# Scottish Marine and Freshwater Science Volume 5 Number 16:The Avoidance Rates of Collision Between Birds and Offshore Turbines

This study reviewed data that have been collected from offshore windfarms and considers how they can be used to derive appropriate avoidance rates for use in the offshore environment.

### Annex 1

**USING A COLLISION RISK MODEL TO ASSESS BIRD COLLISION RISKS
FOR OFFSHORE WINDFARMS**

**(
SOSS
Guidance: March 2012)**

**SUPPLEMENT - AVOIDANCE RATES USING THE BASIC AND EXTENDED
MODELS**

**March 2014 - Bill Band**

This is a supplement to guidance prepared for the Crown Estate
as part of the Strategic Ornithological Support Services programme,
project
SOSS-02
^{[45]}
. That provides guidance for offshore wind developers, and
their ecological consultants, on using a collision risk model to
assess the bird collision risks presented by offshore windfarms.
The March 2012 version of the guidance enabled use to be made of
flight height distribution data.

This supplement is an addition to Stage E - Avoidance and Attraction. That section describes how, having used the collision model to calculate the potential collision rate if birds take no avoiding action, one should then apply an avoidance rate A to allow for the fact that many species of birds do in fact take avoiding action, either at long range (macro) or at close range (micro).

Paragraph 80 notes that

*'if the extended model taking account of flight height
distribution is used, it is important that the calculations on
which avoidance rates are based also start with a no-avoidance
collision rate derived using the extended model'.*

Most of the published literature on avoidance rates is currently
based on using the basic model.
**This supplement shows how such avoidance rates may be
modified to enable their application to the extended
model.**

**Avoidance in the basic and extended models**

The two models - basic and extended - yield different predictions of the rate of collisions before avoidance is taken into account. The extended model is a more refined model which takes into account the effect of flight height distribution. It takes into account the fact that, for a given number of flights at risk height, a flight height distribution skewed towards low altitude leads to a smaller proportion of birds passing through the rotor, and bird passages through parts of the rotor with less risk, than if the distribution were uniform.

The outputs of the two models may be formally compared if the
data input to the basic model on the proportion of flights at risk
height (Q'
_{2R}) is derived from the same flight height distribution
used in the extended model, as in Option 2 of the spreadsheet
accompanying the
SOSS
guidance. That is, the comparison should be made between the
collision rate using the basic model (Option 2) in the spreadsheet,
and the extended model (Option 3).

The collision rates (before avoidance) projected by the two models are:

Basic model (Option 2):

C
_{basic} = v(D
_{A}/2R)(TπR
^{2})t x Q
_{2R}
^{'} x p
_{average} x Q
_{op} (guidance eq.5
^{[46]}
)

i.e. flux factor x Q
_{2R}
^{'} x p
_{average} x Q
_{op}

Extended model (Option 3):

C
_{extended} = v(D
_{A}/2R)(TπR
^{2})t x (2/π) ∫∫ d(y) p(x,y) dxdy x Q
_{op} (guidance eq. 9)

i.e. flux factor x collision integral x
Q
_{op}

Where the bird flight height distribution is skewed towards low
altitude, the extended model prediction C
_{extended} is usually less than C
_{basic}, because this equation takes full account of the
reduction in risk at lower parts of the rotor. Let g be the ratio C
_{extended} / C
_{basic} , g is thus usually less than 1. The value of g
may be obtained by dividing the second of the above equations by
the first:

**g = C
_{extended} / C
_{basic} = collision integral / ( Q'
_{2R} x p
_{average} )** …. eq. S1

and this is readily calculated from the 'Overall collision risk' spreadsheet

g = cell D35 / ( cell D33 x cell D27 )

The expected collision rate must then take into account the proportion A of birds avoiding the turbines ( e.g. by displacement, or by evasive action), by multiplying the above no-avoidance collision rates by the proportion (1-A) which do not avoid. Values of A are typically in the range 90-100%. It is more helpful to think in terms of the non-avoidance rate A' = 1 - A , such that A' is the small proportion of birds which do not avoid the turbines. The expected collision rate is then

A'
_{basic} C
_{basic} in the basic model, or .. eq. S2a

A'
_{extended} C
_{extended} in the extended model. .. eq. S2b

The two models require the use of different non-avoidance rates.
The calculation of C
_{extended} takes account of the effect of a skewed flight
distribution, such that the factor A
_{extended} ( = 1 - A'
_{extended} ) refers only to genuine behavioural avoidance.
The calculation of C
_{basic} in the basic model does not, such that any such
effect, in the basic model, must be covered by the avoidance factor
A
_{basic}.

**Establishing avoidance rates from reference
windfarms**

Values of A'
_{basic} and A'
_{extended} for use in the two models are obtained by
monitoring collisions at one or more reference windfarms, and
working back from the two models. For either model we have

Non-avoidance rate A' = Actual no of collisions / Predicted number of collisions C.

*using basic model*
*using extended model*

Actual no of collisions = A'
_{basic} x C
_{basic}(ref) = A'
_{extended} x C
_{extended}(ref)

thus A'
_{extended} = A'
_{basic} x C
_{basic}(ref) / C
_{extended}(ref)

but g(ref) = C
_{extended} (ref) / C
_{basic} (ref)

so
**A'
_{extended} = A'
_{basic} / g (ref)** … eq. S3

A'
_{extended} is the non-avoidance rate from the reference
windfarm, for use with the extended model. Equation (S3) describes
how it is related to the value of A'
_{basic} derived using the basic model, using the g factor
for this reference windfarm.

Where data from several reference windfarms are used to yield an
average A'
_{basic} , then the value for A'
_{extended} should be the average of A'
_{basic} / g(ref) as calculated for each of the reference
windfarms.

**Applying reference avoidance rates to new or projected
windfarms**

Avoidance rates, derived from collision studies at one or more reference windfarms, may be used to inform the calculation of collision rate at a new or projected windfarm. The assumption in applying such avoidance rates is that the birds' behavioural response to the new windfarm will be similar to their response to the reference windfarm, and hence the proportion of birds avoiding the turbines of the new windfarm, further to the calculation of a no-avoidance collision rate, is likely to be the same as for the turbines of the reference windfarm.

Thus, having established values A'
_{basic} and A'
_{extended} for non-avoidance, as derived from the
reference windfarm, these same values may be assumed to apply to
new or projected windfarms for the same bird species. If C
_{basic}(new) and C
_{extended} (new) are the no-avoidance collision rates
calculated for the new windfarm, the predicted collisions after
avoidance for the new windfarm are:

**basic model: A'
_{basic} C
_{basic} (new)** .. eq. S4a

**extended model: A'
_{extended} C
_{extended} (new)** .. eq. S4b

A'
_{basic} is the avoidance rate established from the
reference windfarm(s) using the basic model, and A'
_{extended} that using the extended model; they are related
as in equation (S3).

**Dealing with lack of information on g(ref)**

Published information on avoidance rates for reference windfarms has not so far included information on avoidance using the extended model, or on g(ref), the ratio between the outputs (before avoidance) of the extended and basic models. Calculation of g(ref) requires information on bird size and speed, turbine parameters, and the flight height distribution at the reference site; however it does not need information on bird density, levels of flight activity, or number of transits. If this limited subset of data is available, then it should be possible to calculate g(ref) for the reference windfarm, for the bird species under assessment.

**It is recommended that any future publication of reference
avoidance rates, derived from collision monitoring studies, should
state both that for use in the basic model and that for use in the
extended model. This will require application of both models to the
reference windfarm.**

### Contact

## There is a problem

**Thanks for your feedback**