The intertidal hydraulics of tide-dominated reef platforms

Abstract

A 2 week field experiment investigated the hydrodynamics of a strongly tidally forced tropical intertidal reef platform in the Kimberley region of northwestern Australia, where the spring tidal range exceeds 8 m. At this site, the flat and wide (∼1.4 km) reef platform is located slightly above mean sea level, such that during low tide the offshore water level can fall 4 m below the platform. While the reef always remained submerged over each tidal cycle, there were dramatic asymmetries in both the water levels and velocities on the reef, i.e., the flood duration lasted only ∼2 h versus ∼10 h for the ebb. These dynamics were investigated using a one‐dimensional numerical model (SWASH) to solve the nonlinear shallow water equations with rapid (sub to supercritical) flow transitions. The numerical model revealed that as water drains off the reef, a critical flow point was established near the reef edge prior to the water discharging down the steep forereef. Despite this hydraulic control, bottom friction on the reef was still found to make a far greater contribution to elevating water levels on the reef platform and keeping it submerged over each tidal cycle. Finally, a simple analytical model more broadly shows how water levels on intertidal reef platforms functionally depend on properties of reef morphology, bottom roughness, and tidal conditions, revealing a set of parameters (a reef draining time‐scale and friction parameter) that can be used to quantify how the water depth will fall on a reef during ebb tide.

Resilience of coral calcification to extreme temperature variations in the Kimberley region, northwest Australia

“…They are also subject to diurnal tidal amplitudes of up to 11 m (Kowalik 2004) that can expose corals to potentially stressful and damaging levels of temperature and light ). Furthermore, water motion can become stagnant during such low tide (Lowe et al 2015), decreasing rates of oxygen export and increasing oxidative stress (Lesser and Farrell 2004;Anthony and Kerswell 2007;Mass et al 2010), as well as increasing the temperature of coral tissue above already elevated ambient levels (Fabricius 2006;Jimenez et al 2008). Thus, intertidal and nearshore environments along the Kimberley coast provide a challenging thermal environment to which corals have adapted, yet to date we have found no record of extensive coral bleaching on a regional scale along the Kimberley coast.…”

“…The presence of these natural barriers combined with largescale (10s-100s m) depressions in the local bathymetry ensures that some water is retained in each pool even when offshore sea levels are several metres lower than the minimum depth of the pool. These types of intertidal environments are commonly found in other shallow coastal environments throughout the Kimberley (Wilson and Blake 2011;Lowe et al 2015;Richards et al 2015). We chose three experimental sites subjected to varying degrees of tidal influence so that we could get the broadest range of light and temperature variability: (1) a shallow intertidal pool with a minimum depth during spring low tide of B0.2 m which gets completely isolated at low tide (herein referred to as 'isolated'), (2) an intertidal pool with a minimum depth of *0.5 m that is only briefly isolated from King Sound at the very peak ebb of low tide (herein referred to as 'intermediate'), and (3) a subtidal site with a minimum depth of *1 m that is never tidally isolated or otherwise disconnected from King Sound (herein referred to as 'subtidal'; Fig.…”

Coral Reef Drag Coefficients – Water Depth Dependence

“…Estimates of drag coefficients for coral reefs are typically one to two orders of magnitude larger than for sandy beaches or continental shelves (e.g., Monismith 2007). The large drag coefficients and energetic flows over shallow coral reefs result in large bottom stresses that are invariably a dominant element of the dynamics (Roberts et al 1975;Symonds et al 1995;Kraines et al 1998;Callaghan et al 2006;Coronado et al 2007;Jago et al 2007;Hench et al 2008;Lowe et al 2009;Vetter et al 2010;Taebi et al 2011;Monismith et al 2013). Consequently, one of the major challenges in modeling currents over coral reefs is determining accurate estimates of drag coefficients.…”

“…The dependence of C da on water depth was included in some early studies of nutrient uptake by corals (e.g., Atkinson and Bilger 1992;Hearn et al 2001), and a more recent laboratory study of flow over coral (McDonald et al 2006) indicates that the drag coefficient depends on water depth. However, studies estimating the drag coefficient over coral reefs using field measurements have not generally considered the dependence of the drag coefficient on water depth [though see Pomeroy et al (2012) and Lowe et al (2015) for two exceptions].…”

“…where t b is bottom stress, r is density, and C da is the drag coefficient for depth-average flow (e.g., LugoFernández et al 1998a,b;Hench et al 2008;Lowe et al 2009;Rosman and Hench 2011;Monismith et al 2013). Depth-average flows are often used because the traditional approach of defining a drag coefficient for the flow at a specific height above bottom is challenging over coral reefs where the precise location of the bottom is ambiguous and because the depth-average flow tends to be a more robust quantity than the velocity at a specific height.…”