29
Figure 3.5. Research framework Spring Tide
Sea Level Rise from BMKG
Highest Sea Level Semarang City Terrain
Model Recent Landuse and
Infrastructure Map
Inundation
Model and Previous Occurrence comparison
Analysis
Expert Consultation
Final Result Submergence Area
I V . R E S U L T A N D D I S C U S S I O N
4.1. Sea Level Change
4.1.1. Tides Projection
Realistic tidal elevation forecasting for 90 day from December, 2011 to February, 2012 in ocean part of Semarang city has been generated using
FVCOM where six components of tides were included in the model S2, M2, N2, K1, P1, and O1. The type of tides in Semarang city is mixed mainly diurnal
tides this type was generated using Formzal number F= 1.46. This tides type shows the same type if it is compared with the result of tide gauge data
processing by Wirasatriya et al. 2006 and Prameswari 2007. The ebb tides and peak tide is occur 2 times a day with different value of height in different
times. The second ebb tide has the higher level that the firs and the first peak tide has the lower level than the second peak tide. The illustration of tides in a
day can be seen in figure 4.1
Figure 4.1. Sea level dynamic in Semarang city based on the day when spring tide is occur
The phenomenon of the spring tide highest peak tide like shown in Figure 4.1, with 0.4730 m of sea level value, has occur in the day 44
th
from total 90 days of model generating tides. This phenomenon has occurred in the full moon
on the second month from total 3 months of model generating tides. It possibly occurs because the first three moon of the year has been known as the periods
when the Earth is closest to the Sun Perihelion if it is compared to other months
http:www.science.nasa.gov
. In this research, the first moon of model generating tides is in December so the full moon occurs in day 15
th
. Revolution period of Moon to the Earth is 29.5 days, so the next full moon in January will
be occur in day 44
th
of total 90 days. In day 15-16 of each month moon calendar Sun-Earth-Moon also will be in one line and it can affect tides to reach
it peaks. NASA
http:www.science.nasa.gov
also states that perigee Moon closest to the Earth generally occur in the first three months of the year
whether it is in January, February, or March. The combination of both phenomenon perigee and perihelion will increase the level of peak tide and
reaches it maximum high in day 44
th
of total 90 days of model generating tides. In spring tide, the highest sea level has found in the coastal area of Semarang
city and Kendal where the height level is constantly decrease toward to the west. It can indicate sea surface in Semarang city was reach it maximum and starting
to recede to lower level. This indication also supported by surface current speed and direction. Speed of surface current is less than 1.5 ms, spreading to all
direction unequally with the majority direction of movement to the part where sea level is lower than the other. Seawater also propagates to land area during
that time following water channelriver and it gives big impact to the land, because it inundate the land area that lower than level of sea level at springpeak
tide. This phenomenon nowadays occurs in Semarang City coastal region, seawater inundates land area as tidal flood rob and affecting people’s life. The
variation of sea surface level at the times when spring tides occur can be seen in figure 4.2.
Figure 4.2. Sea level variation at spring tide In contrary with peak tide, neap tide can’t reach it lowest value at that month
January. Neap tides occur in day 7
th
from total 90 days of model generated tides or in the first quarter revolution of Moon to the Earth with height of sea
level at that time was -0.2310 m or 0.2310 m below mean sea level MSL. By looking at figure 4.3, there are some part in northward research area that has
lower height of sea level than in coastal area of Semarang city so that water in the coastal area of Semarang City move northward.
The direction of surface current in spring and ebb tide near land area 0-10 km shows the different direction. In spring tides, the irregular surface current
mostly toward land area because possibly there are some land area which has lower height than sea level so that water moving to those land area and inundate
land. In ebb tide, the surface current direction also spreading to all direction irregularly but the major direction is to west part of the sea. This area has lower
height of sea level. It means that water moving to that part, not to land area as it is happen in spring tide.
Figure 4.3. Sea level pattern at ebb tide When surface current pattern near land area between spring tide and neap tide
is different, the surface current in open sea shows similar direction, moving from west to east. In December-February, timescale of model generating tides, the
periods has been known as west monsoon periods with it northwest wind propagation. Wind blowing from northwest passing Java Sea to the east,
causing current moves following wind direction. Wind propagation affecting surface current so that it direction was almost similar in open sea. This result
also supported by Sofian 2007 and Widyastuti 2010.
4.1.2. Regional Sea Level Change
