scottish sea farms ltd horse island hydrographic report

SCOTTISH SEA FARMS LTD

HORSE ISLAND

HYDROGRAPHIC REPORT

Report To: Scottish Environment Protection Agency

Report No: H0817- 01

Our Ref: RS

Status: V1

Date: 08 August 2017

Scottish Sea Farms Ltd

South Shian

Connel

PA37 1SB

Tel:

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1. Introduction

A hydrographic survey was carried out by Scottish Sea Farms on the site of an existing shellfish lease located

on the east side of Horse Island, Summer Isles. The survey location is approximately 5.5 km southeast of the

existing SSF fish farm sites of Tanera and Fada. This data has been collected with a view to creating a new

marine cage fish farm on the site of the shellfish lease.

Current speed and direction, depth and meteorological data were collected as part of this survey for use

with SEPA’ s consenting model AutoDEPOMOD to assess this site’ s suitability for development.

2. Site Location

The survey location is at the southeastern end of Horse Island in Horse Sound. The site is sheltered from

west to north with the greatest exposure to the wind from southerly directions. Admiralty charts indicate

that the bathymetry follows the general contour of the island with areas of deeper water to the east.

Figure 1: Locational map showing Horse Island Hydrographic Survey location

3. Methods

A 600 kHz RDI Workhorse ADCP was deployed on 03/ 08/ 2012 and recovered on 09/ 09/ 2012, all of which is

presented in this report. A 15 day subset will be used for AutoDepomod modelling purposes ( 17/ 08/ 2012 –

01/ 09/ 2012). Concurrent weather data was obtained from the station at Aultbea along with a bathymetric

survey of the site.

3.1 Current data

The ADCP was deployed at 57 58.884 N 05 20.281 W (202733, 904007). The meter was mounted in a

fiberglass Seaspider frame and deployed in a u-shaped mooring with ground rope running to weights

attached to surface buoys.

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Current flow and direction were recorded at throughout the water column using 0.75 m bins with an

averaging period of 20 minutes.

Deployment configuration:

Broadband 614.4 kHz

Pings/ Ens 350

Time/ Ping 00:03.42

First Ensemble Date 03/ 08/ 2012

First Ensemble Time 10:36:39:66

Ensemble Interval (s) 1200

1st Bin Range ( m) 1.87

Bin Size ( m) 0.75

Table 1: ADCP configuration for Horse Island Hydrographic Survey

3.2 Bathymetric data

Spot depths were obtained at the survey site using a Hawkeye Digital Sonar hand-held depth sounder. All

depths were later converted to chart datum using predicted tidal heights for Tanera Mor obtained from

Admiralty Total Tide.

3.3 GPS

Position fixes were obtained using a Garmin GPSmap 76S in WGS84. The GPS position accuracy was

compared against a known location and checked for consistency at the end of the survey.

3.4 Weather

Weather data for the survey period was obtained from the station at Aultbea 21 km to the south west.

3. Data processing

3.1 Processing

All bathymetric data collected was corrected to chart datum using tidal heights taken for Admiralty TotalTide

from the Tanera Mor station.

Three depth cells were selected as outlined in Appendix 8 of SEPA’ s Fish Farm manual.

Bins 1, 28 and 38 were selected to represent seabed, cage bottom and sub-surface conditions. Several

ensembles were removed at both the start and end of the recorded to remove data recorded during

deployment and retrieval. Each bin was then checked for missing or erroneous data then analysed using the

SEPA tool HGdata_ analysis_ v7. The summaries for each bin can be found in section 4.3.

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3.2 QC

The RDI Workhorse carries out a series of internal checks on each ping as a measure of quality control. If any

of the returning pings fail the QC criteria ( correlation, echo intensity and error velocity) they are rejected and

not included in the averaged ensemble. Rejected pings have an impact on the expected standard deviation

of the dataset however this can be calculated by using the percentage of good pings.

Depth and Current

Pressure records indicate that the mean depth during the deployment including the height of the frame was

37.7 m, removing the influence of tide gives a depth of 34.3 mCD. This corresponds well with the recorded

depth of 37.3 m (34.4 mCD).

The recorded pressure was also compared with the predicted tidal heights taken from the station at Tanera

Mor (TotalTide). Both range and low water and high water are consistent with those predicted.

Current direction is consistent with local bathymetry and the speeds recorded are within expected range and

are consistent with other sites in the Summer Isles region.

The residual currents at the subsurface, mid and bed bins are 43%, 21% and 19% of the mean current speed

respectively, while the direction appears consistent with that of a weak residual current.

Meter position

Pitch and roll data are somewhat unsettled during the deployment ( figure 2) however heading remains fairly

consistent with no large spikes to indicate a significant disturbance of the meter. These fluctuations remain

within an overall pitch or roll of +/- 5° well within the machine tolerance of 15°. 15 day datasets will be

selected to minimize these fluctuations where possible.

Meter position was recorded during deployment and recovery showing that the position of the meter had

not changed during the survey.

Figure 2: Time series showing pitch, roll, pressure record and heading vs days deployed at Horse Island.

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4. Results

4.1 Bathymetric data

Figure 3: Admiralty chart extract showing location of recorded depths

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STATION ACTUAL WGS84 SOUNDING

DEPTH ( M)

TIDAL HEIGHT

M)

DEPTH

MCD)

A1 57° 59.054' 5° 20.373' 7.5 3.3 4.2

A2 57° 59.001' 5° 20.412' 16.3 3.318 12.982

A3 57° 58.951' 5° 20.450' 19.2 3.336 15.864

A4 57° 58.903' 5° 20.517' 18.2 3.354 14.846

A5 57° 58.856' 5° 20.570' 18.7 3.372 15.328

A6 57° 58.854' 5° 20.571' 29 3.39 25.61

B1 57° 58.805' 5° 20.594' 35.6 3.408 32.192

B2 57° 58.787' 5° 20.516' 28.9 3.426 25.474

B3 57° 58.846' 5° 20.499' 24.2 3.444 20.756

B4 57° 58.942' 5° 20.415' 24.5 3.462 21.038

B5 57° 58.989' 5° 20.355' 26.8 3.48 23.32

B6 57° 59.034' 5° 20.301' 27.9 3.498 24.402

B7 57° 59.077' 5° 20.255' 22.4 3.516 18.884

B8 57° 59.129' 5° 20.203' 28.2 3.534 24.666

C1 57° 59.177' 5° 20.110' 53.5 3.552 49.948

C2 57° 59.128' 5° 20.150' 33.2 3.588 29.612

C3 57° 59.076' 5° 20.190' 35 3.606 31.394

C4 57° 59.026' 5° 20.230' 43 3.624 39.376

C5 57° 58.976' 5° 20.279' 38.8 3.642 35.158

C6 57° 58.932' 5° 20.329' 32.7 3.66 29.04

C7 57° 58.884' 5° 20.377' 29.1 3.678 25.422

C8 57° 58.838' 5° 20.441' 34.1 3.696 30.404

C9 57° 58.786' 5° 20.480' 36.4 3.714 32.686

C10 57° 58.741' 5° 20.498' 30.5 3.57 26.93

D1 57° 58.760' 5° 20.406' 28.6 3.732 24.868

D2 57° 58.813' 5° 20.356' 37.3 3.75 33.55

D3 57° 58.863' 5° 20.335' 35 3.768 31.232

D4 57° 58.917' 5° 20.292' 36.4 3.786 32.614

D5 57° 58.963' 5° 20.247' 44.1 3.804 40.296

D6 57° 58.010' 5° 20.195' 50.8 3.822 46.978

D7 57° 59.042' 5° 20.136' 54.6 3.84 50.76

E1 57° 59.085' 5° 20.015' 47.1 3.858 43.242

E2 57° 59.031' 5° 20.058' 58.2 3.876 54.324

E3 57° 58.997' 5° 20.141' 56 3.894 52.106

E4 57° 58.948' 5° 20.200' 50.7 3.912 46.788

E5 57° 58.903' 5° 20.248' 41.2 3.93 37.27

E6 57° 58.854' 5° 20.311' 37.6 3.948 33.652

E7 57° 58.795' 5° 20.360' 35.6 3.966 31.634

E8 57° 58.744' 5° 20.335' 26.7 3.984 22.716

F1 57° 58.729' 5° 20.253' 34.5 4.002 30.498

F2 57° 58.783' 5° 20.226' 34.1 4.02 30.08

F3 57° 58.835' 5° 20.191' 37.1 4.038 33.062

F4 57° 58.881' 5° 20.138' 52 4.056 47.944

F5 57° 58.927' 5° 20.087' 58 4.074 53.926

Table 2: Depths recorded during Horse Island Hydrographic Survey

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4.2 Meteorological data

Hourly wind speed and direction were obtained from the weather station at Aultbea approximately 21 km

south west of the survey location. As can be seen from the figures below at no point did the wind exceed 10

m/ s for a period of three days. The predominant wind direction during this period was westerly.

Figure 4: Wind rose of wind speed and direction recorded at Aultbea during survey period.

Figure 5: Mean wind speed recorded at Aultbea Weather Station 01/08/ 2012 – 10/ 09/2012

0

20

40

60

80

100N

NNE

NE

ENE

E

ESE

SE

SSE

S

SSW

SW

WSW

W

WNW

NW

NNW

Aultbea Wind Rose

0 m/ s

1 m/ s

2 m/ s

3 m/ s

4 m/ s

5 m/ s

5 m/ s

0

2

4

6

8

10

12

14

16

01/ 08/ 2012 00: 00 11/ 08/ 2012 00: 00 21/ 08/ 2012 00: 00 31/ 08/ 2012 00: 00 10/ 09/ 2012 00:00WindSpeed (m/s)

Date and Time Aultbea Wind Speed 01/ 08/ 2012 -

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4.3 Current data

Bed Cage bottom Subsurface

Bin number 1 28 38

Bin depth ( m) 2.7 22.9 30.4

Mean velocity (m/ s) 0.021 0.039 0.046

Min velocity (m/ s) 0.000 0.000 0.001

Max velocity (m/ s) 0.134 0.197 0.204

Ranked percentage of mean velocity 60 59 58

Ranked percentage of 3 cm/ s 82 46 35

Ranked percentage of 9.5 cm/ s 94 96 93

Residual velocity (m/ s) 0.004 0.009 0.019

Residual direction (°G) 18.4 14 3.6

Major axis 25 35 25

Residual parallel (m/ s) 0.004 0.008 0.017

Residual normal (m/ s) 0.000 - 0.003 - 0.007

Amplitude parallel (m/ s) 0.031 0.058 0.064

Amplitude normal (m/ s) 0.018 0.031 0.035

Longitudinal amplitude anisotropy 8.66 2.57 2.56

Latitudinal amplitude anisotropy 1.71 1.88 1.81

Table 3: Current data statistics of each of the 3 bins

The current speeds and directions recorded at each of the 3 bins are in general agreement with each other

with current speed decreasing with depth as would be would be expected. Current speeds are relatively low

and typical other sites in the region. Maximum current speed recorded during the deployment was 0.204

ms¯¹. Currents at all depths were weakly directional and changes in current speed correspond with the

flooding and ebbing of the tide as indicated by the pressure record although there is some evidence of wind

driven events corresponding to periods of peak wind speed. Summaries from the SEPA tool HG_ Analysis for

the whole survey period are displayed in the figures below:

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Figure 6: HG_Analysis summary page Horse Island 2012 – subsurface

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Figure 7: HG_Analysis summary page Horse Island 2012 – cage bottom

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Figure 6: HG_ Analysis summary page Horse Island 2012 – bed

scottish sea farms ltd horse island hydrographic report

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