Geoscience Reference
In-Depth Information
November the shipping lane was fully restored. Temporary dams and sediment storage pits
were excavated in the winter of 1980 in the Toutle River to hold back sediment delivery from
the upper catchment, and dredging operations were completed in May 1981. These temporary
structures have now been overwhelmed by aggradation of the river (Case Fig. 2.2b).
By 1982 lake waters at Spirit Lake were rising dangerously high behind the eruption debris
dam. By 19 August 1982 a State of Emergency was declared at Spirit Lake and a full-scale
Federal response was initiated. In November 1982 water was being pumped from Spirit Lake to
immediately reduce water levels, however, in order to permanently drain the lake to safe levels
a water transfer tunnel connecting the lake to South Coldwater Creek was built and this was
operational by 6 May 1985.
In order to produce a long-term solution and management strategy for the sediment prob-
lem the Mount St Helens Sediment Retention Structure (SRS) was designed and built. In 1986
work began on the construction of the SRS on the North Fork Toutle River. The purpose of the
structure was to stop the advance of debris avalanche deposits from the 1980 volcanic eruption
moving downstream and causing long-term navigation problems on the Toutle, Columbia and
Cowlitz rivers. This was one element in a three part solution of long-term sediment management;
this also involved river levee construction and a dredging programme. The US Army Corps of
Engineers initiated the project in 1986 and impoundment began in November 1987. The aim
of the sediment retention structure was to dam sediment not water. The structure reduces the
flow to such an extent that sediment is deposited naturally in the upstream side of the structure
and does not migrate downstream causing flooding problems and shipping hazard. The SRS
consists of a 600 m wide embankment standing nearly 56 m above the pre-eruption stream
bed. The embankment is constructed of crushed and fractured rock with an impervious core of
clay. The embankment rests on gravel, and water passes underneath and into the embankment
as the lake rises. The outlet works occupy a large central block, which consists of six rows of
five outlet pipes through which water and fish can pass into the plunge pools and downstream
outlet channels. The pipes are closed off permanently as the level of sediment rises in the
impounded lake upstream. Once the conduits are fully blocked the river will flow continuously
over the wide unlined spillway, which is approximately 38 m above the original stream bed.
The structure is sited about 35 km downstream of the volcano.
The impounded area is estimated to have a sediment retention capacity of approximately
198
10
6
m
3
of sand and gravel and is expected to fill in 50 years. In 2000 the structure was
approximately 30% full. The rate of sediment retention varies greatly from year to year. For
example, in the wet winters of 1996 and 1997, the higher than average discharges resulted
in 23.5
×
10
6
m
3
of sediment being deposited behind the structure. This was nearly four times
the amount trapped in the previous two years.
×
Relevant reading
Major, J.J. (2003) Post-eruption hydrology and sediment transport in volcanic river systems.
Water Resources
Impact
5
, 10 -15.
Major, J.J., Pierson, T.C., Dinehart, R.L., et al. (2000) Sediment yield following severe volcanic disturbance - a
two decade perspective from Mount St Helens.
Geology
28
, 819-22.
Major, J.J., Scott, W.E., Driedger, C., et al. (2005) Mount St. Helens erupts again: activity from September 2004
through March 2005.
U.S. Geological Survey Fact Sheet FS2005-3036
. http://vulcan.wr.usgs.gov/Volcanoes/
MSH/Publications/FS2005-3036/FS2005-3036.pdf (accessed July 2005).
