Journal of Coastal Research 19 0 000±000 West Palm Beach, Florida Season 0000 Longshore Sediment Transport Rates on a Reef-Fronted Beach: Field Data and Empirical Models Kaanapali Beach, Hawaii Dolan Eversole and Charles H. Fletcher Department of Geology and Geophysics School of Ocean and Earth Science and Technology University of Hawaii 1680 East West Rd. POST 721 Honolulu, HI 96822, USA [email protected] ¯[email protected] ABSTRACT EVERSOLE, D. and FLETCHER, C.H., 2003. Longshore sediment transport rates on a reef-fronted beach: ®eld data and empirical models Kaanapali Beach, Hawaii. Journal of Coastal Research, 19(0), 000±000. West Palm Beach (Flor- ida), ISSN 0749-0208. Longshore sediment transport (LST) measured at monthly beach pro®les on Kaanapali Beach, Maui is compared to three predictive models. We observe cumulative net sediment transport rates of approximately 29,379 64400 m3/yr to the north and 22,358 61300 m3/yr to the south for summer and winter respectively. Kaanapali Beach experiences a net annual rate of 7,021 6700 m3/yr to the north and a gross annual rate of 51,736 65100 m3/yr. Transport models, namely CERC (1984), CERC, 1991 (GENESIS) and KAMPHIUS (1991) predict net annual LST rates at 3 3 103 percent, 77 percent and 6 3 103 percent of the observed rates respectively. The success of the Genesis model is attributed to its ability to account for short-term changes in near-shore parameters. The use of CERC (1984) is prone to practical errors in its application including use of the recommended K coef®cient and wave averaging that may signi®cantly overestimate LST. The use of KAMPHIUS (1991) is more sensitive to beach slope and wave period than CERC (1984) and may over-predict transport on steep sloped beaches with high wave energy. Presence of fringing reef signi®cantly affects the ability of LST models to accurately predict sediment transport. When applying CERC (1984, 1991) and KAMPHIUS (1991) formulas, functional beach pro®le area available for sediment transport is assumed much larger than actually exists in Kaanapali. None of the models evaluated account for the presence of a reef system. This may contribute to overestimations of LST as they assume the entire pro®le is mobile sediment. However, the fact that CERC (1991) underestimates the observed transport implies that environmental parameters employed in these models (such as wave height, direction and period) play a more substantial role than the in¯uence of the reef in model results. ADDITIONAL INDEX WORDS: Longshore sediment transport, sediment transport modeling, fringing reef, beach pro- ®les, coastal erosion, Hawaii, beaches. INTRODUCTION We describe the dominant spatial and temporal patterns of sediment transport and volume variability and evaluate Many coastal science and engineering studies attempt to three commonly used Longshore Sediment Transport (LST) predict rates of longshore and cross-shore sediment trans- formulas: (CERC, 1984; KAMPHIUS, 1991; and The Army port. The scope of predictive formulas are largely empirical Corps of Engineers (ACOE) Generalized Model For Simulat- and re¯ect results based on ®eld studies from around the ing Shoreline Change (GENESIS) model (CERC, 1991). We world (KOMAR and INMAN, 1970; DEAN, 1989; BODGE and ®nd the CERC (1991) model ®ts observations of LST best KRAUS, 1991; KRAUS et al., 1991; SHORT, 1999). Researchers while the CERC (1984) and KAMPHIUS (1991) models are have found that sediment concentration and transport at the prone to overestimate the observed longshore transport by breaker line is strongly in¯uenced by breaker type and thus roughly an order of magnitude. wave energy (KANA and WARD, 1980; NIELSEN, 1984; VAN RIJN, 1993). Field techniques for measuring total and sus- pended longshore sediment transport include sediment trac- ENVIRONMENTAL SETTING er, impoundment and streamer traps. Here we employ the Kaanapali Beach is located on the west coast of the island impoundment technique for comparison with three predictive of Maui, Hawaii in the lee of the dominant northeast trade longshore transport models. winds. Meteorological conditions of this coast are variable but The near-shore sediment transport system of Kaanapali typically calm with moderate trade winds and infrequent but Beach, Maui is examined using 13 monthly beach surveys. strong onshore storm winds (Kona Storms). The surrounding islands shelter the area from most swells except for three 02120 received and accepted in revision 21 November 2002. pronounced swell windows. The southern swell window rang- Longshore Sediment Transport Rates on a Reef-Fronted Beach Figure 1. Kaanapali Location Map and Swell Windows. es from approximately 1808 to 2208, west swells 2608 to 2808 spur and groove features in the reef slope at depths of 10±20 while the northern swell window extends from 3508 to 308 m. At approximately 500 m intervals, the fringing reef is bro- (Figure 1). ken by shore-normal channels (Aawa) that direct the ¯ow of The area is exposed to an opposing bi-modal swell regime nearshore water and sediment seaward (Figure 2). The fring- that subjects the beach system to seasonal wave forcing from ing reef constitutes a geologic framework that plays a signif- opposite directions, while west swells rarely enter their re- icant role in the stability and replenishment of the beach sys- spective swell window. North Paci®c deep-water swells in the tem in this area. The southern portion of the study area is winter months can exceed 10 m in height with periods of up largely fronted by fringing fossil coral reef that restricts the to 25 s. Similarly south swells are commonly 1 to 3 m but sub-aqueous beach pro®le area actively involved in sediment can exceed 6 m in height with periods of up to 22 s ARMS- transport and can be idealized as a perched beach atop a TRONG (1983). Kona storms are locally produced low-pressure fossil reef. This shallow fringing reef truncates the surface systems that approach from the south or southwest. Kona area of the beach pro®le, reducing the total area that is avail- storms can generate wave heights up of 3 to 5 m and periods able for sediment exchange and mobilization. of 8 to 14 s. Although these storms occur infrequently, they The study area consists of a 4.6 km continuous carbonate are the cause of extensive coastal damage to south and west beach that is bisected by a prominent basalt headland, Kekaa facing shorelines (MAKAI OCEAN ENGINEERING,INC. and Point. Kekaa Point divides the area into two distinct littoral SEA ENGINEERING INC., 1991; ROONEY and FLETCHER, cells, the Honokowai cell to the north and the Kaanapali cell 2000). to the south with seasonal sediment impoundment occurring Shallow fringing reef (,1 m depth) dominates the northern on both sides. The beach is composed of moderately-sorted and southern extents of the study area with deeper outcrops carbonate sand with a minor basalt component (, 10 percent) of fossil reef (5±10 m depth) observed intermittently in the and a median grain size diameter of 0.23 mm. The beach central area. The fringing reef is composed of fossil reefal generally displays a steep foreshore slope (vertical: horizon- limestone and beachrock that dominate the reef ¯at and shal- tal) mean 1:8, and a gentler backshore (sub-aerial) slope low reef segments. Encrusting coralline algae and branching mean of 1:11. The foreshore slope was applied as input to the corals are found at deeper regions of the reef front forming LST models described. Journal of Coastal Research, Vol. 19, No. 0, 0000 Eversole and Fletcher Figure 3. Cumulative pro®le volume change. Cumulative alongshore Figure 2. Kaanapali Beach fringing reef (gray shading), reef channels, volume change derived from pro®le volumes (vol , vol , etc.) Winter cu- beach survey locations and transport study area. 1 2 mulative volume change calculated from pro®le vol1 to vol5, while summer is calculated from vol5 to vol1. Journal of Coastal Research, Vol. 19, No. 0, 0000 Longshore Sediment Transport Rates on a Reef-Fronted Beach Table 1. Beach pro®les March, 2000 to April 2001. Note the larger volumes and volume ranges of pro®les 5 and 6. Maximum Minimum Volume Mean Mean Volume Net Volume Volume Volume Range Volume Rate Change Change Pro®le (m3/m) (m3/m) (m3/m) (m3/m) (m3/m/month) (m3/m) 1 29.23 9.26 19.97 18.26 20.22 22.83 2 94.71 83.37 11.34 87.64 20.54 26.97 3 137.80 112.17 25.63 128.35 0.99 12.91 4 476.42 349.83 126.59 396.96 27.50 297.49 5 707.66 552.36 155.30 651.81 5.47 71.16 6 733.00 625.62 107.38 685.14 7.00 90.96 7 379.11 244.52 134.59 317.91 0.33 4.24 8 222.70 165.94 56.76 185.64 21.42 218.44 9 206.82 45.88 160.94 126.47 24.70 261.15 10 107.13 92.16 14.97 98.50 20.48 26.18 11 305.80 261.58 44.22 283.22 0.35 4.50 Mean 309.13 231.15 77.97 270.90 20.06 20.84 METHODOLOGY integrate over the entire area and reduce the effect of sea- sonal outliers, we calculate the cumulative net sum along- Monthly Beach Surveys shore as a proxy for longshore transport rates (Figure 3). Observations of monthly beach pro®le changes were col- lected at a series of 11 shore-normal beach pro®le transects Wave Data situated along the length of the study area. Thirteen monthly Wave parameters such as height, period and direction are surveys were performed from March, 2000 to April, 2001.