway geometry is capable of passing large floods, 

due in large part to the huge conveyance of the Toutle River valley upstream. The valley feeding 

into the SRS is broad enough to absorb a transient peak flood discharge exceeding 40,000 cubic 

meters per second without generating an output discharge at the SRS large enough to overtop the 

crest of the dam. Unlike floods, simulated debris flows do not reach the dam and thus have no 

impact on the spillway. Debris flows with volumes up to 28 million cubic meters originating in 

the crater of Mount St. Helens or at Castle Lake spread out and fill the braided channels in the 

broad Toutle River valley upstream of the dam, stopping before they reach the dam. The debrisflow deposits reduce the sediment storage capacity of the SRS, and thus its ability to pass large 

floods in subsequent years. 

References Cited

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13

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Flight Oct 21, 2006), data collection by Watershed Sciences, report date November 16. 

Wolfe, E.W., and Pierson, T.C., 1995, Volcanic-hazard zonation for Mount St. Helens, 

Washington: U.S. Geological Survey Open-File Report 95–497, 12 p. 
14

Figure 5. Initial flooding conditions associated with breaching of the blockage forming Castle Lake and 

spillway. Portrayed are model results at 100 seconds after failure where it was assumed that the failure

forms a slot through the blockage, incorporates part of the blockage, and drains the entire lake. See text for 

explanation of UTM datum.
15

Figure 6. Initial flooding conditions associated with breaching of the blockage forming Castle Lake and 

spillway. This case represents the enhanced flooding conditions associated with a total volume of 56 106

cubic meters. Portrayed are model results at 100 seconds after failure where it was assumed that the 

failure forms a slot through the blockage, incorporates part of the blockage, and drains the entire lake. See 

text for explanation of UTM datum.
16

Figure 7. Water depth at peak stage at the dam, as steady flow is achieved through the Sediment 

Retention Structure. See text for explanation of UTM datum.
17

Figure 8. Peak water surface elevation (stage), as steady flow is achieved through the Sediment Retention 

Structure for a partial dambreak flood from Castle Lake and a total flow volume of 29 million cubic meters 

(23 million cubic meters of lake water plus 6 million cubic meters of blockage material). See text for 

explanation of UTM datum.
18

Figure 9. Water depth at peak stage at the dam, as steady flow is achieved through the Sediment 

Retention Structure for a dambreak flood from complete failure of the blockage of Castle Lake. Here, 46 

million cubic meters comes from the lake and 10 million cubic meters from fluidized blockage volume. See 

text for explanation of UTM datum.
19

Figure 10. Peak water surface elevation (stage), as steady flow is achieved through the Sediment 

Retention Structure for a full dambreak flood from Castle Lake and a total flow volume of 56 million cubic 

meters (46 million cubic meters of lake water plus 10 million cubic meters of blockage material). See text 

for explanation of UTM datum.
20

Figure 11. Hydrographs for both partial and complete failure of the blockage of Castle Lake, producing a 

dambreak flood that impacts the Sediment Retention Structure. The complete failure produces a much 

larger flow volume and a huge discharge into the channel of the North Toutle River upstream of the 

Sediment Retention Structure. 
21

Figure 12. Initial conditions for a debris flow formed from the blockage and lake volume, where outlet is 

blocked and lake volume is assumed to be fluidized debris to a depth of 41 meters, giving a total volume of 

56 million cubic meters. See text for explanation of UTM datum.
22

Figure 13. Final deposit for a debris flow with the initial configuration in figure 12. Despite the huge initial 

volume for the debris flow, the flow comes to rest before reaching the Sediment Retention Structure, and 

fills the channels of the North Fork Toutle River upstream of the dam. See text for explanation of UTM 

datum.
23

Figure 14A. Snapshot of flow depths of hypothetical crater debris flow 1,500 seconds after release. The 

debris flow has split into two parts; one part forms a delta on Spirit Lake, the other part continues down the 

North Fork of the Toutle River towards the Sediment Retention Structure. See text for explanation of UTM 

datum.
24

Figure 14B. Snapshot of flow depth of hypothetical crater debris flow 1,500 seconds after release. The

dark red coloration shows which portions of the flow are still moving: the remainder of the debris is 

stagnant. See text for explanation of UTM datum.
25

Figure 15. Deposit created by a debris flow originating in the crater of Mount St. Helens nearly 3 hours 

after its release. Debris is still moving in the channel feeding the deposit, but most of the material has come 

to rest. Note the levees and portions of the flow remaining stranded. Debris flows from the crater reduce the 

storage of the Sediment Retention Structure far more than large floods, as all the sediment volume is 

deposited in low areas behind the dam. See text for explanation of UTM datum.