Some irrigation tanks have two bisokotuwas. The water
released from the 1st bisokotuwa to the 2nd bisokotuwa
gives a much smoother flow through the sluice.
THE DAM
Source -udithawijesena.blogspot.com
Lanka is almost flat (except for the scattered mountains).
However, when there were no suitable mountains to
build a dam, they designed the dam locations intercon-
necting contour lines of a specific height.
Yoda Ela (Yoda canal) was built by king Dathusena in
5th century A.D. Yoda ela starts from Kala wewa and
travels a 87km journey to Thissa Wewa. The elevation
from Kala Wewa to Thissa wewa would have been
measured precisely for this construction. The gradient of
Yoda ela is 10-20cm per 1km. This gradient was found
uniform along the entire length 87km of the canal. This
proves that the ancient irrigation engineers had sophis-
ticated and precise instrumentation in use.
Ancient dams were built using clay. Therefore, errosion con-
trol was a huge challenge. To avoid this problem they intro-
duced the ‘ralapanawa’. It is a stone masonry on the bund
of the tank. It prevents wave errosion.
Minimizing the damage caused to the dam by seepage is
also another challenge. Even nowadays, engineers consider
seepage as a major threat. Ancient engineers prevented this
by digging a trench at suitable places at a horizontal direc-
tion to the bund and then filling it with clay (mostly using ant
hills, or puddle clay). They got the elephants to trample the
clay into a thick mass effectively, in order to prevent seepage.
Ancient Sri Lankan engineers were also skillful survey-
ors. Some landmark irrigation works are prove this fact.
Normally, a dam of a reservoir is built connecting mountains
to block the water way. Geographically, the dry zone in Sri
Gauge Magazine University of Peradeniya 11