Cyclone and Storm Surge - Iczmpwb.org

6.3
Beside these differences in level because of the different patterns of sedimentation in the past and
the differences in subsidence, the population has excavated ponds and canals; sometimes for the
purpose of water conservation or the improvement of the drainage system but often these excavation
are the so-called "borrow areas" for the construction of the embankments or for rising parts of their
home yards to build their houses on. Many of these ponds and other excavation are situated close to
the embankment. This means that in case the embankment needs to be shifted, the sub-base of the
dike at more than one location will be significantly lower than field level and consequently the fill
needs to be higher. The required crest level needs to be related to some decisive high water level
with a statistical chance to be reached or exceeded once in a specific number of years.
For industrialised areas in Europe the decisive high water level has a chance of occurrence of once
in 3000 years, in agricultural areas in developing countries this can be once in 10 or 15 years. The
choice what should be the decisive high water level is to a very large extent a matter of policy, taking
in consideration the costs of the protection works on the one hand and the potential damage caused
by incidental flooding on the other hand.
The Irrigation Department adopted a design crest level of 4.80+GTS. Ac-cording to information
collected from the office of the Irrigation Department in Gosaba, there are no Benchmarks on the
islands of their sub-division of which the levels has been established in relation to the Reference
Level of the Sundarban.
For that reason the actual levels of the crests of the existing embankments and those of the
embankments that need to be reconstructed cannot be established properly. Reliable and long
ranging data about water levels in the rivers are also not available. Because of this a discussion
about whether a crest level of 4.80+ is sufficient is not relevant. According to information from the
inhabitants the crest levels of the undamaged embankments are considered to be high enough.
According to the Irrigation Department, the standard width of the crest is 1.50 m, in cases no roads
with a pavement of bricks need to be built over that section of the embankment.
From a technical point of view this width is certainly safe enough and for practical reasons the width
is well chosen as it facilitates transportation along the alignment of the embankment of people and
material, which might be an advantage in cases of a calamity. At many locations and over considerable
lengths however the actual width of the crests is not according to this standard. This is not
because the embankments have been damaged by wave-dash or have been affected by slips
initiated by bank failures including parts of the embankment, but simply because they are not
constructed according to the standard specifications; for whatever reason.