NewDEPOMOD MODELLING
Method Statement
Caerthann House
Grosvenor Cr escent
Connel
Argyll PA37 1 PQ
R e g i s t e r e d i n S c o t l a n d , N o : S C 1 7 5 0 8 7Tel: +44 (0)1631 720699 E-mail: [email protected]
TransTech Limitedwww.transtechltd. com
Prepared for
Scottish Environment Protection Agency
6, Parklands Avenue
Eurocentral
Holytown
North Lanarkshire
ML1 4WQ
NewDEPOMOD MODELLING: Method Statement p2/5
Document Details
Author:
PhD, BSc (hons)
Issue Date: 2 September 2019
Issue No: 2019v1
Contents
1. INTRODUCTION .................................................................................................................. 32. NewDEPOMOD .................................................................................................................... 33. MODEL SET-UP AND MODELLING METHODOLOGY ....................................................... 3
3.1 Bathymetry .................................................................................................................. 33.2 Pen group.................................................................................................................... 33.3 Current meter data ...................................................................................................... 43.4 Calibration of modelling predictions against good quality seabed data ........................ 4
4. MATLAB ............................................................................................................................... 45. QUALITY ASSURANCE ....................................................................................................... 5REFERENCES ............................................................................................................................ 5
List of Abbreviations
CMD CommandGIS Geographic Information SystemGPS Global Positioning SystemOSGB 1936 Ordnance Survey Great Britain 1936SAMS Scottish Association for Marine ScienceSEPA Scottish Environment Protection Agency
NewDEPOMOD MODELLING: Method Statement p3/5
1. INTRODUCTION
This statement provides a general overview of the methods TransTech will use forNewDEPOMOD modelling of marine finfish pen sites.
This method statement is a general overview and given that the modelling is site-specific, fulldetails of the methodology employed at each site will be provided to the Scottish EnvironmentProtection Agency (SEPA) in the report(s) submitted for approval.
2. NewDEPOMOD
NewDEPOMOD is a computer model developed by the Scottish Association for Marine ScienceSAMS) in collaboration with the aquaculture industry and SEPA. The model predicts the impact
of marine pen site discharges on the seabed in order to optimise the operation of aquaculturesites to match environmental capacity.
NewDEPOMOD and its predecessors ( DEPOMOD and AutoDEPOMOD) are particle trackingmodels. AutoDEPOMOD has been used for the regulatory monitoring of Scottish aquaculturesites since 2006 and NewDEPOMOD provides a range of improvements on the predictions itcan make in terms of particle deposition, erosion, resuspension and transport.
The model produces conservative estimates of the holding capacity of a site that can be tunedusing benthic survey monitoring data if an existing site is to be modified. For new site locationsthe predictions can be improved once the site is operational and benthic survey data is collected.
Therefore, depending on the nature of the modelling being undertaken, tuning and calibrationexercises can determine:
the most suitable portrayal of the physical dispersion processes in the model; the most suitable representation of deposition for assessing risks; or both.
3. MODEL SET- UP AND MODELLING METHODOLOGY
The construction of the model and the modelling methodology undertaken by TransTech willfollow the guidance given in Aquaculture Modelling: Regulatory Modelling Guidance For TheAquaculture Sector(1) and subsequent updates.
3.1 Bathymetry
If an existing site is to be modelled, depths will be collected at the existing pen group cornersand if the site is to be modified at the corners of the proposed location of the corner pens. Atnew sites a bathymetric survey will be carried out at the proposed pen group location. For themodelling project, uniform bathymetry, based on the average of the group corners, will begenerated over the chosen model domain which is likely to be 2 km2 in most cases.
The methodology for the creation of the model’s sitename.depomodbathymetryproperties file willfollow that described in the guidelines(1). This will include formatting it as a regular grid of 79 x 79elements georeferenced to OSGB 1936 datum.
3.2 Pen group
For existing sites, the pen group will be set-up from GPS coordinates taken at the grid buoys. The site will then be mapped out in GIS software to determine pen centre positions and the pen
NewDEPOMOD MODELLING: Method Statement p4/5
group’s angle of orientation. For new sites the pen centre positions and the group’s orientationwill be determined using GIS.
For both methods the coordinates of the existing or proposed pen positions will be checkedagainst those within the sitename-cages.depomodcagesxml file to ensure they are correct.
3.3 Current meter data
Current meter data (90 days) will be processed using SEPA’s HG-analysis tool. The flowmetryfile (sitename.depomodflowmetryproperties) required by NewDEPOMOD will be generated asper current guidance.
It is noted that there are some issues with the use of flow data obtained from a single location. This data may not necessarily be representative of the wider model domain or even the flowcharacteristics at other times than that recorded. This can lead to a positional offset of thepredicted impact relative to the location of the farm’s pens. However, first and foremost themethod has been developed to predict magnitudes of deposition and overall scales of impactwith positional accuracy a secondary concern. As such, TransTech intends to use good qualitydata so that seabed impacts within the mixing zone will tend to be centred predominantly underthe farm pens, even in cases with highly dynamic and complex flow.
It is appreciated that at some sites the flow datasets may have significant residual currents, particularly at the bed. When this occurs, the model can under-estimate the levels of deposition. Thus, for sites with appreciably large residual speeds ( i.e. those 35-40% of the mean flowspeed) this risk will be mitigated as per the recommendations as laid out in SEPA’s currentguidance(1) and future updates to this.
Given that the flow data has a high degree of influence on modelling predictions, extremes in thedata ( e.g. storms or other sporadic events or, at highly tidal locations, significant differencesbetween the neap and spring tides or flood and ebb tides) will be considered by TransTech. Should the flow data display these characteristics the model will be run to output spatial impactresults at a number of specified time points and the results will be contextualised, aggregated orsummarised in an appropriate manner.
3.4 Calibration of modelling predictions against good quality seabed data
Where good quality seabed data is available, the model will be be calibrated and validated with aview to providing more accurate, site-specific predictions. As such, for sites for which this dataexists, we intend to adjust parameters within the model in order to generate a footprint thatmatches as closely as possible to the observational ( benthic survey) data.
For this approach TransTech will typically carry out several types of simulation. As per thecurrent guidance(1), in the first instance simulations will be made in order to identify the optimummodelling methods and configuration and to measure their accuracy based on comparisons withthe benthic survey data.
For new sites SEPA’s default procedures will be followed.
4. MATLAB
It is TransTech’s intention to run NewDEPOMOD predominantly in CMD mode using MathWorksMATLAB.
SEPA scripts along with those provided from other sources and those compiled by TransTechwill be used to process the model’s output.
NewDEPOMOD MODELLING: Method Statement p5/5
5. QUALITY ASSURANCE
All input data, modelling, processing of results and their reporting will undergo a qualityassurance review which will follow established TransTech Ltd procedures.
REFERENCES
1) Aquaculture Modelling: Regulatory Modelling Guidance For The Aquaculture Sector. Scottish Environment Protection Agency. June 2019.
