Site and Hydrographic Survey Report East Tarbert Bay, Isle of Gigha
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Site and Hydrographic Survey Report East Tarbert Bay, Isle of Gigha August 2017 Written By: Status: Final Date: 17th Aug 2017 The Scottish Salmon Company 8 Melville Crescent Edinburgh EH3 7JA Contents ____________________________________________________________ 1 Introduction 3 2 Site description 3 3 Survey programme 4 3.1 Current meter set up 4 3.2 Weather station set up 4 3.3 Current meter configuration file 4 3.4 Mooring system 5 4 Bathymetry and site survey 5 5 Data processing 6 5.1 RDCP data 6 5.2 Magnetic heading corrections 7 5.3 Hydrographic survey results 7 5.4 Summary statistics 7 5.5 Meterological results 8 6 Discussion 8 Annex HG7 analysis summaries and Contour Signal Strength 10-14 2 The Scottish Salmon Company 2017 1 INTRODUCTION This report describes the methods used to acquire current speed and direction data and spot depths at a site in the Sound of Gigha and presents the outcomes of the survey exercise. This deployment was carried out using an Aanderaa RDCP600 mounted in a weighted seabed frame. Analyses were carried out in accordance with SEPA guidelines ( Regulation and monitoring of marine cage fish farming in Scotland - a procedures manual. Attachment VIII Site and Hydrographic Survey Requirements. Version 2.7, 31st October 2008). 2 SITE DESCRIPTION The hydrographic survey site is located on the east coast of the Isle of Gigha, Argyll, in the vicinity of the existing Scottish Salmon Company (SSC) East Tarbert Bay farm site. There were 12 cages on-site at the time of survey. In addition, the existing SSC Druimyeon Bay cage site lies around 1.2km to the south of the deployment position. Within the survey area, the seabed slopes eastwards to water depths >30m. No depths exceeding 43m were recorded. East Tarbert Bay 5km Sound of Gigha Figure 1: Location of East Tarbert Bay 3 The Scottish Salmon Company 2017 3 SURVEY PROGRAMME 3.1 Current meter set up The deployment position was (WGS84) 55º42. 931’ N 005º42. 577’ W NR ( 67094 53135) which is within 150m of the existing and proposed East Tarbert Bay site centre. The survey period comprised a half-lunar 15-day period; from 11th July to 26th July 2016. Positioning for the survey was provided by GPS and compatible chart plotter as installed on TSSC landing craft and was backed up using a Garmin handheld GPS with minimum 8 satellite coverage. 3.2 Weather station set up Meteorological data was obtained from the Machrihanish weather station at WGS84)( 55º25. 983’ N 005º42. 564’W (NR 65428 21722),approximately 30km due south of the site. The data was provided via the website www.windfinder. com 3.3 Current meter configuration file Figure 2:RDCP600 configuration files The deployment was configured at the time of deployment having established water depth and expected tidal range on site.This was carried out on the instrument internal set up touch-screen. 4 The Scottish Salmon Company 2017 3.4 Mooring system The RDCP was fixed in a gimballed, weighted sea bed frame. The head of the RDCP was 600mm from the seabed throughout the deployment. Internal parameters pitch, roll and heading were checked post-deployment to ensure a stable position was maintained throughout. 4 BATHYMETRY AND SITE SURVEY A full bathymetric survey was carried out using a hand held acoustic sounder on 21/04/17. Boat GPS was used at the start of the survey to verify accuracy to return positional information (WGS84). This was cross-checked against a hand held Garmin GPS receiving a minimum of 8 satellite coverage. Recorded depths were corrected to CD using Admiralty Total Tide software and referenced against tide predictions for the secondary port Sound Of Gigha. The bathymetric survey concluded that chart data was reliable throughout the development area. The bathymetry plot and selected depth soundings are tabulated below: RCM Figure 3: Bathymetric plot 5 The Scottish Salmon Company 2017 GPS Position Time Measured Tide Corrected Sounding UTC) depth ( m) height ( m) depth ( m) Latitude ( N) Longitude ( W) Eastings Northings 1 55 43.101 005 42.641 167044 653455 1235 23.1 0.5 22.6 2 55 43.093 005 42.451 167242 653429 1233 32.4 0.5 32.0 3 55 43.091 005 42.260 167442 653415 1232 20.3 0.5 19.8 4 55 42.934 005 42.748 166915 653151 1054 11.6 0.5 11.1 5 55 42.938 005 42.575 167097 653148 1043 38.4 0.5 37.9 6 55 42.926 005 42.411 167267 653117 0939 37.5 0.6 36.9 7 55 42.932 005 42.308 167375 653122 1256 25.2 0.5 24.7 8 55 42.777 005 42.692 166958 652856 1210 20.7 0.5 20.2 9 55 42.765 005 42.522 167134 652825 1211 33.2 0.5 32.8 10 55 42.753 005 42.317 167348 652791 1258 17.5 0.5 17.1 5 DATA PROCESSING 5.1 RDCP data Data was extracted from the RDCP and viewed using RDCP Studio. From there the data was extracted to Microsoft Notepad then later to Excel. SEPA specify that data should be presented for specified depths so the data was selected against these requirements: Sub-surface – from a depth 5m below the lowest predicted spring tide during the deployment period Cage-bottom – at a depth corresponding to the bottom of the cages at mean sea level ±1m Near-bottom – as close to the bed as practicable Mean tidal height recorded by the RDCP during the deployment (MSL) was 34.8m, and minimum tidal height for the deployment was 34.2m. Cage depth was assumed to be 10m. As such near-bottom depth was taken at a range of 3m (allowing for a 1m blanking distance and 0.6m height of the mooring frame), cage-bottom at 25.3m and sub- surface at 29.8m. Specified and deployed depths of data retrieval are compared below: 6 The Scottish Salmon Company 2017 Measured depth at deployment 34.7m Predicted tidal height at deployment (from 0.6m Admiralty tide tables) Corrected depth at deployment (CD) 34.1m Lowest predicted tide during deployment 0.1m Charted mean sea level (range) 1.0m Average tidal height 0.6m Mean sea level height above bed 34.7m Assumed cage depth 10.0m RDCP mooring height 0.6m Mean RDCP recorded pressure 34.8m Height above bed Bed cell 3.0m Mid cell 25.3m Surface cell 29.8m Depth below MSL Bed cell 31.7m Mid cell 9.4m Surface cell 4.9m Hydrographic data were analysed using the SEPA tool HGdata_analysis_v7, running in Excel. Pre and post-deployment values were rejected from the dataset, restricting the full data records to 1081 records. 5.2 Magnetic heading corrections A Grid Magnetic Angle of 0.502º W (from http://www.geomag.bgs.ac.uk/ data_service/charts_navig/gma_calc.html) was applied and grid convergence ( from HGdata_analysis_v7) was 3.06º W to give overall declination of –3.562º. Date Lat Long NGR_X NGR_Y mag. dec grid conv. GMA 11/07/2016 55°42.931N 005°42.577W 167094 653135 3.562W 3.06W 0.502W Notes: positions recorded to WGS84, and converted to OSGB36 using OSTN97 magnetic declination from US-NOAA Geomagnetism website grid convergence from OS corrections tool grid magnetic angle = magnetic declination – grid convergence 5.3 Hydrographic survey results Pitch and roll exhibited some variation during the deployment from 3.356 to 3.748º and 2.031 to -1.586º respectively. However these results are within acceptable limits for the instrument and indicate a stable deployment. 7 The Scottish Salmon Company 2017 An overview of data quality is displayed graphically as a plot showing signal strength throughout Column 1 (see Annex) – configured to report on the whole water column. This plot shows reliable data collected across all cells below the surface at which point interference due to turbulence become apparent. Data cells representative to bed, cage depth and sub-surface were extracted from Column 1. 5.4 Summary statistics Bed Mid Surface No. of records 1081 1081 1081 Start date / time 11/07/16 10:21 11/07/16 10:21 11/07/16 10:21 End date / time 26/07/16 10:21 26/07/16 10:21 26/07/16 10:21 Mean velocity (m/s) 0.117 0.124 0.073 Minimum velocity (m/s) 0.002 0.004 0.002 Maximum velocity (m/s) 0.311 0.329 0.275 Ranked % of mean velocity 55 51 56 Ranked % of 3 cm/s 6 5 16 Ranked % of 4.5 cm/s 13 10 28 Ranked % of 9.5 cm/s 42 335 72 Residual velocity (m/s) 0.047 0.018 0.041 Residual direction °G 178 259 26 Major axis °G 185 200 20 Residual parallel (m/s) 0.047 0.009 0.041 Residual normal (m/s) - 0.006 0.015 0.004 Amplitude parallel (m/s) 0.171 0.185 0.095 Amplitude normal (m/s) 0.052 0.055 0.045 Amplitude anisotropy 3.30 3.36 2.08 5.5 Meteorological results The data from the weather station deployed at Machrihanish was analysed using the SEPA tool HGdata_analysis_v7. The data shows the 24hr average did not exceed 10m/sec over the survey period. The data from this weather station exhibits no wind speeds recorded in excess of 10m/s. Mean wind speed over the whole deployment was 5.1m/s. The deployment period therefore met the requirement of mean daily wind speed <10m/s.