Dhaka City Flooding
Inappropriate land development increases risk
Dhaka city is located on the floodplains of the distributaries of the Brahmaputra, and the topography is almost flat because of deltaic formation. Change in the floodplain water regime due to land development inconsistent with the floodplain topography is the main cause of rising flood hazard in Dhaka city. Some factors that are responsible for flood hazard, and steps necessary to address the issue, are briefly discussed below. Factors responsible
During monsoon, the river water level remains high and results in fluvial flood in the floodplain due to spill from the rivers. Flood proofing is the appropriate flood management approach in floodplain landscape. This was the traditional practice in Dhaka, as evident from old buildings -- e.g. Ahsan Manzil. In this practice, road levels and plinth levels are set above flood level. The practice of flood proofing has stopped because of the embankment constructed in the western part of the city to provide protection against fluvial flood. Lakes, khals (floodplain channel) and ponds act as detention reservoirs in the western part by storing runoff generated from local rainfall, which is drained by gravity when the river water level decreases. The storm water flood hazard in the protected western part of the city is on the rise because of increasing imperviousness and decreasing storm runoff detention capacity. Land development activities and associated land use changes that are not consistent with floodplain landscape functions are among the main factors responsible for rising flood hazard. Deposition of solid waste in, and lack of proper maintenance of, storm sewers and khals make the situation worse. Increase in imperviousness due to decrease in green areas has resulted in an increase in storm runoff and decline in groundwater recharge. Decrease in groundwater recharge due to reduced infiltration during monsoon is a concern for the city's water supply. Uncoordinated road construction, ignoring the storm runoff passage, is also an important factor contributing to storm water hazard because road networks modify drainage catchments on the floodplain topography. Because of the rise in storm water hazard, some drainage zones in Dhaka have been provided with pumped drainage systems. Reduction in retention areas at the pump intake because of encroachment is causing storm water flooding in areas with pumped drainage system. Pumped drainage is not appropriate in floodplain topography in delta setting since it causes increase in flood level in the adjacent floodplain areas. The government has decided to construct a road-cum-embankment in the eastern part of Dhaka to provide protection against fluvial flood. Meanwhile land development activities are going on in the eastern part without much consideration to the storm water management issues, increasing the intensity of flood hazard. Steps needed
Urban area planning should ensure that land development activities stick to the following principle regarding storm water management: Storm-runoff that would be generated after land development should not exceed the pre-development magnitude. This can be achieved by making provision for inlet control within individual property to reduce storm runoff, by maintaining green area in sub-catchments to facilitate infiltration and by providing on-site detention storage to trap excess runoff. Floodplain wetlands perform crucial detention function in storm drainage process. They are also storehouses of ecological resources and means of recreation. Preservation and restoration of wetlands is a must for sustainable storm drainage system in floodplain landscape. It is essential to maintain green areas so that groundwater is recharged adequately through infiltration during monsoon. As per Dhaka Metropolitan Development Plan of Rajuk, storm water management boards will oversee the protection of the required retention pond area (12%) within their management areas. Land development activities are going on without fulfilling this requirement. This policy will be effective if at least 12% on-site storm water retention area is ensured in land development projects. Detention reservoir based gravity drainage system is the appropriate storm water drainage system in floodplain landscape. An example of such a detention system is the Hatir Jheel (wetland), that is regulated by a regulator at Rampura. This detention system is located in the western part of Dhaka City, and it's performance is declining because of encroachment on the Hatir Jheel itself. Water storage and drainage capacities of urban catchments are to be increased by recovering encroached wetlands and khals. Making provision for water transport within Dhaka will usher in great benefit for the city dwellers. It is to be ensured in the eastern part of Dhaka City that road alignment and culvert locations are consistent with the storm water runoff process in the floodplain landscape. Landscape consistent passage of storm runoff should be shown in the urban area plan. By taking into consideration the effects of proposed land developments on floodplain water regime, appropriate plinth level and road level for every zone also should be set in the urban area plan.
For all latest news, follow The Daily Star's Google News channel.
Comments