Project Overview

The Problem

Objectives

Background

Approach

Progress

Project Overview: Background

Atolls are unique, poorly understood settings:

1. Very low topography
2. Very steep bathymetry
3. Limited freshwater lens
4. Unconsolidated and consolidated stratigraphy
5. Changing biogeomorphic system

Problems for accurate predictions:

1. Extreme — and often unknown — morphology
2. Limited field data — no empirical relationships nor data to calibrate and validate models
3. Unclear if existing models developed for other environments can work in these settings

At right, schematic drawing showing the features of a typical atoll (Fletcher and Richmond, 2010). The exposed ocean-side beaches on the atoll islet are commonly steep and gravel rich, whereas the lagoon-side beaches are more protected and tend to be gently sloping and sand rich. Shown are saltwater, transition, and freshwater zones. [Larger version.]

The projected rates of sea-level rise through 2100 are 2 to 10 times greater than optimal net vertical reef flat accretion rates — thus sea level will rise faster than coral reef flats can grow upward. Waves in shallow water break when their height (H) is approximately 125% of the water depth. Run-up (R), the maximum elevation of wave uprush on the coast, increases with wave height.

At right, sea-level rise will result in larger waves (H), higher run-up (R), and greater inundation and resulting impacts.

1) GIS-based passive "bathtub" model

At right, is the model typically used for sea-level rise studies. The model is applicable to low-energy water bodies, such as estuaries and marshes. [Larger version]

2) Dynamic model that includes wave-driven run-up

At right, is the model we’re using because islands in the Pacific Ocean are exposed each year to waves as high as 5 to 7 meters (16 to 22 feet). [Larger version]