Reefs, beaches, and dunes are Maui’s underrated environmental heroes
While not obvious to the casual visitor, Maui’s sandy beaches, wind, and waves are engaged in a seasonal tug of war where sand builds up on shore and then gets pulled back to sea year after year. It is clear to most that the playing field extends up and down the coast. Less obviously, it extends some distance offshore and back behind the dunes into low-lying areas. The slow ebb and flow will be noticed by attentive and repeat beachgoers, though these processes may come dramatically accelerated and snap into focus during extreme weather events. Today, as human-induced climate change accelerates sea-level rise, the old boundaries of where waves and sand intermingle are shifting inland, and roads and houses built near the beach are increasingly vulnerable to damage.
Storm surge, Tides, and Wave Runup
There are several ways that ocean water interacts with the coast, and depending on tides, wind, and wave conditions, this interaction can build up a beach or start wearing it away. Periods of calm tend to build up beaches while periods of strong weather and high tides will wear them down.
The skimboard zone on a beach is the area of wave runup. This is where the water surges up after a wave has crashed and then flows back down the beach to meet the next wave and its swash of runup. Depending on the tide, wind, and waves, runup can create a nice environment for a toddler to play. Under calm weather conditions, wave runup can help carry sand up the beach, and wind will build up protective berms or dunes behind the beach. At the other extreme, when the waves start getting larger and more aggressive, runup can start pulling sand away from the beach. Larger waves, with more extensive runup, can push saltwater past dunes and over lava banks or manmade seawalls resulting in sand or debris overwash, erosion and undermining, or saltwater flooding.
Tides also affect how far up a beach a wave’s runup will reach. In the summer, the year’s highest tides or “King Tides” are produced when pull of gravity from the sun, moon, and earth are aligned and all three are at their closest. On Maui, this can add inches to tide height and cause flooding on its own, and may allow wave runup to reach further up the beach.
Along with the everyday effects of tides and waves pushing water up our beaches, storms can have a dramatic effect on water height in addition to bringing larger and more destructive waves. Storm surge is the buildup of water ahead of an approaching storm such as a hurricane or kona storm. It acts more like a surge in the height of the tide than a wind-generated wave and can add feet above even the height of a spring or king tide. Flooding from storm surge can push saltwater past protective beaches and dunes and into homes and across fields to flood low-lying areas such as wetlands.
Reefs, Beaches, and Dunes
While water is pulling and pushing at sand along the coast, building up and tearing down beaches, the system does not go for all or none. Reefs, beaches, and dunes all remove some of the energy brought to our shores by waves and limit how far they can push water up onto shore.
The importance of reefs in breaking the energy of waves is appreciated not just by surfers, but by anyone that owns property along the shoreline. This has been demonstrated by the loss of beaches along Maui’s Kalama Park. About four soccer fields’ worth of reef were blasted away in front of the park immediately after World War II. The thought was to improve the quality of the park’s beaches, but it quickly resulted in the beaches eroding. The lava rock revetments were installed in the early 1970s to stop the erosion and, while they protect the park, the sandy beach has been permanently banished. Prior to demolition, the reefs off of Kalama Park slowed the water reaching the beach enough to keep the sand protected and stable. When treated kindly, reefs and beach systems are self-healing and expected to be able to keep pace with sea-level rise. Free shoreline protection that is also self-maintaining provides an amazing value that developers have learned the hard way through lessons like those of Kalama Park’s dead beaches.
While today reefs are generally recognized as important coastal protectors that are fairly static, beaches are much more dynamic. Tara Owens, Maui’s Coastal Processes & Hazards Specialist with the University of Hawaii Sea Grant explained that the beach and dunes are coastal protection that works a bit like a bank account that accepts deposits and withdrawals of coastal protection credits.
“During storm events, beach and dune erosion are withdrawals from the bank account as they absorb the brunt of the energy from waves hitting the coast,” she explained. Under normal conditions, the sand is pulled offshore during a storm, creating an offshore beach that further dampens the effects of waves. The same sand is then slowly brought back to the beach and dune system by wind, waves, and currents during less extreme times. This is a typical seasonal cycle for many of Maui’s beaches, with the exact local patterns dictated by coastline orientation and exposure to either winter or summer swells or both.
In order to act as wave-shock absorbers, beaches and dunes need to be able to move and they need to be able to exchange the currency of freely moving sand at will. When we build houses or roads in the coastal zone and start preventing sand from moving as it should, or when waves start withdrawing the coastal protection budget from under poorly located structures, dollar-cost damages are incurred.
Beach Hardening or Retreat
If the beach in front of your house, hotel, or road starts washing out to sea, it is an understandable reaction to want to do something to protect it. Beach hardening, putting revetments or seawalls in place is one approach to preventing erosion, but it is now well known that this disrupts or stops the beach-building process. The change in how wave energy is distributed and the interruption of the natural flow of sand in the system frequently push the wave energy and erosion problems down the coast to adjacent properties. After the construction of the Kalama Park revetment, properties along Halama street experienced increased beach loss. Today, many properties along the former beach have installed seawalls to protect the houses there while causing the loss of the beach to move ever further north.
While it is understandable that a homeowner would want to keep their house from falling into the ocean, beach hardening protects private property (the house) with a resulting loss of public property (the beach). For new development, the County of Maui Planning Department has setback requirements that are based on historical erosion rates for a particular site. Seawall construction on sandy shorelines is no longer allowed by state law. New activities or development along the shoreline may trigger an assessment of “managed retreat” as an option that must be considered as well. This could involve re-configuring or relocating buildings or infrastructure out of hazard zones and out of harm’s way.
Sea-Level Rise and the Future of our Coasts
As sea-level rise continues in the coming decades, Maui will continue to experience greater and worse shoreline erosion. The 2011 National Assessment of Shoreline Change: Historical Shoreline Change in the Hawaiian Islands, reported that 85 percent of Maui beaches are likely to continue eroding. The report also noted that Maui has lost the most beaches (11 percent, primarily due to armoring) and is experiencing the highest rate of beach erosion in the state.
Looking to the future, the state and county are taking the threat of sea-level rise seriously. Owens, who assists the Maui County Planning Department noted that Maui County was the first in the state to adopt erosion setback requirements and that all Maui County government units have done sea-level rise vulnerability assessments. While the initial setback rules were based on historical erosion trends, as the County continues to strive for future resilience, they are proposing to incorporate an expected 3.2-foot sea-level rise “erosion hazard line” plus a 40-foot minimum setback buffer, which would require new construction or redevelopment to be sited safely landward of that location.
Owens has also been involved in efforts with researchers at the Pacific Islands Ocean Observing Systems (PacIOOS) and the University of Hawaii to improve Maui’s ability to predict today’s run-up impacts in real time as well as model and visualize future flooding events. West Maui has been a particular area of focus since it has been one of the hardest hit regions from erosion and high waves. The online West Maui Wave Runup Forecast Tool can be used to provide near real-time advance warning for areas under threat from the combination of high waves and high water levels over the coming six days. Additionally, the online West Maui Wave Flooding tool visualizes future scenarios of wave flooding over land under various sea level rise thresholds. It is available for use by anyone, from planners and developers to the general public, to consider for longer term planning. Although future construction can be built with consideration for maintaining the ability of beaches and dune systems to move as they must to provide coastal protection, existing structures present a challenging problem. Sea-level rise not only threatens structures right on the coast but can also increase the risk of flooding in low-lying areas. The costs of relocating homes and businesses away from the coast and restoring the protective function of beaches for the public good versus the costs incurred by private citizens and corporations is as much a political issue as an environmental issue. The concept of planned retreat to relocate structures to safer areas and potentially restore the buffering function of areas prone to flooding, such as wetlands, is sure to be a long-term and challenging journey for all involved.
- Hawaii SLR Viewer: hawaiisealevelriseviewer.org
- West Maui Wave Run-up Forecast: pacioos.org/shoreline-category/runup-westmaui/
- West Maui Wave Flooding Tool: pacioos.hawaii.edu/shoreline/slr-westmaui/
- Maui Beach Parks Vulnerability and Adaptation Tool: mauicounty.gov/BPVAStudy