Shoreline Change in the Hawaiian Islands Bradley M. Romine1, Charles H. Fletcher1, Matthew M. Barbee1, L. Neil Frazer1, Tiffany R. Anderson2, Bruce M. Richmond3 [email protected] [email protected] [email protected] [email protected] [email protected] [email protected] 1University of Hawaii, Dept. of Geology and Geophysics, http://www.soest.hawaii.edu/coasts/ 2University of Hawaii SeaGrant College Program, http://seagrant.soest.hawaii.edu/ 3USGS Pacific Coastal & Marine Science Center, http://walrus.wr.usgs.gov/ North Kauai INTRODUCTION Shoreline Change Trends Haena Pt Hanalei Bay -0.7 ± 0.6 m/yr 0.7 ± 0.7 m/yr Historical shoreline studies are used by coastal zone managers to mitigate coastal Shoreline Change Bands erosion impacts (Fletcher and others, 2003). Beaches on the Hawaiian Islands of outer: short-term trend (1940’s-) short-term Kauai, Oahu, and Maui are investigated for chronic beach erosion, defined as shore- inner: long-term trend (all yrs) long-term line change occurring over 10’s to 100 years. We map shoreline positions using or- thorectified aerial photographs and topographic survey charts (T-sheets). Shoreline Erosion Aliomanu movement is measured at 12,203 transects spaced every 20 m (244 km). Uncer- erosion very likely* -0.7 ± 0.4 m/yr erosion likely** Anahola tainties are calculated for shoreline positions using data from seasonal beach profile 0.7 ± 0.4 m/yr measurements and the mapping process (Romine and others, 2009). Accretion accretion very likely* accretion likely** beach lost to erosion undetermined*** Kauai East Kauai CALCULATING SHORELINE CHANGE RATES *conclusive trend at 95% C.I. **models agree on trend, but are not conclusive at 95% C.I. West Kauai ***models do not agree on trend Major’s Bay Transect Plot 1.6 ± 1.8 m/yr 160°W 158° 156° 50 rate = −0.4 ± 0.1 m/yr Hawaiian 22° Oomano Kaua‘i N -1.4 ± 0.2 m/yr O‘ahu Islands Waimea 0 1.4 ± 0.7 m/yr meters Koki Pt -1.5 ± 0.4 m/yr −50 Maui 1920 1940 1960 1980 2000 0 100 km 20° year shoreline position South Kauai ± positional uncertainty shoreline trend Single Transect Method Rates are calculated independently at each transect using weighted least squares regression (e.g., Genz and others, 2007). KAUAI SHORELINE CHANGE shoreline position shoreline change Kauai beaches are erosional in the long-term with an model (EX) 10 average rate of all transects of -0.08 ± 0.01 m/yr and 0 65% of transects suggesting a trend of erosion. Results Rocky Pt 0.8 ± 0.8 m/yr −10 for Kauai are somewhat inconclusive in the short-term. North Oahu −20 The average rate of all transects (0.03 ± 0.02 m/yr) sug- Shoreline Position (m) (cross-shore distance) −30 gests accretion but slightly more transects (51%) indi- 2000 cate erosion than accretion (36%). Strongly accreting 1980 90 80 Haleiwa 1960 70 -0.7 ± 0.3 m/yr 60 beaches are found at Hanalei Bay (north) and at west Shoreline 1940 50 1920 40 short-term Time (yr) 30 Transect # Kauai. Six km or 8% of the total length of Kauai (alongshore distance) long-term EX beaches were lost to erosion. Model Eiegenbeaches Method Rates are calculated using shoreline positions from all East Oahu transects on a beach. Rates are modeled using a linear combination of Eigenvec- tors (principle components) calculated from a matrix of the shoreline position data. OAHU SHORELINE CHANGE An optimal model (best-fit, fewest parameters) is selected using Information Criteria Kualoa Pt Oahu has the lowest average long-term erosion rate -1.8 ± 0.3 m/yr (IC) scores (Frazer and others, 2009; Genz and others, 2009). (-0.01 ± 0.01 m/yr) and percent of erosional transects Kualoa (48%). In spite of relatively low overall rates of erosion, 1.5 ± 0.4 m/yr 9 km of Oahu beach was lost to erosion in the time Oahu span of analysis – the highest of the three islands. Strongly accreting beaches are found at east Oahu with Pokai Bay 1.7 ± 0.6 m/yr 54% of the beach accretional and an average long-term Maili rate of 0.08 ± 0.01 m/yr. North and west Oahu are -1.3 ± 2.2 m/yr HAWAII SHORELINE CHANGE dominantly erosional in the long and short-term. West Oahu Twenty-two km (9%) of the total length of beach studied was lost to erosion in the time span of our study. The majority of transects with beach remaining have a sig- nificant or likely trend of erosion (61% long-term, 56% short-term). Averages of all long and short-term rates are erosional at -0.06 ± 0.01 m/yr and -0.02 ± 0.01 m/yr, MAUI SHORELINE CHANGE resp. Percents of transects with statistically significant trends are low (24% total, long-term; 18% total, short-term) due to sparse data (often < 10 historical shore- Maui beaches are the most erosional of the three is- Keahi Pt lines) and high noise (positional uncertainty), typical of historical shoreline studies. lands based on average rates (-0.13 ± 0.01 m/yr, long- -1.6 ± 0.4 m/yr Shoreline change behavior is spatially variable along Hawaii beaches with cells of term) and percents of eroding transects (77%, long- erosion and accretion often separated by only a few hundred meters on a continu- Kaimana term). All regions of Maui are dominantly erosional in 0.8 ± 0.2 m/yr ous beach, or by short headlands that separate the coast into many small embay- the long and short-term with erosional trends at over ments and pocket beaches. 70% of transects. Seven km or 11% of the total length South Oahu of Maui beaches were completely lost in the time span of analysis – the highest percent loss of the three is- lands. 0 10 km short-term Shoreline changes for the beaches of Kauai, Oahu, and Maui. long-term North Maui Beach Beach Long-Term (all years) Short-Term (1940s -) Baldwin Loss, km Loss, % Avg Rate, m/yr ET% Es% AT % As% Avg Rate (m/yr) ET % Es% AT % As% -1.5 ± 1.1 m/yr Kanaha All 21.5 9 -0.06 ± 0.01 61 16 31 8 -0.02 ± 0.01 56 11 35 7 1.5 ± 1.3 m/yr Kauai 6.0 8 -0.08 ± 0.01 65 14 26 3 0.03 ± 0.02 51 6 36 4 north 1.7 8 -0.10 ± 0.01 71 12 20 2 -0.05 ± 0.02 53 5 31 3 east 0.9 6 -0.15 ± 0.02 77 21 19 2 -0.05 ± 0.02 57 6 28 4 Puunoa Pt south 1.9 14 -0.14 ± 0.02 70 20 16 1 -0.11 ± 0.04 60 11 24 1 0.6 ± 0.2 m/yr west 1.5 7 0.02 ± 0.03 47 6 42 7 0.27 ± 0.07 39 6 54 8 Oahu 8.7 8 -0.01 ± 0.01 48 16 42 13 -0.01 ± 0.01 49 12 44 11 West Maui north 0.2 1 -0.08 ± 0.01 63 23 25 4 -0.06 ± 0.01 63 14 29 4 east 5.5 13 0.08 ± 0.01 38 10 54 22 0.06 ± 0.01 42 8 51 13 south 3.0 11 -0.02 ± 0.01 41 15 50 15 0.00 ± 0.01 41 14 53 17 west 0.0 0 -0.09 ± 0.03 62 21 30 5 -0.08 ± 0.03 57 19 34 5 Maui Ukumehame Maui 6.8 11 -0.13 ± 0.01 77 20 17 3 -0.12 ± 0.01 75 13 19 3 -0.9 ± 0.6 m/yr Kawililipoa north 0.9 6 -0.20 ± 0.03 77 25 19 3 -0.19 ± 0.03 72 21 24 3 -1.1 ± 0.6 m/yr Kawililipoa kihei 2.1 11 -0.09 ± 0.01 80 12 17 5 -0.07 ± 0.01 76 11 18 4 1.6 ± 0.4 m/yr west 3.8 14 -0.12 ± 0.01 74 22 16 1 -0.11 ± 0.01 77 10 18 2 Avg Rate = avg of rates at all transects ± RSS(all uncertainties)/n ET%: total pct of transects with significant or likely trend of erosion; Es%: pct of transects with significant trend of erosion Kihei, Maui AT%: total pct of transects with significant or likely trend of accretion; As%: pct of transects with significant trend of accretion REFERENCES Fletcher, C.H., Rooney, J.J.B., Barbee, M., Lim, S.-C., and Richmond, B.M., 2003, Mapping Shoreline Change Using Digital Orthophotogrammetry on Maui, Hawaii: Journal of Coastal Research, v. Special Issue 38, p. 106-124. Frazer, L.N., Genz, A.S., and Fletcher, C.H., 2009, Toward Parsimony in Shoreline Change Prediction (I): Basis Function Methods: Journal of Coastal Research, v. 25, no. 2, p. 366-379. Genz, A.S., Fletcher, C.H., Dunn, R.A., Frazer, L.N., and Rooney, J.J., 2007, The Predictive Accuracy of Shoreline Change Rate Methods and Alongshore Beach ACKNOWLEDGEMENTS Variation on Maui, Hawaii: Journal of Coastal Research, v. 23, no. 1, p. 87-105. The research described in this poster was carried out by the University of Hawaii Coastal Geology Group under contract with the State of Hawaii; Counties Genz, A.S., Frazer, L.N., and Fletcher, C.H., 2009, Toward Parsimony in Shoreline Change Prediction (II): Applying Basis Function Methods to Real and Synthetic of Kauai, Oahu, and Maui; USGS National Assessment Project; US Army Corps of Engineers; NOAA; Hawaii CZM; Hawaii Sea Grant; and the Harold K.L.