The output maps of this study identify areas in Oahu and Kauai that are high in human activity/development that contribute to coastal erosion. A few intermediate steps were crucial in coming to this result. All shapefiles for each of the socioeconomic variables were converted to raster files in order for the variables to be aggregated. The dams variable, however, had to be preprocessed with supplementary data, such as a DEM, in order to determine the extent to which they contributed to coastal erosion. Below is an outline of these intermediate steps and a summary of the final four vulnerability maps.
The data acquired for each socioeconomic variable were all in shapefile format. The inset maps in Figure 8 display each variable before they have been classified and aggregated together in raster format. The insets show examples of zoomed in areas of Oahu for each specific variable. These same variables were also acquired for Kauai.
Figure 8. A map of Oahu with five inset maps that show a close up area of each socioeconomic variable used in this study.
Flow Accumulation Model
A major part of this study was to classify the dams on Oahu and Kauai according to the level of potential interference they had with sediment flow. As previously mentioned, dams have significant impacts on a coast’s sediment budget. The amount of sediment flow that occurs throughout each island was based on the flow accumulation models developed from DEMs of both of the islands. Figure 9 displays a close up of the flow accumulation model for Kauai. The inset map shows Kauai’s DEM which is what that the accumulation model is based off of. The dams on this map lie at various points on the flow accumulation network within specific watersheds. The same process was used for the island of Oahu.
Figure 9. A close-up of Kauai’s flow accumulation model used to determine the impact of each dam on sediment flowing to the coast. The inset map displays Kauai’s digital elevation model. Both of the dams shown on this map lie on a flow accumulation stream. Due to the wide range of accumulation values, some accumulation streams are not as visible as others.
Watershed Vulnerability and Coastal Vulnerability Maps
The watershed vulnerability maps (see Figure 11 and Figure 12) and coastal vulnerability maps (see Figure 13 and Figure 14) for Oahu and Kauai display areas of human activity/development that contribute to coastal erosion. The colour of these maps become darker as areas of human activity/development increase. It is worth noting that Oahu’s coastal vulnerability map shows higher frequencies of human activity than Kauai’s, but Kauai’s watershed vulnerability map has more affected areas than Oahu’s. Therefore, despite Oahu’s higher population and greater areas of urban development, the dams around Kauai’s coast may be a source of human development that is equally as impactful to the coast.
Figure 10. Watershed vulnerability map of Oahu displaying watershed areas which are vulnerable to decreased sediment flow to the coast because of dams that are present upstream.
Figure 11. Watershed vulnerability map of Kauai displaying watershed areas which are vulnerable to decreased sediment flow to the coast because of dams that are present upstream.
Figure 12. Coastal vulnerability map of Oahu that displays areas of human activity that render the coast more vulnerable to erosion. The inset map displays an area that is at higher risk for coastal erosion due to human activity/development.
Figure 13. Coastal vulnerability map for Kauai that displays areas of human activity that render the coast more vulnerable to erosion. The inset map displays an area that is at higher risk for coastal erosion due to human activity/development.
The Coastal Geology Group of the University of Hawaii has compiled records of historic maps and aerial photos from as late as 1910 in order to produce an interactive, mosaiced maps of Oahu and Kauai that indicate areas of shoreline change. Therefore, these changes represent coastal erosion and accretion that have occurred due to natural changes as well as human impacts. The findings of the study combined with these historical records can provide greater insight as to which areas could use further research. Figure 14 displays a close up image of the historic record of coastline change for an area in Oahu that is high in urban development. As indicated by the red bars, much coastal erosion has occured around this urban area. This validates the importance of focusing on how humans may be contributing to this change. Figure 15 displays a close up image from the historic records of an area in Kauai where there is an impacted watershed from upstream dams. As seen in the image, there is no built-up urban areas, but coastal erosion is occurring. It would be worth further investigation to determine the extent to which this erosion may be occurring due to the upstream dams.
Figure 14. Aerial photo of Kahala, Oahu that depicts historical shoreline change along an urban area. Red bars represent the extent of coastal erosion, while blue bars represent coastal accretion since 1925. Data retrieved from The Coastal Geology Group of The University of Hawaii (Hawaii Coastal Erosion Website, 2016).
Figure 15. Aerial photo of Kipu Kai, Kauai that depicts historical shoreline change along an urban area. Red bars represent the extent of coastal erosion since 1927. This is a coastal area at the outset of one of the vulnerable watersheds from upstream dams. Data retrieved from The Coastal Geology Group of The University of Hawaii (Hawaii Coastal Erosion Website, 2016).
A few strengths and weaknesses of this study project are identified. A strength of this analysis is that the socioeconomic effects on coastline erosion are compared and explored as an alternative to the usual coastline analysis which focuses on natural causes of erosion. This model can be applied to other coastal areas that are experiencing increasing amounts of human activity on top of coastal erosion to provide a deeper understanding of the processes at hand. A weakness of this analysis is the difficulty of incorporating tourism activities that contribute to coastal erosion. More sufficient data such as areas of tourist attractions or beaches would have more accurately demonstrated areas of tourism, but such data was unavailable. Buffers for hotels were used to account for tourism, but the area covered by the buffers appeared to be irrelevant in comparison to the actual effects that tourists may have on beaches. Another weakness is that the original harbors data were only points, thus, circular buffers had to be created using the area data found in the harbors’ attribute table. Therefore, despite its weaknesses, this study provides a good starting point for analyzing socioeconomic factors that contribute to coastal erosion, and points to areas that could use further research and analysis.