Glider observations of sediment resuspension during storm conditions

<p>Sediment resuspension and transport on continental shelves are primarily driven by episodic energetic events, such as storm. Unfortunately, resuspension processes remain poorly quantified using traditional sampling techniques due to the intermittency and the intensity of these events. The recent integration of Acoustic Doppler Current Profilers (ADCPs) onto underwater gliders changes the way current and sediment dynamics in the coastal zone can be monitored. Their endurance and ability to measure in all weather conditions increase the probability of capturing sporadic meteorological events. We used a Slocum glider equipped with a CTD (Conductivity, Temperature, Depth), an optical payload and a RDI 600 kHz phased array ADCP to examine storm-induced sediment resuspension in the Gulf of Lion’s shelf (NW Mediterranean). Observations show that early in the storm, when the waves are highest, resuspension is limited by stratification. During the storm, erosion of the pycnocline through thickening of the bottom and surface mixed layers lead to resuspension in the full water column. Coincident optical and acoustic backscatter measurements indicate that the resuspended particulate assemblage is homogeneous and composed of large particles. Glider-ADCP observations showed for the first time that waves may be the predominant forcing which drive the resuspension on the outer shelf (> 80 m) during the winter storm. While, in the Gulf of Lions, which is considered as a relatively low energy continental shelf, modeling studies consider that only current drive resuspension in the outer shelf. This study highlights the usefulness of glider-ADCP to describe episodic processes and to support validation and improvement of regional hydrodynamic models.</p>

Detecting plastic items in the water column using an Acoustic Doppler Current Profiler (ADCP)

<p>An Acoustic Doppler Current Profiler (ADCP) is commonly used to monitor flow velocity. An accurate method to obtain discharge in a river or a channel is to mount an ADCP to a boat and sail transects across the channel. Additionally, these surveys may also be used to obtain the amount of plastic items in the water column. The transport of plastic items suspended in the water column may be substantial and is more challenging to monitor than the transport of floating items. We carried out a feasibility test in a harbour of a river. We deployed the ADCP horizontally at 1.0 m depth and released plastic items (and similarly shaped organic items for comparison) 5 times at 1.0, 3.0 and 5.0 m from the ADCP. We compared the signal strength in a 5 s period after release with the background signal strength.</p><p>The item was steady within the detection volume for the majority of the 5 s periods. Three out of five plastic items had signal strengths a least 5 dB higher than the background strength (at several distances). We conclude that at least these items were detected. The similarly shaped organic items generally had a lower signal strength. Although the response of each item as a function of orientation, distance along and across the beam should be investigated further, the feasibility study shows the potential to additionally determine the amount of plastic items in the water column from ADCP observations.  </p>

Latitudinal distributions of 234Th in the upper western Indian Ocean

<p>We conducted an onboard measurement of dissolved- and particulate <sup>234</sup>Th in seawater of upper Indian Ocean. The study region covers the meridional section of upper (<500 m depth) Indian Ocean (3⁰N to 15⁰S at 67⁰E in July 2017, and 5⁰S to 13⁰S at 60⁰E and 5⁰S to 24⁰S 67⁰E in April 2018). Dissolved and particulate (>1.2 μm) <sup>234</sup>Th ranged 0.8 – 2.7 dpm L<sup>-1</sup> and 0.05 – 0.7 dpm L<sup>-1</sup>, respectively. In July 2017, the large deficiency of dissolved <sup>234</sup>Th were consistently observed at ~50m depth where the subsurface chlorophyll maximum (SCM) present, along the entire section (5⁰S to 13⁰S). After then, the <sup>234</sup>Th/<sup>238</sup>U were almost ~1 in ≥100 m depths. In contrast, in April 2018, the significant deficits of dissolved <sup>234</sup>Th were observed in entire upper water columns, 0 – 200m depths. This difference in distribution patterns between two years appears to be related to the annual-/seasonal- variations of SCM patterns. In 2018, SCM were shown in 70 – 80 m depths near equator (5⁰S degree), and gradually deepens in lower latitude (SCM presents in 130 m depths in 24⁰S). Interestingly, the unusually lowest dissolved <sup>234</sup>Th (and very low particulate <sup>234</sup>Th also,) were observed in 5⁰S 60⁰E, near the Seychelles–Chagos thermocline ridge (SCTR) region. There are two hypotheses to explain this extremely lower concentrations of <sup>234</sup>Th. The one is that the large input of lithogenic particles from SCTR, seems to be due to largest <sup>234</sup>Th removal in the water column of extremely shallow area (<300 m of bottom depth). The other is that unusually strong eastward currents (>1 m/s of zonal velocity, based on ADCP observations) can laterally transport the <sup>234</sup>Th. In this presentation, we will also present the preliminary results of vertical export fluxes of some particulate trace elements (Al, Fe, Mn, Cu, Zn, Ni, Pb, and etc.) in the upper Indian Ocean estimated by using this <sup>234</sup>Th tracer.</p>

Numerical Modeling of Beach Morphological Change: A Case Study of Kaanapali Beach, Maui, Hawaii

This case study reports results from field observations and numerical simulations of waves and morphological changes along a portion of Kaanapali Beach on West Maui, Hawaii, which is protected by a hard coral reef and experiences shoreline changes from season to season. The SWAN spectral wave model shows reasonable agreement with ADCP observations of wave-heights for the winter months. Simulated beach profile change over one-month time frame was able to reasonably capture the trend of beach face migration (accretion or erosion); the modeled shoreline also shows satisfactory agreement with beach survey data. This case study suggests that Delft3D is able to capture key features of sediment transport along a narrow beach protected by a fringing reef.

Modulation of Near-Inertial Oscillations by Low-Frequency Current Variations on the Inner Scotian Shelf

Near-inertial oscillations (NIOs) on the inner Scotian shelf are studied using observations, a simple slab model, and two operational shelf circulation models. High-frequency radar and ADCP observations from December 2015 to February 2016 show that individual NIO events forced by time-varying wind stress typically lasted for three to four inertial periods. NIOs with speeds exceeding 0.25 m s−1 were observed in the offshore part of the study region, but their amplitudes decreased shoreward within ~40 km of the coast. The NIOs had spatial scales of ~80 and ~40 km in the alongshore and cross-shore directions, respectively. The NIO phases varied moving from west to east, consistent with the typical movement of winter storms across the study region. Evolving rotary spectral analysis reveals that the peak frequency fp of the NIOs varied with time by ~7% of the local inertial frequency. The variation in fp can be explained in part by local wind forcing as demonstrated by the slab model. The remaining variation in fp can be explained in part by variations in the background vorticity associated with changes in the strength and position of the Nova Scotia Current, an unstable baroclinic boundary current that runs along the coast to the southwest. Two operational shelf circulation models are used to examine the abovementioned features in the high-frequency-radar and ADCP observations. The models reproduce the spatial structure of the NIOs and, in a qualitative sense, the temporal variations of fp.