• Application Of Natural Channel Design Principles In Conversion Of A Multiple-Thread To Single-Thread Channel On An Alluvial Fan: Mccallum Creek, Alaska

      Roach, Christopher Hume; Carlson, Robert F. (2002)
      McCallum Creek crosses the Trans Alaska Pipeline in the Central Alaska Range near Paxson, Alaska. It was converted from a braided channel on an alluvial fan to a stable single thread channel at the pipeline crossing. Objectives included controlling the position of the channel at the pipeline and minimizing aufeis accumulations through the converted reach. A natural channel design methodology was used, which involves determining the dimension, pattern, and profile of a stable reference reach for the associated valley type and stream type and emulating those characteristics in the design reach. A failure mode analysis was conducted. Mitigative measures were identified for each potential failure mode. The channel was lowered to reduce surface energy loss and associated aufeis accumulations. Performance was good during the 2000 and 2001 seasons. Aufeis accumulations have not recurred. Lessons learned are presented from this and several similar projects constructed along TAPS in 2000 and 2001. <p>
    • Subsurface structure of the volcanoes in Katmai National Park, Alaska

      Jolly, Arthur D.; McNutt, Steve; Wyss, Max; Eichelberger, John; Stone, David (2000)
      The three-dimensional velocity, attenuation and b-value structure is mapped beneath the Katmai group volcanoes, located in south-central Alaska. Data for these studies include 4320 earthquakes recorded in the period July 26, 1995 to November 30, 1999 on a 5--18 station short-period seismograph array. The velocity structure is determined by inversion of P-wave travel-times for 8041 rays from 815 earthquakes. The inversion revealed the lowest velocities (3.6--5.0 km/s) centered beneath Novarupta, Trident and Mageik volcanoes between the surface and 4 km below sea level and moderately lower velocities at 0--6 km depth between Martin volcano and Katmai caldera (4.5--6.0 km/s). Higher relative velocities (5.0--6.5 km/s) prevail outboard of the volcanic axis and at Griggs volcano. The attenuation structure is determined by inversion of the amplitude spectra roll off to obtain t* for 1301 rays from 230 earthquakes in the magnitude range (0.8 < ML < 1.8). The inversion, which is well constrained in the depth range 0--6 km, reveals higher attenuation along the volcanic axis 1/Q = 0.008-0.018 (55 < Q < 125) and lower attenuation in non-volcanic regions of the study area 1/Q = 0.01--0.000 (100 < Q < infinity). The attenuation is greatest beneath Mageik, Trident and Novarupta (1/Q = 0.018; Q = 55) between the surface and 6 km below sea level. Frequency-magnitude distributions are determined by mapping b-values for ~1300 earthquakes larger than the magnitude of completeness (0.7 ML). The analysis reveals high b -values at Mageik volcano (1.2--2.2), intermediate b-values at Martin (1.0--1.6) and Katmai caldera (1.2--1.4) and low b-values at Trident (0.6--1.2). Results point to the existence of a large region of partially molten rock centered beneath Mageik, Novarupta and Trident volcanoes at 0--4 km depth. The localized nature of the high b-value zone at Mageik volcano suggests that the magma is discontinuous, occurring as several distinct bodies. The deeper high attenuation anomaly might mark the now solidified but highly fractured plumbing system associated with the 1912 Novarupta eruption.