Seawater-flooding events and impact on freshwater lenses of low-lying islands: Controlling factors, basic management and mitigation

Gingerich, Stephen B.; Voss, Clifford I.; Johnson, Adam G.

doi:10.1016/j.jhydrol.2017.03.001

Cited by 79 publications

(66 citation statements)

References 16 publications

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“…The reef flat is fully exposed (dry) at low tide, is about 250-350-m wide, and covers an area of about 1 km 2 . Most of the groundwater the water supply system utilizes originates from a horizontal, 1000-m long, skimming well lying just below ground surface, parallel to the runway [13]. This type of well-pumping system limits upconing of the deeper saline water during groundwater withdrawals [16].…”

Section: Study Areamentioning

confidence: 99%

“…Aquifer horizontal permeability (k) has previously been calculated to be 1 × 10 −11 -2 × 10 −10 m 2 (hydraulic conductivity [K]: 1 × 10 −4 -1.6 × 10 −3 m/s) in the upper Holocene layers and about 3.5 × 10 −10 m 2 (K: 3.2 × 10 −3 m/s) in the lower Pleistocene layer [17]. Roi-Namur's FWL thickness has been shown to vary according to levels of recharge, ranging from 5-12 m thick [13,16]. The groundwater on Roi-Namur is artificially recharged using stored rainwater; this artificial recharge amounts to approximately 3.5% of the natural recharge from rain (66 × 10 6 L/year for the years 2000-2012) and started in 2009 [13].…”

Section: Study Areamentioning

confidence: 99%

“…Roi-Namur's FWL thickness has been shown to vary according to levels of recharge, ranging from 5-12 m thick [13,16]. The groundwater on Roi-Namur is artificially recharged using stored rainwater; this artificial recharge amounts to approximately 3.5% of the natural recharge from rain (66 × 10 6 L/year for the years 2000-2012) and started in 2009 [13]. The available potable freshwater supply has been estimated to 86 × 10 7 L for Roi and 16 × 10 6 L for Namur [16].…”

Section: Study Areamentioning

confidence: 99%

“…In all cases, the excess inventory per time (i.e., differences between source and sink fluxes) was divided by radon concentration in groundwater (i.e., groundwater endmember) to calculate groundwater discharge. Site locations for these surface water time-series deployments were strategically placed based on previous data e.g., [13,16,42], and assumptions on gradients in oceanographic, geologic, and hydrologic controls (Figure 1). …”

Section: Submarine Groundwater Discharge (Sgd)mentioning

confidence: 99%

“…A number of threats to the FWLs have been identified, including: (a) infiltration of anthropogenic contaminants [5]; (b) upconing of saline water due to excessive freshwater pumping [6]; (c) reduction in reef and island size due to coastal erosion, leading in turn to a reduction in the size of the FWL [7]; (d) droughts that hinder successful recharge of the FWLs [5,[8][9][10]; and (e) storm surges that cause large waves to wash over the atolls resulting in saline intrusion [11][12][13]. Nonetheless, a better understanding of the processes that influence these FWLs-especially in light of expected climate change scenarios on low-lying atoll islands is essential to better assess atoll water resources management challenges in the near future.…”

Section: Introductionmentioning

confidence: 99%

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Atoll Groundwater Movement and Its Response to Climatic and Sea-Level Fluctuations

Oberle

Swarzenski

Storlazzi

2017

Water

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Groundwater resources of low-lying atoll islands are threatened due to short-term and long-term changes in rainfall, wave climate, and sea level. A better understanding of how these forcings affect the limited groundwater resources was explored on Roi-Namur in the Republic of the Marshall Islands. As part of a 16-month study, a rarely recorded island-overwash event occurred and the island's aquifer's response was measured. The findings suggest that small-scale overwash events cause an increase in salinity of the freshwater lens that returns to pre-overwash conditions within one month. The overwash event is addressed in the context of climate-related local sea-level change, which suggests that overwash events and associated degradations in freshwater resources are likely to increase in severity in the future due to projected rises in sea level. Other forcings, such as severe rainfall events, were shown to have caused a sudden freshening of the aquifer, with salinity levels retuning to pre-rainfall levels within three months. Tidal forcing of the freshwater lens was observed in electrical resistivity profiles, high-resolution conductivity, groundwater-level well measurements and through submarine groundwater discharge calculations. Depth-specific geochemical pore water measurements further assessed and confirmed the distinct boundaries between fresh and saline water masses in the aquifer. The identification of the freshwater lens' saline boundaries is essential for a quantitative evaluation of the aquifers freshwater resources and help understand how these resources may be impacted by climate change and anthropogenic activities.

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Section: Study Areamentioning

confidence: 99%

Section: Study Areamentioning

confidence: 99%

Section: Study Areamentioning

confidence: 99%

Section: Submarine Groundwater Discharge (Sgd)mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Atoll Groundwater Movement and Its Response to Climatic and Sea-Level Fluctuations

Oberle

Swarzenski

Storlazzi

2017

Water

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A Bayesian‐Based System to Assess Wave‐Driven Flooding Hazards on Coral Reef‐Lined Coasts

et al. 2017

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Many low‐elevation, coral reef‐lined, tropical coasts are vulnerable to the effects of climate change, sea level rise, and wave‐induced flooding. The considerable morphological diversity of these coasts and the variability of the hydrodynamic forcing that they are exposed to make predicting wave‐induced flooding a challenge. A process‐based wave‐resolving hydrodynamic model (XBeach Non‐Hydrostatic, “XBNH”) was used to create a large synthetic database for use in a “Bayesian Estimator for Wave Attack in Reef Environments” (BEWARE), relating incident hydrodynamics and coral reef geomorphology to coastal flooding hazards on reef‐lined coasts. Building on previous work, BEWARE improves system understanding of reef hydrodynamics by examining the intrinsic reef and extrinsic forcing factors controlling runup and flooding on reef‐lined coasts. The Bayesian estimator has high predictive skill for the XBNH model outputs that are flooding indicators, and was validated for a number of available field cases. It was found that, in order to accurately predict flooding hazards, water depth over the reef flat, incident wave conditions, and reef flat width are the most essential factors, whereas other factors such as beach slope and bed friction due to the presence or absence of corals are less important. BEWARE is a potentially powerful tool for use in early warning systems or risk assessment studies, and can be used to make projections about how wave‐induced flooding on coral reef‐lined coasts may change due to climate change.

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Quantifying threats to groundwater resources in the Republic of Maldives Part II: Recovery from tsunami marine overwash events

Alsumaiei

Bailey

2018

Hydrological Processes

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Marine overwash events are among the most serious short‐term threats to groundwater supply of small coral islands. During such events, seawater can inundate small islands partially or completely, causing salinization of the aquifer. A comprehensive knowledge of freshwater lens recovery is essential for water planners on these islands. In this study, a numerical modelling approach is used to quantify recovery of the freshwater lens on 4 islands of the Maldives after a tsunami‐induced overwash event similar to that experienced from the Indian Ocean earthquake in December 2004. The islands vary in size (0.2 to 10.1 km2) and span the climatic regions of the Maldives. A tested 3‐dimensional SEAWAT groundwater model for each island is used to simulate the recovery process. Recharge rates from historical rainfall data and from global climate models are imposed on each island during the post‐overwash recovery period. The effect of groundwater pumping on lens recovery also is examined. Results show abrupt decrease in fresh groundwater volumes for each island, followed by recovery that is significantly influenced by island size and recharge patterns. Overall, salinization is more widespread on small islands (<1 km2), but recovery is more rapid than for large islands. Between 50% and 90% of lens recovery occurs after 2 years for small islands (<1 km2) whereas only 35% and 55% for large islands. Imposing pumping rates required to sustain the local population lengthened the recovery time between 5% and 15%, with smaller islands having the higher percentage. However, the governing factor on recovery time is the spatial extent of land surface inundation by the overwash event, with wave height and duration of the event having a negligible impact. A strong relationship exists between required recovery time and island surface area, thereby providing a method to determine recovery time for other atoll islands not investigated in this study with similar geologic structure. Our results can be used to aid in managing water resources during the post‐overwash period.

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Seawater-flooding events and impact on freshwater lenses of low-lying islands: Controlling factors, basic management and mitigation

Cited by 79 publications

References 16 publications

Atoll Groundwater Movement and Its Response to Climatic and Sea-Level Fluctuations

Atoll Groundwater Movement and Its Response to Climatic and Sea-Level Fluctuations

A Bayesian‐Based System to Assess Wave‐Driven Flooding Hazards on Coral Reef‐Lined Coasts

Quantifying threats to groundwater resources in the Republic of Maldives Part II: Recovery from tsunami marine overwash events

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