Kahana Beach Regional Beach Nourishment Feasibility Study

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2 Kahana Beach Regional Beach Nourishment Feasibility Study Prepared For: The County of Maui 200 S. High Street Kalana O Maui Building, 6th Floor Wailuku, HI Prepared By: MOFFATT & NICHOL 3780 Kilroy Airport Way, Suite 600 Long Beach, CA September 2016 M&N File 9245

3 Kahana Beach Regional Beach Nourishment Feasibility Study September Table of Contents 1. Executive Summary Introduction Literature Review Erosion at Kahana Bay Sand Search Summary Sand Search Results Oceanographic Design Parameters Preliminary Design Concepts Concept Project Pilot Project Triggers for Renourishment Constructability Offshore Dredging Pipeline Corridor Sand Placement Coastal Structures Conclusions Next Steps References ATTACHMENTS Attachment A Concept Nourishment Project Attachment B Pilot Nourishment Project APPENDICES Appendix 1 Literature Review Appendix 2 Synopsis of Data Collection Appendix 3 Results of Sub-bottom Profiling Appendix 4 Environmental Document Elements Appendix 5 Presentation to Maui Planning Commission Final iii

4 Kahana Beach Regional Beach Nourishment Feasibility Study September TABLE OF TABLES Table 4-1: Results of Shoreline Change Analysis... 7 Table 6-1: Estimated Sediment Volumes in Focus Area Deposits Table 7-1: Tidal Datums for Maui Table 7-2: Extreme Water Levels for Maui Table 7-3: Combined Water Depth for 50-year Condition TABLE OF FIGURES Figure 2-1: Location of Kahana Bay on West Maui... 3 Figure 2-2: Project Site Map... 4 Figure 4-1: Post-storm Beach Conditions along Kahana Bay... 8 Figure 4-2: Pre-storm (February 2016, left) and post-storm (May 2016, right) beach conditions... 8 Figure 5-1: Kahana Sand Search Study Area Figure 5-2: Kahana Sand Source Focus Area Figure 6-1: Screen Capture of Sub-bottom Processing Software with Annotations Figure 6-2: Side Scan Sonar Image with Annotations Figure 6-3: Estimated Sediment Thickness in Four Deposits Figure 6-4: Sediment Sample Locations in Kahana Bay Figure 6-5: Comparison of Offshore Sediment to Beach Fill Envelope Figure 6-6: Photograph of Offshore Sand Figure 8-1: Conceptual 50,000 CY Beach Nourishment with T-head Groins Figure 8-2: Conceptual T-head Groin Layout Figure 8-3: Iroquois Point T-Groin Project Figure 9-1: Theoretical Barge-mounted Hydraulic Pump System Figure 9-2: Potential Dredge Pipeline Routes from Sand Deposits 19 and 22 to Kahana Bay Figure 9-3: Geotube Use in Beach Nourishment Projects Figure 9-4: Initial Settling Basin Configuration Final iv

5 Kahana Beach Regional Beach Nourishment Feasibility Study September Executive Summary This Feasibility Report presents an assessment of the technical feasibility of a regional beach nourishment project at Kahana Bay, West Maui. The work includes a review of existing studies and data, an identification of potential sand sources, dredging and construction methods, and preliminary concept development. The analyses included herein do not include a detailed environmental or engineering study. Developing the actual concepts referenced here will require additional investigation of the compatibility of the offshore sediment with Kahana Beach, environmental assessments, and detailed engineering design. However, this report can be used to assist in making decisions regarding the options available for shoreline restoration and protection on a regional basis as opposed to an individual property basis. Numerous studies have estimated the long-term rate of erosion at Kahana Bay. Based on these studies, the beach loses 0.5 to 1 foot of beach width per year. This chronic loss of beach reduces recreational opportunities as well as diminishes the natural shoreline protection throughout the region. Due to its reduced beach width, Kahana Beach is susceptible to damage from regular and extreme storm events. The high-wave climate during March and April 2016 led to loss of beach, failure and undermining of hardened structures, and water quality impairment from clay soils leaching into Kahana Bay. Damages from this period required both emergency and long-term shoreline improvements. Individual properties have applied for both emergency and long term permits to harden the shoreline that are potentially inconsistent with Maui County s long term plan for the shoreline. M&N and a team of subcontractors (Rising Tide Engineering and Golder & Associates) performed an offshore sand search from April 18 to April 22, 2016, and presented the preliminary results to the Maui Planning Commission on April 26, The purpose of the search was to identify and categorize offshore sand deposits. Various instruments were used to collect bathymetry readings, delineate material layers, and collect offshore and onshore sediment samples. The sand search identified four sand deposits, two of which are within one mile of Kahana Beach and contain a combined sand volume exceeding 275,000 cubic yards (CY). An initial analysis of the sediment quality revealed that the offshore sand was potentially compatible with the existing beach sand. In addition, the sand search identified potential dredge pipeline routes that would allow for hydraulic dredging. Based on the quantity of offshore sand available, concept designs for long-term 50,000 to 100,000 CY nourishments were prepared; these alternatives would add 50 to 110 feet of initial beach width. Sand retention structures, such as armor stone T-head groins, are proposed to stabilize the fill and retard the loss of beach to erosion. While these long-term projects may take between 3 to 5 years to design, permit and construct, the County may also consider smaller, individual emergency projects that could be permitted under the Small-Scale Beach Nourishment (SSBN) regulation. These projects would address the urgent need to protect the coastline, as well as serve as pilot projects to demonstrate the use of nourishment along with geo-bag T-head groins. These pilot projects would be confined to a smaller portion of the coastline, and require less fill. At this point, a regional beach nourishment project for restoring the beach at Kahana Bay is a technically feasible and constructible approach to mitigating erosion, protecting shoreline property, and restoring the beach on a regional scale. Final 1

6 Kahana Beach Regional Beach Nourishment Feasibility Study September The next steps in the process would require the individual properties to form a binding entity to contract, perform, monitor, and maintain the project, as well as the following tasks: Project Planning Field Investigations Preliminary Engineering Environmental Document Preparation and Permitting Final Engineering Bidding and Construction. Final 2

7 Kahana Beach Regional Beach Nourishment Feasibility Study September Introduction In February 2016, the County of Maui ( the County ) and consultant Moffatt & Nichol (M&N) launched a study to investigate the feasibility of using regional beach nourishment to provide coastal protection and to restore the beach at Kahana Bay (Figure 2-1). The geographic area comprising Kahana Bay, for the purposes of this study, includes the 3,500-foot-long beach cell from Pohaku Park also known as "S-Turns" at the south end to Kahana Stream at the north end, and encompassing nine (9) condominium complexes and one (1) Kuleana parcel (Figure 2-2). Figure 2-1: Location of Kahana Bay on West Maui Final 3

8 Kahana Beach Regional Beach Nourishment Feasibility Study September Figure 2-2: Project Site Map The purpose for this study is two-fold. First, this study investigates a regional solution to the continuing and worsening erosion problem throughout West Maui. Beach nourishment represents a holistic approach to coastal restoration and protection that improves beach resources, and enhances recreational activity and property values. Historically, the threat of shoreline erosion has been addressed by individual condominium associations armoring their shorelines. Reports indicate that the existing shoreline armoring fronting four (4) of nine (9) condominium complexes in the Kahana Bay beach cell may have contributed to changes in the nearshore littoral processes that can contribute to beach loss and end effect erosion at neighboring properties. Second, this study seeks to demonstrate that shoreline protection options involving beach nourishment are feasible. The Hololani Resort condominiums within Kahana Bay has recently obtained a permit to construct a hybrid seawall revetment fronting their property, and adjacent property owners are concerned that the seawall may exacerbate the erosion trend in the littoral cell. Final 4

9 Kahana Beach Regional Beach Nourishment Feasibility Study September This study will assess alternative erosion mitigation strategies that can protect threatened buildings while restoring and conserving the natural environment in an effort to discontinue the cumulative impact of armoring. This regional beach nourishment study consists of a literature and data review, site surveys, an offshore sand search, and the development of preliminary nourishment designs. In addition to the information presented in this report, M&N also presented the results of the sand search, the proposed beach nourishment solution, and preliminary designs to the Maui Planning Commission on June 14, This presentation was based primarily on this report, and the slideshow is included as Appendix 5. Final 5

10 Kahana Beach Regional Beach Nourishment Feasibility Study September Literature Review The study was initiated by a literature review, which provided various information pertinent to environmental data at Kahana Bay, historic shoreline conditions and erosional events, and preliminary engineering for other beach nourishment projects in West Maui. An Annotated Bibliography that details the various pieces of literature reviewed is included as Appendix 1. The United States Army Corps of Engineers (USACE) initiated a Regional Sediment Management (RSM) program for the West Maui region in The purpose of this program is to delineate coastal erosion, damage to coral reefs, and other coastal issues that threaten the West Maui coastline. Key components of the RSM include a compilation of historic shorelines and numerical modeling of sediment transport. The purpose of the sediment transport modeling was to estimate sediment transport pathways, but not to predict the sediment transport rates. The results of this modeling demonstrated that longshore transport is far more significant than cross shore transport in this region; sediment is removed from the beach via offshore streams; in Kahana Bay, these streams are located at the edges of the littoral cell. As a consequence, sediment retention structures that are perpendicular to the beach may be effective in limiting this longshore transport. The benthic mapping and LiDAR collection were useful for designing the sand search and for preliminary engineering. The benthic mapping identified potential sand deposits along the West Maui coastline, which were considered to be preliminary search areas. The LiDAR provided high quality topography and bathymetry along the beach and nearshore regions of Kahana Bay. This data was digitized to create an existing ground that was used for preliminary engineering design. This study also provided high quality aerial images. Fletcher et al. (2012) provides a record of historic shorelines dating back to These historic shorelines were used to estimate a long-term erosion rate, and intermediate erosion rates during pre-development and post-development beach conditions. Long-term erosional trends were augmented by beach management plans at Kaanapali, which is just south of the study area; these plans note extreme storms that have occurred in the region, and the corresponding seasonal erosion along portions of the beach. Two studies investigated potential beach nourishment projects at Kapalua and Kaanapali (north and south of Kahana Bay, respectively, along the West Maui coastline). These studies identified offshore sand deposits to the north and south of the project site, assessed potential construction methods and outlined the State Department of Land and Natural Resources (DLNR) Beach Nourishment Guidelines. These guidelines state that the fill material: Must not contain more than 6% silt material (sand grain size smaller than mm). Must not contain more than 10% coarse material (sand grain size greater than 4.76 mm). Must have a grain size distribution that falls within 20% of the existing beach grain size distribution. The overfill ratio of the fill sand to existing sand shall not exceed 1.5. No more than 50% of the fill sand shall have a grain diameter less than mm. Must be free of contaminants such as silt, clay, sludge, organic matter, turbidity, grease, pollutants, and others. Must be dominantly composed of naturally occurring carbonate beach or dune sand. Final 6

11 Kahana Beach Regional Beach Nourishment Feasibility Study September Erosion at Kahana Bay Erosion is a significant problem throughout Hawaii, and has been a subject of much research; it has been attributed to several causes, including sea-level rise, reduced reef productivity, and man-made structures, which are primary factors in the changing size and shape of Hawaii s shorelines. According to University of Hawaii research, Kahana Beach loses more than half a foot of beach width each year (ft/yr) on average since Fletcher et al., 2003 estimates the erosion rate to be 0.72 ft/yr. M&N analyzed historic shorelines from 1912, 1949, 1960, 1975, 1988, and 1997 (shorelines were developed by the University of Hawaii Coastal Geology Group and provided as an attachment to Fletcher et al., 2012) to estimate long-term erosion and short-term erosion over those periods of time; the results are presented in Table 4-1. During the period from 1949, before any shoreline armoring had occurred, the average rate of erosion was 0.9 ft/yr; this likely represents the background rate of erosion for a wide, nonarmored beach. Shoreline armoring began in the early 1950s; during this time period (i.e., ), the shoreline experienced an average erosion rate of 1.9 ft/yr. Year Change in Beach Width since 1912 (ft) Table 4-1: Results of Shoreline Change Analysis Average Long-term Erosion (ft/yr) Change in Beach Width Since Previous Shoreline (ft) Average Short-Term Erosion (ft/yr) While erosion is a chronic problem, event-based phenomenon adds significantly to beach erosion. Large storm waves combined with elevated water levels can transport large quantities of sediment offshore over a course of days to weeks. While long-term erosion rates are helpful for planning purposes, it is important to recognize that episodic storm events cause the most significant damage to the coastline and coastal infrastructure. During March and April 2016, Kahana Bay was subject to several large storm events. Damage from these storms included loss of beach, failure and undermining of hardened structures, and water quality impairment from clay soils leaching into Kahana Bay. Figure 4-1 shows the state of various parts of the coastline after the storm damage. The damage from these storms required emergency action to prevent additional loss of property or structural failure. Moreover, the resultant state of the shoreline called for emergency shoreline improvements to protect against such future events. Figure 4-2 shows two photographs taken from the same location, before (February 2016) and after (May 2016) these storms. This figure shows that the entire beach in its existing state can be lost in three months. Final 7

12 Kahana Beach Regional Beach Nourishment Feasibility Study September Figure 4-1: Post-storm Beach Conditions along Kahana Bay Figure 4-2: Pre-storm (February 2016, left) and post-storm (May 2016, right) beach conditions Final 8

13 Kahana Beach Regional Beach Nourishment Feasibility Study September Longshore transport causes seasonal beach width changes within the littoral cell. These seasonal beach variations have been well documented in the Kaanapali littoral cell between Kekaa Point and Hanakaoo Point. Over this 3,600-foot stretch of coastline, the beach width can change by over 150 feet at either end during one season. From May through August 1998, two transects located at the southern end of this littoral cell decreased by 161 and 128 feet; this erosion corresponded to nearly 70,000 CY of sand (Sea Engineering, 2012). A sandy Kahana Beach may be subject to similar seasonal fluctuation. Final 9

14 Kahana Beach Regional Beach Nourishment Feasibility Study September Sand Search Summary M&N performed a sand search with two sub-contractors, Golder Associates and Rising Tide Engineering, from April 18 to April 22, The field team consisted of Dave Hrutford (Golder Associates), Peter Fahringer (Golder Associates), Cathy Smith (Golder Associates), Ian Horswill (Rising Tide Engineering), Kyle Aveni-Deforge (Rising Tide Engineering), and Robert Sloop (Moffatt & Nichol). The purpose of this sand search was to investigate the West Maui nearshore area, from depths between 10 to 50 feet, for potential sand deposits that could be used for a regional beach nourishment program. This search used several methods: acoustic sub-bottom profiling to collect bathymetry and delineate material layer properties, side scan sonar to determine material type at the seafloor, jet-probing to verify sediment layer thickness, and vibracoring to verify sediment layer thickness and to perform physical sediment analyses. In addition, sand samples were collected at the proposed sand deposits, the project site, and adjacent beaches and analyzed for grain size. Appendix 2 includes a synopsis of the data collection activities and the data inventory, including the actions taken by the field team, and the outcomes of these daily activities. The additional data noted in the Data Inventory section is available upon request. Appendix 3 describes the sub-bottom profiler and side scan sonar methods in detail. The study area for the sand search can be seen in Figure 5-1. The study area was determined based on previous sand searches, aerial imagery, and benthic mapping/lidar data performed by the U.S. Army Corps of Engineers (USACE, 2015) in 2014, and LiDAR data (USACE, 2015). This review identified potential sediment deposits offshore of West Maui (polygons in Figure 5-1). This entire area was surveyed during the first day of field work, as indicated by the trackline that extends the entire length of shoreline (Subbottom Survey Track 1, orange). The first day of surveying revealed four polygons close to Kahana Bay that were selected for further investigation these four polygons comprise the focus area. Figure 5-2 shows the four polygons that comprise the sand source focus area. This focus area was investigated via closely spaced ( feet) side-scan sonar and sub-bottom profiler transects, vibracore samples, and jet-probes on the second through fifth days of the sand search. As this figure shows, these sites are located reasonably close (0.5 to 3 miles) to Kahana Beach. The sand search also investigated potential dredge pipelines from these deposits to the shoreline. Sediment samples were collected from various beaches along the coastline to categorize native beach sand. Six samples were collected over 1.7 miles of coastline; these sample locations can be seen in Figure 5-1. Sediment samples were collected at adjacent sites along the coastline in an attempt to characterize the natural beach along West Maui, as opposed to only collecting samples from the highly eroded and sorted beach at Kahana Bay. Final 10

15 Kahana Beach Regional Beach Nourishment Feasibility Study September Kahana Beach Figure 5-1: Kahana Sand Search Study Area Final 11

16 Kahana Beach Regional Beach Nourishment Feasibility Study September Kahana Beach Figure 5-2: Kahana Sand Source Focus Area Final 12

17 Kahana Beach Regional Beach Nourishment Feasibility Study September Sand Search Results The sand search results consisted of an estimate of potential offshore sediment volume and a grain size analysis. As mentioned above, the sand search mapped the bottom bathymetry, material type, and layer thickness. Sample output from the sub-bottom profiler and side scan sonar can be seen in Figure 6-1 and Figure 6-2. Figure 6-1: Screen Capture of Sub-bottom Processing Software with Annotations Final 13

18 Kahana Beach Regional Beach Nourishment Feasibility Study September Figure 6-2: Side Scan Sonar Image with Annotations The sub-bottom profiler and side scan sonar results were used to estimate the quantity of sand in the four deposits. Figure 6-3 shows the focus area with sediment thickness contours superimposed onto an aerial image (this figure refers to each deposit as a GRID AREA ); additional detail of these potential sand deposits can be seen in Appendix 3. Estimated sediment volumes in each deposit are tabulated in Table 6-1. As this table shows, the total volume of these four deposits is over 400,000 CY. The two deposits that are most feasible, based on their proximity to the site and shallow depth, sites 19 and 22, have a combined estimated volume of over 275,000 CY. These two deposits are separated by a narrow ridge composed of hard material. Final 14

19 Kahana Beach Regional Beach Nourishment Feasibility Study September Figure 6-3: Estimated Sediment Thickness in Four Deposits Final 15

20 Kahana Beach Regional Beach Nourishment Feasibility Study September Table 6-1 lists the estimated sediment volumes in the focus area deposits. Table 6-1: Estimated Sediment Volumes in Focus Area Deposits Sand Deposit Estimated Volume (cy) Area (acres) 18 69, , , , Total 405, Note that these volumes represent the total volume of sand available in the area. The amount of sand that would be available for use as beach nourishment would be less due to dredging inefficiencies, limitations on the extent of the dredging area due to adjacent benthic assets, and potential requirements to minimize impacts to local wave patterns. Vibracore samples and sediment samples were collected from the proposed borrow areas and the coastline. One of the samples from sand deposit 22 and the coastline samples were subject to a grain size analysis as a preliminary compatibility determination based on the physical criteria of the DLNR guidelines (Section 3). In addition, grain size analysis from previous investigations in Kahana Bay (Rising Tide, 2015) were provided for sand deposit 19. The locations of these samples can be seen in Figure 6-4. Final 16

21 Kahana Beach Regional Beach Nourishment Feasibility Study September Figure 6-4: Sediment Sample Locations in Kahana Bay Figure 6-5 compares the grain size curve of sediment analyzed from sand deposits to the DLNR fill envelope defined by the native beach sand. The DLNR grain size envelope was developed by plotting the beach sand grain size envelope and expanding it to include sediment that is 20% finer and 20% coarser. As this figure shows, the offshore sediment fits within the limits of the DLNR fill envelope. A photograph Final 17

22 Kahana Beach Regional Beach Nourishment Feasibility Study September of the offshore sand is presented in Figure 6-6. Based on visual inspection, this sand is composed of carbonate sand free of sludge, organic matter, grease, and other pollutants. These preliminary tests indicate that the offshore sediment is compatible with beach sand. Additional compatibility assessment would be required to successfully implement a beach nourishment program; this would require collecting and analyzing additional shoreline and borrow locations for physical, chemical, and biological properties to test for compliance with local, state, and federal requirements for beach nourishment. Figure 6-5: Comparison of Offshore Sediment to Beach Fill Envelope Figure 6-6: Photograph of Offshore Sand Final 18

23 Kahana Beach Regional Beach Nourishment Feasibility Study September Oceanographic Design Parameters Oceanographic parameters at Kahana Bay include water levels and the incoming wave climate. These data were extracted from buoys on Maui and wave evaluations at nearby locations. Ocean water levels at Kahana Bay are primarily influenced by tides, storm surge, and sea level rise (SLR). Tides are semi-diurnal and the tidal range is 2.25 feet between Mean Higher-High Water (MHHW) and Mean Lower-Low Water (MLLW). Tidal datums are based on the NOAA buoy located at Kahului Harbor, Maui (NOAA, 2016). These datums, based on the present tidal epoch ( ), can be seen in Table 7-1. All values are in feet, relative to Mean Sea Level (ft MSL). The highest observed water level at this site was 2.37 ft MSL, observed on 12/20/1968. Table 7-1: Tidal Datums for Maui Datum Value (ft MSL) Description MHHW 1.13 Mean Higher-High Water MHW 0.78 Mean High Water MTL Mean Tide Level MSL 0 Mean Sea Level DTL 0.01 Mean Diurnal Tide Level MLW Mean Low Water MLLW Mean Lower-Low Water Statistical analysis of water levels at this buoy was performed in order to estimate extreme water levels; these values incorporate each water level component that contributes to observed data. Extreme water levels are presented in Table 7-2. As these values show, extreme water levels are dominated by tides (relative to other water level factors) Table 7-2: Extreme Water Levels for Maui Return Period Extreme Water Level (ft MSL) 2 years years years 1.65 Several studies predict future SLR may increase considerably (Fletcher 1992 and Fletcher et al. 2002). The median sea level increase on Maui is predicted to be 9.1 inches over the next 50 years and over 19 inches over the next century. More recent work by Fletcher indicates that a value of 40 inches by 2100 is highly likely (Fletcher, 2008). The risk of seal level rise to Maui is highlighted in Norcross-Nu u, et al. (2008). The paper cites the potential for rates that range as high as 1.6 m (63 inches) per century. The large variations in these potential predictions warrants serious consideration for shore protection systems throughout the island. Final 19

24 Kahana Beach Regional Beach Nourishment Feasibility Study September The wave climate at Kahana Bay may be reasonably approximated from a wave modeling study performed by Marc M. Siah & Associates, Inc. (2011) and from the design conditions stated in the Hololani Environmental Assessment (EA) (Hololani Resort Condominiums, 2013). The wave modeling study simulates nearshore wave climate at Kahana Sunset, a condominium association located a mile north of the project site that experiences erosion similar to the resorts located on Kahana Bay. Also, similar to Kahana Bay, the Kahana Sunset is fronted by a nearshore reef that extends about 2,000 feet offshore. The wave modeling study found that waves from the north refract and break over the reef, well offshore of the entrance to the Kahana Sunset embayment; this phenomenon may also be present at Kahana Bay, as aerial imagery shows waves breaking primarily at the outer extent of the reef. At the entrance of the Kahana Sunset embayment, which is located 500 feet offshore (and therefore landward of the reef edge), wave heights range from 1.5 to 3 feet under extreme swells. These results, not only indicate breaking of larger waves over the reef as they enter the embayment and approach the coastline, [they] also show dissipation of some wave energy due to the effects of nearshore bathymetry and topography (Marc M. Siah & Associates, Inc., 2011). Due to the proximity to Kahana Sunset and the similar bathymetry, the wave heights at Kahana Bay may similar to the wave heights at Kahana Sunset. However, three differences exist between the conditions at Kahana Bay and the conditions simulated in this study. Firstly, the inlet to the Kahana Sunset embayment is much narrower than the inlet at Kahana Bay, providing additional protection from incoming swells. Secondly, the Kahana Sunset embayment is sheltered from south swells, which significantly affect sediment transport at Kahana Bay. Thirdly, this study also omits sea level rise, therefore may not adequately characterize future wave conditions. The Hololani EA estimates the significant wave height based on a depth limited wave at the existing shoreline. Water levels are based on a 50-year sea level change condition, and include tides, storm surge, wave setup, other phenomena (mesoscale eddies, sea level rise), and a nominal depth of 3 feet. These factors are summarized in ns at the Kahana Bay shoreline. Table 7-3, below. Based on these factors, the design water depth is 7 feet, and the design wave height is 5.5 feet. Additional study is required to validate the parameter values and the use of depth to breaking wave height ratio (d/h b = 0.78) for the conditions at the Kahana Bay shoreline. Table 7-3: Combined Water Depth for 50-year Condition (Source: Hololani Resort Condominiums, 2013) Parameter Water Depth Contribution (feet) Tide (Mean Higher-High Water) 1.2 Storm Surge 0.5 Wave Setup 1.5 Other Phenomena (Mesoscale Eddies, Sea Level Rise) 0.8 Nominal Water Depth at Mean Sea Level 3.0 Design Water Depth 7.0 The wave heights reported by Marc M. Siah & Associates, Inc. (2011) Final 20

25 Kahana Beach Regional Beach Nourishment Feasibility Study September Preliminary Design Concepts Review of the previous work in this area indicates that there is a large potential for both longshore and cross-shore sediment transport that must be addressed in any erosion control project. The Kahana Beach area does have an advantage for beach placement concepts in that is and independent littoral cell with headlands at each end that limit longshore losses out of the littoral system. Potential project alternatives include beach nourishment only, beach nourishment with groins, beach nourishment with T-head groins, beach nourishment with backstop protection, and beach nourishment with an offshore breakwater. The advantages and disadvantages are discussed below. Beach nourishment alone would provide the recreational and shoreline protection benefits of moving the wave breaking zone seaward providing additional beach width. However, the combination of longshore and cross shore sediment transport in the area could result in very high background erosion rates as well as large losses in storms events. Due to the lack of sand retention structures, this option is the lowest-cost option for initial construction, and may be the least challenging for construction and permitting. Moreover, this option avoids potential environmental impacts of rock or geotextile. The primary disadvantage of this option is the limited project longevity and potential for frequent renourishment requirements. Beach nourishment with groins could enhance sand retention on the beach by reducing the longshore transport of sand. A review of seasonal shoreline positions on West Maui revealed that longshore transport has a significant impact on beach width; stub groins would reduce the longshore transport rate, retaining sediment on the updrift side of the groins. As a consequence of this updrift accretion, armoring on the downdrift side of the groin may be necessary. Since longshore transport occurs in both directions, both sides of the groin may require this armoring. In addition, certain groin configurations can lead to rip currents adjacent to the downdrift side of the groins; these currents may create a safety hazard to recreation, and may also lead to sediment being transported offshore. Including groins in the project adds a requirement for rock; because these structures need to be placed relatively close together, the tonnage required can be relatively large per linear foot of coastline. This rock leads to higher materials and construction costs, as well as additional permitting requirements. Beach nourishment with T-head groins provides the similar longshore current reduction as stub groins, plus additional benefits from the T-head configuration. These T-heads protect the beach from storms that would induce cross-shore transport, and prevent the formation of rip currents adjacent to the groins. The T-heads also reduce incoming wave energy, and produce diffraction/refraction patterns that help stabilize the shoreline between any two groins. Also, T-head groins have been demonstrated to be more effective at sand retention than conventional groins on shorelines that exhibit significant bi-directional sand transport, as is the case within Kahana Bay. Ultimately, this configuration offers the highest degree of sand retention. An additional advantage is that T-head groins are spaced further apart than stub groins, therefore providing longer rock-free stretches of beach. Disadvantages of T-head groins are the larger environmental footprint and the potential complication of in-water construction. These increase the permitting and the construction efforts. Backstop protection is an effective way of protecting individual properties, and may be considered with or without beach nourishment. Backstop protection effectively offers a last line of defense that protects Final 21

26 Kahana Beach Regional Beach Nourishment Feasibility Study September the shoreline against scour and potential to infrastructure if the beach has been eroded. Alternative types of backstop protection include revetments, seawalls, or geotubes. Backstop protection may be considered on a property by property basis; at Kahana Bay, several properties have already hardened their shorelines. Beach nourishment with an offshore breakwater or series of breakwaters was also considered at the screening level. Whereas groins function as sand retention structures by the physical blocking of alongshore sand transport, offshore breakwaters retain sand by blocking wave energy which transports sand along the shoreline. The result is deposition of littoral drift sand in the shadow, or lee, of the offshore breakwater or series of breakwaters. Negative aspects of offshore breakwaters include the fact that they tend to require significantly great volume of stone to construct due to greater water depth. They also require more expensive water-based construction. Furthermore, they have a greater tendency to have a negative impact of the quality of nearby surf sites. Finally, they cover much greater area of seafloor resulting in potentially greater negative environmental impact. For these reasons, the offshore breakwater alternative was not considered further in this study. For the purposes of this feasibility study, a concept plan that incorporates beach nourishment and T-head groins have been prepared. This alternative was selected for the concept plan because it offers the highest degree of sediment retention. Moreover, it includes both shore-parallel and shore-perpendicular structures that would require construction on the dry beach and in the water. A more detailed alternatives analysis should be performed to optimize the concepts based on the level of protection required, performance expectations, schedule, regulatory, and financial considerations. This alternatives analysis is a fundamental part of the environmental documentation. 8.1 Concept Project The sand search identified the presence of a significant amount of high-quality beach sand in the nearshore area of Kahana Beach. Although the two primary sand deposits contain over 275,000 CY of sand, only 50, ,000 CY may be available for regional beach nourishment. Based on these findings, M&N has investigated concept designs to protect the infrastructure located in Kahana Bay and to restore the beach through beach nourishment. Long-term concepts include 50,000 CY and 100,000 CY beach fills, stabilized by armor stone T-head groins; these projects would result in 54 feet and 110 feet of added beach width, respectively. The T-head groins would be necessary to retain the fill sand in place, given the highly erosive conditions at Kahana Bay. The preliminary plans can be seen in Attachment A. A conceptual layout of a 50,000 CY beach nourishment with T-head groins is depicted in Figure 8-1. It should be noted that the equilibrated shoreline will fluctuate seasonally. The design approach for T-head groins were based on a combination of property considerations (i.e., property boundaries) and technical literature. Groin spacing for the 50,000 CY and 100,000 CY beach fills are proposed to be placed at each property boundary and at every other property boundary, respectively. Based on this groin spacing, the design approach for the groin and T lengths were adopted from Bodge (2003); the dimensions used in this approach are presented in Figure 8-2. Final 22

27 Kahana Beach Regional Beach Nourishment Feasibility Study September Figure 8-1: Conceptual 50,000 CY Beach Nourishment with T-head Groins Final 23

28 Kahana Beach Regional Beach Nourishment Feasibility Study September Figure 8-2: Conceptual T-head Groin Layout According to this approach, the gap (G) between two T-heads is determined by: G = ( L 2X)/(2γ + 1) n where L/n is the distance between adjacent groins, X is the horizontal distance between the mean low water and mean high water shorelines (12.5 feet), and γ is the indentation ratio, generally between 0.35 and 0.65 (for this application γ ~ 0.5). Because L/n is more than 10 times greater than 2X for all properties, the gap between two T-heads was estimated to be half of the length between groins. Due to the irregular spacing between adjacent groins, the T-head length (H) was not symmetrical across the groin. Each portion of the T was equal to half of the adjacent gap. The Bodge (2003) guidance states that the tip of the T-head should be located a distance S+γG from the shoreline. S represents the distance between the back of the berm and the mean low water (MLS) shoreline. Due to the significant erosion experienced during recent storms, these preliminary concepts assume the back of the berm will be located approximately at the edge of vegetation or the existing seawall (based on the property), and that S is equal to the distance by which the beach will be widened. Although the gap between the groins varies along the shoreline, groins are most effective when the opening between two T-heads is oriented perpendicular to wave climate. Because both north and south swells can cause significant erosion along West Maui, the gap should be oriented parallel to the shoreline. Therefore, the length of the groin was designed such that the average length satisfies the S+γG parameter and the gap between two groins is parallel to the existing shoreline. In order to satisfy these conditions, the angle of the T legs differs from groin to groin. Final 24

29 Kahana Beach Regional Beach Nourishment Feasibility Study September Finally, the guidance presents guidelines that relates the gap between the T-head groins to S. A simple rule is that the MLW shoreline will be located about one-third to two-thirds of the structures gap behind the structures seaward face. Therefore, the gap length between each T-head groin is a function of the volume of sand to be placed on the beach. The concept designs proposed would offer several benefits. First, they would advance the shoreline seaward, protecting the structures and infrastructure on the shoreline from incoming waves. Second, in addition to protecting the actual properties and structural foundations, they would also improve water quality by buffering the upland clay-rich soils being released into Kahana Bay. Third, they would improve the beach resources along this stretch of coastline, drastically enhancing recreational activity and property values. Beach nourishment project with T-head groin sand retention structures have been successful in other locations throughout Hawaii. Specifically, Iroquois Point (constructed in May 2013) on O ahu consisted of a beach nourishment of 90,000 cubic yards of sand along with 9 T-head groins. Even after the recent El Niño winter, the sediment appears to be well maintained on the beach. A photograph of this site, taken on February 22, 2016 can be seen in Figure 8-3. Figure 8-3: Iroquois Point T-Groin Project Final 25

30 Kahana Beach Regional Beach Nourishment Feasibility Study September Pilot Project To address the immediate erosion concerns and determine the feasibility of such a project, certain condominium associations may want to pursue a pilot project under the Small-Scale Beach Nourishment (SSBN) regulation in order to expedite the permitting process and achieve protection sooner. This project would entail a beach nourishment confined to certain portions of the coastline, using geobag T-head groins to stabilize this fill. Such a project would provide emergency protection while also testing future designs for a larger, long-term project. The SSBN limits beach nourishment to 10,000 CY of sand. In order to go forward with a streamlined process that falls under this regulation, a condominium association would need to apply independently. Each project would consist of beach nourishment, along with T- or L-head groins at the property boundaries of the condominium association conducting the project. Concept designs for this pilot project are contained in Attachment B. These designs assume an eroded beach with a width of less than 10 feet, and no beach in front of properties that have constructed seawalls. Because the beach width added varies based on the linear footage of shoreline at each property, the beach width added ranges from 55 to 130 feet. The purpose of the drawings in Attachment B is solely to present a conceptual image of how the properties would look with 10,000 cubic yards in front of each one. However, jetty placement, size, and orientation would depend on which properties pursue such a project and may differ from Attachment B. Further investigation of the SSBN policy is required to determine the feasibility of implementing SSBN projects at several properties within Kahana Bay. 8.3 Triggers for Renourishment The proposed T-head groins will retard erosion at Kahana Bay; however, a combination of rising sea levels, enhanced recreational use, and extreme storms will continue to erode sediment from the beach. The nourished beach will be considered adequate for protection of shoreline infrastructure so long as the beach is not susceptible to being lost in one storm. While little data is available to quantify the minimum beach width that may be lost during one storm at Kahana Bay, such data does exist at Kaanapali. As stated in Section 3, the shoreline at the southern portion of the 3,600-foot long littoral cell retreated by 161 feet in two months, or 4.5% of the beach length. Assuming that the same proportional beach recession would be possible in the littoral cell created by T- head groins, the potential seasonal retreat could be 25 feet and 35 feet for groins spaced at each and at every other property line, respectively; therefore, beach widths of 25 feet and of 35 feet represent the triggers for nourishment for the 50,000 CY and 100,000 CY nourishment projects with T-head groins. A beach monitoring plan would be necessary to indicate when the relevant trigger would be reached, indicating that nourishment is necessary. As presented in Table 4-1, the average erosion rate from 1912 to 1940 is estimated to be 0.9 foot per year. This represents the erosion rate for a wide, undeveloped beach condition. Because of the beach widths that would be achieved by a nourishment project, combined with the sand retention provided by groin structures, this represents a conservative value for which to estimate future erosion. Based on 0.9 foot per year of beach loss, renourishment would be necessary after about 30 and 75 years for the 50,000 CY and 100,000 CY nourishment projects. Final 26

31 Kahana Beach Regional Beach Nourishment Feasibility Study September Constructability The following sections provide a general description of construction methods typically used on this type of project and some potential challenges associated with each project component. Ultimately, the methods will be selected by the contractor and will depend on the material/equipment availability, permit requirements, and the contractor s experience. However, a preliminary review of the site and of potential construction options indicates that such a project is feasible. 9.1 Offshore Dredging Based on the potential volume of sand required for nourishment, the depth of the sand deposits, and the distance between the sand deposits and the shoreline, hydraulic dredging is the most effective option for the beach nourishment component of the project. This system would entail staging a hydraulic pump system on a barge that is anchored above the sand deposit (Figure 9-1); this system would use a booster pump with a downsized hose to maintain adequate pressure and flow velocities. Similar projects on Hawaii, using an 8-inch diameter suction head that downsized to a 6-inch hose, achieved a maximum dredge rate of 1,000 cy/day and an average rate of 330 cy/day over the same distance. The wind and wave climate off Kahana Beach could pose some constructability challenges depending on the size and type of dredging equipment used. These challenges could result in delays or lower production rates but are not expected to affect project feasibility. Timing of the project to occur in low swell seasons will be important. Protection of the marine environment is another key element of the offshore construction process. Environmental protection measures and Best Management Practices (BMPs) should be implemented during the dredging process to minimize impacts to sea life and reef habitat in the area. These BMPs may include designated observers for Endangered Species Act listed species, careful location of spuds and anchors to avoid sensitive habitat, and turbidity curtains. Final 27

32 Kahana Beach Regional Beach Nourishment Feasibility Study September Figure 9-1: Theoretical Barge-mounted Hydraulic Pump System (Source: Final 28

33 Kahana Beach Regional Beach Nourishment Feasibility Study September Pipeline Corridor The sand search confirmed the feasibility of constructing a pipeline from the sand deposits 19 and 22 to the shoreline near Pohaku Park. From each deposit, a submarine channel was identified that was sufficiently wide and free of obstructions to route the submerged pipeline; these potential pipeline routes can be seen in Figure 9-2. An additional sub-bottom profile and scuba survey should be performed to finalize the pipeline routes used to deliver sand to the beach areas. Figure 9-2: Potential Dredge Pipeline Routes from Sand Deposits 19 and 22 to Kahana Bay 9.3 Sand Placement Generally, beach fill projects pump the sand onto the beach and use earthwork equipment such as bulldozers, excavators, loaders and dump trucks to shape the beach fill template. The dredge pipeline typically discharges into a shore parallel settling area which is surrounded by berms on three sides. As the water/sand slurry travels along this settling area the sand falls out of suspension and the water returns to the ocean. After a settling area fills with sediment the process will shift to another settling area up the beach, or use earthwork equipment to distribute the material from the same settling area. Turbidity curtains are often used to contain turbidity from the discharge water during sand placement activities. Final 29

34 Kahana Beach Regional Beach Nourishment Feasibility Study September The narrow width of the existing beach may pose a challenge during initial sand placement since there may be limited sand available to grade a temporary settling area. A contractor would have several options to overcome this challenge: 1) If there is sufficient sand elsewhere along Kahana Beach, the contractor could borrow sand to form a settling area with temporary berms to receive the dredged material and initiate nourishment. 2) Another strategy for retaining sediment would be to directly fill large geotextile tubes (geotubes) with the dredged material. The fluid portion of the slurry seeps out of the geotextile fabric and ports in the top of the tube, leaving only the sediment contained within the geotextile. There are several variations of this method that could be employed. The geotubes, once filled, could be arranged to create a settling basin, essentially a more robust version of the conventional method described above in which the water/sand slurry flows along the length of the basin allowing sediment to fall out of suspension. The use of geotubes in beach nourishment projects can be seen in Figure 9-3. Using this strategy, consideration should be given to leave the geotubes adjacent to the vegetation line (i.e., the landward barrier of the settling basin) in place after dredging is complete. These geotubes could function as additional large storm protection while the beach is present, or a protected stockpile of sediment if the beach erodes. The other geotextile used to form temporary settling basins should be removed as a part of the final beach grading to leave sediment in place. 3) Rock imported for use in the sand retention structures could also be used in lieu of geotubes to create a temporary settling basin during initial sand placement activities. Final 30