Flood control through best management practices
Saturday, 13 September 2008
Md. Khalequzzaman
Flooding and the solutions to it can or should be analysed in the context of three fundamental parameters: run-off, water carrying capacity, and land elevation changes. What is needed is analysis of land use practices in watersheds over the last few decades that have the potentials to impact hydrodynamic behaviour of rivers, affecting three vital parameters mentioned above.
Unplanned urbanisation can adversely impact flooding in a watershed. Prior to urbanisation there exists a greater lag time between intense rainfall and peak stream flow. After urbanisation the lag time is shortened, peak flow is greatly increased, and the total run-off is compressed into a shorter time interval, creating favourable conditions for intense flooding. For example, in a city, fully covered by storm drains with 60 per cent of the land surface accounts for roads and buildings (like Dhaka City), floods are almost six times more numerous than before urbanisation.
It is necessary to adjust drainage capacity in a watershed, taking into account the "basin development factor (BDF)" to accommodate the extra runoff that results due to urbanisation. However, no attempts have been taken to increase the drainage capacity of these rivers to adjust to the BDF. On the contrary, many of the rivers, including the Buriganga, are being filled up by people. The illegal encroachment of the rivers is reducing their water carrying capacity during floods. Moreover, unplanned land use has diminished the internal drainage system consisting of tributaries to the Buriganga and Sitalakkhya rivers. For instance, the reduced Dhanmondi Lake and Baridhara Lake, the remnants of tributaries to the Buriganga-Sitalakkhya rivers as well as the filled-up Dholaikhali channel has drastically reduced the drainage capacity from Dhaka City. The lack of an efficient storm sewer system in Dhaka City also reduces its drainage capacity, causing water logging throughout the monsoon.
Riverbed elevation is most pronounced for the Ganges and its distributaries. From the border point with India where it enters Bangladesh to the point where the Ganges meets the Brahmaputra, the riverbed rose by five to seven meters in recent years. The average width of the Ganges decreased from 1.27 km in 1973 to 1.01 km in 1985. Riverbed elevation has been so pronounced in Bangladesh that one cannot but notice the change during one's lifetime. For example, the Old Brahmaputra river, navigable for steamers 30 years ago, is now an abandoned channel. This is true for many distributaries of the Ganges and Meghna rivers, such as the Madhumati, Bhairab, Chitra, Gorai and the like. Riverbed elevation reduces the water carrying capacity of rivers, causing bank overflow and flooding. This recent riverbed elevation has undoubtedly contributed to the increased flooding propensity in Bangladesh.
Heavy monsoon showers remove the surface soil through run-off. Eroded sediments are deposited on riverbeds, reducing the water carrying capacity and increasing flooding propensity in a watershed. Soil erosion also reduces land elevations and increases elevations of riverbeds, contributing to increased flood depths. The land elevations in other parts of Bangladesh must have reduced over time due to soil erosion. Besides this, the tilling on the mountain slopes of the Himalayas is thought to be responsible for massive soil erosion in Nepal, which eventually causes rapid riverbed elevation in Bangladesh.
Deforestation accelerates soil erosion during monsoon precipitation. This, in turn, is believed to contribute to floods. Deforestation within Bangladesh contributes to the soil erosion. The amount of forest cover in Bangladesh was reduced from 15.6 per cent in 1973 to 14.6 per cent in 1985-86, and eventually to 13.4 per cent in 1987. And the pressure for further reduction thereof has been on. A minimum of 25 per cent forest cover is suggested for a healthy ecosystem. The forest cover in Bangladesh now is estimated at less than 10 per cent.
A delta can gain in elevation if sediment accumulation rates are greater than the rates of local relative sea-level rise. Limited data show that the average sediment accumulation rates in the coastal areas of Bangladesh is five to six mm/year for the last few hundred years, which is not enough to keep pace with the rising sea level at 7.0mm/year. As a result, land elevations must have been decreasing over time in Bangladesh, resulting in more flooding inundation.
A greater understanding of the processes that contribute to increased flooding propensity, however, can help us mitigate the adverse effects on human lives, the environment, and the economy. To mitigate flooding propensity in Bangladesh, both the government and the people will have to shift their paradigms, to adopt best management practices (BMP) in agriculture, forestry, landuse planning, water resources management, and urbanisation. The flood control actions through the BMPs will help reduce the run-off as well as increase the of drainage capacity and land elevations with respect to sea level or riverbeds.
The BMPs to reduce flooding in Bangladesh should include dredging of rivers, re-excavation of abandoned channels, ponds and lakes, dispersion of dredged sediments to raise to higher lands, village platforms and road elevations, conservation tillage, establishment of vegetated buffer zones along rivers. They should also include removal of polders in the coastal areas to enhance sedimentation on delta plain, efficient storm sewer systems in cities, planned urbanisation, watershed-scale land use zoning maps, reforestation, good governance, integrated regional water resources development plans with participation of India, Nepal, and Bhutan.
Formulating solutions to flooding problems requires a comprehensive understanding of the geologic settings of the region, and a better knowledge of hydrodynamic processes that are active in watersheds. Only solutions that take into account the underlying long-term factors contributing to flooding problems can prevail. Such contributing factors are as follows: unplanned urbanisation, soil erosion, local relative sea-level rise, inadequate sediment accumulation, subsidence and compaction of sediments, riverbed aggradation, and deforestation.
Structural solutions, such as the building of embankments along the rivers and polders in coastal regions in Bangladesh, will not solve the flooding problems, but will result in many adverse environmental, hydrologic, economic, ecological, and geologic consequences. Solutions to flooding problems can be achieved by adopting and exercising watershed-scale best management practices in agriculture, construction, forestry, governance, river and water resources management, urbanisation, flood forecasting, and regional cooperation.
.............................................................
By courtesy: Global Amitech, News From Bangladesh (NFB). The writer is a university teacher in the USA
Flooding and the solutions to it can or should be analysed in the context of three fundamental parameters: run-off, water carrying capacity, and land elevation changes. What is needed is analysis of land use practices in watersheds over the last few decades that have the potentials to impact hydrodynamic behaviour of rivers, affecting three vital parameters mentioned above.
Unplanned urbanisation can adversely impact flooding in a watershed. Prior to urbanisation there exists a greater lag time between intense rainfall and peak stream flow. After urbanisation the lag time is shortened, peak flow is greatly increased, and the total run-off is compressed into a shorter time interval, creating favourable conditions for intense flooding. For example, in a city, fully covered by storm drains with 60 per cent of the land surface accounts for roads and buildings (like Dhaka City), floods are almost six times more numerous than before urbanisation.
It is necessary to adjust drainage capacity in a watershed, taking into account the "basin development factor (BDF)" to accommodate the extra runoff that results due to urbanisation. However, no attempts have been taken to increase the drainage capacity of these rivers to adjust to the BDF. On the contrary, many of the rivers, including the Buriganga, are being filled up by people. The illegal encroachment of the rivers is reducing their water carrying capacity during floods. Moreover, unplanned land use has diminished the internal drainage system consisting of tributaries to the Buriganga and Sitalakkhya rivers. For instance, the reduced Dhanmondi Lake and Baridhara Lake, the remnants of tributaries to the Buriganga-Sitalakkhya rivers as well as the filled-up Dholaikhali channel has drastically reduced the drainage capacity from Dhaka City. The lack of an efficient storm sewer system in Dhaka City also reduces its drainage capacity, causing water logging throughout the monsoon.
Riverbed elevation is most pronounced for the Ganges and its distributaries. From the border point with India where it enters Bangladesh to the point where the Ganges meets the Brahmaputra, the riverbed rose by five to seven meters in recent years. The average width of the Ganges decreased from 1.27 km in 1973 to 1.01 km in 1985. Riverbed elevation has been so pronounced in Bangladesh that one cannot but notice the change during one's lifetime. For example, the Old Brahmaputra river, navigable for steamers 30 years ago, is now an abandoned channel. This is true for many distributaries of the Ganges and Meghna rivers, such as the Madhumati, Bhairab, Chitra, Gorai and the like. Riverbed elevation reduces the water carrying capacity of rivers, causing bank overflow and flooding. This recent riverbed elevation has undoubtedly contributed to the increased flooding propensity in Bangladesh.
Heavy monsoon showers remove the surface soil through run-off. Eroded sediments are deposited on riverbeds, reducing the water carrying capacity and increasing flooding propensity in a watershed. Soil erosion also reduces land elevations and increases elevations of riverbeds, contributing to increased flood depths. The land elevations in other parts of Bangladesh must have reduced over time due to soil erosion. Besides this, the tilling on the mountain slopes of the Himalayas is thought to be responsible for massive soil erosion in Nepal, which eventually causes rapid riverbed elevation in Bangladesh.
Deforestation accelerates soil erosion during monsoon precipitation. This, in turn, is believed to contribute to floods. Deforestation within Bangladesh contributes to the soil erosion. The amount of forest cover in Bangladesh was reduced from 15.6 per cent in 1973 to 14.6 per cent in 1985-86, and eventually to 13.4 per cent in 1987. And the pressure for further reduction thereof has been on. A minimum of 25 per cent forest cover is suggested for a healthy ecosystem. The forest cover in Bangladesh now is estimated at less than 10 per cent.
A delta can gain in elevation if sediment accumulation rates are greater than the rates of local relative sea-level rise. Limited data show that the average sediment accumulation rates in the coastal areas of Bangladesh is five to six mm/year for the last few hundred years, which is not enough to keep pace with the rising sea level at 7.0mm/year. As a result, land elevations must have been decreasing over time in Bangladesh, resulting in more flooding inundation.
A greater understanding of the processes that contribute to increased flooding propensity, however, can help us mitigate the adverse effects on human lives, the environment, and the economy. To mitigate flooding propensity in Bangladesh, both the government and the people will have to shift their paradigms, to adopt best management practices (BMP) in agriculture, forestry, landuse planning, water resources management, and urbanisation. The flood control actions through the BMPs will help reduce the run-off as well as increase the of drainage capacity and land elevations with respect to sea level or riverbeds.
The BMPs to reduce flooding in Bangladesh should include dredging of rivers, re-excavation of abandoned channels, ponds and lakes, dispersion of dredged sediments to raise to higher lands, village platforms and road elevations, conservation tillage, establishment of vegetated buffer zones along rivers. They should also include removal of polders in the coastal areas to enhance sedimentation on delta plain, efficient storm sewer systems in cities, planned urbanisation, watershed-scale land use zoning maps, reforestation, good governance, integrated regional water resources development plans with participation of India, Nepal, and Bhutan.
Formulating solutions to flooding problems requires a comprehensive understanding of the geologic settings of the region, and a better knowledge of hydrodynamic processes that are active in watersheds. Only solutions that take into account the underlying long-term factors contributing to flooding problems can prevail. Such contributing factors are as follows: unplanned urbanisation, soil erosion, local relative sea-level rise, inadequate sediment accumulation, subsidence and compaction of sediments, riverbed aggradation, and deforestation.
Structural solutions, such as the building of embankments along the rivers and polders in coastal regions in Bangladesh, will not solve the flooding problems, but will result in many adverse environmental, hydrologic, economic, ecological, and geologic consequences. Solutions to flooding problems can be achieved by adopting and exercising watershed-scale best management practices in agriculture, construction, forestry, governance, river and water resources management, urbanisation, flood forecasting, and regional cooperation.
.............................................................
By courtesy: Global Amitech, News From Bangladesh (NFB). The writer is a university teacher in the USA