Coastal Engineering Report for Lower Honoapiilani Road Erosion at Kaopala Bay Lahaina, Maui, Hawaii September 2018 Prepared for: County of Maui, DPW Engineering Division 200 S. High Street Kalana O Maui Bldg. 4 th floor Wailuku, HI 96793 Prepared by: Sea Engineering, Inc. Makai Research Pier Waimanalo, HI 96795 Job No. 25619
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Coastal Engineering Report for Lower Honoapiilani Road ...
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Equipment and material access would necessarily be from Lower Honoapiilani Road.
Equipment and materials would be staged on the road and would require at least a one lane
closure along 300 feet of Lower Honoapiilani Road.
Installation
ElcoRock containers would be filled on-site, using the ElcoRock J-Bin filling
apparatus.
Any remaining trees fronting the road in the project area would be cut or removed
to allow unobstructed installation of the ElcoRock. Beach cobbles should be
graded to create a stable, flat surface for ElcoRock placement.
Geotextile underlayment would be rolled out by hand. Installation of the
ElcoRock containers would be accomplished using an excavator. Road plates are
recommended to protect the road surface.
Construction Duration
Construction is anticipated to take approximately 1 week.
Permits for Temporary Emergency Shore Protection
Temporary installations typically require administrative authorization from both the County
Department of Planning and State DLNR-OCCL. As the structure is usually placed seaward of
the upper reach of the wash of the waves, nominal jurisdiction is held by the state. However, the
shoreline zone is designated a special management area regulated by the county. The county
Special Management Area Emergency Permit is designated as SM3 and is issued when there is a
threat to a legally habitable structure, or when public infrastructure is at risk of failure which
would substantially affect public health and safety. DLNR-OCCL operates under similar
emergency criteria, and will issue a letter of authorization for the emergency structure.
If the emergency structure is located below the high water mark, it is considered to also be in
“Waters of the United States”, and will need a letter of authorization from the U.S. Army Corps
of Engineers.
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Figure 4-2. ElcoRock revetment layout
Figure 4-3. ElcoRock revetment section
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4.2.2 Seawall
A seawall is a vertical or sloping concrete, cement-rubble-masonry (CRM), cement-masonry unit
(CMU), or sheet pile wall used to protect the land from wave damage and erosion. A seawall, if
properly designed and constructed, is a proven, long lasting, and relatively low maintenance
shore protection method. Seawalls also have the advantage of having a relatively small footprint
on the shore.
The impervious and vertical face of a seawall results in very little wave energy dissipation.
Hence, wave energy is deflected both upward and downward, and also a large amount of wave
energy is reflected seaward. The downward energy component can cause scour at the base of the
wall - therefore the foundation of a seawall is critical for its stability, particularly on an eroding
shoreline. Ideally, a seawall should be constructed on a solid, non-erodible substrate. Seawalls
are not flexible structures, and their structural stability is dependent on the stability of their
foundations.
If the foundation of the seawall is breached, hydraulic processes can erode fill material behind
the wall. With the loss of enough fill, the ground surface behind the seawall will collapse into a
sink hole. When a sink hole is observed, repairs should be made as soon as possible or the wall
may eventually fail. Repair methods vary; one method is to excavate behind the wall, reinforce
the foundation with concrete, and replacing the fill with appropriately graded material. To avoid
foundation problems, the seawall foundation should be well below the potential scour level.
The presence of a visible rock substrate at a relatively high elevation near the most at-risk
portion of the shoreline embankment gives strong support for the use of a seawall as a shore
protection option (see Figures 2-4, 2-5, and 2-13).
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Figure 4-4 illustrates a potential design cross-section for a CRM seawall structure at Kaopala
Bay. The rock substrate simplifies wall requirements for both a foundation and anchoring,
allowing use of a gravity wall such as the CRM structure shown. This type of structure will also
allow some expansion of the road shoulder by pushing the wall seaward and adding fill behind
the wall.
Advantages of a CRM wall at Kaopala Bay include:
Potential for good foundation conditions;
High foundation elevation minimizes size of the wall;
Relatively simple design and construction;
Compatible with drainage outlets;
Aesthetically well matched with surroundings.
The presence of a high energy cobble beach mitigates some of the negative qualities of a seawall.
Wave reflection will tend to rapidly attenuate due to the high friction properties of the cobbles,
and these also have excellent resistance to scour.
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A seawall is particularly applicable to the area of concern shown in Figure 2-3, and as shown in
Figure 4-4. However, it can also be used to protect the revetment reaches by placing the wall
against the vertical clay/silt escarpment as long as a solid foundation is still accessible.
Geotechnical borings are recommended at approximately 100-ft intervals along the reach of the
bay in order to have a complete assessment of the foundation conditions. The remnant stones
that form the over-steepened section against the clay/silt scarp should be removed prior to wall
construction, and place at lower elevations on the beach (Figure 4-5).
Figure 4-4. Typical seawall section
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Figure 4-5. Schematic of CRM wall used on revetment protected reaches
4.2.3 Rock Revetment
A rock rubblemound revetment is a sloping uncemented structure built using boulder-sized rock.
The most common method of revetment construction is to place an armor layer of stone, sized
according to the design wave height, over an underlayer and filter designed to distribute the
weight of the armor layer and to prevent loss of fine shoreline material through voids in the
revetment. The armor layer is typically two stone diameters in thickness.
Armor Stone Size
Typical revetment structures are designed as rock rubble mounds with a slope of up to 1.5
Horizontal to 1Vertical (1.5H:1V), which is the steepest slope recommended by the Coastal
Engineering Manual (2006). Stone sizing for the design wave height is given by the Hudson
Formula (Coastal Engineering Manual, 2006):
where,
W = weight in pounds of an individual armor stone
wr = unit weight of the stone, 160 lb/ft3
H = wave height, 6.9 feet (see Section 3.4.8)
KD = armor stone stability coefficient, 2
cot)1( 3
3
rD
r
SK
HwW
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Sr = specific gravity of the stone relative to seawater, use 2.5
cot θ = cotangent of the groin side slope, use 1.5
The resultant armor stone weight is approximately 5,000 pounds with a corresponding nominal
diameter of 3.2 feet. A range of ± 25% of the median weight is typically utilized, which yields a
stone weight range of 3,750 to 6,500 pounds. These are preliminary values, and may be
conservative. A more rigorous design wave development including numerical modeling is
recommended for final design parameters.
Revetment Underlayer
Underlayer stone is utilized to transition between the large armor stone and small filter stone or
filter layer. Sizing of the underlayer stone is important for providing sufficient porosity for
energy dissipation rather than reflection, to achieve interlocking between the armor and
underlayer, and to insure that the underlayer material cannot be dislodged through voids in the
armor layer. Underlayer stone is sized at approximately one-tenth the armor stone weight, which
in this case is approximately 500 lbs.
The underlayer stone should be placed over a geotextile filter fabric layer. The geotextile
prevents the migration of fine soil particles through voids in the structure, and permits relief of
hydrostatic pressures within the soils. The underlayer stone protects the geotextile from damage
during placement of the armor layer, and together with the geotextile helps distribute the weight
of the armor stone to provide for more uniform settling. The existing slope should be graded and
dressed prior to revetment construction to provide a 1.5H:1V slope. Rocks and other debris
which might puncture or tear the geotextile should be removed from the prepared slope.
Figure 4-6 shows a potential rock revetment design cross section, assuming a rock substrate for
embedment of the revetment toe.
One major advantage of a rock revetment versus a vertical seawall is that the rough porous rock
surface and relatively flat slope of the structure will tend to absorb wave energy and reduce wave
reflection. Properly designed and constructed rock revetments are durable, flexible, and highly
resistant to wave damage. Should toe scour occur, the structure can settle and readjust without
major failure. Damage from large waves is typically not catastrophic, and the revetment can still
function effectively even if damage occurs. From a coastal engineering perspective, a rock
rubblemound revetment is often the most suitable shore protection alternative for coastal
shorelines.
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The project area contains reaches with existing rock revetment protection. The stone is mostly
undersized, and down-slope movement has caused over-steepening of the top portions of the
structure. While new revetment construction is probably not practical for the at-risk portions of
the shoreline due to the wide footprint of the structure, repair of the existing revetment reaches is
a feasible option. It is recommended that additional geotechnical information be acquired in
order decide if a wall or revetment is the best approach.
Figure 4-6. Typical rock revetment section
4.2.4 Beach Nourishment
When sand loss is gradual and the beach has a high economic value for recreation and tourism, it
is sometimes good coastal management policy to replenish the littoral cell with sand from
offshore or other sources. Massive beach nourishment projects have taken place on the eastern
seaboard, Gulf coasts (USACE CEM, 2006), and Southern California (SANDAG, 2000). In the
past ten years “Hawaii sized” beach nourishment projects have taken place on Oahu, Kuhio
Beach Park (2006), Waikiki Beach (2012) and Iroquois Point (2013).
Beach nourishment is expensive, and containment features or structures such as T- head groins
are sometimes necessary to keep the sand from disappearing. Some areas have features such as
headlands or reefs that naturally stabilize the sand. On open ocean or otherwise unprotected
coasts T-head groins both decrease the amount of wave energy reaching the beach, and act as
artificial littoral cells to stabilize the sand.
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Beach nourishment requires a supply of sand that is similar in character to the native beach sand.
While sand in Hawaii may seem like a plentiful commodity, the reality is that good quality beach
sand is in very short supply. Inland dune deposits have been used for some nourishment efforts,
but the process of transport by wind preferentially selects a naturally finer grain size, and dune
sand therefore tends to be composed of grains that are too fine for many applications. Offshore
sand deposits also tend to have grain sizes that are finer than many beaches, and many reef-top
deposits are thin and of insufficient volume for meaningful use. However, offshore deposits
have been found that are in some cases suitable. Dredging and recovery operations are
expensive, but have been shown to be effective. The Waikiki Beach Maintenance project (2012)
recovered 24,000 cy of sand from and offshore deposit and pumped it to shore to nourish 1,730
feet of beach in the center of Waikiki, at a cost of $2.4M.
Beaches can be stabilized by beach nourishment, and the addition of sand can slow the erosion of
fast lands. However, in dynamic environments sand that is not held in place by natural
headlands or containment structure can rapidly disappear during adverse conditions. Recent
beach monitoring in Waikiki shows that approximately half of the 2012 nourishment project
sand is no longer on the beach.
In Kaopala Bay, there appears to be a stable beach in the lee of Haukoe Point (see Figure 2-6).
However, the beach along the rest of the bay is composed of rock cobbles with little to no sand.
This morphology is a strong indication that, in general, sand is not stable along this reach of
coast. Implementing a beach nourishment project for the purpose of protecting the shoreline
scarp and protecting the roadway is not likely to be successful.
4.2.5 Shore Protection Impacts
Shore protection is a controversial topic because of both real and perceived impacts to the coastal
environment. Following are brief descriptions of typical impacts that may occur due to
construction of shore protection.
Impoundment
Impoundment is the sequestration of sediment behind a shore protection structure. By
preventing erosion of the upland behind the structure, an eroded beach can be starved of new
sand. Although there are some beach morphologies that may behave in this way, beaches on the
West Coast (California) and Hawaii generally are not directly supplied with sand from coastal
upland erosion. Studies have shown that California beaches derive about 10% of their sand from
the erosion of coastal bluffs, with the majority of beach sand being sourced from river and
stream flow and then transported laterally along the coast (Patsch & Griggs, 2006). In contrast,
Hawaii beaches are primarily composed of carbonate sand that is derived from biogenic
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production on fringing reefs and transported to shore by wave action (Moberly & Chamberlain,
1964; Inman et al, 1963).
Placement Loss
Placement loss is the term used to describe the loss of beach due to the footprint of a structure
encroaching on the beach area. The amount of placement loss depends on the structure type and
where it is located. A vertical seawall placed landward of the shoreline would result in virtually
no placement loss, for example, while loss due to revetment construction (see Figure 4-6) may be
20 to 30 ft. However, this figure is misleading since much of the revetment is buried, especially
during seasonal periods when sand is present on a beach.
Flanking Erosion (End Effects)
Flanking erosion is when erosion occurs behind a shore protection structure, and can be a
mechanism for structure failure. Flanking can be accelerated by end effects. End effects are
caused by the radiation of waves from the edge of coastal structure. The waves radiate in a more
or less arcing pattern due to an expanding wave front (i.e. like waves in a pond). Because of this
pattern, as well as attenuation due to wave breaking and bottom friction, the waves lose energy
with distance from the source and results in end effects being a near field process, meaning that
the effects are most pronounced close to the source, in this case the end or edge of the structure.
Figure 4-7 is a photograph of end effect processes, showing waves radiating from the edge of a
temporary structure. The soft sand on the adjacent shoreline is easily eroded. The near-field
property of the phenomenon can be seen by inspection of the eroded shoreline in the photograph.
Passive Erosion
Passive erosion is a term used to describe the effect of erosion that occurs next to a protected
shoreline. With time the unprotected shoreline can erode sufficiently to create an offset and
change the geometry of the coast. Geometry is important for coastal processes, and the offset
can create a de-facto headland that provides a barrier to sand transport and prevents deposition in
front of the protected area. Figure 4-8 shows an example of passive erosion at Kahana Bay:
sand is prevented from being transported and deposited by the offset between the protected and
unprotected shorelines.
Wave Reflection and Scour
Wave reflection from vertical structures is perceived to inhibit the accretion of sand and have a
generally negative impact on beaches, although evidence of this effect has not been clearly
documented (Griggs et al, 1994, CEM, 2006). Nevertheless, it is accepted coastal engineering
practice to minimize wave reflection as much as possible. Wave reflection can also cause a soft
substrate to scour and deepen into a trough in front of the reflecting surface, thereby affecting the
nearshore morphology.
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Figure 4-7. An example of end effects
Figure 4-8. Example of passive erosion at Kahana Bay; note shoreline offset – shoreline on the
right was unprotected until recently
4.2.6 Shore Protection Impact Mitigation
Much of the impacts from shore protection can be minimized by understanding the existing
environmental conditions and designing accordingly. For example, a revetment is often a
preferred structure as it tends to reduce wave reflection. However, the substantial cobble beach
that exists at Kaopala Bay is a natural and extremely effective wave absorption mechanism that
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will minimize reflection and scour in front of a vertical wall such that the ameliorating effects of
a revetment are redundant. Passive erosion and flanking due to end effects is typically countered
by extending the structure to where it is naturally protected such that these effects are minimize
or eliminated.
4.3 Regulatory Environment
4.3.1 Overview
Hawaii shorelines and coastal waters are governed by a complex array of Federal, State, and
County jurisdictions, rules and regulations. Figure 4-9 illustrates the approximate jurisdictional
boundaries that will determine the scope of the environmental review and regulatory permitting
requirements for a shore protection structure at Kaopala Bay. The U.S. Army Corps of
Engineers (USACE) has jurisdiction seaward of the mean higher high water (MHHW) line and
offshore for 200 nautical miles. The Hawaii Department of Land and Natural Resources (DLNR)
has jurisdiction seaward of the certified shoreline1 to 3 nautical miles offshore, which overlaps
with portions of the USACE jurisdiction. The area landward of the certified shoreline is
managed by the County of Maui.
Figure 4-9. Approximate jurisdictional boundaries at Kaopala Bay
4.3.2 Federal Requirements
The USACE is the designated lead agency with regulatory authority over Navigable Waters of
the United States, which includes the oceans and coastal waters seaward of MHHW.
1 “Shoreline” means the upper reaches of the wash of the waves, other than storm or seismic waves, at high tide during the season
of the year in which the highest wash of the waves occurs, usually evidenced by the edge of vegetation growth, or the upper limit of debris left by the wash of the waves (Hawaii Administrative Rules (HAR) §13-222).”
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Construction seaward of MHHW typically requires a Department of the Army (DA) permit
pursuant to Section 10 of the Rivers and Harbors Act of 1899 (33 USC 403) and Section 404 of
the Clean Water Act (33 USC 1344). All work or structures in or affecting the course, condition,
location or capacity of navigable waters, including tidal wetlands, require DA authorization
pursuant to Section 10. In addition, activities involving the discharge of dredged or fill material
(i.e., armor stones) into Waters of the United States requires a DA permit pursuant to Section
404. Activities which require federal permits must also meet requirements of the Coastal Zone
Management Act (CZMA). The MHHW elevation for the project site is approximately 1.2 feet
above mean sea level (MSL). The horizontal location of the MHHW line varies depending on
the beach profile. The rock revetment structure would likely extend seaward of MHHW. Thus,
Federal permits would be required. Federal permits may not be required for the seawall
alternative, assuming it can be located landward of the MHHW line on the shoreline.
4.3.3 State of Hawaii Requirements
The Hawaii Department of Land and Natural Resources (DLNR) is the designated lead agency
with regulatory authority over Conservation District and Public Trust lands, which includes
unencumbered land and submerged land. All lands in Hawaii are classified into four land use
districts: urban, rural, agricultural, and conservation. Hawaii Revised Statutes (HRS) § 205-2
(Coastal Zone Management , or CZM) is the overarching policy that designates the types of uses
permitted in the four land use districts. The area landward of the shoreline at Kaopala Bay is
located in the Urban District. The area seaward of the shoreline in Kaopala Bay is considered
submerged State land and is located in the Conservation District, Resource Subzone, which
extends 3-nautical miles seaward from the shoreline.
The certified shoreline establishes the landward limits of the Conservation District. The Board
of Land and Natural Resources (BLNR) has exclusive jurisdiction to govern land uses in the
Conservation District. Shore protection would be considered an identified land use in the
Resource Subzone of the Conservation District pursuant to Hawaii Administrative Rules (HAR)
§13-5-24-P-15 Shoreline Erosion Control (D-1); therefore, the project will require a
Conservation District Use Permit (CDUP) from the DLNR Office of Conservation and Coastal
Lands (DLNR-OCCL).
The Conservation District Use Application (CDUA) will require submittal of an Environmental
Assessment (EA) and a Finding of No Significant Impact (FONSI) pursuant to Hawaii Revised
Statutes (HRS), Chapter 343, and its implementing regulations. Hawaii Administrative Rules
(HAR) Title 11, Chapter 200, addresses the determination of significance and contents of an
environmental assessment.
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Water quality standards and regulations are administered by the Hawaii Department of Health,
Clean Water Branch (DOH-CWB). A National Pollutant Discharge Elimination System
(NPDES) Permit is required any time construction activity covers an area one (1) acre in size or
greater, and is intended to prevent pollutants from reaching coastal waters as a result of storm
water runoff. Information required to obtain a permit includes project specific details and
construction drawings, receiving state water information, storm and non-storm water discharge, a
Best Management Practices Plan (BMPP), and Post-Construction Pollutant Control Measures.
Should any part of the project be constructed below the MHHW mark, a DA Section 404 permit
would be required, which may in turn trigger a requirement for a Section 401 Water Quality
Certification (WQC), which is administered by the Hawaii Department of Health. The WQC
requires the applicant to conduct water quality monitoring before, during, and after construction.
4.3.4 County of Maui Requirements
The County of Maui Department of Planning is the designated lead agency with regulatory
authority over the area landward of the certified shoreline within the Special Management Area
(SMA). Hawaii Revised Statutes (HRS) §205-2 establishes the types of land uses that are
permissible within the SMA, which is considered the most sensitive area of the coastal zone.
The certified shoreline establishes the seaward limits of the SMA and is the baseline that
counties use to calculate shoreline setbacks in Hawaii. It is likely that some portion of proposed
shore protection will be mauka of the certified shoreline, and will therefore require an SMA
permit and a Shoreline Setback Variance (SSV). As with the CDUP, the SMA requires an
Environmental Assessment.
Lower Honoapiilani Road is located in the VE Flood Zone with a Base Flood Elevation of 17
feet, therefore a Flood Development Permit and Coastal High Hazard Area Certificate will likely
be required. The activity may involve ground disturbing activities and/or stockpiling of
materials, so a Grubbing and Grading Permit may be required.
The anticipated environmental review and regulatory permitting requirements for a shore
protection structure at Kaopala Bay are summarized in
Table 4-1, below.
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Table 4-1. Anticipated Environmental Review and Regulatory Permitting Requirements
FEDERAL
Department of the Army Permit (DA)
Lead Agency: US Army Corps of Engineers (USACE)
Authority: Section 10 of the Rivers and Harbors Act of 1899 (33 USC 403) and
Section 404 of the Clean Water Act (33 USC 1344)
STATE OF HAWAII
Conservation District Use Permit (CDUP)
Lead Agency: Hawaii Department of Land and Natural Resources (DLNR)
Authority: Hawaii Administrative Rules (HAR) 13-5
Environmental Assessment (EA)
Lead Agency: Hawaii Office of Environmental Quality Control (OEQC)
Authority: Hawaii Revised Statutes (HRS) Chapter 343
National Pollution Discharge Elimination System Permit (NPDES)
Lead Agency: Hawaii Department of Health (DOH)
Authority: Hawaii Administrative Rules, Chapter 11-55
Water Quality Certification (WQC)
Lead Agency: Hawaii Department of Health (DOH)
Authority: Section 401 of the Clean Water Act (33 USC 1344)
COUNTY OF MAUI
Special Management Area Minor Permit (SMA)
Lead Agency: County of Maui Department of Planning
Authority: Hawaii Revised Statutes (HRS) Chapter 205-A
Special Flood Hazard Area Development Permit
Lead Agency: County of Maui Department of Planning
Authority: Maui County Code (MCC), Chapter 19.62
Coastal High Hazard Area Certification
Lead Agency: County of Maui Department of Planning
Authority: Maui County Code (MCC), Section 19.62.060.G.6.a
Grubbing and Grading Permit
Lead Agency: County of Maui Department of Public Works
Authority: Maui County Code (MCC), Chapter 20.08
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5. RECOMMENDATIONS
The site investigations by SEI engineers determined that there is an immediate coastal threat to
Lower Honoapiilani Road. An emergency revetment constructed with ElcoRock 2.5m³
geotextile containers is recommended to mitigate the immediate threat. Planning for long term
shoreline management solution should begin immediately, as design and permitting for
permanent structures can be a multi-year process.
The presence of a hard rock substrate in the area of concern and along much or all of the
shoreline reach makes the CRM seawall option attractive (see Figure 4-4). The wall would have
a relatively small footprint, have an uncomplicated design, and would be aesthetically
compatible with the local environment. Geotechnical borings are recommended to map the
elevation of the hard rock substrate. The environmental effects of a seawall at this location are
negligible. A seawall built close to the existing vertical erosion scarp is physically similar in
terms of coastal processes. Wave reflection off a nearly vertical clay scarp is virtually identical
to reflection off a near vertical seawall, and the narrow footprint of a seawall minimizes
placement loss. There is little to no sand visible in the coastal escarpment, so impoundment of
beach quality sand by the wall structure is extremely unlikely. The effects of passive erosion,
and flanking erosion can be minimized or eliminated by proper treatment of the ends of the
proposed structure.
The existing beach is primarily composed of rock cobbles. These will mostly remain in place,
and tend to collect as a berm in front of the new CRM seawall. The sand beach at the north end
of the project reach exists because that area is somewhat protected from direct wave exposure by
Haukoe Point. The new seawall is also not likely to cause substantial changes in this area.
Figure 5-1 is a schematic representation of a new seawall along the project reach. At minimum,
it is recommended that an approximate 170’ reach along the existing area of concern be
protected with a permanent wall as soon as possible. The wall should extend from the protected
area in the lee of the Robinson revetment and butt into the existing rock outcrop (Figure 2-5).
Eventually the entire Kaopala Bay reach should be protected for a combined distance of about
680 ft. The walls would exist in two reaches (390 ft and 290 ft) that abut either side of the
headwall for the 54” drainage pipe.
A rubblemound rock revetment is not recommended for the project reach due to the wide
footprint and consequent placement loss. This is especially compelling because the smaller
seawall option is relatively straightforward due to the rock substrate. However, if the substrate is
found to be too deep in places for an adequate wall foundation, a rubble mound rock revetment
could be substituted in those locations.
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Managed retreat consisting of road and utility relocation is feasible in this location due to the
potential availability of land for a new transit corridor, but is certainly not the most cost effective
solution and would cause extensive disruption to the community.
Beach nourishment is not considered a feasible option for this area. Based on existing
conditions, sand is generally not stable along the project reach and any beach nourishment effort
would need to be accompanied with substantial retention structures such as T-groins. Even then,
beach nourishment would not guarantee stability of the shoreline and protection of the road.
Table 5-1 is a comparison of selected protection options. Project costs listed in the table are
based on recent projects on Maui and Kauai. Costing for shore protection is uncertain as each
project has its own difficulties, and is dependant on the availability of stone and access for heavy
equipment.
5.1 Recommendations for follow-on work
Follow-on work for protection of the Kaopala Bay shoreline include additional design,
environmental review and permitting.
5.1.1 Additional Design
Additional design needs include a new topographic survey of the entire reach, geotechnical
borings (e.g., at 100’ intervals or less) to determine the rock substrate conditions and elevation,
additional oceanographic parameter analysis, and structural engineering design input. These
elements can be presented together in a Basis of Design Report. The report should be prepared
by an engineering firm with experience in coastal structure design.
5.1.2 Environmental Review
An environmental assessment will be required for county and state permits. The assessment will
likely require a marine biological and water quality survey, cultural impact assessment,
archaeological monitoring plan, and a preliminary drainage plan. The environmental assessment
can be prepared by an engineering firm with experience in coastal design, or by a planning firm
with experience in Maui County with assistance from the design engineers and other
subcontractors.
5.1.3 Permits
As indicated in Section 4.3, the project will probably require state (CDUP) and county (SMA,
SSV) permits at minimum. Federal permits may or may not be required, depending on the final
design configuration and location. If the rock substrate is sufficient in elevation, the CRM wall
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design may be out of federal jurisdiction. Permit application can either be prepared by a
planning firm with assistance from the design engineers, or by the design engineers.
Figure 5-1. Schematic representation of new CRM seawall.
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Table 5-1. Comparison of Alternatives
Alternative SEI Rating Pros Cons
1. CRM Seawall Preferred
Shoreline armoring (maximum protection);
Minimize coastal footprint;
Minimal impact on coastal processes;
Rugged, adaptable structure
Low impact on marine environment
Reflective (but equivalent to existing morphology)
Cost: $5,000 – $6,000 per linear ft
2. Revetment Second Preferred
Shoreline armoring (maximum protection);
Minimize reflection;
Minimal impact on coastal processes;
Rugged, adaptable structure
Wide footprint / placement loss
Cost: $6,000 – $9,000 per linear ft
3. Emergency ElcoRock
Containers
Preferred
(Temporary)
Proven robust temporary protection
Quickly implemented
Installed with Emergency permits
Temporary only – will degrade with time (e.g. 2 yrs) and will need repairs
4. Beach Nourishment Not Appropriate
None
Sand is not stable, would require extensive additional retention structures.
Will not protect roadway
Cost (with structures): $4,000 - $9,000 per linear ft
8. Managed Retreat Not Appropriate Feasible due to available land
No additional structures required
Will allow coastal erosion
Will require relocation of transit corridor
Will require relocation of buried utilities
Very high cost (to be determined)
Coastal Engineering Report for Lower Honoapiilani
Road Erosion at Kaopala Bay
Sea Engineering, Inc. 66
6. REFERENCES
Anderson, Tiffany R., C.H. Fletcher, M.M. Barbee, I.N. Frazer, B.M. Romine, 2015; Doubling of
coastal erosion under rising sea level by mid-century in Hawaii; Natural Hazards: Journal of the
International Society of the Prevention and Mitigation of Natural Hazards, Springer Science DOI
10.1007/s11069-015-1698-6
Davidson-Arnott R., 2005; Conceptual model of the effects of sea level rise on sandy coasts.
Journal of Coastal Research 21(6):1166–1172
Eversole, D., Fletcher, C.H., 2003. Longshore Sediment Transport Rates on a Reef-
Fronted Beach: Field Data and Empirical Models Kaanapali Beach, Hawaii, Journal of
Coastal Research: Vol. 19, No. 3, pp. 649–663.
Firing, Y.L. and M.A. Merrifield, 2004. Extreme sea level events at Hawaii: Influence of mesoscale
eddies. Geophysical Research Letters, Vol. 31(24).
Fletcher, C.H. III, E.E. Grossman, B.M. Richmond, and A.E. Gibbs, 2002. Atlas of Natural Hazards
in the Hawaiian Coastal Zone. Investigations Series I-2761. U.S. Geological Survey: Washington,
D.C.
Fletcher, C., R. Boyd, W.J. Neal, and V. Tice, 2010. Living on the Shores of Hawaii. University of
Hawaii Press. Honolulu, HI.
Griggs, G.B, J. Tait, W. Corona, 1994; The Interaction of Seawalls and Beaches: Seven Years of
Monitoring Monterey Bay, California; Shore and Beach, July 1994
Hawaii Climate Change Mitigation and Adaptation Commission, 2017. Hawaii Sea Level Rise
Vulnerability and Adaptation Report. Prepared by Tetra Tech, Inc. and the State of Hawaii
Department of Land and Natural Resources, Office of Conservation and Coastal Lands
Moberly, R., and T. Chamberlain, 1964; Hawaiian Beach Systems; Hawaii Institute of
Geophysics, HIG-64-2
Inman, D.L., W.R. Gayman, D.C. Cox, 1963; Littoral Sedimentary Processes on Kauai, a
Subtropical High Island; Pacific Science, Vol. XVII, January 1963
NASA. Climate Change: Vital Signs of the Planet: Sea Level Change website.