An official website of the United States government
Abstract Using observations, numerical models, and theory, we explore a framework to classify reefs as open or closed based on their dynamics. While the concepts of open and closed reefs are used widely in studies of coral reef hydrodynamics and are generally based on geometry, there is no consensus on what qualifies as open and closed. With observations from Ofu, American Samoa, we show that the reef flat exhibits two different dynamical regimes depending on tidal and wave forcing. Flow over this reef flat resembles a classic one‐dimensional barrier reef flow during low tide, where wave setup creates a cross‐reef pressure gradient which forces flow on the flat. On high tide, however, flow on the flat is oblique to the crest, and at times directed offshore. We reproduce this behavior in an idealized numerical model of a fringing reef. We classify open reefs as a condition where an onshore, wave‐generated pressure gradient is balanced by friction, and closed reefs as a condition where an onshore radiation stress gradient is opposed by an offshore pressure gradient. Results from the fringing reef model show that the system transitions between open and closed behavior over a tidal cycle. Results from an additional barrier reef numerical model exhibits almost exclusively open reef behavior, for which we derive a simple theoretical model. We argue that classifying reefs as open or closed based on their dynamics, rather than geometry, is a more meaningful approach to comparing reefs and predicting their dynamical response to wave and tidal forcing.
more »
« less
Reef Flat Flow Dynamics for a Nearly Closed Fringing Reef Lagoon: Ofu, American Samoa
Abstract We discuss observations of tidally varying wave‐forced flows in the reef system on Ofu, American Samoa, a barrier reef and lagoon system that appears open at low tide and closed at high tide. At high tide, the free‐surface pressure gradient nearly balances the radiation stress gradient in the depth‐integrated momentum equation. At depth, there is an imbalance between these two forces, generating an undertow and flows that turn alongshore, and for some of the time, offshore, behavior similar to rip currents observed on beaches. At low tides, the wave forcing drives purely onshore flows. In general, wave transport is important to determining the total net transport. While the dynamically closed nature of the lagoon mostly suppresses cross‐reef transport, there is always some flow through the lagoon with the strongest flows occurring at high tides and when the wave forcing is strongest.
more »
« less
- Award ID(s):
- 1948189
- PAR ID:
- 10376410
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Oceans
- Volume:
- 127
- Issue:
- 10
- ISSN:
- 2169-9275
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Internal waves can influence water properties in coastal ecosystems through the shoreward transport and mixing of subthermocline water into the nearshore region. In June 2014, a field experiment was conducted at Dongsha Atoll in the northern South China Sea to study the impact of internal waves on a coral reef. Instrumentation included a distributed temperature sensing system, which resolved spatially and temporally continuous temperature measurements over a 4‐km cross‐reef section from the lagoon to 50‐m depth on the fore reef. Our observations show that during summer, internal waves shoaling on the shallow atoll regularly transport cold, nutrient‐rich water shoreward, altering near‐surface water properties on the fore reef. This water is transported shoreward of the reef crest by tides, breaking surface waves and wind‐driven flow, where it significantly alters the water temperature and nutrient concentrations on the reef flat. We find that without internal wave forcing on the fore reef, temperatures on the reef flat could be up to 2.0°C ± 0.2°C warmer. Additionally, we estimate a change in degree heating weeks of 0.7°C‐weeks warmer without internal waves, which significantly increases the probability of a more severe bleaching event occurring at Dongsha Atoll. Furthermore, using nutrient samples collected on the fore reef during the study, we estimated that instantaneous onshore nitrate flux is about four‐fold higher with internal waves than without internal waves. This work highlights the importance of internal waves as a physical mechanism shaping the nearshore environment, and likely supporting resilience of the reef.more » « less
-
Abstract Through idealized, numerical models this paper investigates flows on a reef geometry which has received significant attention in the literature; a shallow, fringing reef with deeper, shore-ward pools or lagoons. Given identical model geometries and varying only reef flat drag coefficients between model runs ( $$C_D = [0.001,0.005,0.01,0.05,0.1]$$ C D = [ 0.001 , 0.005 , 0.01 , 0.05 , 0.1 ] ), two distinct circulation patterns emerge. One is related to low reef water levels and high roughness, and efficiently flushes the entire reef system resulting in low residence times (an ‘open reef’). The other is related to high reef water levels and low roughness, and in spite of the development of an offshore undertow, this dynamic is inefficient at flushing the reef-pool system and facilitating exchange flow with offshore waters (a ‘closed reef’). This paper shows that even given indistinguishable geometry and offshore conditions, this information is insufficient to predict reef dynamics, and suggests that reef roughness (and thus reef health) plays a comparable role in determining circulation patterns and residence times. Furthermore, a transition from open to closed or vice versa caused by e.g., a loss of reef roughness or increase in mean sea level could have implications for transport and mixing of nutrients and water masses, as well as larval dispersal.more » « less
-
Abstract Despite its relatively small magnitude, cross-channel circulation in estuaries can influence the along-channel momentum balance, dispersion, and transport. We investigate spatial and temporal variation in cross-channel circulation at two contrasting sites in the Hudson River estuary. The two sites differ in the relative strength and direction of Coriolis and curvature forcing. We contrast the patterns and magnitudes of flow at the two sites during varying conditions in stratification driven by tidal amplitude and river discharge. We found well-defined flows during flood tides at both sites, characterized by mainly two-layer structures when the water column was more homogeneous and structures with three or more layers when the water column was more stratified. Ebb tides had generally weaker and less definite flows, except at one site where curvature and Coriolis reinforced each other during spring tide ebbs. Cross-channel currents had similar patterns, but were oppositely directed at the two sites, demonstrating the importance of curvature even in channels with relatively gradual curves. Coriolis and curvature dominated the measured terms in the cross-channel momentum balance. Their combination was generally consistent with driving the observed patterns and directions of flow, but local acceleration and cross-channel advection made some notable contributions. A large residual in the momentum balance indicates that some combination of vertical stress divergence, baroclinic pressure gradients, and along-channel and vertical advection must play an essential role, but data limitations prevented an accurate estimation of these terms. Cross-channel advection affected the along-channel momentum balance at times, with implications for the exchange flow’s strength. Significance StatementCurrents that flow across the channel in an estuary move slower than those flowing along the channel, but they can transport materials and change water properties in important ways, affecting human uses of estuaries such as shipping, aquaculture, and recreation. We wanted to better understand cross-channel currents in the Hudson River estuary. We found that larger tides produced the strongest cross-channel currents with a two-layer pattern, compared to weaker currents with three layers during smaller tides. Higher or lower river flow also affected current strength. Comparing two locations, we saw cross-channel currents moving in opposite directions because of differences in the curvature of the river channel. Our results show how channel curvature and Earth’s rotation combine to produce cross-channel currents.more » « less
-
The drivers of the tidal and residual flows in estuaries can vary spatially and temporally due to geomorphic complexities, fortnightly tides, and climatic influences. In this paper, we explore the mechanisms that give rise to the circulation patterns in Frenchman Bay, Maine, on the Eastern Coast of the USA, under varying freshwater input conditions and fortnightly tidal phases, using idealized simulations from a high-resolution, three-dimensional numerical model. The results of the simulations at the tidal timescale reveal a tidal asymmetry in vorticity, where vorticity generated during flood tide is not spun-down during the subsequent ebb. This asymmetry prompts the investigation of the residual circulation in the bay which is characterized by large tidal residual eddies. These eddies are found to persist in the depth-averaged residual flow regardless of the freshwater input or tidal phase, leading to the conclusion that the eddies are “geomorphically-constrained” in the bay. Analysis of the horizontal momentum terms and a simulation performed without Coriolis forcing demonstrates that the tidal stress terms predominantly balance the barotropic pressure gradient to give rise to the eddy patterns, while the Coriolis force acts to strengthen their vorticity. The eddies create a laterally sheared residual flow structure with depth, however the flow is more vertically sheared during the neap tide when the baroclinic pressure gradient plays a larger role. These findings demonstrate the persistence of tidal residual eddies regardless of freshwater input or fortnightly tidal phase in a geomorphically complex deglaciated coastal bay with low freshwater input.more » « less
