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Achill Sound & Clew Bay

Overview

Location of Achill Sound & Clew Bay in Co. Mayo, west coast of Ireland.

Achill Island is the largest island off the coast of Ireland. Achill is attached to the mainland by a road bridge at Achill Sound. Achill Sound is located south of Blacksod Bay, across Tullaghan Bay. The Sound separates the island of Achill from the mainland and is open to the North Atlantic at both its northern and southern ends. At the northern end the Sound has a broad mouth whilst at its southern end the narrower mouth is protected from the open sea by Achillbeg Island.

Clew Bay is a large bay situated in Co. Mayo in the west of Ireland. It is approximately 27 km in an east west direction from Newport to Clare Island and 13 km from Mulranny to Louisburgh in a north south direction. The eastern end of the bay is divided into Westport Bay to the south and Newport Bay to the north. Both of these smaller bays are dominated by a spectacular example of a drowned drumlin field. These elongate hills are composed of glacially derived sediments which were inundated by rising sea levels at the end of the last glaciation.

Folklore maintains that there is an island for every day of the year in the bay. There may be fewer than 365 visible above the water however, this INFOMAR survey reveals that there are many more drumlins that have been completely submerged beneath the water surface.

Photograph of drumlins in Clew Bay taken from the aeroplane during the INFOMAR lidar survey in 2003. (Click image for more detail)

At the western end of the bay lies Clare Island which is famous for an extensive biological survey that was under taken 1909 to 1911 to investigate the natural history of the island. In 1991, a new survey of the island began to examine the changes to the island over the course of the last century. This multidisciplinary survey includes broad subjects such as ecology, geology and heritage.

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History of INFOMAR Surveys

INFOMAR surveys of Clew Bay began in the summer of 2002. Tenix LADS performed a survey over a test area in the centre of the bay to determine the suitability of the lidar method to map the bay. Due to the shallow water in much of the bay, LiDAR was found to be a cost effective and safe method to survey the inner bay. In 2003, Tenix LADS returned to survey a large portion of the inner bay, totaling 198 km2.

Another Lidar survey was completed in Achill Sound in 2010 by Pelydryn. The seabed is predominantly sand. Good laser penetration was achieved throughout, although the narrow and deeper channel at Doonyier Point proved difficult to survey, probably due to the fast tidal stream through the gap. Depths in excess of 10m were recorded throught the Sound, although at the southern entrance depths greater than 20m were achieved.

The RV Geo and RV Keary focused their efforts on the waters on the south side of Achill Island in 2011.

Coverage from survey legs undertaken to survey Achill Sound & Clew Bay. (Click image for more detail)

Survey Coverage polygons can be downloaded as a shapefile from our Interactive Web Data Delivery System (IWDDS).

Select your area of interest. Vector datasets - Offshore - Offshore Shapefiles

Read more about the Lidar survey on our Blog by clicking on the link below.

2010 LiDAR Surveying

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Bathymetry (Water Depth)

Bathymetry of Achill Sound & Clew Bay (Click image for more detailed map).

For a more detailed chart of the bathymetry of Achill Sound & Clew Bay click here

You can download Bathymetry xyz data from IWDDS

(Data Type: Vector Datasets, Region: Offshore, Theme: Bathymetry (Survey Leg) entire survey leg or Bathymetry (Survey Line) for individual tracklines. Take note of the survey names in INFOMAR Survey History.

You can also download the bathymetry data in ArcGIS GRID format

You can download Lidar xyz data from IWDDS

(Data Type: Vector Datasets, Region: Offshore, Theme: Lidar.

You can also download the Lidar data in ArcGIS GRID format

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Shaded Relief

The 3D appearance is achieved using software called Fledermaus. By using some vertical exaggeration, artificial sun-shading (usually as if there is a light source in the nw 315°) and colouring the depths using various colour maps, it is possible to highlight the subtle relief of the seabed. This helps us to quickly understand the variation in depths.

Shaded relief image of data collected in 2003 lidar survey showing both exposed and submerged drumlins in Clew Bay. Gaps (white areas) in the data occurred where the laser technology used in lidar surveys could not penetrate through the entire water column due to the water being too deep or decreased clarity in the water. (Click image for more detail)

Detail from shaded relief image showing area of drumlins with the area of drumlins above sea-level masked by the grey areas. The image clearly shows submerged drumlins to the west and also remnants of drumlins that have had their relief reduced by erosion. (Click image for more detail)

 

All of our Charts for Achill Sound & Clew Bay can be downloaded in pdf format from this page.

Download Shaded Relief Bathymetry Geotiffs from our Interactive Web Data Delivery System (IWDDS)

Select your area of interest.Offshore Geotiffs - Bathymetry Shaded Relief Geotiffs. Take note of the survey names in INFOMAR Survey History.

Select your area of interest. Offshore Geotiffs - Lidar Shaded Relief Geotiffs.

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Backscatter

Multibeam Systems also collect additional information, including the strength of the acoustic signal (or return) from the seafloor. This is known as Backscatter. Differing seafloor types, such as mud, sand, gravel and rock will have different Backscatter values depending on the amount of energy they return to the sonar head. Rocky areas will typically have high returns while soft sediments like mud are more likely to absorb energy and have low Backscatter returns. These differing values are used to generate a grey-order image (i.e. dark for high returns, bright for low returns) of the seabed which can be used to examine the nature of the seafloor.

Click image for more detailed map.

Detailed .pdf backscatter charts can be downloaded by clicking here

Download Backscatter Geotiffs from our Interactive Web Data Delivery System (IWDDS)

Select your area of interest.Offshore Geotiffs - Backscatter Geotiffs. Take note of the survey names in INFOMAR Survey History.

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Ground Truthing/Seabed Sampling

To verify the results of remotely sensed sonar data, it is important to collect physical sediment samples from the seabed. These may be from the surface where grabs are used or may penetrate through the seabed and retain the vertical structure of the sediment by using various coring methods. These samples are also critical for the confirming seabed classifications.

INFOMAR has not taken any samples in this area.

10 samples (pink triangles) were taken around Achill head in 2012 by MERC Consultants. Most of the samples were Sand. Full PSA was carried out on the samples.

48 samples (pink triangles) were taken around Clew Bay, Newport Bay & Westport Bay in 2011 by Aquafact. Full PSA was carried out on the samples.

33 samples (brown triangles) were taken around Clew Bay in 2009 by the Marine Institute. Full PSA was carried out on the samples.

Click image for more detailed map.

Sample locations can be downloaded as a shapefile from our Interactive Web Data Delivery System (IWDDS).

On the IWDDS Select your area of interest. Vector datasets - Offshore - Offshore Shapefiles

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Geology

Achill Island Geology
The rocks of Achill belong to an ancient geological group called the Dalradian, composed of metamorphic rocks like schist and quartzite, that have experienced dramatic tectonic upheavals in the past.

The map shows numerous fault lines carving the island – these also separate groups of rock that were formed at different times and in different environments. Note the southern tip of Achill, where we see fault lines running from north-west to south-east. These same faults can be seen to continue onto the seabed, visible in the INFOMAR data as fissures and cracks in the rock outcrop.


Further to the north-west, the seabed is smoother with little rock outcrop – note that this is in line with an indent in the coastline, where we also see a change in the rock type (from orange to green on the legend). Perhaps the rock unit (the Grampian Group) at this part of the coast is generally softer than the surrounding rocks and has been eroded preferentially by the sea?

By combining the seabed data with the geological maps on land, we can learn a lot about the history of the island.

Onshore Bedrock Geology shapefiles can be downloaded from our Interactive Web Data Delivery System (IWDDS).

On the IWDDS Select your area of interest. Vector datasets - Onshore - Bedrock

Offshore Geology shapefiles can be downloaded from our Interactive Web Data Delivery System (IWDDS).

On the IWDDS Select your area of interest. Vector datasets - Offshore - Offshore Shapefiles

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INFOMAR in Google Earth

Image of Lidar data for Blacksod Bay in Google

Download the bathy lidar Achill Blacksod Broadhaven 10m dataset.

Clew LiDAR

 

To view additional datasets in Google Earth please click here

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View data in the free iView4D

Download the free viewer here

Download the Achill Blacksod Broadhaven 10m dataset here

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Data Access

Full details outlining the process to gain access to datasets for the bay above or all INFOMAR data can be found in the INFOMAR | Data page of this website.

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Return to Survey Details Map

Follow these links to your area of interest on the INFOMAR website:

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