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Student Advising M. Shah Alam Khan |
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Name of the student: SOHELI NASRIN Program: M. Sc. (WRD)
Title: DETENTION-BASED STORMWATER DRAINAGE MANAGEMENT USING A DYNAMIC PROGRAMMING APPROACH IN A SELECTED AREA IN DHAKA
Background and present state of the problem:
Historically, stormwater drainage in Dhaka city largely depended on natural depressions, detention areas and low-lands where stormwater would be temporarily stored while the connecting khals drained water to the rivers around the city. With time, these detention areas and khals have diminished, mostly due to unplanned development and encroachment, resulting in frequent stormwater flooding in the city. Unplanned development has also caused significant deviation in land use in the catchments from the projected land use, which is likely to alter previous runoff estimates. A master plan for flood protection and drainage improvement for greater Dhaka (JICA, 1991) has been partially implemented, leaving integrated stormwater management in Dhaka as a pending task. Efficient stormwater drainage is also considered to be essential in Dhaka Metropolitan Development Plan (RAJUK, 1997).
About one-third of the stormwater in western Dhaka passes through the Hatirjheel detention area toward the Balu river through the Begunbari khal (BUET, 2005). At present, the gates of the Rampura regulator at the outlet of Hatirjheel are closed when water level rises to a critical level. Temporary 5-cusec pumps at Rampura drain out rainfall-runoff accumulated in Hatirjheel and the adjoining Gulshan and Banani lakes while the regulator gates remain closed. This make-shift pumping operation is unable to drain out internal stormwater efficiently, particularly during high-intensity rainfall events later in the season when the internal water level is high. By closing the gates earlier in the rainy season at a specified river water level, it is possible to leave room in the Hatirjheel system for high-intensity rainfall-runoff later in the season. To ensure no overflow of the detention area at any time for a maximum allowable water level, the closed-gate period can be sub-divided into shorter periods, and pumping requirements during these periods can be optimized by dynamic programming. Similar dynamic programming formulations have been examined in other countries (Rathnam et al., 2004; Ormsbee et al., 1987).
Objectives with specific aims and possible outcome:
The specific objectives of the study are to:
(i) classify land use in the catchments of the Hatirjheel system and estimate generated runoff; (ii) determine optimum pumping requirements during shorter periods while the regulator gates at Rampura are closed; and (iii) assess damage in the surrounding areas of Hatirjheel from probable overflow of the detention area.
Possible Outcome: This study is expected to provide a dynamic programming tool that can be used for stormwater management in urban detention basins.
Outline of Methodology:
The catchment of the Hatirjheel system will be delineated from DWASA maps. Land use classes in the catchment, and areas of Hatirjheel and Gulshan-Banani lakes will be estimated by digitizing satellite pictures of Dhaka available on the internet. Runoff ratio for different land use classes will be estimated from literature. Relevant officials of DWASA will be interviewed for additional information on the constraints in pumping operation and the current practices for stormwater management in the Hatirjheel system.
The proposed dynamic programming formulation will use forward recursion where stages would be 15-day periods in closed-gate condition, and states would be the possible water levels that can be maintained at the beginning of a given stage. Feasible range of states in a stage would depend on the constraints such as maximum pumping capacity, and maximum and minimum allowable water levels. The pumping requirement in each stage would be computed from the decision of moving from a given state in the current stage to a state in the next stage, the dry weather flow, and the water balance from rainfall-runoff, evaporation and infiltration. The overall optimal decision or pumping requirement would be a set of decisions during each stage. The dynamic programming problem will be solved for several gate-closing water levels. The maximum allowable water level will be set based on the perimeter elevation of the detention area. For each gate-closing water level, the problem will be solved for several rainfall frequencies. From an analysis of these results, guidelines will be proposed for gate closing and pumping operation. Probable damage caused by overflow of the detention area will be estimated from runoff generated for a specified rainfall frequency, optimal pumping requirements, estimated unit land value and approximate topography of the catchment.
References:
BUET (2005). "Techno-economic and Environmental Assessment of the Proposed Commercial Development of Lowlands Adjacent to Sonargaon Hotel", Report prepared for Rajdhani Unnayan Kartripakkha, Dhaka.
JICA (1991). "Master Plan Study for Greater Dhaka", FAP 8A, Main Report and Supporting Reports I & II, Flood Plan Coordination Organization, Dhaka.
Ormsbee, L.E., Houck, M.H., and Delleur, J.W. (1987). "Design of dual-purpose detention systems using dynamic programming", Journal of Water Resources Planning and Management, ASCE, Vol. 113, No. 4, pp. 471-484.
RAJUK (1997). "Dhaka Metropolitan Development Plan (1995-2015), Vol-II: Urban Area Plan (1995-2005)", DMDP Project, Rajdhani Unnayan Kartripakkha, Dhaka.
Rathnam, E.V., Abdulla, S.M., and Jayakumar, K.V. (2004). "Urban runoff estimation and optimization of stormwater detention systems - a case study for Hyderabad city, India", In: Global Solutions for Urban Drainage, E.W. Strecker and W.C. Huber, eds., ASCE, Reston, VA, USA.
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