Gravir Outer, Isle of Lewis Site and Hydrographic survey report

Transcription

1 Gravir Outer, Isle of Lewis Site and Hydrographic survey report November 2013 The Scottish Salmon Company 8 Melville Crescent Edinburgh EH3 7JA Report No: Issued By: Mathew Laughton Checked By: Mark Edmonds Status: Final Date: December 2013

2 Contents 1 Introduction 3 2 Site description 3 3 Survey programme Current meter set up Weather station set up Current meter configuration file Mooring system 5 4 Bathymetry and site survey 7 5 Data processing RDCP data Magnetic heading corrections Hydrographic survey results Summary statistics Meterological results 9 6 Discussion 10 Appendices

3 1 Introduction This report describes the methods used to acquire current speed and direction data and spot depths at a site on the east coast of Lewis and presents the outcomes of the survey exercise. This is the first hydrographic survey of a prospective site for cage site development. This deployment was carried out using an Aanderaa RDCP600 mounted in a 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 site is located on the outer Loch Odhairn in northern area of Isle of Lewis. The existing site lies about 150m west from the current meter location. The survey area comprises on the centre of the loch & the coastline north & south of the survey area. Water depth around the survey site ranges between circa m

4 3 Survey programme 3.1 Current meter set up The deployment position was (WGS84) 58º N W (NB ). The survey period comprised a half-lunar 15-day period; from 07 th June 2013 to 22 nd June 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 Simultaneous weather station deployment was carried out at TSSC shorebase at Gravir, Loch Odhairn (NB ). The weather station used was a Skye Instruments DataHog2 collecting records for wind speed and direction. The sensor was pole-mounted at a height above sea level of ~5m in an area unaffected by local topography and of similar exposure to the survey site. 3.3 Current meter configuration file Figure 2: RDCP600 configuration files - 4 -

5 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 windows screen. 3.4 Mooring system The RDCP was fixed in a gimbled, weighted sea bed frame. The head of the RDCP was 600mm from the seabed throughout the deployment. Internal parameters pitch, roll and heading were used 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 16/02/12. Boat GPS at the start of the survey to verify accuracy was used to return positional information (WGS84). This was cross-checked against a hand held Garmin GPS receiving minimum of 8 satellite coverage and later converted to OSGB36 using Recorded depths were corrected to CD using Neptune Tide v.7 ( tide software and referenced against tide predictions for the secondary port of Loch Shell. The bathymetric survey concluded that chart data was limited throughout the survey area although the information collected will prove suitable for modelling purposes. RDCP Figure 3: Bathymetric plot

6 Date Time Latitude Longitude Easting Northing OSGB32 Depth (m) 2/17/ :48: N W NB /17/ :49: N W NB /17/ :49: N W NB /17/ :49: N W NB /17/ :49: N W NB /17/ :49: N W NB /17/ :50: N W NB /17/ :50: N W NB /17/ :50: N W NB /17/ :50: N W NB /17/ :51: N W NB /17/ :51: N W NB /17/ :51: N W NB /17/ :51: N W NB /17/ :51: N W NB /17/ :52: N W NB /17/ :52: N W NB /17/ :52: N W NB /17/ :52: N W NB /17/ :53: N W NB /17/ :53: N W NB /17/ :53: N W NB /17/ :55: N W NB /17/ :56: N W NB /17/ :56: N W NB /17/ :56: N W NB /17/ :56: N W NB /17/ :56: N W NB /17/ :57: N W NB /17/ :57: N W NB /17/ :57: N W NB /17/ :57: N W NB /17/ :57: N W NB /17/ :58: N W NB /17/ :58: N W NB /17/ :58: N W NB /17/ :58: N W NB /17/ :58: N W NB /17/ :59: N W NB /17/ :59: N W NB /17/ :59: N W NB /17/ :59: N W NB /17/ :00: N W NB

7 2/17/ :00: N W NB /17/ :00: N W NB /17/ :00: N W NB /17/ :00: N W NB /17/ :01: N W NB /17/ :01: N W NB /17/ :04: N W NB /17/ :04: N W NB /17/ :04: N W NB /17/ :04: N W NB /17/ :04: N W NB /17/ :05: N W NB /17/ :05: N W NB /17/ :05: N W NB /17/ :05: N W NB /17/ :06: N W NB /17/ :06: N W NB /17/ :06: N W NB /17/ :06: N W NB /17/ :06: N W NB /17/ :07: N W NB /17/ :07: N W NB /17/ :07: N W NB /17/ :07: N W NB Data Processing 5.1 RDCP data Data was extracted from the RDCP and viewed using RDCP Studio and from there extracted as to Microsoft Notepad and later 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 37.9m, and minimum tidal height for the deployment was 36.4m. Cage depth was assumed to be 15m. As such near-bottom depth was taken at a range of 2-4m (cell 1) (allowing for a 1m blanking distance and 0.6m height of the mooring frame), cage-bottom at 22.9m (cell 21) and sub-surface at 31.4m (cell 30). All figures are distance from bed

8 Specified and deployed depths of data retrieval are compared below: Measured depth at 37.3m deployment Predicted tidal height at 2.1m deployment (from Admiralty tide tables) Corrected depth at 35.2m deployment (CD) Lowest predicted tide 1.2m during deployment Charted mean sea level 3.0m (range) Average tidal ht 2.8m Mean sea level height 37.9m above bed Assumed cage depth 15m RDCP mooring height 0.6m Mean RDCP recorded 37.9m pressure Height above bed - Bed cell - Mid cell - Surface cell Depth below MSL - Bed cell - Mid cell - Surface cell 3.0m 22.9m 31.4m 34.9m 15m 6.5m 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 1.18º W (from was applied and grid convergence (from HGdata_analysis_v7) was 4.2º W to give overall declination of 5.38º - 8 -

9 Date Lat Long NGR_X NGR_Y mag. dec grid conv. GMA 07/06/ W 4.2W 1.8W 78N.727W Notes: grid magnetic angle = magnetic declination grid convergence 5.3 Hydrographic survey results Pitch and roll exhibited some variation during the deployment from 0.21 to 0.79 and to respectively. These results are within acceptable limits for the instrument and indicate a stable deployment. An overview of data quality is displayed graphically as a plot showing signal strength throughout Column 1 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 becomes apparent. Data cells representative to bed, cage depth and sub surface were extracted from Column Summary statistics Bed Mid Surface No. of records Start date / time 07/06/13 12:00 07/06/13 12:00 07/06/13 12:00 End date / time 22/06/13 12:00 22/06/13 12:00 22/06/13 12:00 Mean velocity (m/s) Minimum velocity (m/s) Maximum velocity (m/s) Ranked % of mean velocity Ranked % of 3 cm/s Ranked % of 4.5 cm/s Ranked % of 9.5 cm/s Residual velocity (m/s) Residual direction G Major axis G Residual parallel (m/s) Residual normal (m/s) Amplitude parallel (m/s) Amplitude normal (m/s) Amplitude anistrophy

10 5.5 Meterological results The data from the weather station deployed at TSSC Gravir shorebase. The data 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 the weather station exhibits 24h rolling averages between 1.1. and 5.7m/s, with no wind speeds recorded in excess of 10m/s. Mean wind speed over the whole deployment was 2.4m/s. The deployment period therefore met the requirement with mean daily wind speed <10m/s. 6.0 Discussion Pressure and attitude records showed no evidence of problems with the deployment such as mooring dragging or excessive movement of the seabed mooring frame. The tidal height range and time series are consistent with predicted tidal heights. In general, the site was characterised by high tidal energy. All depths exhibited broadly similar current velocities although generally increasing with depth. However, vertical differentiation was minor. Current direction showed greater differentiation between different depths with the near bed major axis being directly opposed to the net depth and surface cells. Velocities were moderate, with a maximum recorded velocity of 0.240m/s, although the re-suspension threshold of 9.5cm/s was exceeded 24% of the time for the near bed data. The site is therefore highly resuspensive and likely to result in a high export of released solids in AutoDepomod modelling. Velocity and direction showed clear 12.5h periodicity at all depths and velocity showed clear spring neap variation. Observed depth data were shown to be generally greater than charted depths although limited detailed chart data was available for the surveys site. This is unsurprising given the limited amount of detailed surveying which has been carried out at remote locations such as being looked at here. Where limited detailed survey information exists, chart data is generally conservative with its representation of bathymetry. Chart bathymetry is considered suitable for use in modelling although due to the lack of detailed chart bathymetry, spot depths will be necessary to ground through the model output. Overall, the recorded data are indicative of a well flushed site, typical of an open loch location and is considered suitable for development in terms of hydrography and bathymetry. This dataset is considered to provide a good basis for AutoDepomod modelling

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