Ocean Networks Canada - Fraser River Delta

Expedition 2019: Highlights Story Map

duncanlowrie@uvic.ca — Thu, 14 Nov 2019 19:36:10 +0000

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Introduction to Fraser Delta

cbonnett@uvic.ca — Thu, 15 Aug 2013 18:03:19 +0000

Fraser River Delta at a Glance:

Region: Located at the mouth of the Fraser River, near the major population centre of B.C.’s Lower Mainland, where fresh, silt-laden water meets the salty water of the Strait of Georgia.
Number of Instrument Platforms: 3
Depth: 108 m
Location: Latitude: 49^o.50'N, Longitude: 123^o.20'W
Seafloor Composition: Soft, sediment-covered and unstable slope.
Principal Research: Submarine Slope Failure and Sediment Stability

Map of Ocean Networks Canada installations in the Salish Sea. Click to enlarge.

Environment/Ecosystems

The Fraser River drains the coastal mountains of southern British Columbia. The winter snow pack melts in the spring, causing the river volume to more than double, with peak discharge in May and June of nearly 10,000 cubic metres per second. This massive fresh-water input dramatically alters the near-surface and regional circulation patterns in the southern Strait of Georgia. Enhanced by tidal mixing, the deep estuarine circulation causes a net surface flow out through the Juan de Fuca Strait, and a deep in-flow of Pacific water, rich in nutrients.

The river discharge is also ladened with silt, washed down through the Fraser Valley. As the river empties into the Strait of Georgia, the slowing and mixing with salt water precipitates a settling of new sediments along the entire Delta front. Tidal resuspension and transport further move the sediments along the Delta slope. Rapid accumulation often results in unstable slopes, which can fail catastrophically as under-water landslides. Located near important coastal infrastructure such as the Vancouver Airport, the deep-sea Deltaport or the Tsawwassen ferry terminal, a proper understanding of this dynamic coastal region is imperative.

What Makes the Fraser River Delta Observatory Unique?

While in the past it has been difficult to study these long-time scale failure events using short-term moored instruments, Ocean Networks Canada’s deployment of a new instrument array along the Delta slope makes detailed, real-time, and long-term data available for researchers.

With the slope failure events themselves occurring over a span of only a few seconds, but with conditioning mechanisms taking months, or even years to form, the instrument array at ONC’s Fraser River Delta observatory is uniquely capable of handling such a demanding task.

One type of instrument currently used by scientists at the Fraser River Delta Observatory, known as a Seismic Liquefaction In Situ Penetrometer (SLIP), uses piezometers (to measure pressure in the water column and seabed), accelerometers (to measure seismic activity), and inclinometers (to measure sediment movement through strain) to gather data on the conditions associated with slope failure.

Another, the Delta Dynamics Laboratory (DDL), provide additional environmental information on water properties, turbidity, and currents for the slope stability studies. A mini-node supports the entire Delta site, including hydrophones to listen for undersea landslides and earthquakes.

It is the integration then of multiple instrument and data types present within the Fraser River Delta Observatory which makes it unique, and particularly important, to the coastal Lower Mainland and Vancouver Island.

The Submarine Slope Failure and Sediment Stability project, a project led by Gwyn Lintern of Natural Resources Canada, is helping to understand and predict submarine instability along the Fraser River Delta Slope, hoping to protect coastal infrastructure from the consequences of such a slope failure. Associated research can be found in the following study briefs:

An Underwater Laboratory at the Fraser River Delta.
Turbidity measurements taken at the Fraser River Delta observatory help indicate the start of the spring freshet.
Imagenex 881ASector Sonar Imaging takes hourly images of the ocean bottom to detect any changes which occur in the bedforms.
Echosounder identifies bubbles rising from biogeochemically active delta sediments.
Delta Dynamics Laboratory (DDL) uses its echosounder to identify internal waves at interface between the salt- and freshwater boundary characteristic of Fraser River Delta .
CODAR (Coastal Ocean Dynamics Applications Radar) allows the measurement of surface current velocities at a distance.
DDL’s echosounder has been used to illustrate that a ‘sediment waterfall’ occurs at low tide, when receding ocean water allows a flood of fresh water from the Fraser River to carry silt far out into the Straight of Georgia.

Videos

Introduction to Fraser River Delta Observatory by Dr. Gwyn Lintern

Sediment Cascade From Fraser River

rlat@uvic.ca — Mon, 14 May 2012 07:00:00 +0000

The Fraser River freshet is starting (snow pack melt in the coastal mountains), and the suspended sediment loads coming out of the river mouth into the Strait of Georgia are also increasing. This is especially evident at our Fraser Delta site, located just south of the mouth of the Fraser River. In these data plots we can see that at low tide, when the river runs fastest, there is a spike in the near bottom turbidity at our Fraser Delta site, associated with the cascade of suspended sediment, as evidenced in the 200 kHz ZAP echogram. During this first week of May (2012), the low tides have been occurring late in the day (16:00-17:00 PDT, 23:00-24:00 UTC), and we have seen a steady increase in the near bottom turbidity signal just after low tide.

Bubbles Rising From the Delta Sediments

rlat@uvic.ca — Mon, 07 Nov 2011 08:00:00 +0000

Dynamic sediments and sediments with active biogeochemistry often generate a variety of chemical compounds, some of which will be gaseous. This inverted echo-sounder image from 30 October 2011 at the Delta Dynamics Laboratory in the Strait of Georgia at 108m water depth near the mouth of the Fraser River has captured numerous clouds of rising bubbles. Such bubbles have several distinct characteristics.

First, we can see the uniform rise velocity, suggesting nearly constant bubble size. Bubbles have been detected that dissolve with height (common in unsaturated water conditions, as the bubbles shrink they slow and the traces arc to the right) or bubbles that grow as a result of reduced hydrostatic pressure (the traces curve upwards). This image coincides with low tide, another condition that is known to encourage gas release as the water pressure is at a minimum. We have also detected bubbles rising from schools of fish.

One final observation: the ensonified volume of water is an 8 degree cone expanding upwards from the transducer head. The bubbles may appear in the acoustic back-scatter at any height as they enter the beam, and with the strong tidal currents in the Strait of Georgia, they can easily be advected horizontally both into and out of the beam, thus the random appearance of bubble clouds into and out of the ensonified volume.

Ocean Networks Canada - Fraser River Delta

Expedition 2019: Highlights Story Map

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Introduction to Fraser Delta

Fraser River Delta at a Glance:

Environment/Ecosystems

What Makes the Fraser River Delta Observatory Unique?

Videos

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Sediment Cascade From Fraser River

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Bubbles Rising From the Delta Sediments

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