Document Type

Thesis

Date of Degree Completion

Spring 2026

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Dr. Breanyn MacInnes

Second Committee Member

Dr. Lisa Ely

Third Committee Member

Dr. Bretwood Higman

Abstract

Catastrophic flood events such as landslide tsunamis and jökulhlaups in glaciated areas can transport and strand icebergs, which obstruct the flow of floodwaters and leave a unique and recognizable grounded iceberg wake pattern in the sediment surface. Grounded iceberg wake patterns (GIWs) were identified at the two study areas in Southcentral Alaska; the outwash plain of a 1967 landslide tsunami at the Grewingk Glacier and proglacial lake, and the Matanuska River valley which experienced an unrecorded jökulhlaup in recent geological times. From these sites, 110 GIWs were identified through visual interpretation of lidar imagery and characterized by measuring their geometry, position, and spatial relationships. Results show most GIWs point in the same direction and exist in clusters, with a depression which was wider than it was long, an obstacle shadow ridge downstream that was higher than the elevation of the surrounding surface, two wake lines measuring 20-60ft long on either side of the shadow ridge defining an angle between 50° and 80°. Despite having different flood origins, the source of the flood waters did not change the characteristics of the GIWs, and their features scaled with their size.

Grounded iceberg wakes can assist in estimating flow characteristics, including direction of flow, sediment load, and velocity of floodwaters. Direction of flow, determined from the position of the obstacle shadow ridge and wakes relative to the iceberg depression, showed flow away from the flood source, as well as backwash in the case of the tsunami. Sediment deposition can be estimated from the depth of the iceberg depression and by the obstacle shadow ridge. Iceberg depression depths averaged 1.76 ft in Grewingk and 4.8 ft in Matanuska, which represents a minimum estimate of both flow depth and the thickness of the sediment deposited by the floods. Velocity of floodwaters, estimated by calculating the Froude number from the wake angle, ranged from 10.1-27.2 f/s for water depths of 3.3 and 6.6 ft. This thesis shows how GIWs are valuable tools for predicting future flood events and understanding previous catastrophic flood events in glaciated areas.

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