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Open-File Report O-13-07, Oregon Beach Shoreline Mapping and Analysis Program: Quantifying Short- to Long-Term Beach and Shoreline Changes in the Gold Beach, Nesika Beach, and Netarts Littoral Cells, Curry and Tillamook Counties, Oregon, by Jonathan C. Allan and Laura L. Stimely
—from the report:
This report describes the procedures used to establish new beach observation sites along the Gold Beach, Nesika Beach, and Netarts littoral cells on the Oregon coast. On the basis of these efforts, a total of 21 beach profile sites were established in the Gold Beach littoral cell, which extends from Cape Sebastian in the south to Otter Point in the north. An additional 14 profile sites were established in the Nesika Beach cell, just north of the Gold Beach cell. On the north coast in Tillamook County, 24 beach profile sites were established in the Netarts littoral cell. In addition to real-time kinematic (RTK) differential Global Positioning System (RTK-DGPS) surveys of 59 new beach profile sites, analyses were undertaken to compare these results to surveys carried out using airborne lidar. In each cell, new tidal datum-based shorelines were measured and compared against both recent historical (lidar) shorelines and older historical shorelines (e.g., 1920s, 1950s, and 1960s era). Our beach monitoring efforts completed thus far have identified the following large-scale beach responses:
- Erosion is occurring immediately north and south of the Rogue River jetties, while much of the shore south of Hunter Creek remains relatively unchanged when compared to historical shoreline information.
- Significant erosion has occurred adjacent to Hunter Creek due to northward migration of the creek coupled with ocean wave attack. This recent phase of erosion now threatens several homes built adjacent to the creek and ocean. Analyses of aerial photos and lidar data indicate that the response has occurred as recently as in 1985 and hence is not unique. We speculate that the recent northward migration may be due to the occurrence of the 2009-2010 El Niño, which likely shifted significant volumes of sand along the beach to the north, preventing Hunter Creek from draining out along its more typical westerly or southwesterly course. In the absence of high flows to punch an outlet, the creek simply began to migrate northward.
- At the north end of the littoral cell (north of the community of Rogue Shores) the beach has been gaining sand, which has resulted in seaward progradation of the shore.
- Significant erosion is occurring along the coastal bluffs that front the community of Nesika Beach. As indicated in Figure 23, the mean change in the toe of the bluffs between 1967 and 2008 was determined to be -15.4 m (-50.5 ft), with a standard deviation (σ) of ±7.1 m; ±1σ about the mean gives an erosion range of -8.3 to -22.5 m (-27.2 to -73.8 ft). (±1σ equates to 68.2% of all measured values and provides a good measure of the typical range of responses along a given shore.) The total excursion over which the shoreline has varied was found to range from +2.4 m to -30 m (+7.9 to -98.4 ft).
- Estimates of the bluff erosion rate indicate that the bluffs are receding at an average rate of -0.38 m/year (-1.25 ft/year); mean ±1σ gives an erosion range of -0.20 to -0.55 m/year (-0.66 to -1.8 ft/year). These values are slightly lower than the erosion rates determined by Priest and others (2004), who identified an average erosion rate of ~-0.58 m/yr (-1.9 ft/yr).
- Recent mapping (2011) of the bluff toe and top indicates little erosion has occurred along the bluff top since the lidar was flown in 2008. For the most part, this finding applies to measurements of the bluff toe. However, in a few discrete shore sections, we observed some 2 to 3 m (6.6 to 9.8 ft) of additional retreat, causing the bluffs to become oversteepened in those areas.
- At the north end of the cell the beaches are actively advancing (prograding) seaward.
- Analyses of historical shorelines indicate that the beach along Netarts Spit was in its most accreted state in the 1920s and 1960s.
- Since the 1960s, and particularly in the last decade, coastal erosion has come to dominate the overall response along essentially the full length of the spit. Lidar data derived changes in the position of the dune toe between 1997 and 2009 indicate a mean net retreat of -21.8 m (-71.5 ft) (Figure 26, left); the mean ±1σ gives an erosion range of -13 to -30.6 m (-42.7 to -100.4 ft) since 1997, while the absolute range of measured response varied from +4.4 m to -35.9 m (+14.6 to -117.8 ft).
- The estimate of the mean erosion rate for the past decade is -2.0 m/year (mean ±1σ indicates that 68.2% of the variability ranges from -1.2 to -2.8 m/year [-3.9 to -9.2 ft/year]) (Figure 26, right). This reflects the highest erosion rate presently known for the dune-backed beaches on the Oregon coast.
- Unless conditions change soon, continued erosion along Netarts Spit will lead to spit breaching and could eventually impact bay hydrodynamics.
- In the north adjacent to the community of Oceanside, the beach appears to be in a state of quasi-equilibrium, responding to periodic shifts in sediment to the north due to effects from El Niño winter storms, followed by reversals where the sand is shifted back to the south by storm waves.
- Shoreline measurements undertaken between Happy Camp and Oceanside appear to capture the effects of the 2009-2010 El Niño, which caused the bay mouth to migrate northward, significantly lowering sand elevations in front of the Capes landslide and eventually removing a large sand wedge that had accumulated north of the mouth to Netarts Bay.