Instrumentation for direct measurements of wave‐driven flow over a fringing reef crest

Abstract

Shallow dynamic flows are very important processes in environmental systems, yet they are notoriously difficult to measure. For example, on coral reefs, shallow flows over the reef crest typically range from 0 to 1 m depth and velocity up to 1–2 m s−1 including oscillatory motion of waves. To directly measure depth and velocity in this challenging environment, we retrofitted a vertical acoustic Doppler current profiler (ADCP, Teledyne, RDI) designed for streams into a new shallow water acoustic Doppler current profiler system (SW‐ADCP) to expand memory and power limitations and allow for deployment in wave‐dominated environments. We captured variations in shallow reef crest depth and flow over a period of 3 weeks in the reef/lagoon system of Ofu Island, American Samoa. The new SW‐ADCPs recorded water depth and three‐dimensional velocity in 3 cm bins every 3 s. We then used velocity profiles to estimate volumetric flow and drag coefficients and verified these estimates using boundary layer theory. We observe that the mean velocity profile is well approximated by a log‐layer formulation with z0 of 3.5 cm, despite the shallow depths, strong flows, and breaking waves. Our observations validated the use of SW‐ADCPs as a tool for measuring flows in shallow (0.1–1 m), dynamic coastal marine environments.

Wave Modulation of Flows on Open and Closed Reefs

“…A comprehensive field program was deployed in Ofu, American Samoa between March 10-28, 2017 (Figure 2; see Hefner et al, 2019;Maticka, 2019;Rogers et al, 2018). Instrument specifications, locations, and sampling schemes are listed in Table 1 Mean depth on the reef flat was about 0.6 m, with a 1 m tidal range (Figure 3).…”

“…To illustrate a classification based on dynamics rather than geometry, we employ observations from Ofu, American Samoa, where an extensive field program was carried out in 2016-2017 (see Hefner et al, 2019;Maticka, 2019;Rogers et al, 2018). The Ofu reef is a shallow (0-2 m depth), fringing reef with small pools and narrow channels connecting the pools to the open ocean.…”

Reef Flat Flow Dynamics for a Nearly Closed Fringing Reef Lagoon: Ofu, American Samoa

“…In addition to the vADCP observation of undertow, Lagrangian drifter tracks (see Maticka, 2019) show that besides the main channel, offshore flows were also consistently present near station H-1, a behavior seen in the vADCP data taken there and discussed in Hefner et al (2019). However, at high tides, offshore flows also appeared near the D-transect, a behavior that is similar to rip currents that develop on beaches (MacMahan et al, 2006) (see Figure 1 in Rogers et al (2018)).…”

“…However, the presence of outflows on the reef flat near sta. H-1 (the vADCP data shown in Hefner et al (2019)) suggests that for closed systems, shallow outflow channels may develop where the reef crest is locally lower than adjacent sections. Whether or not these depressions could ultimately develop into "full fledged" channels is an open question.…”

“…Additionally, 2 MHz Nortek ADPs made similar flow and wave measurements at stations B0, F0, and H0 in the Lagoon. Detailed velocity profiles on 3 s intervals were obtained at D‐4 using a Teledyne RD Instruments vADCP configured to run autonomously (Hefner et al., 2019). Unfortunately besides the failure of the vADCP after 5 days due to instrument flooding, the RBR pressure sensor deployed at D‐4 also failed to record any data.…”