Geology of proposed Blue Lake dam site and tunnel near Sitka, Alaska

Twenhofel, William Stephens

doi:10.3133/cir147

Cited by 5 publications

(5 citation statements)

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“…Ice retreated from lowland coastal areas like Baranof Island by 12.1–12.4 ka (Ager et al, 2010; Hamilton, 1986; Kaufman & Manley, 2004; Mann, 1986; Mann & Hamilton, 1995). The region near Sitka is characterized by well‐defined glacial valleys (Twenhofel, 1951) with steep walls and low‐gradient valley bottoms. This high‐relief terrain extends to the coastal margin of most of Baranof Island.…”

Section: Study Area: Sitka Southeast Alaskamentioning

confidence: 99%

Debris flow initiation in postglacial terrain: Insights from shallow landslide initiation models and geomorphic mapping in Southeast Alaska

Patton

Roering

Orland

2022

Earth Surf Processes Landf

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Debris flows pose persistent hazards and shape high-relief landscapes in diverse physiographic settings, but predicting the spatiotemporal occurrence of debris flows in postglacial topography remains challenging. To evaluate the debris flow process in high-relief postglacial terrain, we conducted a geomorphic investigation to characterize geologic, glacial, volcanic, and land use contributions to landslide initiation across Southeast Alaska. To evaluate controls on landslide (esp. debris flow) occurrence in Sitka, we used field observation, geomorphic mapping, landslide characteristics as documented in the Tongass National Forest inventory, and a novel application of the shallow landslide model SHALSTAB to postglacial terrain. A complex geomorphic history of glaciation and volcanic activity provides a template for spatially heterogeneous landslide occurrence. Landslide density across the region is highly variable, but debris flow density is high on south-or southeast-facing hillslopes where volcanic tephra soils are present and/or where timber harvest has occurred since 1900. High landslide density along the western coast of Baranof and Kruzof islands coincides with deposition of glacial sediment and thick tephra and exposure to extreme rainfall from atmospheric rivers on south-facing aspects but the relative contributions of these controls are unclear. Timber harvest has also been identified as an important control on landslide occurrence in the region. Focusing on a subset of geo-referenced landslides near Sitka, we used the SHALSTAB shallow landslide initiation model, which has been frequently applied in non-glacial terrain, to identify areas of high landslide potential in steep, convergent terrain. In a validation against mapped landslide polygons, the model significantly outperformed random guessing, with area under the curve (AUC) = 0.709 on a performance classification curve of true positives vs. false positives. This successful application of SHALSTAB demonstrates practical utility for hazards analysis in postglacial landscapes to mitigate risk to people and infrastructure. | INTRODUCTIONDebris flows are recognized as a key process that shapes high-relief areas and can have devasting consequences for human safety, infrastructure, and the natural environment (Schuster & Highland, 2007).Debris flows travel rapidly through steep terrain, increasing their volume through entrainment, which can expand downslope areas of inundation and hazard zones (Hungr et al., 2005). Globally, debris flows have been documented in a wide array of settings, including forested steeplands, volcano flanks, breached moraine-dammed lakes, and postglacial alpine hillsides. As a result, the topographic and sedimentary signature of past debris flow activity can be highly diverse, emphasizing the need to characterize zones of debris flow initiation, runout, and deposition for hazards mitigation.In soil-mantled steeplands (unglaciated), the repetitive action of shallow landslides that translate into debris flows imparts a distinctive and...

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Section: Study Area: Sitka Southeast Alaskamentioning

confidence: 99%

Debris flow initiation in postglacial terrain: Insights from shallow landslide initiation models and geomorphic mapping in Southeast Alaska

Patton

Roering

Orland

2022

Earth Surf Processes Landf

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“…Several reconnaissance studies and detailed site studies have been completed on various aspects of the geology of the Sitka area since the turn of the century (see Knopf, 1912;Buddington and Chapin, 1929;Twenhofel, 1951;Berg and Hinckley, 1963;others, 1963, 1964;Brew and others, 1969). However, much work remains to be done.…”

Section: Regional Settingmentioning

confidence: 99%

Reconnaissance engineering geology of Sitka and vicinity, Alaska, with emphasis on evaluation of earthquake and other geologic hazards

Yehle¹

1974

Open-File Report

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Inferred effects from future earthquakes^-Continued Ground shaking Continued Geologic units in category 1 (strongest expectable shaking)-^Continued Embankment fill (Qfe) overlying alluvial deposits (Qa) and modern delta deposits of the intertidal zone (Qi) Volcanic ash deposits (Qv) Geologic units in category 2 (intermediate expectable shaking) Modern beach deposits (Qb) Embankment fill (Qfe) overlying modern beach deposits (Qb), elevated shore and delta deposits (Qeb), and glacial drift deposits (Qd) Elevated shore and delta deposits (Qeb) Modified ground (Qfm) Geologic units in category 3 (least expectable shaking)-Glacial drift deposits (Qd) Bedrock of Sitka Group (KJs) Compaction Liquefaction Reaction of sensitive and quick clays Water-sediment ejection and associated subsidence and ground fracturing Earthquake-induced subaerial landslides-Earthquake-induced subaqueous landslides 70 Effects on glaciers and related features Effects on ground water and streamflow Tsunamis, seiches, and other earthquake-induced water waves Inferred future effects from geological hazards other than earthquakes Subaerial and subaqueous landslides Stream floods 78 High water waves Volcanic activity Recommendations for additional studies 82 Glossary 84 References cited 87 Appendix I Preliminary examination of volcanic ash and lapilli samples from near Sitka, Alaska 103 Recommended future geologic studies in the Sitka area could provide additional information needed for land-use planning. Detailed geologic mapping and collection of data on geologic materials, joints, faults, and stability of slopes are strongly recommended. Extensive offshore marine geophysical studies are needed to determine the position of nearby branches of the Fairweather fault; sensitive seismometers are needed to detect very small earthquakes in order to obtain an indication of activity along the fault branches. Naturally occurring volcanic ash apparently will not liquefy during earthquakes, but the ability of excavated ash to liquefy within a range of vibrational conditions suggests the need for further study. The program of volcanic surveillance being conducted on selected volcanoes elsewhere in Alaska should be expanded to include the volcanoes on Kruzof Island. Determination of the natural periods of oscillation of large lakes, fiords, bays, and sounds would assist in the prediction of heights of water waves caused by earthquake ground shaking. correspondence with Federal, State, and city-borough officials, private citizens, and personnel of engineering and construction companies that have worked at Sitka. Especially acknowledged is the help of Fermin (Rocky) Gutierrez, Administrator of the City and Borough of Sitka, and

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“…The Edgecumbe Volcanics of Kruzof Island are undeformed. The northeast and southwest sides of northern Baranof Island are marked by two great northwest-trending lineaments that probably are the traces of major faults (Twenhofel andSainsbury, 1958, p. 1436). The lineaments are developed along zones of intense crushing and shearing that are as much as half a mile wide on Baranof Island.…”

Section: Structural Geologymentioning

confidence: 99%

Reconnaissance geology of northern Baranof Island, Alaska

Berg¹,

Hinckley²

1963

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The rocks of northern Baranof Island range in age from Paleozoic (?) to Mesozoic ; they are tightly folded, intricately faulted, and regionally metamorphosed. "*' They are also contact metamorphosed near the borders of dioritic intrusive rocks. that are probably related to the Coast Range batholithic complex. To the west, on Kruzof Island, rocks of Mesozoic age are unconformably overlain by volcanic rocks that were extruded from Mount Edgecumbe and other vents during Quaternary time. On Baranof Island the strike of most of the contacts, bedding, and Js niineralogical layering, as well as the trend of the lineations, is northwest; however, between Rodman Bay and Saook Bay where strata of Triassic(?) age show two episodes of folding, the rocks commonly strike north and east. Fieldwork combined with the study of aerial photographs shows that many well-defined ""* lineaments are probably faults and shear zones. For mapping purposes the bedded rocks are herein divided into seven lithologic-" stratigraphic units. Although no fossils have been found in or reported from the rocks of northern Baranof Island, the ages of many of the units are tentatively established by lithologic correlation with rocks that are better dated else-"' where in the region, particularly those on Chichagof Island.

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Geology of proposed Blue Lake dam site and tunnel near Sitka, Alaska

Cited by 5 publications

References 0 publications

Debris flow initiation in postglacial terrain: Insights from shallow landslide initiation models and geomorphic mapping in Southeast Alaska

Debris flow initiation in postglacial terrain: Insights from shallow landslide initiation models and geomorphic mapping in Southeast Alaska

Reconnaissance engineering geology of Sitka and vicinity, Alaska, with emphasis on evaluation of earthquake and other geologic hazards

Reconnaissance geology of northern Baranof Island, Alaska

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