Environmental Engineering Reference
In-Depth Information
Table 24.1
Classification of land types based on depth of
flooding (after MPO 1986)
which are protected by embankments, flood con-
trol and drainageworks, are situated in F0 lands, or
high lands. In fact in urban areas and especially for
Dhaka the major economic benefits are measured
as the increase in land values and protection of
existing economic infrastructure. This is reflected
in the design of the embankments surrounding the
four major cities, Dhaka, Chittagong, Khulna and
Rajshahi urban areas. A 100-year flood frequency
is adopted as the design water level for urban
settlements, whereas the embankments protect-
ing agricultural areas are normally designed for a
flood of 25-year frequency.
In rural areas the scenario is somewhat differ-
ent. While even slight flooding can disrupt urban
life and normal activities, inundations are not
always unwelcome in rural areas. The Bengali
language distinguishes between the normal floods
of the rainy season, which are locally known as
barsha, and the more harmful floods of abnormal
depth and timing, which are termed bonna. The
borsha, which occurs more frequently than bon-
na, is not considered (in rural areas) to be a hazard
at all, but rather a necessity for survival. In fact for
rice farmers too little water is a greater threat than
too much. The relation between the cropping
calendar and the seasonal flooding is documented
in Figure 24.3. In different seasons of the year
different varieties of rice dominate, adapted to the
hydrological conditions of the respective season.
The cropping calendar is not only adapted to the
different seasons but also to the different levels of
the land.
In rural Bangladesh, the farmers decide which
crops to plant. In their decision-making, the rela-
tive profitability of crops and amount that can be
safely harvested play a major and in many cases
decisive role, though their decision-making is of-
ten constrained by resources. The total harvest
depends on the agroecological environment, i.e.
the land type according to flood depth. Depending
on the type of land and soil characteristics the
prevalent cropping calendars are described in
Figure 24.3.
Although around 50 different crops are grown
on the agricultural lands the calendar mainly
evolves around rice plantation. This is because
Land
type
Flood
depth (cm)
Nature of
ooding
Description
F0
High land
30
Intermittent
<
F1
Medium-high land
30
90
Seasonal
-
F2
Medium-low
90
180
Seasonal
-
F3
Low land
180
360
Seasonal
-
F4
Low to very low
360
Seasonal/
perennial
>
.
F4, or low to very low land, is land in depressions
that normally stays wet for all or most of the year
even during the dry season. Most of this type of
land is deeply flooded to more than 360 cm in the
rainy season. The important difference between
very low land and other types of land is that it stays
wet for all or most of the dry season, so cannot be
used for most crops.
Figure 24.2 shows the broad distribution of
these land types in the different regions in Bangla-
desh. It must be remembered, however, that
a range of flood depths can occur even within the
same village. In general, normal seasonal flooding
is shallow in the northwest, west, east and south of
the country, and is deep in the centre and north.
In modern flood protection and drainage pro-
jects embankments raised for flood control have
been found to have caused appreciable changes in
land types. Assessments of various FCDI project
areas show that former F2 (medium-low land) and
F3 (low land) were converted into F0 (high land)
and F1 (medium-high land) (Brammar 2002). Since
F0 and F1 provide areas where the richest diversity
of crops can be grown, the production strategy
concentrates on the requirement for these areas.
Table 24.2 gives the percentage of land types prev-
alent in 1990.
Land Use Planning According to Land Types
Flood protection for major cities, important com-
mercial and industrial areas and key transport
and communication infrastructure is given the
highest priority. Most urban areas in the country,
