Rapp. Comm. int. Mer Médit., 36,2001
81
Introduction
Split harbour is placed on the east coast of the Middle Adriatic. The
harbour is covering 0.6 km
2
, with a mean depth of 5.4 m. The south
entrance communicates with Bra_ Channel with maximum depths of
about 70 m. The depth falls from the entrance (12-15 m), to shallow
parts on the northern side (less than 2 m).
In the Adriatic Sea the diffusion studies were rare, and in one [1] the
experiment was held in the area near Dubrovnik (South Adriatic).
There were used two models to parametrize the diffusion: i - Joseph-
Sendner approximation [2] gave the values of diffusion parameter
between 0.07 and 0.10 cm, while ii - Neuman-Pierson [3] approxima-
tion gave the diffusion parameter between 0.9 and 1.9 cm/s.
Data collection
The experiment was organized in 1996 during three different sea-
sons: in February when typical vertical isotherm is formed, in July
with strong vertical stratification, and in November/December when
the sea cooling is in progress. The measurements comprised the col-
lection of CTD at 23 stations, oxygen at 7 stations and current data at
2 stations, whereas diffusion measurements were performed at three
stations (D1, D2 and D3, located inside the harbour) in the way that 50
l of sea water, containing 30 g of the tracer RHODAMINE-B and
diluted by 300 ml methanol and 100 ml acetone, were ejected into the
sea. After the ejection, the oil spill was monitored and measured using
3 theodolites stationed on the high positions on the buildings in the
harbour. Moreover, the oil spill was photographed regularly in time.
The experiments were held during calm winds in order to minimize
the in?uence of winds, waves and currents.
It should be pointed out that the experiment was organized with lots
of difficulties, because the Split harbour is the second major port in the
Croatian part of the Adriatic. Consequently, the ship and ferry traffic
was very intensive and in?uenced the experiment organization. For
example, the first idea was to lay twoAANDERAA current meters at
the entrance of the harbour, what was done during February. But, local
ferries didn't pay attention to them at all, so they simply destroyed one
of them and moved the other one for almost 50 km by the propeller
(fortunately we have found it).
Results
The analysis of diffusion will be based on the theory developed by
Okubov [1,3], and it is based on the equation for instantaneous source.
It introduced diffusion coefficient a by using the parametrization done
by Joseph and Sendner [2]. Thus, the diffusion is stronger the diffusion
parameter is lower and vice versa.
The diffusion experiment was performed three times: on 13
February, 4 July and 9 December 1996. The diffusion parameter is cal-
culated from the data of the spot area from two near-time geodetic
measurements, and is given in Table 1.
In February at station D1 spot stretched very quickly and disap-
peared after 11 minutes, while at D2 and D3 remained at least 20 min-
utes. The in?uence of currents was manifested as extensive along-cur-
rent stretching. Diffusion parameter had values from 0.28 cm/s at D3
(strong diffusion caused by strong currents) to 0.77 cm/s at D2 (the
currents were weaker).
In July the oil spot remained between 14 and 17 minutes, with dif-
fusion parameter values between 0.34 cm/s (at D1) and 1.12 cm/s (at
D2 and D3). The in?uence of currents is obvious at D1, increasing the
diffusion process, while at D2 the values are more realistic and repre-
sentative for the summer period.
In December the diffusion had the values between 0.60 cm/s and
0.95 cm/s. The currents at measuring points were relatively weak, so
that the quality of data was high with the diffusion parameter span
lower than during February and July.The oil spot remained at least 17
minutes at all stations.
The span of diffusion parameter is lowest at station D2, which is
positioned inside the small shallow sub-basin at the top of the harbour,
so the currents were weaker here. Consequently, the quality of the
results is the best. On the contrary, at D1 and D3 the ventilation is
higher as a result of stronger current fields.
The diffusion was directly in?uenced by currents, being higher
when stronger currents occurred (lower diffusion parameter a). During
winter it had the values between 0.28 and 0.95 cm/s, while the sum-
mer values were between 0.34 and 1.12 cm/s. Currents were lowest at
station D2, so there was the lowest diffusion span (between 0.51 and
1.12 cm/s). These results can be useful for the future works on diffu-
sion, especially in the Adriatic Sea where only a few such experiments
were made.
References
1 - Bone M., 1971. Diffusion and sea - colouring experiments (in
Croatian), Hidrografski godi_njak, Split, 61-73.
2 - Joseph J. and Sendner H., 1958. Über die horizontale diffusion im
Meere, Dt. Hydrogr. Z., 11, 2, 49-77.
3 - Neumann G. and Pierson W., 1966. Principles of Physical
Oceanography, Prentice - Hall, NewYork, 541 pp.
AN ANALYSIS OF DIFFUSION CHARACTERISTICS IN THE HARBOUR AREA
(SPLIT HARBOUR, ADRIATIC SEA)
Ante Smircic*, Nenad Leder, Ivica Vilibic
Hydrographic Institute of the Republic of Croatia, Split, Croatia - dhi-oco@dhi.tel.hr
Abstract
An experiment, including measurements of physical, chemical and biological properties, was conducted in the Split harbour area (Middle
Adriatic) in order to investigate the diffusion there. After the tracer (RHODAMINE-B) had been ejected into the sea, the oil spill was fol-
lowed using three theodolites from the land and by photographing the area. The horizontal diffusion coefficient, calculated from the sim-
ple one-dimensional Okubov model, had values between 0.3 and 1.1 cm/s. Seasonally, the diffusion coefficient had higher values during
summertime, while during winter it had lower values as a result of the in?uence of current field.
Keywords: Adriatic Sea, coastal process, sewage pollution
STATIONTime slice
a
February
a
July
a
December
(min)(cm/s)(cm/s)(cm/s)
3 - 7
0.47
0.390.71
D17 - 13
-
0.340.65
13 - 17
-
-0.60
3 - 7
0.77
1.120.88
D27 - 130.51
0.810.80
13 - 17
-
-0.73
3 - 7
0.28
1.040.95
D37 - 130.28
0.820.7
13 - 17
-
0.74-
Table 1. Diffusion coefficient a measured in February, July and
December 1996 at stations D1, D2 and D3.
