Full text: Transactions of the Symposium on Photo Interpretation

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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