II. FLOOD LOSS REDUCTION AND REVIEW OF PAST EXPERIENCES
E. Environmental impacts of flood control projects
1. Impact on physical environmentSome of the impacts of flood control projects upon the environment are briefly discussed below.
FCD facilities have been provided for about one quarter of the land area of the country. There has been an average growth of approximately 120,000 hectares per year in the area covered by FCD projects, which in turn shows a decreasing trend of approximately 80,000 hectares per year in the flooded area during the last 30 years as can be seen from Figures 5(a) and (b). Cumulative effects of progressive reduction in the inundated floodplain areas and delinking of rivers with its floodplains result in the following impacts on water, land and soil resources:
- Increase in downstream flood risks.
- Increase in risks from extreme flood events in schemes.
- Closed system needs flushing for pollution control.
- Increased problems of agro-chemical and sewage pollution.
- Changing bank erosion.
The flood control polders in the lower part of the Atrai river basin in the NW region can be mentioned as examples of severe hydraulic impacts. A characteristic topographical feature in the area is the presence of several clusters of depressions adjacent to the river. These depressions are locally known as Beels and are an important wetland resource (see Section I.C.1). Storage of flood water in Beels moderates peak flood flows while augmenting the post-monsoon flows. Several projects were constructed in the 1970's and 1980's in order to prevent flooding of the Beels as can be seen from Figure 8(a). As a consequence, Chowdhury et al. (1996) shows that there has been a rising trend in the annual maximum water level series with a decreasing trend in the annual minimum water level series for the Atrai river as shown in Figure 8(b). The embankment was cut by the people during the high flood in reaction to increased flood risks outside the project area.
Another example is the hydraulic and morphological impacts of flood control embankments along both banks of the Gumti river in the SE region as illustrated in Figures 9(a) and (b). These embankments were constructed and subsequently raised in order to protect Comilla town from flash floods. The confinement of Gumti river by embankments has caused almost equal rising trends in the annual maximum, average and minimum water level series as shown in Figure 9(b) which suggests a rise in the river bed. The protected area is at huge catastrophic risk if the earthen embankment fails during extreme floods.
The Coastal Embankment Project in the SW, SC and Chittagong regions is the biggest flood control project in Bangladesh with the project area covering approximately 6 per cent of the area of Bangladesh. The general ground level in the coastal area is below high tide level. The objective of the project is to prevent flooding of agricultural land during high tide. Without the embankment, an enormous volume of tidal water would have been stored in the floodplain during rising tides which would have allowed sediment in suspension to be deposited over the floodplain. During ebb tide, the stored water is drained through the tidal rivers providing a flushing action. This process of tidal flooding and drainage occurs approximately twice a day. As a consequence of the embankments in SW and SC regions, around 2000 million cubic metres of flooding volume has been lost during spring tide (Halcrow, 1993b). The result of this enormous decrease in tidal volume is that the water level drops quickly during ebb tide, and both the ebb tide velocity and the low water depth have decreased. This has resulted in siltation in the river causing a rise in the river bed [Figure 10(b)] which is the main cause of the deterioration of waterways in the SW and SC regions (Jansen et al., 1994; DHV, 1989). As a result of the rise in the channel bed relative to land levels in the flood protected area, serious problems of drainage congestion and waterlogging have developed in the Polder-25 (Beel Dakatia) in the SW region [Figure 10(a)]. Some of the channels are almost dead due to siltation. The siltation of tidal rivers is spreading progressively southward i.e. seaward (Halcrow, 1993b).