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The Tidal Model
Shellfish leasing grounds are located on large tidal mudflats and therefore are
submitted to a variable immersion duration from sea to shore. Due to their filtering
activity, oyster growth depends directly on this parameter. A tidal model is written
under the
programming language. The script running on ArcView provides
water level estimates at given times. The calculation is made by using formulae given
by the EPSHOM (Hydrographic Survey) manual. The results were compared to tables
given by EPSHOM and show very little discrepancy. However, this calculation is not
available when the distance with the reference harbour is too large.
This model is used to calculate the immersion duration at each depth. Water
level calculations are recorded in a ASCII file with a six minute time lag. By reading
this file between two given dates
another script permits to generate a DBF
file where the frequencies of water level values at a 0,1 meter accuracy
are
recorded. The immersion durations
for each depth Z are calculated by summing
the frequencies over the values deeper than Z.
The DBF file is joined to every depth table to provide maps of immersion duration,
of oysters is a isometric function of Dtz
where
X is a complex function for determination of growth per time unit. SFGz represents the
gain in flesh dry weight for an oyster. In this way, Fig. 4 illustrates the effect of a
theoretical siltation of 0,5 metres on the spatial distribution of immersion duration, in
per cent of loss of immersion time. This calculation provides direct coefficient
for calculation of
at two different depths.