Collaborative Research: Glacial Valley Profile Evolution

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This is a collaborative proposal by the Principal Investigators at the University of California-Santa Cruz and University of Washington. The Division of Earth Sciences (Geology and Paleontology) is contributing about one-third of the funding. Our understanding of how glaciers carve and sculpture alpine landscapes is poor. Although significant progress has been made on developing the physics of abrasion and sculpturing of glacial beds, glacial erosion into landscape models has largely been ignored. This void in our knowledge base is due to the difficulty in developing a model that captures the: 1) growth and decay of the glacier; 2) generation and temporary storage of meltwater within the glacier; and 3) spatial distribution of erosion over the bed. The Principal Investigators will make field measurements on the Bench Glacier in the Chugach Range of south-central Alaska and develop a model to determine how glaciers function as agents of landscape erosion. They will focus on the evolution of the long valley profile which is a logical step after the modeling of cross-valley profile evolution recently done by Harbor. The Principal Investigators' field program will provide data sets relevant to the timing, spatial distribution and magnitude of erosion over the glacier bed. They will use a straight one-dimensional model and incrementally add more complex factors such as climate forcing and topology of the glacier. They will: 1) document the mass balance profile; 2) measure the pattern of surface speed in time and space using multiple reflectors on the ice; 3) measure the water and sediment output using acoustic sensors for river stage; and 4) make a combination of sampling and turbidity measurements for suspended sediments. Sliding speed and water balance data will guide modeling of timing and pattern of erosion, while annual sediment yield will provide an integral constraint. The sediment yield complicates the relationships, but a three-year monitoring program will minimize variations in storage.The staggered finite one-dimensional glacier model will vary in extent and thickness through time, driven by variations in the mass balance profile or the meteorological constituents that dictate precipitation and melt patterns. This approach will allow the Principal Investigators to incorporate important feedbacks in the system, including evolution of the valley shading as the valley deepens, and reduction of melt rate if the ice surface becomes mantled with debris. They will focus on water balance in the glacial system as water pressure modulates sliding. The instantaneous erosion rate is dictated by a glacial erosion rule that is tied to the calculated local sliding rate and the erodibility of the bed. The Principal Investigators will use their model to explore the formation of overdeepening valleys and hanging valleys. This glacier valley evolution model will have broad applications because it is critically needed to address climate-erosion-uplift feedbacks in mountain range evolution. It will also provide the history of sediment and water flux from glaciated terrain, which modulates chemical weathering, terrace development and basin filling in regions downstream. A model of glacier-tributary interactions, required for the hanging valley problem, will provide the basis for studying jokulhlaups which are poorly understood hazardous phenomena.