longer the fetch of a longshore current, the more numerous are the tidal open
ings. The location of these inlets also bears a certain relation to the direction
of the longshore currents. The underlying bar has only one tidal opening in the
northern end of the lagoon.
The continuation of the bar as
it plunges into the sea is seen very
clearly on the air photos (fig. 2).
The tidal inlets allow the tide to
sweep in and out of the lagoon,
transporting an appreciable a-
mount of material as it does so. At
times a lagoon may be completely
filled up or it may be deepened.
Although the tides are not very
strong in the Negombo area, a
fair amount of sand appears to
be deposited on the inner side of
the lagoon forming a finger shaped
tidal lagoon (see fig. 3). Some of
the lagoon deposits, particularly
in the south, are derived from the
river which empties itself into the
lagoon.
The predominant wind, which
lasts for about six months of the
year, is from the south-west. Of
the remaining six months, four
months are occupied by a relativ- Fig. 3
ely weak north-east monsoon while
the remaining two are the inter-
monsoonal months. The material transported by the rivers in the south should
have a dominant influence on the material deposited on the beaches due to
the wind direction.
It is evident from the above study that the main factors governing the sand
economy are:
1. Tidal suction at the inlet.
2. Transportation of sand by longshore currents.
3. Material originating from the southern rivers.
The sand balance of Negombo Beach is diagrammatically shown in fig. 4.
If we call the amount of sand deposited near Negombo coast SD, it appears
from the diagram that SD = SW + T-f-N —P—S. All possible steps should be
taken to prevent the loss of P and S or to increase SW, T and N.
The conclusion arrived at after the study of the photographs is that the area
south of Negombo is a coast of emergence. The double truncation of the beach
ridges north of Negombo suggests that the coast has been seaward of its present
WORKING GROUP 8
HERATH
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