Cyclone and Storm Surge - Iczmpwb.org

2.44
• Transport the water to the sluice gates with an acceptable loss of head to allow for a
maximal head at these gates,
• To store the excess water that needs to be spilled through the sluice gates during the
periods that this drainage is hampered by high water levels in the rivers.
Well sized sluices are required to evacuate the excess water during ebb, with proper gates which,
provided that they are adequately operated and maintained, can keep the water level in the system
within acceptable limits and can prevent intrusion of saline water through the gates during high tides.
If drainage in tidal regions is not performing satisfactorily often people are inclined to assume that the
sluice gates are too small and are as often inclined to neglect the other components of the system.
An enlarged storage in the main system does not reduce directly the total amount of water that needs
to be spilled but it reduces the required capacity of the sluices be-cause the evacuation of water can
be arranged over a longer series of tides. When however the enlarged storage should lead to an
extended area to be cultivated in the dry season, the volume of water that needs to be spilled will
reduce. Excess water is "promoted" to useful water.
Without detailed information it is impossible to determine which package of measures should be
taken to establish the best possible drainage infra-structure. This does not mean that no action can
be taken to improve the present situation without a detailed investigation. An inventory mobilizing the
experiences of the farmers might lead to indications about the "bottle-necks" in the drainage process.
Nevertheless, the absence of representative climatological data and reliable topographical data about
the levels of the different plots, levels and sizes of the canals and ponds, levels of the sluices and
data about the water levels in the rivers is a constraint for the implementation of the best possible
drainage system.
2.7.1. River / channel bank erosion
As mentioned before, the banks fail due to either an undercutting by strong flows in the channels
parallel to the bank or by slope failure, which may be termed as geotechnical failure. Figure 6 shows
the mode of failure and bank retreat due to parallel river current and Figure 2-10 shows the modes of
geotechnical failure.
As may be observed from Figure 2-11, there could be one mode of failure of the river bank, in which
strong river currents erode away the toe material from the bottom of the banks. In this way, the base
becomes weaker, and gradually the entire bank line recedes.