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SPATIAL ANALYSIS OF COASTAL EROSION OVER FIVE DECADES NEAR BARROW, ALASKA
MANLEY, WILLIAM F. INSTAAR.
There has been increasing interest in recent years in processes affecting Arctic coastlines, including shoreline erosion and the release of both inorganic and organic sediment to the nearshore marine environment. Beyond scientific interest, the prospect of continued -- and possibly accelerated -- coastal erosion is a major concern for many Arctic communities. Documenting and understanding spatial variability in erosion rates are increasingly attainable as high-resolution imagery becomes available, and as GIS and remote-sensing tools are more widely accepted. This study presents such an analysis for a broad area near Barrow, Alaska (Fig. 1).
Shoreline erosion and accretion were quantified by comparison of co-registered datasets and imagery. Orthorectified Radar Imagery (ORRI) was acquired by Intermap Technologies in July, 2002 at 1.25 m horizontal resolution. Twenty frames of black and white aerial photos from August, 1955 were scanned and georectified to the ORRI using an image-to-image polynomial transformation in ArcGIS, with resulting resolution of about 1.4 m and an RMS error of 2.6 m. The 2002 and 1955 shorelines were digitized with points spaced every 20 m along the 250 km of mainland coastline. For barrier islands and the Barrow Spit, the 1955 coastline was digitized from DRG files depicting the USGS 15-minute topographic maps. Using a variety of vector ArcInfo commands, horizontal displacement of the mainland shoreline was converted to erosion and accretion rates for the intervening 47 years. (Note that time-averaged rates will underrepresent episodically high rates during storm events). Overall error considering georectification, digitizing, and transient waterline shifts due to microtidal fluctuation and wave-set up is approx. 3.1 m for the mainland coast, equating with 0.07 m/yr. For barrier features, where the DRG's are less accurate, error is about 28 m (0.6 m/yr).
Nearly all of the mainland coast (91%) has experienced erosion (Fig. 1). Highly variable across the study area, rates average -0.91 m/yr, with an average horizontal shoreline displacement of -42.5 m. (Rates and displacements are negative for erosion). Relatively low rates of about -0.3 m/yr occur along the Chukchi coast, where sand- and gravel-dominated beaches are backed by bluffs up to 15 m high. Rates are higher along the low coastal plain facing Elson Lagoon, exceeding -5 m/yr near Scott, Ross, and Christie Points (e.g., Fig. 2), before decreasing again in the sheltered waters of inner Admiralty Bay. Rates also decrease within small bays and inlets, on a local scale. Lateral accretion from 1955 to 2002 is uncommon, limited to short stretches of widening beach along the Chukchi coast, and isolated progradation or shifting of small nearshore spits and bars. Immediately adjacent to Barrow, the shoreline has eroded -0.2 to -0.8 m/yr, in agreement with a higher-resolution, related study, whereas the beach near the NARL/UIC complex has prograded on average +0.3 m/yr. The narrow offshore barrier islands have migrated considerably, with an average horizontal shift of 205 m.
Although erosion over five decades has been locally variable, a few patterns emerge. High bluffs and coarse beach sediment protect the Chukchi shoreline, whereas very low coastal bluffs exposing ice-rich, peaty soils are susceptible along the Beaufort mainland coast. Beyond bluff height and shoreface lithologies, fetch plays an important role, with the inner portions of bays and inlets protected at a variety of scales. It appears also that erosion is more pronounced where ice-wedge polygons are strongly developed within mature thaw-lake basins. Near Barrow, human activities in the nearshore zone may have played a role, and erosion is a concern -- even though it occurs there more slowly than the region as a whole. The importance of extreme weather events, and the possibility of accelerated change due to warming and decreasing summer sea ice, will be examined as other imagery improves the temporal resolution for analysis.
Figure 1. Orthorectified Radar Imagery from 2002 for the "Barrow Peninsula", also showing erosion rates calculated by comparison with 1955 aerial photographs. Along the mainland coast, coastal change is calculated as either erosion (blue to red) or accretion (white). For narrow offshore features that have commonly migrated farther than they are wide, results are presented in terms of distance of change per year. For color, please see the online figure or an available PDF file.
Figure 2. Close-up of Ross Point, showing: A) the black and white 1955 aerial photography, with the digitized 1955 shoreline in white, and the superimposed 2002 shoreline color-coded by coastal change; and B) the 2002 radar imagery, similarly showing the 1955 and 2002 shorelines. Numbers in panel A indicate rates of erosion or accretion in m/yr.
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