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
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 5 3.4 Mooring system 5 4 Bathymetry and site survey 7 5 Data processing 7 5.1 RDCP data 8 5.2 Magnetic heading corrections 8 5.3 Hydrographic survey results 8 5.4 Summary statistics 9 5.5 Meterological results 9 6 Discussion 10 Appendices 11-15 2
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. 25 40m. - 3 -
3 Survey programme 3.1 Current meter set up The deployment position was (WGS84) 58º02.678 N 006 22.727 W (NB 41698 14553). The survey period comprised a half-lunar 15-day period; from 07 th June 2013 to 22 nd June 2013. 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 41698 14553). 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 -
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 www.nearby.org.uk. Recorded depths were corrected to CD using Neptune Tide v.7 (www.neptunenavigation.co.uk) 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. - 5 -
Date Time Latitude Longitude Easting Northing OSGB32 Depth (m) 2/17/2012 11:48:46 58 2.6629 N 6 22.1178 W 142299 914475 NB4229914475 42.8 2/17/2012 11:49:01 58 2.6691 N 6 22.1542 W 142264 914489 NB4226414489 42.7 2/17/2012 11:49:15 58 2.6732 N 6 22.1787 W 142240 914498 NB4224014498 42.8 2/17/2012 11:49:30 58 2.676 N 6 22.2073 W 142212 914505 NB4221314505 42.8 2/17/2012 11:49:45 58 2.6744 N 6 22.2608 W 142160 914506 NB4216014506 42.8 2/17/2012 11:49:59 58 2.6711 N 6 22.3163 W 142105 914503 NB4210514503 42.5 2/17/2012 11:50:14 58 2.6678 N 6 22.3777 W 142044 914501 NB4204414501 42 2/17/2012 11:50:29 58 2.6637 N 6 22.443 W 141979 914497 NB4198014497 41.7 2/17/2012 11:50:43 58 2.6605 N 6 22.5005 W 141922 914495 NB4192314495 41.3 2/17/2012 11:50:55 58 2.6576 N 6 22.5492 W 141874 914493 NB4187514493 41.1 2/17/2012 11:51:07 58 2.6596 N 6 22.5968 W 141828 914500 NB4182814500 40.8 2/17/2012 11:51:19 58 2.6801 N 6 22.6142 W 141813 914539 NB4181314539 39.2 2/17/2012 11:51:31 58 2.6998 N 6 22.5932 W 141836 914574 NB4183614574 39.2 2/17/2012 11:51:43 58 2.7072 N 6 22.5516 W 141878 914585 NB4187814585 39.4 2/17/2012 11:51:55 58 2.7105 N 6 22.5053 W 141924 914588 NB4192414588 39.7 2/17/2012 11:52:07 58 2.7121 N 6 22.4562 W 141972 914588 NB4197314588 40.3 2/17/2012 11:52:19 58 2.7133 N 6 22.4069 W 142021 914587 NB4202114587 40.8 2/17/2012 11:52:31 58 2.7158 N 6 22.3575 W 142070 914588 NB4207014588 41.4 2/17/2012 11:52:46 58 2.7186 N 6 22.2957 W 142131 914590 NB4213114590 42.5 2/17/2012 11:53:01 58 2.724 N 6 22.2338 W 142192 914596 NB4219314596 42.8 2/17/2012 11:53:15 58 2.7285 N 6 22.1766 W 142249 914600 NB4224914600 42.8 2/17/2012 11:53:27 58 2.7318 N 6 22.1272 W 142298 914603 NB4229814603 42.5 2/17/2012 11:55:48 58 2.7887 N 6 22.1775 W 142255 914712 NB4225514712 42 2/17/2012 11:56:01 58 2.783 N 6 22.2223 W 142211 914704 NB4221114704 42.5 2/17/2012 11:56:14 58 2.7768 N 6 22.2712 W 142162 914696 NB4216214696 42.5 2/17/2012 11:56:26 58 2.7719 N 6 22.3198 W 142113 914690 NB4211414690 41.9 2/17/2012 11:56:39 58 2.7678 N 6 22.3653 W 142068 914685 NB4206814685 41 2/17/2012 11:56:52 58 2.7641 N 6 22.4149 W 142019 914682 NB4202014682 39.7 2/17/2012 11:57:04 58 2.7613 N 6 22.4602 W 141974 914679 NB4197414679 38.9 2/17/2012 11:57:17 58 2.7563 N 6 22.5128 W 141922 914673 NB4192214673 37.8 2/17/2012 11:57:30 58 2.7531 N 6 22.5622 W 141873 914671 NB4187314671 36.7 2/17/2012 11:57:42 58 2.751 N 6 22.6081 W 141828 914670 NB4182814670 36.1 2/17/2012 11:57:55 58 2.7387 N 6 22.6513 W 141784 914650 NB4178414650 36.4 2/17/2012 11:58:07 58 2.7191 N 6 22.678 W 141755 914615 NB4175514615 37.2 2/17/2012 11:58:20 58 2.6941 N 6 22.6959 W 141734 914570 NB4173514570 37.5 2/17/2012 11:58:33 58 2.667 N 6 22.7 W 141727 914520 NB4172714520 38.1 2/17/2012 11:58:45 58 2.6424 N 6 22.6918 W 141732 914474 NB4173314474 39.2 2/17/2012 11:58:58 58 2.6178 N 6 22.6689 W 141752 914427 NB4175214427 39.4 2/17/2012 11:59:11 58 2.5998 N 6 22.6301 W 141788 914391 NB4178814391 35 2/17/2012 11:59:25 58 2.5912 N 6 22.5755 W 141840 914371 NB4184114371 32.6 2/17/2012 11:59:36 58 2.5928 N 6 22.5307 W 141885 914372 NB4188514372 33.9 2/17/2012 11:59:49 58 2.5953 N 6 22.4777 W 141937 914373 NB4193714373 35.6 2/17/2012 12:00:01 58 2.5978 N 6 22.4288 W 141985 914374 NB4198514374 37.2-6 -
2/17/2012 12:00:15 58 2.6019 N 6 22.372 W 142042 914378 NB4204214378 38.6 2/17/2012 12:00:26 58 2.6047 N 6 22.3268 W 142086 914381 NB4208614381 38.9 2/17/2012 12:00:40 58 2.6088 N 6 22.27 W 142143 914385 NB4214314385 40 2/17/2012 12:00:52 58 2.6117 N 6 22.2215 W 142191 914387 NB4219114387 41.1 2/17/2012 12:01:05 58 2.617 N 6 22.169 W 142243 914393 NB4224414393 41.9 2/17/2012 12:01:18 58 2.6228 N 6 22.1166 W 142295 914401 NB4229614401 42.8 2/17/2012 12:04:00 58 2.6994 N 6 22.1712 W 142251 914546 NB4225114546 42.8 2/17/2012 12:04:13 58 2.7014 N 6 22.2226 W 142200 914553 NB4220114553 42.8 2/17/2012 12:04:27 58 2.701 N 6 22.2779 W 142146 914556 NB4214614556 42.8 2/17/2012 12:04:39 58 2.6994 N 6 22.3253 W 142099 914556 NB4210014556 42.1 2/17/2012 12:04:51 58 2.6965 N 6 22.3725 W 142053 914554 NB4205314554 41.9 2/17/2012 12:05:06 58 2.6924 N 6 22.4318 W 141994 914550 NB4199414550 41.4 2/17/2012 12:05:20 58 2.6883 N 6 22.4865 W 141940 914546 NB4194014546 40.6 2/17/2012 12:05:35 58 2.6818 N 6 22.545 W 141881 914537 NB4188214537 40.4 2/17/2012 12:05:50 58 2.6748 N 6 22.6035 W 141823 914528 NB4182314528 40 2/17/2012 12:06:05 58 2.6678 N 6 22.6617 W 141765 914519 NB4176514519 39 2/17/2012 12:06:17 58 2.6617 N 6 22.7101 W 141717 914511 NB4171714511 38 2/17/2012 12:06:30 58 2.6518 N 6 22.7583 W 141668 914495 NB4166814495 37.4 2/17/2012 12:06:43 58 2.6412 N 6 22.8155 W 141611 914479 NB4161114479 36.7 2/17/2012 12:06:58 58 2.6301 N 6 22.8775 W 141548 914463 NB4154914463 35.3 2/17/2012 12:07:13 58 2.6195 N 6 22.9394 W 141486 914447 NB4148614447 33.5 2/17/2012 12:07:28 58 2.6125 N 6 23.0033 W 141423 914438 NB4142314438 32.2 2/17/2012 12:07:43 58 2.6113 N 6 23.0683 W 141359 914440 NB4135914440 30.8 2/17/2012 12:07:56 58 2.6146 N 6 23.1241 W 141304 914450 NB4130514450 30 5 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. - 7 -
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 http://www.geomag.bgs.ac.uk/data_service/charts_navig/gma_calc.html) was applied and grid convergence (from HGdata_analysis_v7) was 4.2º W to give overall declination of 5.38º - 8 -
Date Lat Long NGR_X NGR_Y mag. dec grid conv. GMA 07/06/2013 58 02.6 006 22 141698 914698 5.38W 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 -0.95 to -0.24 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 1 5.4 Summary statistics Bed Mid Surface No. of records 1081 1081 1081 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) 0.069 0.050 0.059 Minimum velocity (m/s) 0.001 0.003 0.002 Maximum velocity (m/s) 0.221 0.219 0.240 Ranked % of mean 58 58 56 velocity Ranked % of 3 cm/s 18 29 23 Ranked % of 4.5 cm/s 36 53 40 Ranked % of 9.5 cm/s 76 91 85 Residual velocity (m/s) 0.031 0.018 0.015 Residual direction G 120 287 237 Major axis G 95 300 285 Residual parallel (m/s) 0.028 0.018 0.010 Residual normal (m/s) 0.013-0.004-0.011 Amplitude parallel (m/s) 0.099 0.068 0.084 Amplitude normal (m/s) 0.036 0.040 0.044 Amplitude anistrophy 2.74 1.70 1.93-9 -
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. - 10 -
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