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Freeware: Collisions model
Collisions between fast vessels and large cetaceans have emerged as a problem
since speeds above 14 knots became common. The free program
Collision.exe (325 KB) provides a simple 2
dimensional model of maximum possible collision rates based on vessel size
and track lengths, plus whale size, population density and mean surface time.
The basis of the model is:
Assume:
- The vulnerable parts of the whale can be represented as a line of the
same length as the whale.
- The whale's orientation relative to the direction of travel of the ferry
is random.
- The whale does not tend to move into or out of the ferry's path.
- The ferry transect has an overall density of whales that is the same as
some larger area from which a survey has given a density estimate.
- Ferries do not avoid whales.
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Specify or measure:
L |
whale length, m. |
T |
fraction of whale time at surface. |
W |
damaging width of the ferry, m. |
P |
whale population density – animals per sq. km.
in a survey area including the ferry transect. |
D |
distance travelled by the ferry within the
population survey area, km. |
Y |
yearly number of transects by the ferries. |
Then:
The whale as a horizontal linear target at random orientation to the
ferry's line of travel will present an average 'target size' of 0.64 *
whale's length. The whale could be viewed as a point and half the 'target
size' of the whale can then be added to both sides of the 'damaging width' of
the ferry to give a 'collision strip width' of W + 1.27L. From the length of
the ferry transect a 'collision area' can then be derived - (W+ 1.27L)*D/1000
sq km. With the number of transects per year and the density of whales at
risk this gives:
Annual collisions = (W + 0.64L)*D/1000*Y*T*P
Sources of Error
Errors in estimating L, T, W and P could be significant, as could errors
in the assumptions above:
Assumption |
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1 |
Reasonable, could overestimate slightly. |
2 |
Ferries crossing migratory pathways between islands
would have up to 56% higher risks; whales orientating way from noisy
ferries would have lower risks – any evidence? |
3 |
The whale does not tend to move into or out of the
ferry's path. Probably reasonable for high speed ferries because the
difficulty for a whale of estimating the track of a fast ferry even if it
tried to do this and avoid it. Diving might occur? |
4 |
Errors possible. |
5 |
Species with a highly visible blow might often be
avoided by ferries in daylight, but not in darkness. |
L |
The tail may be at lower risk, making the 'risk
length' shorter than this. |
T |
Time just below surface will also be at risk. |
W |
Displacement of the whale with the water flow around
the hull may reduce this factor. |
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