Presentation title

Natasha Rowlands, Dr Mark Rehfisch & Dr Stephanie McGovern

with the support of Bronagh Byrne and Emma Heywood

18th March 2015

A Plea for Survey Precision To Grid or Transect Post-Consent?

Offshore wind post-consent monitoring seminar

Introduction to London Array Offshore Windfarm Surveys

• London Array Ltd was one of the first organisations to use grid-based survey

designs.

• Currently finalising data for the second post-construction survey period (2014 / 15).

Year Survey

2009 / 2010 Pre-construction

2010 / 2011 Pre-construction

2011 / 2012 Construction

2012 / 2013 Construction

2013 / 2014 Post-construction

2014 / 2015 Post-construction (ongoing)

• Winter surveys of the London Array wind farm development have been undertaken

by APEM using a grid-based design since 2009.

• The main species of concern is red-throated diver, a species known to be highly

sensitive to disturbance (King et al. 2009 etc).

• Grid scale was selected to achieve a target precision or CV of 0.16 for this key

species. A CV of 0.16 makes it possible to detect a halving or doubling in numbers

between surveys.

Introduction to London Array Offshore Windfarm Surveys

Why use grid-based survey design?

• APEM’s grid-based survey methodology is based on first principles:

- Aim to collect sufficient independent estimates of seabird density to achieve a

pre-defined level of confidence (London Array target precision 0.16).

• Fundamental approach is that the sampling unit is at the image level.

Survey accuracy and precision

• Without suitable confidence in the estimates it is not possible to assess if

construction has had a significant impact on the population

• High confidence = small confidence intervals

• Low confidence = large confidence intervals

Low confidence – overlapping

confidence intervals

High confidence – non-overlapping

confidence intervals

Survey accuracy

• Grids can present distinct advantages when surveying species with clumped

distributions, such as wintering red-throated diver aggregating in the Thames

Estuary.

• The following simulations are for demonstration purposes only; the birds are not

drawn to scale; 11% average coverage targeted for both survey approaches.

Population = 282 Grid estimate = 217 (11.1% coverage) Transect estimate = 121 (14% coverage)

Population = 282 Grid estimate = 426 (9.9% coverage) Transect estimate = 657 (14% coverage).

Population = 282 Grid estimate = 295 (9.9% coverage) Transect estimate = 658 (14% coverage).

Population = 282 Grid estimate = 337 Transect estimate = 0

Population = 282 Grid estimate = 337 (11.3% coverage) Transect estimate = 0 (7.2% coverage).

Survey accuracy

Estimate 1 2 3 4 Average

n-1

Population Error

Grid 217 426 295 337 318.8

43.5

282 -36.8

Transect 121 657 658 0 348.2

174.1

282 -77

• The above table is based on a few simulations. Had very many

simulations been carried out for both sampling strategies the average

estimated population size (column 6) would have been close to the real

population size (column 7), but the confidence intervals for the grid-based

approach would have been smaller. The smaller the confidence the more

reliable the population estimate.

• Transect methods provide less reliable population estimates with real

world sampling effort.

Survey accuracy – Markov Chain Monte Carlo Simulation

Grids achieved the target precision and tighter confidence

intervals at a much lower percentage coverage than transects.

A simulation based on Carmarthen Bay SPA common scoters, a difficult to survey

clumped species

Dashed grey lines show where the grid method achieved the same confidence intervals as the transect method with 10.5% coverage

Grid Coverage

• Often, the exact area to be developed for a wind farm is not finalised until surveying

has commenced or been completed.

• If coarse scale transects are used to achieve the desired coverage across the study

area, transects may not be ideally placed if the wind farm boundaries move.

• Surveying with a grid ensures even coverage even if boundaries move.

Wind farm coverage Grid = 11.10% Transect = 17.47 %

Wider area coverage Grid = 10.16% Transect = 14%

Wind farm coverage Grid = 11.10% Transect = 0 %

Wider area coverage Grid = 10.16% Transect = 14%

Habitat coverage

• Without knowledge of the habitat (yellow

bar) prior to surveying, areas of interest

may be over- or under-sampled by a

transect leading to skewed estimates.

Such as transient sandbars in the Outer

Thames Estuary.

• Oblique image taken

during survey of

transient sandbank,

often utilised by seals

as haul out sites.

Habitat coverage

• With the regular sampling of a grid

spatial coverage of habitats (yellow bar)

should be sampled in proportion to its

presence even with no prior knowledge

of the area

Assessment of displacement

• Transect-based options may only provide 1 or

2 transects on each side of the buffer area.

This makes it difficult to estimate any bird

displacement due to the wind farm in one

direction as displacement will only be

measured at 1 or 2 distances to the nearest

turbine.

• As data on a grid are collected at a finer spatial

scale, samples are collected at multiple

distances from the turbines, allowing a more

detailed assessment of displacement to be

undertaken.

Assessment of diver displacement at London Array

• The use of a grid-based survey design allowed for the assessment of the density

profile of divers during pre-, during-, and post-construction within the London Array

Offshore Windfarm and surrounding buffer (Zone 1).

• Diver density has changed within Zone 1 over the last 5 years of winter survey.

• Although the period of construction appears to have led to a change in diver

distribution, there is no clear evidence that this change in distribution continued

post-construction.

Assessment of diver displacement at London Array

New data subject to review

• Results of analysis carried out following the first year of post-construction

monitoring for red-throated diver are as follows:

• The repeated measures ANOVA showed a significant effect of construction

(p<0.05, F=30.94).

• This suggested a significant decrease between pre- and during-construction diver

density (p<0.05) and a significant increase between during- and post-construction

diver density (p<0.05).

• There was no significant difference between pre- and post-construction

diver density (p>0.05).

• Surveying using a grid design provided accurate population estimates and

sufficient statistical power to successfully detect any possible change in numbers.

Assessing the effect of construction

Windfarm

development stage

Pre-construction

Construction p < 0.05 Significant difference

Post-construction p > 0.05 No significant difference

New data subject to review

Summary

• The London Array dataset provides an important longitudinal dataset for

the assessment of bird populations post-construction.

• Grid sampling has provided accurate population estimates (smaller

confidence intervals) of red-throated divers and sufficient statistical

power to detect change in estimates and distributions during the

construction stages of London Array.

• Habitats have been sampled in proportion to presence producing more

reliable population estimates.

• Grid-sampling has allowed more detailed assessment of displacement of

red-throated divers during-construction.

• Future proofing – surveying with a grid ensures even coverage if wind

farm boundaries move, allowing more reliable comparison post-

construction.

Why are girds important for post-consent monitoring?

Thank you for listening – any questions?

Special thanks to London Array Limited and DONG Energy