of this 
it of 
have 
retion 
9 and 
spits. 
upur, 
arine 
aches 
been 
ed to 
, the 
uted 
zone 
athi- 
Port 
truc- 
drift 
- 
Bhat H, Gangadhara 
  
4 DISCUSSION 
Of the spits noticed on either side of the three estuaries, the Bengre and the Udyavara spits are spectacular with length of 
more than 8 km. The Bengre spit is pointing towards south, whereas the Udyavara spit is pointing northwards. The other 
spits like Pavanje, Mulki and Ullal are less than 2-3 km inlength. According to Evans (1942); Schwartz, (1984); and Bird 
(1984) the growth of the spit is largely due to long-shore drift and length of spits above the water increases only when the 
waves and shoredrift move sand from the direction of their land connection. The study carried out by Kunte and Wagle 
(1991) along the southern Karnataka coast based on the spit growth direction indicates that long-shore drift is either 
towards north or south depending upon monsoon season. According to them the bi-directional shore drift could be the 
primary agent for sediment redistribution and confinement within the region, thereby keeping the prograding coast straight, 
smooth and stable. According to Reddy et al., (1978, 1982) the predominance of southerly drift of the coastal currents 
during major part of the year could be responsible for the growth of Bengre spit. It is obvious that littoral drift cannot 
explain the disposition of Bengre and Udyavara spits as they are oriented in opposite directions. Further the Gurupur river 
had a separate exit to the sea just north of the present New Mangalore Port, as indicated by the presence of paleochannels 
which have been identified in the satellite imagery and aerial photographs. Subrahmanya and Jayappa (1987) opine that 
the river Gurupur has migrated southerly in four stages. The river started migrating to the south and its migration was 
temporarily arrested by a fracture which is the continuation of a NNE-SSW trending lineament. The continued southerly 
migration was assisted by littoral drift which made the river to turn south and finally join the river Nethravathi near 
Bengre to form a common estuary. 
Comparison of the oldest and recent data products reveals that there is a shift in the erosional as well as accretional sites 
from time to time. According to Jayappa (1996), the migration of the eroding sites could be due to the shifting of waves 
convergence zones or due to man made structures. Based on the multidated data products study Gangadhara Bhat (1995) 
indicated that from 1910 to 1990 period Hejamadi Kodi (northern spit of Pavanje-Mulki river) has experienced loss of land 
(about 2.5km?) due to shifting nature of the river mouth and the southern Mulki spit has gained about 3km" of land. 
Considering that the dominant littoral drift is southwards, the growth of the Udyavara and Pavanje spits to the north 
needs an alternate explanation. Based on the regional remote sesnsing study Ramasamy et al., (1995) have conjectured two 
major E-W trending cymatogenic arches in southern India, of which one falls along Mangalore-Madras. It was suggested 
by Subrahmanya (1996a) that the Indian peninsula is undergoing deformation which has resulted in buckling and uplift along 
anear E-W trending line extending from Mulki on the west coast and Pulicat Lake on the east coast. Because of this uplift, 
streams on either side of the Mulki-Pulicat lake Axis (MPA) have been migrating in opposite directions. The uplift is 
confirmed by several other lines of evidence. Considerable amount of fluviatile sediments occur at higher levels as river 
terraces along the upstream of the Nethravathi river course (Gangadhara Bhat and Subrahmanya, 1993) which points to 
deepening of the valley indicating rejuvenation of the river. An evaluation of depositional and erosional sites in the coastal 
zone indicates that the area covered by depositional features marginally exceed those under erosion. The presence of beach 
ridges in the northern sector, presence of oyster shells above the present high tide line near Suratkal (Subrahmanya, 1996b) 
, presence of paleo- tidal flats around Nethravathi- Gurupur estuary, the shallowing of the nearshore zone to the north of 
Mangalore, all these point to emergence of land in this sector. Tide gauge records of Mangalore area which indicate a 
relative fall in sea-level of about Imm/year substantiate this inference. 
Although southerly drift and uplift along MPA can explain the origin of most of the features noticed , one aspect i.e. 
migration of Nethravathi-Gurupur estuarine mouth towards north, in the past few decades has been an enigma. To under- 
stand this phenomenon, bathymetric charts have been made use of. Comparison of the bathymetric map of 1910-1912 with 
the extrapolated bathymetric map of 1967 shows seaward movement of the 5 fathom contours to the north of Nethravathi- 
Gurupur estuary thereby pointing to shallowing of the nearshore. Landward migration of the same contour to the south 
of Nethravathi-Gurupur estuary indicates deepening of the nearshore Fig. 1). The shallowing/deepening occurs along a 
line which when extended coincides with a cliffed shoreline and lineament on the land. The rocky outcrops are to the north 
of the lineament which also marks the upthrown side (shallowing of the nearshore). Under the influence of this movement, 
the Nethravathi river has migrated slightly northwards there by building up the Ullal spit. A careful study of the bathymetry 
also reveals the region through which Nethravathi river could have discharged to the sea. 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B7. Amsterdam 2000. 195