Cyclone and Storm Surge - Iczmpwb.org

6.4
The standard gradient of the outer slope and the inner slope is according to the specifications of the
Irrigation Department: 2 (horizontal): 1 (vertical). For the riverside slope this gradient is most likely a
practical compromise. Steeper gradients reduce the wave run-up but make the slopes more vulnerable
to erosion by wave-dash. Slopes of 2:1 are usually stable if they are properly constructed.
The soils of the embankments at their landsides will gradually become less saline and grass will start
to grow on the inner slopes, "promoting" these slopes to grazing grounds for cattle and goats. If the
gradient is too steep the cattle will damage the slopes while grazing as can be noticed at many
locations. At a less steep angle the hoofs of the animals act as efficient "compactors", improving the
quality of the embankment, which is important even if it only happens at the inner slope. The gradient
of the landside slope should therefore not be steeper than 2:1 and preferably less. After all, the zone
of the inner slope is reasonably productive when used as grazing ground.
In addition to the remarks made about the actual width of the embankments, which have been
visited, needs to be stated that the gradients of the side slopes of nearly all the inspected
embankments are steeper than required by the specifications of the Department. At the riverside this
was sometimes the result of wave-dash which was not yet repaired, but over considerable lengths
the embankments have been constructed with side slopes of 1:1 or even steeper. For all these
reasons the earthen embankments are less stable than they should be and could be.
6.2.2. Alignment and Berm
Like any other structure made by mankind, an embankment needs to be built on a stable base; if the
foundation fails the structure will be damaged, how solid the structure might have been constructed.
If the base of an embankment is weakened, whatever might be the cause, reinforcement of the
embankment itself will be ineffective.
To guarantee this solid base, the embankment needs to be constructed at a safe distance from the
edge of the river especially if the river banks are not stable and show cavings and slides.
The zone between the embankment and the river, usually indicated as the “outer berm”, is important
for a number of reasons:
• to guarantee the embankment a proper foundation,
• to reduce the velocity of the water flowing close to the embankment during the periods when
the water levels are high,