Coastal erosion threatens to damage nearly 87,000 homes along the shoreline of the United States in the next 60 years, according to a report released in 2000 by FEMA. Population growth, rising sea levels, hurricanes, dams (that block the resupply of sand to beaches) and severe storms associated with El Niño events are escalating concerns that damage from shoreline erosion could cost hundreds of millions in coming decades. In California, these concerns are intensified by demo-graphics and geography: about 80 percent of the state’s 34 million residents live within 50 kilometers of the ocean and about 86 percent of the state’s shoreline is classified as actively eroding. Coastal bluff erosion is of particular concern in counties such as San Diego and Santa Cruz, where homes have been built on landslide-prone bluffs and where heavy surf scours the shore.

Using ArcGIS, a sea-level rise inundation comparison was conducted using four different techniques under five sea-level rise conditions for the Kauai, Hawaii, towns of Hanalei Bay, Kapa'a, and Waimea. Sea-level rise was mapped in 0.5 m increments from 0.0 m of rise to 2.0 m of rise. Datasets used in the analysis include a digital elevation model (DEM) layer, wave height data, tidal elevation data, and land cover data. The four techniques illustrating projected inundation serve as a comparison of passive versus dynamic models. The primary goals of this study were to not only compare passive and dynamic sea-level rise inundation models, but also to provide a realistic representation of what future sea-level rise will look like on Kauai, and which areas would be inundated at specific future water surface levels. The results of this analysis can be used to aid Kauai government officials in planning for the future and to aid in prioritizing where and what infrastructure and development will need to be considered before actual sea-level rise impacts occur.

The native Inupiaq community of Shaktoolik, in northwestern Alaska, is located on a low-lying barrier spit in Norton Sound. The inhabited portion of the spit is 7.1 m above MLLW at its highest and only ~200 m across. The community is vulnerable to marine flooding on both the open ocean and lagoon sides of the spit during large storms. Storm events in this region typically occur during the fall and winter months, often when the coastline is protected from flooding and erosion by shorefast ice. High latitudes are experiencing the greatest increases in temperature due to global warming and the reduced duration and extent of sea ice is affecting Alaskan coastal communities. Continued reduction of sea ice, which typically protects the coastline from exposure during large storm events in late fall and winter, may result in the need to relocate many Native Alaskan coastal settlements.

The goal of this study was to quantify changes in storm surge and wave runup during ice-free months for the mid- to late-21st century. A combination of field data, including beach profiles, offshore bathymetry, debris line position, and sediment grain size, as well as modeled meteorological data were used to address the research goals. An analytical approach was developed to quantify storm surge in Alaskan coastal communities with historical meteorological data from the North American Regional Reanalysis. This analytical model was used to calculate projected storm surge flooding levels in Shaktoolik for the mid- and late-21st century for both a moderate and high greenhouse gas emissions scenario (RCP 4.5 and RCP 8.5 respectively), with meteorological output from the MIROC5 global climate model. Additionally, projected wave runup heights during storm events were determined numerically using WAVEWATCH III to calculate wave height and period from the projected MIROC5 meteorological data and SBEACH to model the maximum runup heights along the Shaktoolik shoreline.

Total storm water levels (storm surge plus wave runup height) were calculated for 236 projected storm events in the Bering Sea and used to find the return periods of flooding for each emissions scenario. When compared to modeled historic storm surge, the results show that the moderate emissions scenario is similar to the historical. The 100-yr surge level is +6.3 m above MLLW for the moderate emissions scenario and +6.1 m historically. The high emissions scenario produces lower storm frequency than the moderate emissions scenario and the return periods of flooding for storm surge are lower than the historical values, the 100-yr surge level is +5.0 m. When wave setup and runup are included in the total flood levels the 100-yr flood level is +10.2 m above MLLW for the moderate emissions scenario and +8.7 m for the high emissions scenario, compared to +10.4 m historically. All three of these flooding levels overtop the highest ground elevation in the community.

California's sandy shorelines are important resources in terms of ecological habitat, recreation and tourism, and aesthetic beauty. They also serve as important natural buffers, often protecting back-beach development and infrastructure from wave attack. Encroachment or fixing of the back beach in conjunction with sea-level rise will reduce the accommodation space of beaches and subsequently increase the vulnerability of the back beach to extreme water level events associated with storms. Understanding the present state of California's sandy shorelines in terms of landward migration potential, vulnerability to extreme water events, and local morphodynamic variability is important for coastal policymakers and managers to make informed decisions regarding the future of the California coastline. In this study, the landward migration potential of California's beaches was assessed through a statewide cataloging and classification of all sandy shorelines in the context of back beach immobility and restrictions to landward migration. Beach inundation vulnerability from increased frequency of high water events with rising sea level was determined with extreme water level exceedance distributions from historic tide gauge records and sea-level rise projections of 18 cm, 79 cm, and 140 cm by 2100. Finally, alongshore variability and the influence of high wave events and berm overtopping on beach profile morphodynamics were analyzed through seasonal beach surveying of a coastal lagoon pocket beach. Fifty-eight percent of the total length of sandy shoreline in California is restricted in the back beach from landward migration. This is primarily due (in Central and Southern California) to anthropogenic coastal development and infrastructure. Beach inundation potential from extreme high water events is highly variable and is a function of berm height and backshore morphology. Backshore morphology influences foreshore and berm morphodynamics by modifying swash sediment transport. The growth and persistence of a berm on a bar-built coastal lagoon beach is dependent on berm overtopping; however, overtopping beyond a threshold frequency becomes destructive and can lead to berm erosion. While statewide more than half of California's sandy shorelines are being threatened by reduced accommodation space through terrestrial and marine encroachment, understanding local variability is essential for determining the future of these highly valued resources.

Climate change threatens established communities on low-lying Pacific atoll islands by increasing their vulnerability to wave-driven flooding and shoreline instability. Atoll islands are dynamic features whose morphology can quickly respond to changing climate, sea level, and wave conditions. Results from this thesis provide new information about how future wave climates may change near Pacific atoll islands, specific projections for flooding and erosion at two particular atolls, and insights into how atoll morphology affects shoreline susceptibility to erosion. The first study projects how wave climates may change in the western tropical Pacific by 2100 at 25 island locations under two greenhouse gas emissions scenarios. Output from four global climate models were used to run a dynamically-downscaled numerical wave model. The mean of the top 5% of future boreal winter and summer wave heights, and their associated directions and frequencies, were calculated. Large winter wave heights were projected to decrease throughout the study area, and summer large wave directions were projected to change by as much as 30°. These changes were incorporated into the second study, which modeled run-up and erosion along island shorelines at Wake and Midway Atolls under differing oceanographic forcing conditions. Sea-level rise was projected to be the dominant control on flooding and morphological change on these two atolls, indicating that atoll islands are at great risk even if future wave heights decrease. Small changes in the direction of large incident waves also had a significant impact on shoreline flooding and erosion. Better-protected shorelines (e.g., lagoon shorelines) were disproportionately affected by increasing sea level and changing wave directions. While these findings were site specific for two atypical atolls, the third study involved modeling shoreline erosional change on a schematic atoll and evaluating how initial morphology affects shoreline response. Bathymetry for a generic atoll was generated with varying morphological parameters and wave-driven erosion was calculated. The parameters that had a significant impact on shoreline erosion were: reef flat width, reef flat depth, island width, and atoll diameter. Atolls with narrower, deeper reef flats, narrower islands, and smaller diameters were most susceptible to shoreline instability with sea level rise. Windward islands are projected to lengthen and migrate toward the lagoon, leeward islands are projected to lengthen and migrate toward the reef rim, and oblique islands are projected to migrate leeward and toward the lagoon. Findings from all three studies elucidate shoreline processes along atoll islands and provide critical information that can guide aid allocation and help communities plan for future hazards.

Abstract

Bethany M. Nagid

Historical Shoreline Evolution as a Response to Dam Placement on the Elwha River, Washington

Morphological changes of the Elwha River delta shoreline in Washington are analyzed from 1870-2015, revealing year-by-year as well as location-based geomorphological evolution. Change in the Elwha shoreline prior to the placement of two dams is shown as accretion in two of three areas of the delta with overall change of up to ~20-30m. To the east of the Elwha River mouth, annualized erosion rates during the lifetime of the dams averaged ~1m/year, but increased in recent years, regularly exceeding 4m/year between 2009 and dam removal in 2012. Other areas showed no significant trends during the overall time period of dam-use, but exhibit a wide variety of year-to year changes. Shoreline changes following dam removal (2012-2015) have shown a wide variety of responses: (1) an accreted shoreline beyond the extent of any previous year (west of the Elwha mouth); (2) an accreted shoreline not yet returned to the spatial extent prior to dam placement (east of the Elwha mouth), and (3) an eroded shoreline beyond previous shore configurations (east of Point Angeles).

This survey of over 130 years of shoreline data at the Elwha delta is intended to identify differences in shoreline morphology during distinct periods of dam placement, use, and removal. The large-scale changes in shoreline morphology associated with dam placement and removal will be important processes to understand as traditionally dammed river ecosystems transition to dam removal in coming years.

Global mean sea level increased by ~20 cm during the 20th century and the rate is expected to accelerate during this century. Many major cities are already exposed to damaging coastal storms and sea-level rise (SLR) will magnify storm impacts. SLR adaptation can reduce harm, but this new concept is complicated because adaptation plans must be tailored to each community's specifications, due to differences in geologic setting, development, etc. This study designed a process for local SLR adaptation planning, in part through the assessment of the city of Santa Barbara's vulnerability to SLR, including evaluations of shoreline topography and development, historical storm damage, and exposure to SLR. The risk of wave damage to Santa Barbara's shoreline development and infrastructure will be high by 2050 but very high by 2100. The risk of flooding and inundation of low-lying areas will be moderate by 2050 but very high by 2100. The risk of increased cliff erosion rates will be moderate by 2050 but very high by 2100. The threat of beach inundation will be low by 2050 but high by 2100.

Most of the flooding and erosion along the U.S. West Coast are caused by storm surges and wind-driven waves, particularly during strong El Niño events. There is a need to predict El Niño occurrences for planning purposes, but forecasts from most of the best El Niño Southern Oscillation (ENSO) prediction models have plateaued at a moderate level, leaving room for improvement in ENSO observing systems, models, and data assimilation methods. While the effects of ENSO on wave heights along the U.S. West Coast are well known, no prior studies have examined whether wave heights are also predictive of the phenomenon. This study finds that significant wave heights (Hsig) along the U.S. West Coast are slightly suppressed during the summers preceding El Niño winters, but the trend is weak and the data are noisy, so contributions to ENSO forecasts are negligible. The summer Hsig trend is strongly associated with the summer North Pacific (NP) Index, which measures the area-weighted sea-level pressure over the Gulf of Alaska.