Draft - Bangladesh University of Engineering and Technologymsalamkhan.buet.ac.bd/teaching_msk_files/Hatirjheel_project_3.pdf · full stage, the retention capacities of the Gulshan

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Draft

Hatirjheel Development and Peripheral Roadway and Walkway ProjectFinal Report

Chapter FourDRAINAGE AND DETENTION ANALYSIS

4.1 Introduction

Hatirjheel and the Begunbari khal drain stormwater from approximately one-third area of Dhaka city. Hatirjheel is the largest stormwater detention basin in the city which is hydraulically linked with the Gulshan and Banani lakes. Banani lake is linked with Hatirjheel through a canal which also receives stormwater from the Mohakhali box culvert. The outfall of this combined system covering the DWASA Drainage Zones F and G is located at Rampura where a regulator was constructed after the flood in 1988. The current practice of DWASA for stormwater management in Zones F and G is to allow stormwater detention in Hatirjheel and gravity drainage through Rampura until the external river water level rises above +5.0 m PWD. The regulator gates are kept closed during the period the external water level is above +5.0 m PWD, and approximately 50 five-cusec temporary pumps are used to drain out runoff generated by internal rainfall.

The master plan for flood protection and drainage improvement of Dhaka city (JICA, 1991) includes construction of a permanent pumping station for Compartment DC 3, including Zones F and G, at its outfall on the Balu river. Because of this plan, the scenario with a permanent pumping station at Rampura for the Hatirjheel gravity drainage system was not considered by BUET (2005).

The stormwater detention capacity of the Hatirjheel system has been reducing in the recent years because of encroachment and land development in the detention areas. Although construction of the proposed walkway and roadway will reduce the available detention capacity, it is hoped that further reduction in the present detention capacity will be avoided. The top elevation of the proposed roadway and walkway will be +7.0 m PWD whereas most built up areas around the system are at a land elevation above +6.0 m PWD (JICA, 1991). Therefore stormwater detention in the system above +6.0 m PWD may retard stormwater inflow into the system from relatively low-lying catchment areas and increase the possibility of flooding in the catchment.

The Hatirjheel-Gulshan lake-Banani lake combined system is also proposed to serve as an integrated lake for possible future recreational and navigational purposes. The proposed minimum water level in the integrated system is +2.5 m PWD, which is the present dry season minimum water level in the lakes. The combined system receives a considerable amount of domestic wastewater which is proposed to be diverted by CSOstructures and bypass sewers. Therefore the dry weather flow is not considered in the detention and drainage analysis.

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4.2 Sub-catchments and Cross-drainage

The Hatirjheel combined system receives stormwater from DWASA Drainage Zones F and G in western Dhaka city. Zones F and G are further divided into 8 and 4 sub-catchments, respectively. In a meeting on June 02, 2008, DWASA officials demarcated the sub-catchment boundaries and located the major outfalls to update the drainage map of the area (see Figure 4.1). Table 4.1 gives the areas of the sub-catchments and lakes. The area of the Drainage Zones F and G combined is 27.468 km2. Runoff coefficients for the sub-catchments are estimated from land use patterns observed in satellite pictures, and the values suggested by JICA (1991).

Table 4.1: Area of sub-catchments in Drainage Zones F and G.

Sub-CatchmentName

Area (km2)

G1 5.783G2 2.890G3 2.500G4 3.782

Gulshan and Banani lakes 0.921F1 4.340F2 0.625F3 1.291F4 1.083F5 1.666F6 0.457F7 0.665F8 0.250

Hatirjheel 1.215Total area 11.592 15.876

The main storm sewer outfall into the system is the Panthapath-Paribag combined box culvert outlet adjacent to Hotel Sonargaon. During field visits, several other major outfalls and several smaller inflowing streams were identified. The Banani lake is linked with Hatirjheel through a canal whereas flow through the outlet of the Gulshan lake is controlled by a gate. Dry weather flow through all the storm sewer outlets is proposed to be diverted by CSO structures and bypass sewers.

4.3 Detention Capacity of the Combined Hatirjheel System

Most built up areas around the combined Hatirjheel system are above +6.0 m PWD elevation (JICA, 1991). There will be a risk of flooding from internal rainfall if stormwater detention in the system is allowed above this level. To avert this risk of flooding in the catchment areas, the allowable maximum water level in the system is

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decided to be +5.5 m PWD by the study team in consultation with DWASA officials (in a meeting on September 17, 2008).

The minimum dry season water levels in the Gulshan and Banani lakes are found to be approximately +2.5 m PWD from topographic surveys. Therefore the minimum water level in the proposed combined system, which will also serve as a multi-purpose lake, is set to be at +2.5 m PWD. Considering a water loss of approximately 1.5 m from the system by evaporation and infiltration during the three dry months, December through February, excavation of the Hatirjheel bed to a depth of -1.5 m PWD is proposed by the study team.

Detention capacity of the combined Hatirjheel system was estimated based on topographic surveys and design cross-sections. A cross-section survey was conducted to estimate the water retention capacities of the Gulshan-Banani lakes. Figure 4.2 shows the surveyed cross-section locations. Four sections in the Gulshan lake, G1 through G4, and four sections in the Banani lake, B1 through B4, were surveyed.

Figures 4.3 and 4.4 show the surveyed cross-sections in the Gulshan and Banani lakes, respectively. Analysis of the cross-section data indicate that the dry season water levels in the Gulshan and Banani lakes are approximately 2.5 m PWD and 4.0 m PWD, respectively. This difference is most likely caused by the gate control and difference in invert levels at the two outlets. Assuming that the available storm runoff storage capacity would be the volume between the dry season water level and the water level at the bank full stage, the retention capacities of the Gulshan and Banani lakes are estimated to be 1,320,939 m3 and 656,843 m3, respectively.

Figure 4.5 shows the total detention volume at different water levels in Hatirjheel after project completion. The total detention volumes at +5.5 m PWD and +2.5 m PWD levels will be 5.74 Mm3 and 3.07 Mm3, respectively. Since the dry season minimum water level required to be maintained in the combined system is +2.5 m PWD, the effective detention volume in Hatirjheel only will be 2.67 Mm3. A volume of approximately 3.07 Mm3

below the +2.5 m PWD level will remain as dead storage in Hatirjheel. Cross-section surveys in the Gulshan and Banani lakes indicate that the effective detention volume between +2.5 m PWD +5.5 m PWD levels is approximately 3.06 Mm3. Therefore the effective detention capacity of the combined Hatirjheel-Banani lake-Gulshan lake system is approximately 5.73 Mm3.

4.4 Critical Periods for Runoff Detention

Historical rainfall and river water level data were analyzed for storm runoff and drainage analysis. Daily rainfall data from BMD were analyzed to determine the mean daily, weekly, bi-weekly and monthly rainfall amounts for a design rainfall frequency of 1:5 yr. Water levels at the outfall of the Begunbari khal on the Balu river were estimated from the historical water level records of two BWDB stations at Mirpur and Demra. Figure 4.6shows the mean weekly river water level and 1:5 yr weekly rainfall.

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Two factors influence stormwater detention and drainage through the Hatirjheel combined system. First, the water level in the Balu river, which varies with inflow from the upper catchments and trans-boundary flow. During the wet season, water level in the Balu river rises and stormwater is discharged through the Rampura regulator against a relatively high downstream water level. Above a critical external water level of approximately +5.0 m PWD, internal stormwater outflow through the Rampura regulator is significantly retarded. Second, although significant rainfall occurs during the pre-monsoon period, the generated internal runoff may drain out under gravity because of the relatively low external river water level. The internal storm runoff accumulates in the detention area when the regulator gates are kept closed to protect the internal areas from river flood.

Runoff detention in the Hatirjheel system is critical when the regulator gates are closed due to relatively high external water level, and the internal rainfall amount is also high. Figure 4.7 shows that on an average, the river water rises to +5.0 m PWD level in the second week of July and falls below this level in the last week of September. Figure 4.8shows that in a flood year, the first peak arrives in late July to early August and the second peak arrives in early September. Rainfall amounts are relatively high in mid July and late September in the flood years. These observations suggest that the critical periodsfor stormwater detention in the Hatirjheel system are from July 10 – 20 and from September 10 – 25. During these periods additional pumping may be required, and the temporary pumps should operate at the specified rates immediately after the regulator gates are closed.

4.5 Numerical Studies for Stormwater Drainage and Detention

A numerical water balance model has been developed to analyze different scenarios of detention and drainage in the Hatirjheel system. The model can be used to determine the water levels in the detention areas and temporary pumping requirement under different scenarios of rainfall-runoff with diverse cases of external river water level dictating the boundary condition. Observed rainfall and river water level data for 21 years (1985 to 2007, except 2000 and 2001) have been used for simulation of the present situation. This period includes several extreme river flood and internal rainfall events.

4.5.1 Modeling Approach

The combined Hatirjheel system has been conceptualized as a reservoir with an outlet at the Rampura regulator. Water level in this reservoir fluctuates due to inflow of the rainfall-runoff from the surrounding catchments, direct precipitation, water losses by evaporation and infiltration, and inflow or outflow through the outlet. This reservoir is connected through the outlet to the Begunbari khal and the Balu river. Therefore water level inside Hatirjheel is influenced by the water level in the Balu river. A minimum water level of +2.5 m PWD is to be maintained in this reservoir by raising the invert level of the regulator. So, even if the external water level in the Begunbari khal falls below +2.5 m PWD, water level inside the Hatirjheel system will not fall below this level. When the water level inside the Hatirjheel system is higher than the external water level, water

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flows out under gravity through the open regulator, and the reverse occurs when the external water level is higher. During the wet season, from July through September, water level in the Hatirjheel system needs to be controlled by the regulator to avoid inundation of the surrounding areas. A maximum water level of +5.5 m PWD is to be allowed in the Hatirjheel system during this period. So, if the external water level rises to +5.5 m PWD, the regulator gates are closed and temporary pumps are operated to drain out runoff from internal rainfall during this ‘closed-gate’ period. During two ‘critical periods’, July 10 –20 and September 10 – 25, the regulator gates are to be closed at +5.0 m PWD and temporary pumps are to be operated at the specified weekly pumping rates. However, the regulator gates are to be opened to drain out the internal water under gravity when the external water level is below +5.5 m PWD and lower than the internal water level.

In this model, after every time step water level inside the reservoir is calculated using a water balance equation taking into consideration the internal rainfall, catchment area, water surface area, evaporation, infiltration and runoff ratio. The calculated internal water level is then compared with the external water level to decide whether the regulator gates are to be kept open or closed, and whether temporary pumping is required.

4.5.2 Governing Equations

The water balance equation used in the model is as follows:

wifwwwwc A*)LL(A*IA*EA*Rr*)AA(*R (4.1)

where R = rainfall, Ac = catchment area, Aw = water surface area of the combined Hatirjheel system, r = runoff ratio (= total runoff / total rainfall), E = evaporation, I = infiltration, Li = initial water level in the system, and Lf = final water level in the system.

If temporary pumping is required, the pumping rate is calculated using the following equation:

e*t

A*)LL(P wif (4.2)

where P = pumping rate, t = time period, and e = pumping efficiency.

4.5.3 Simulation of the Present Scenario

Detention and drainage in the combined Hatirjheel system is dependent on both external river water level and internal rainfall. However, since these two factors are mutually independent a simple probabilistic analysis of the water level or rainfall would not provide a clear understanding of the performance of the system. Therefore performance of the system from 1985 to 2007 was simulated using observed daily river water level and rainfall data, and existing temporary pumping capability. Table 4.2 gives the parameters and their values used in the model.

Simulation results for different years are given in Appendix A. Figure 4.9 shows the simulation results for the moderate flood in 2007. This simulation is conservative since the gates are closed and pumping starts at +5.5 m PWD instead of +5.0 m PWD, and the ‘critical periods’ are not considered. In Figure 4.9, the firm line indicates the observed

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water level in the river and the dotted line shows the simulated water level in the Hatirjheel system. Temporary pumping has been provided on a daily basis, when required, to maintain the water level below +5.5 m PWD. Figure 4.9 also shows the observed rainfall (mm/day) with the vertical bars at the top, and the required daily pumping rate (m3/s) with the vertical columns at the bottom.

Table 4.2: Model parameters for simulation of daily performance of the systemfrom 1985 through 2007.

Water surface area of the combined system 2,185,365 m2

Combined catchment area 26.16 km2

Gate closed at external river water level +5.5 m PWDMaximum allowable water level +5.5 m PWDMinimum water level maintained in lake +2.5 m PWDPumping starts at water level +5.5 m PWDInfiltration 0.010 m/dayEvaporation 0.015 m/dayRunoff ratio 0.80Number of 5-cusec pumps 50Pumping efficiency 75%Time step 1 day

A summary of analysis of the simulation results is given in Table 4.3. Temporary 5-cusec pumps required in addition to the existing 50 pumps are called ‘excess pumps’. The existing capacity of these fifty 5-cusec pumps is 7.1 m3/s. In practice, DWASA also installs temporary 15-cusec dredgers during extreme events. Table 4.3, Figure 4.9 and Appendix A indicate that during moderate to extreme floods, when the regulator gates arekept closed for a prolonged period, pumping may be required at relatively high rates only for a few days. Temporary pumping is required at much lower rates during most of the wet season. In a normal year, the system may even function fully under gravity drainage without any pumping. During the extreme events like those in September 2004 or August-September 1998, the design drainage capabilities of the system are unlikely to sustain (Hossain, 2004; Khan, 2006).

Figure 4.10 shows the observed water levels inside and outside the Rampura regulator during the floods in 1998, 2004 and 2007. During the 1998 flood, the regulator gates were kept closed due to relatively high river water level and pumping was required for as long as 41 days. However, during this period the required daily temporary pumping rates were relatively low since the number of high-intensity internal rainfall events was low. On the contrary, although temporary pumping was required for 25 days in 2004, the required maximum daily pumping rate was much higher than that in 1998. This was caused by high-intensity rainfall events, as high as 341 mm/day at approximately 1:100 year frequency, during this period.

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Table 4.3: Summary of analysis of the simulation results from 1985 through 2007.

Yea

r

Max

imum

riv

erW

ater

Lev

el (

m)

Max

imum

rai

nfa

llin

a d

ay (

mm

)

Max

imum

pu

mpi

ng

rate

in a

day

(m

3 /s)

Tot

al v

olum

e to

be

pu

mp

ed (

Mm

3 )

Nu

mb

er o

f d

ays

pu

mp

ing

req

uire

d

Nu

mb

er o

f d

ays

‘Exc

ess

pu

mp

s’ r

equi

red

Nu

mb

er o

f d

ays

wit

hp

um

pin

g ra

te >

=10

m3 /s

Nu

mb

er o

f d

ays

wit

hp

um

pin

g ra

te >

=15

m3 /s

Nu

mb

er o

f d

ays

wit

hp

um

pin

g ra

te >

=25

m3 /s

Nu

mb

er o

f d

ays

wit

hp

um

pin

g ra

te >

=50

m3 /s

Nu

mb

er o

f d

ays

wit

hp

um

pin

g ra

te >

=10

0 m

3 /s

1985 5.7 92.0 5.4 0.801 5 0 0 0 0 0 01986 5.3 176.0 0.0 0.000 0 0 0 0 0 0 01987 6.8 138.0 44.6 14.347 25 9 6 4 3 0 01988 7.7 135.0 12.2 4.858 21 3 1 0 0 0 01989 5.4 118.0 0.0 0.000 0 0 0 0 0 0 01990 5.4 94.0 0.0 0.000 0 0 0 0 0 0 01991 5.9 123.0 38.7 10.814 17 7 6 3 2 0 01992 4.6 90.0 0.0 0.000 0 0 0 0 0 0 01993 5.9 140.0 45.3 11.752 16 9 6 3 2 0 01994 5.1 140.0 0.0 0.000 0 0 0 0 0 0 01995 6.3 83.0 15.9 4.769 14 4 3 1 0 0 01996 6.1 150.0 16.6 8.105 21 10 3 1 0 0 01997 5.7 121.0 13.7 1.825 4 2 2 0 0 0 01998 8.0 122.0 39.4 21.050 41 11 11 7 4 0 01999 6.3 141.0 40.4 14.112 26 9 7 4 2 0 02000 6.0 - - - - - - - - - -2001 5.5 - - - - - - - - - -2002 6.0 88.0 13.1 3.641 12 3 2 0 0 0 02003 6.1 93.0 6.4 1.483 7 0 0 0 0 0 02004 7.1 341.0 111.6 16.120 25 5 4 4 2 1 12005 5.6 128.0 9.2 0.596 1 1 0 0 0 0 02006 5.3 185.0 0.0 0.000 0 0 0 0 0 0 02007 6.4 152.0 19.3 5.383 16 4 4 2 0 0 0

4.5.4 Analysis of Detention Scenarios

Several detention scenarios were simulated to determine the effect of allowable maximum water level on the detention capacity of the system, and to minimize the temporary pumping requirement while maintaining the allowable maximum water level. Raising the allowable maximum water level from +5.5 m PWD to +6.0 m PWD will lower the total pumped volume and maximum pumping rate by approximately 66% and 49%, respectively, in an average year. If the gates are closed early in the season, at about +4.5 m PWD external water level, the system will have more storage capacity for internal runoff. However, during the pre-monsoon period the internal storm runoff will have to be

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evacuated by temporary pumping for a much longer period to keep this storage available for later detention. On the contrary, since the external water level is relatively low during the pre-monsoon, most of the internal runoff maybe evacuated by gravity drainage. If the gates are closed late in the season, above +5.0 m PWD external water level, to take more advantage of gravity drainage, the available detention capacity for runoff generated from high-intensity rainfall events may not be sufficient and temporary pumping at much higher rates would be frequently required.

Figure 4.11 shows the simulated water levels and weekly temporary pumping rates for a normal flood year (1996) and two extreme flood years 1998 and 2004. Observed rainfall and external water level were used in the simulations. Although the system performs reasonably well in 1996 requiring relatively low temporary pumping, the system requires much higher pumping rates during a few extreme rainfall events in 1998 and 2004 to maintain the maximum internal water level below +5.5 m PWD. Figure 4.12 shows the simulated water levels and weekly temporary pumping rates at two frequencies of external water level, 1:5 year and 1:50 year, and for an internal rainfall frequency of 1:5 year. Figure 4.12(a) represents a normal detention scenario whereas Figure 4.12(b) represents an extreme detention scenario. The maximum pumping rate and pumped volume are 8.02 m3/s and 39.17 Mm3, respectively, in a normal scenario, and 9.39 m3/s and 50.85 Mm3, respectively, in an extreme scenario. The regulator gates are closed at +5.0 m PWD during the two critical periods, and gravity drainage is allowed through the gates when the external water level is relatively low.

4.6 Stormwater Management in the Combined Hatirjheel System

The combined Hatirjheel system should be efficiently managed for effective detention and gravity drainage of stormwater. The allowable maximum water level in the system is proposed to be +5.5 m PWD. Figure 4.12 indicates that most of the internal storm runoff may be detained in the system or gravity-drained through the Rampura regulator until the external water level rises to +5.0 m PWD, normally in early July, when the gates should be closed. Temporary pumping will be required after closing the gates to maintain the maximum water level below +5.5 m PWD. The regulator gates should be opened whenever the external water level falls below +5.5 m PWD and the internal water level is higher than the external water level. During two ‘critical periods’, July 10 – 20 and September 10 – 25, this level should be +5.0 m PWD instead of +5.5 m PWD. In a normal flood year, the gates may be required to be kept closed most of the time from July to September. The approximate temporary pumping rates for normal and extreme detention scenarios are shown in Figure 4.12. Temporary pumping at relatively high rates may be required during the two ‘critical periods’.

During the dry season the minimum water level in the system should be maintained at +2.5 m PWD. This can be done by raising the invert level of the Rampura regulator to +2.5 m PWD. However, the water level in the system may decrease by about 1.0 m because of infiltration and evaporation during the three dry months, December through February.

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4.7 Modification of the Rampura Regulator and Lake Outlets

The Rampura regulator plays a vital role in stormwater management in the Hatirjheel system. Efficient operation of the regulator is essential for protection of the internal areas from river flood, and drainage of internal storm runoff when the external water level is relatively low. The internal areas around Hatirjheel were inundated by intrusion of river flood in 1998, because of the failure to close the regulator gates in time.

DWASA officials communicated to the study team on several occasions that the gates do not properly lock into place while closing and need to be repaired or rehabilitated. Field observations also suggest an inefficient performance of the gates. Figure 4.10 shows water levels observed at two gauges, one upstream and one downstream of the regulator, during the floods in 1998, 2004 (July) and 2007. Although the internal water level is required to be maintained below +5.5 m PWD, in all occasions the difference between the internal and external water levels was insignificant, indicating intrusion of external river water through the gates.

The minimum water level in the combined Hatirjheel system is proposed to be maintained at +2.5 m PWD, which is higher than the present invert level of the Rampura regulator. Therefore the invert will have to be raised to +2.5 m PWD level to maintain the minimum water level in the proposed lake without closing the regulator gates during the dry season.

Excess water from the Banani lake flows into Hatirjheel through a canal whereas a gate controls the flow from the Gulshan lake. While it is essential to maintain the hydraulic connectivity between Hatirjheel and the lakes, it is also important that Hatirjheel, Banani lake and Gulshan lake independently function as detention areas in their respective sub-catchments. It is proposed that the outlets of the lakes into Hatirjheel be replaced with culverts similar to that under the Tongi Diversion Road. The invert level of these culverts should be set at their present levels or +0.0 m PWD, whichever is higher.

4.8 Conveyance of Peak Discharge through the Culverts

There are two culverts in the area behind Hotel Sonargaon, one under the Railway and the other under the Tongi Diversion Road. Conveyance efficiencies of these culverts under peak discharge conditions were checked using standard hydraulic equations.

4.8.1 Peak Discharge Calculation

Peak discharge, Q, through the culverts was calculated using the Rational formula, Q =CIA, where C = runoff coefficient, I = rainfall intensity, and A = catchment area. A composite runoff coefficient of 0.6 is estimated from secondary sources and land use pattern observed in the satellite pictures. The total area of the sub-catchments discharging through the culverts is 4.965 km2. Using the I-D-F relationship proposed by JICA (1991), peak discharge estimates given in Table 4.4 were obtained for different return periods and rainfall durations.

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Table 4.4: Peak discharge to be conveyed through the culverts.

Duration (min)

Frequency (year)

Intensity, I(mm/hr)

Peak discharge, Q (m3/s)

102 125 1035 155 12750 220 181

302 90 745 115 9450 170 140

602 60 495 82 6750 125 103

1202 42 345 58 4850 85 70

4.8.2 Basis for Adequacy Checking

Allowable discharge through the existing culverts is calculated for different head water elevations following the equations given by Henderson (1966):

2 2

3 3BQ C BH gH , (4.3)

when H/D < 1.2 and the water surface at the entrance does not reach the soffit, and

2 ( )h hQ C BD g H C D , (4.4)

when H/D > 1.2 and the water surface at the entrance reaches the soffit,where CB = coefficient of contraction, Ch = coefficient of contraction in the vertical plane, B = width of opening, D = height of opening, and H = head water.

Allowable Q calculated from Eqns. 4.3 and 4.4 are compared with the Q given in Table 4.1 to determine the conveyance adequacy of the culverts at different rainfall frequency and duration.

However, it is also noted that peak runoff from the sub-catchments does not reach the culverts directly from the storm sewer outfalls. Since the runoff accumulates in the area adjacent to Hotel Sonargaon before being discharged through the culverts, the actual peak velocity may be significantly lower than the estimated value at upstream submerged conditions.

4.8.3 Railway Culvert

Present dimensions of the Railway culvert are given in Table 4.5. The opening height, D,is calculated to be 5.28 m from this data. After analyzing all possible flow conditions

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using Eqns. 4.3 and 4.4, allowable Q through the existing culvert is found to be 118 m3/s. A comparison with the Q given in Table 4.4 indicates that discharge through the Railway culvert is obstructed if runoff is generated by a 10-minute duration rainfall having a return period of 5 or 50 years, or a 30-minute duration rainfall having a return period of 50 years (see Table 4.6). Increasing the opening width to 7 m would allow a peak discharge of 185 m3/s, which would be adequate for conveyance of runoff generated by a 10-minute duration rainfall having a return period of 50 years.

Table 4.5: Present dimensions of the Railway culvert.

Length (m) 9.1Upstream width (m) 4.6Downstream width (m) 5.35Upstream Top RL (m) 7.982Downstream Top RL (m) 7.94Upstream Soffit (m) 6.622Downstream Soffit (m) 6.52Upstream Invert (m) 1.342Downstream Invert (m) 1.342Upstream Water Surface Width (m) 4.5Downstream Water Surface Width (m) 4.6

Table 4.6: Allowable peak discharge through the Railway culvert.

Q (m3/s) Return period (yr) Duration (min)34 2 12048 5 12049 2 6067 5 6070 50 12074 2 3094 5 30103 2 10103 50 60127 5 10140 50 30181 50 10

4.8.4 Tongi Diversion Road Culvert

Present dimensions of the Tongi Diversion Road culvert are given in Table 4.7. The opening height, D, is calculated to be 6.21 m from this data. After analyzing all possibleflow conditions using Eqns. 4.3 and 4.4, allowable Q through the existing culvert is found to be 152 m3/s. A comparison with the Q given in Table 4.4 indicates that discharge

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through the Tongi Diversion Road culvert is obstructed if runoff is generated by a 10-minute duration rainfall having a return period of 50 years (see Table 4.8). Increasing the opening width to 7 m would allow a peak discharge of 185 m3/s, which would be adequate for conveyance of runoff generated by a 10-minute duration rainfall having a return period of 50 years.

Table 4.7: Present dimensions of the Tongi Diversion Road culvert.

Length (m) 30.1Upstream width (m) 6.15Downstream width (m) 6.15Upstream Top RL (m) 7.652Downstream Top RL (m) 7.652Upstream Soffit (m) 7.352Downstream Soffit (m) 7.352Upstream Invert (m) 1.142Downstream Invert (m) 1.075Upstream Water Surface Width (m) 5.7Downstream Water Surface Width (m) 5.7

Table 4.8: Allowable peak discharge through the Tongi Diversion Road culvert.

Q (m3/s) Return period (yr) Duration (min)34 2 12048 5 12049 2 6067 5 6070 50 12074 2 3094 5 30103 2 10103 50 60127 5 10140 50 30181 50 10

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References:

BUET (2005), “Techno-economic and Environmental Assessment of the Proposed Commercial Development of Lowlands Adjacent to Sonargaon Hotel”, Rajdhani Unnayan Kartripakkha (RAJUK), Dhaka.

Henderson, H. M. (1966), “Open Channel Flow”, Collier Macmillan Publishers, London.

Hossain, A.N.H.A, Azam, K.A., and Serajuddin, M. (2004), “Impacts of flood on water supply, sanitation, drainage of Dhaka city and mitigation options”, Proc. National Workshop on Options for Flood Risk and Damage Reduction in Bangladesh, Prime Minister’s Office, The People’s Republic of Bangladesh, Dhaka, pp. 1-17.

JICA (Japan International Cooperation Agency), (1991), “Master Plan for Greater Dhaka Protection Project”, FAP 8A, Main Report and Supporting Reports I & II, Flood Plan Coordination Organization (presently WARPO), Dhaka.

Khan, M.S.A. (2006), “Stormwater flooding in Dhaka city: causes and management”, Journal of Hydrology and Meteorology, Nepal, Vol. 3, No. 1, pp. 77-85.

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Figure 4.1: Sub-catchments within DWASA Drainage Zones F and G, and major storm sewer outfalls.

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Figure 4.2: Cross-section survey locations in the Gulshan and Banani lakes.

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Figure 4.3: Cross-sections in the Gulshan lake.

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Figure 4.4: Cross-sections in the Banani lake.

18

0

1

2

3

4

5

6

7

8

1-J

an

15

-Ja

n

29

-Ja

n

12

-Fe

b

26

-Fe

b1

1-M

ar

25

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pr

22

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6-M

ay

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3-J

un

17

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29

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l

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16

-De

c

Me

an

riv

er

wa

ter

leve

l (m

PW

D)

0

100

200

300

400

500

600

700

800

900

We

ekl

y to

tal r

ain

fall

(mm

)

RainfallWater level

0

1000

2000

3000

4000

5000

6000

7000

8000

0 1 2 3 4 5 6 7 8

Water level in Hatirjheel (m PWD)

To

tal

det

enti

on

vo

lum

e (X

10

3 m3 )

Figure 4.5: Total detention volume at different water levels in Hatirjheel.

Figure 4.6: Mean weekly water level in the Balu river and 1:5 yr rainfall in Dhaka.

19

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

1-Ja

n

29-J

an

26-F

eb

26-M

ar

23-A

pr

21-M

ay

18-J

un

16-J

ul

13-A

ug

10-S

ep

8-O

ct

5-N

ov

3-D

ec

31-D

ec

Ob

serv

ed r

iver

wat

er l

evel

(m

PW

D)

0

100

200

300

400

500

600

700

1:5

yr r

ain

fall

(m

m)

0

10

20

30

40

50

1-Ja

n

16-

Jan

31-

Jan

15

-Fe

b

2-M

ar

17

-Ma

r

1-A

pr

16-

Ap

r

1-M

ay

16

-Ma

y

31

-Ma

y

15-

Jun

30-

Jun

15

-Ju

l

30

-Ju

l

14

-Au

g

29

-Au

g

13

-Se

p

28

-Se

p

13-

Oct

28-

Oct

12

-No

v

27

-No

v

12

-De

c

27

-De

c

wa

ter

leve

l (m

) a

nd

pu

mp

ing

ra

te (

m3/s

) 0

60

120

180

240

300

rain

fall

(m

m)

Pumping rate (cumec)

2007 observed rainfall (mm)

2007 observed external WL (m)

Simulated internal WL (m)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

19-D

ec3-

Jan

18-J

an

2-F

eb17

-Feb

4-M

ar19

-Mar

3-A

pr18

-Apr

3-M

ay18

-May

2-Ju

n17

-Jun

2-Ju

l17

-Jul

1-A

ug

16-A

ug31

-Aug

15-S

ep30

-Sep

15-O

ct30

-Oct

14-N

ov29

-Nov

14-D

ec29

-Dec

13-J

an

Week No.

Riv

er w

ater

leve

l (m

PW

D)

1:5 yr1:50 yr199619982004

First peak

Second peak

Figure 4.7: Critical periods for stormwater detention in the Hatirjheel system.

Figure 4.8: External water level in normal and extreme flood years.

Figure 4.9: Simulation result for daily performance of the system in 2007.

20

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

Au

g-0

3

Au

g-0

8

Au

g-1

3

Au

g-1

8

Au

g-2

3

Au

g-2

8

Se

p-0

2

Se

p-0

7

Se

p-1

2

Se

p-1

7

Wa

ter

lev

el (

m P

WD

)

0

100

200

300

400

500

600

Ra

infa

ll (m

m)

RainfallExternal WLInternal WL

(a) Flood 1998

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

Jul-

12

Jul-

14

Jul-

16

Jul-

18

Jul-

20

Jul-

22

Jul-

24

Jul-

26

Jul-

28

Jul-

30

Wa

ter

lev

el (

m P

WD

)

0

100

200

300

400

500

600

Ra

infa

ll (m

m)


(b) Flood 2004

4.5

5.0

5.5

6.0

6.5

7.0

7.5

8.0

Jul-

25

Jul-

27

Jul-

29

Jul-

31

Au

g-0

2

Au

g-0

4

Au

g-0

6

Au

g-0

8

Au

g-1

0

Au

g-1

2

Au

g-1

4

Au

g-1

6

Au

g-1

8

Wa

ter

lev

el (

m P

WD

)

0

100

200

300

400

500

600

Ra

infa

ll (m

m)


(c) Flood 2007

Figure 4.10: Observed water level at Rampura during major floods.

21

0

2

4

6

8

10

12

14

wee

k1

wee

k3

wee

k5

wee

k7

wee

k9

wee

k11

wee

k13

wee

k15

wee

k17

wee

k19

wee

k21

wee

k23

wee

k25

wee

k27

wee

k29

wee

k31

wee

k33

wee

k35

wee

k37

wee

k39

wee

k41

wee

k43

wee

k45

wee

k47

wee

k49

wee

k51

Wat

er l

evel

(m

) an

d P

um

pin

g r

ate

(m3/s

)

0

100

200

300

400

500

600

Rai

nfa

ll (

mm

)

pumping rate

1996 observed rainfall

1996 observed external water level

simulated internal water level

(a) 1996

0

2

4

6

8

10

12

14

wee

k1

wee

k3

wee

k5

wee

k7

wee

k9

wee

k11

wee

k13

wee

k15

wee

k17

wee

k19

wee

k21

wee

k23

wee

k25

wee

k27

wee

k29

wee

k31

wee

k33

wee

k35

wee

k37

wee

k39

wee

k41

wee

k43

wee

k45

wee

k47

wee

k49

wee

k51

Wat

er l

evel

(m

) an

d P

um

pin

g r

ate

(m3/s

)

0

100

200

300

400

500

600

Rai

nfa

ll (

mm

)

pumping rate




(b) 1998

0

2

4

6

8

10

12

14

wee

k1

wee

k3

wee

k5

wee

k7

wee

k9

wee

k11

wee

k13

wee

k15

wee

k17

wee

k19

wee

k21

wee

k23

wee

k25

wee

k27

wee

k29

wee

k31

wee

k33

wee

k35

wee

k37

wee

k39

wee

k41

wee

k43

wee

k45

wee

k47

wee

k49

wee

k51

Wat

er l

evel

(m

) an

d P

um

pin

g r

ate

(m3/s

)

0

100

200

300

400

500

600

Rai

nfa

ll (

mm

)

pumping rate




(c) 2004

Figure 4.11: Detention scenarios for normal and extreme flood years.

22

0

2

4

6

8

10

12

14

wee

k1

wee

k3

wee

k5

wee

k7

wee

k9

wee

k11

wee

k13

wee

k15

wee

k17

wee

k19

wee

k21

wee

k23

wee

k25

wee

k27

wee

k29

wee

k31

wee

k33

wee

k35

wee

k37

wee

k39

wee

k41

wee

k43

wee

k45

wee

k47

wee

k49

wee

k51

Wat

er l

evel

(m

) an

d P

um

pin

g r

ate

(m3/s

)

0

100

200

300

400

500

600

Rai

nfa

ll (

mm

)

pumping rate

1:5 year rainfall

1:5 year external water level


(a) 1:5 yr external water level

0

2

4

6

8

10

12

14

wee

k1

wee

k3

wee

k5

wee

k7

wee

k9

wee

k11

wee

k13

wee

k15

wee

k17

wee

k19

wee

k21

wee

k23

wee

k25

wee

k27

wee

k29

wee

k31

wee

k33

wee

k35

wee

k37

wee

k39

wee

k41

wee

k43

wee

k45

wee

k47

wee

k49

wee

k51

Wat

er l

evel

(m

) an

d P

um

pin

g r

ate

(m3/s

)

0

100

200

300

400

500

600

Rai

nfa

ll (

mm

)

pumping rate

1:5 year rainfall

1:50 year external water level


(b) 1:50 yr external water level

Figure 4.12: Detention scenario for design conditions.

23

APPENDIX ASimulation of Daily Performance from 1985 to 2007

Figure A.1: Simulation results for the year 1985.



0

10

20

30

40

50

1-J

an

16

-Ja

n

31

-Ja

n

15

-Fe

b

2-M

ar

17

-Ma

r

1-A

pr

16

-Ap

r

1-M

ay

16

-Ma

y

31

-Ma

y

15

-Ju

n

30

-Ju

n

15

-Ju

l

30

-Ju

l

14

-Au

g

29

-Au

g

13

-Se

p

28

-Se

p

13

-Oct

28

-Oct

12

-No

v

27

-No

v

12

-De

c

27

-De

c

wa

ter

lev

el

(m)

an

d p

um

pin

g r

ate

(m

3 /s) 0

60

120

180

240

300

rain

fall

(m

m)





0

10

20

30

40

50

1-J

an

16

-Ja

n

31

-Ja

n

15

-Fe

b

2-M

ar

17

-Ma

r

1-A

pr

16

-Ap

r

1-M

ay

16

-Ma

y

31

-Ma

y

15

-Ju

n

30

-Ju

n

15

-Ju

l

30

-Ju

l

14

-Au

g

29

-Au

g

13

-Se

p

28

-Se

p

13

-Oct

28

-Oct

12

-No

v

27

-No

v

12

-De

c

27

-De

c

wa

ter

lev

el

(m)

an

d p

um

pin

g r

ate

(m

3 /s) 0

60

120

180

240

300

rain

fall

(m

m)





0

10

20

30

40

50

1-J

an

16

-Ja

n

31

-Ja

n

15

-Fe

b

2-M

ar

17

-Ma

r

1-A

pr

16

-Ap

r

1-M

ay

16

-Ma

y

31

-Ma

y

15

-Ju

n

30

-Ju

n

15

-Ju

l

30

-Ju

l

14

-Au

g

29

-Au

g

13

-Se

p

28

-Se

p

13

-Oct

28

-Oct

12

-No

v

27

-No

v

12

-De

c

27

-De

c

wa

ter

lev

el

(m)

an

d p

um

pin

g r

ate

(m

3/s

) 0

60

120

180

240

300

rain

fall

(m

m)





24




0

10

20

30

40

50

1-J

an

16

-Ja

n

31

-Ja

n

15

-Fe

b

1-M

ar

16

-Ma

r

31

-Ma

r

15

-Ap

r

30

-Ap

r

15

-Ma

y

30

-Ma

y

14

-Ju

n

29

-Ju

n

14

-Ju

l

29

-Ju

l

13

-Au

g

28

-Au

g

12

-Se

p

27

-Se

p

12

-Oct

27

-Oct

11

-No

v

26

-No

v

11

-De

c

26

-De

c

wa

ter

lev

el

(m)

an

d p

um

pin

g r

ate

(m

3 /s) 0

60

120

180

240

300

rain

fall

(m

m)





0

10

20

30

40

50

1-J

an

16

-Ja

n

31

-Ja

n

15

-Fe

b

2-M

ar

17

-Ma

r

1-A

pr

16

-Ap

r

1-M

ay

16

-Ma

y

31

-Ma

y

15

-Ju

n

30

-Ju

n

15

-Ju

l

30

-Ju

l

14

-Au

g

29

-Au

g

13

-Se

p

28

-Se

p

13

-Oct

28

-Oct

12

-No

v

27

-No

v

12

-De

c

27

-De

c

wa

ter

lev

el

(m)

an

d p

um

pin

g r

ate

(m

3 /s) 0

60

120

180

240

300

rain

fall

(m

m)





0

10

20

30

40

50

1-J

an

16

-Ja

n

31

-Ja

n

15

-Fe

b

2-M

ar

17

-Ma

r

1-A

pr

16

-Ap

r

1-M

ay

16

-Ma

y

31

-Ma

y

15

-Ju

n

30

-Ju

n

15

-Ju

l

30

-Ju

l

14

-Au

g

29

-Au

g

13

-Se

p

28

-Se

p

13

-Oct

28

-Oct

12

-No

v

27

-No

v

12

-De

c

27

-De

c

wa

ter

lev

el

(m)

an

d p

um

pin

g r

ate

(m

3 /s) 0

60

120

180

240

300

rain

fall

(m

m)





25




0

10

20

30

40

50

1-J

an

16

-Ja

n

31

-Ja

n

15

-Fe

b

2-M

ar

17

-Ma

r

1-A

pr

16

-Ap

r

1-M

ay

16

-Ma

y

31

-Ma

y

15

-Ju

n

30

-Ju

n

15

-Ju

l

30

-Ju

l

14

-Au

g

29

-Au

g

13

-Se

p

28

-Se

p

13

-Oct

28

-Oct

12

-No

v

27

-No

v

12

-De

c

27

-De

c

wa

ter

lev

el

(m)

an

d p

um

pin

g r

ate

(m

3 /s) 0

60

120

180

240

300

rain

fall

(m

m)





0

10

20

30

40

50

1-J

an

16

-Ja

n

31

-Ja

n

15

-Fe

b

1-M

ar

16

-Ma

r

31

-Ma

r

15

-Ap

r

30

-Ap

r

15

-Ma

y

30

-Ma

y

14

-Ju

n

29

-Ju

n

14

-Ju

l

29

-Ju

l

13

-Au

g

28

-Au

g

12

-Se

p

27

-Se

p

12

-Oct

27

-Oct

11

-No

v

26

-No

v

11

-De

c

26

-De

c

wa

ter

lev

el

(m)

an

d p

um

pin

g r

ate

(m

3 /s) 0

60

120

180

240

300

rain

fall

(m

m)





0

10

20

30

40

50

1-J

an

16

-Ja

n

31

-Ja

n

15

-Fe

b

2-M

ar

17

-Ma

r

1-A

pr

16

-Ap

r

1-M

ay

16

-Ma

y

31

-Ma

y

15

-Ju

n

30

-Ju

n

15

-Ju

l

30

-Ju

l

14

-Au

g

29

-Au

g

13

-Se

p

28

-Se

p

13

-Oct

28

-Oct

12

-No

v

27

-No

v

12

-De

c

27

-De

c

wa

ter

lev

el

(m)

an

d p

um

pin

g r

ate

(m

3/s

) 0

60

120

180

240

300

rain

fall

(m

m)





26




0

10

20

30

40

50

1-J

an

16

-Ja

n

31

-Ja

n

15

-Fe

b

2-M

ar

17

-Ma

r

1-A

pr

16

-Ap

r

1-M

ay

16

-Ma

y

31

-Ma

y

15

-Ju

n

30

-Ju

n

15

-Ju

l

30

-Ju

l

14

-Au

g

29

-Au

g

13

-Se

p

28

-Se

p

13

-Oct

28

-Oct

12

-No

v

27

-No

v

12

-De

c

27

-De

c

wa

ter

lev

el

(m)

an

d p

um

pin

g r

ate

(m

3 /s) 0

60

120

180

240

300

rain

fall

(m

m)





0

10

20

30

40

50

1-J

an

16

-Ja

n

31

-Ja

n

15

-Fe

b

2-M

ar

17

-Ma

r

1-A

pr

16

-Ap

r

1-M

ay

16

-Ma

y

31

-Ma

y

15

-Ju

n

30

-Ju

n

15

-Ju

l

30

-Ju

l

14

-Au

g

29

-Au

g

13

-Se

p

28

-Se

p

13

-Oct

28

-Oct

12

-No

v

27

-No

v

12

-De

c

27

-De

c

wa

ter

lev

el

(m)

an

d p

um

pin

g r

ate

(m

3 /s) 0

60

120

180

240

300

rain

fall

(m

m)





0

10

20

30

40

50

1-J

an

16

-Ja

n

31

-Ja

n

15

-Fe

b

1-M

ar

16

-Ma

r

31

-Ma

r

15

-Ap

r

30

-Ap

r

15

-Ma

y

30

-Ma

y

14

-Ju

n

29

-Ju

n

14

-Ju

l

29

-Ju

l

13

-Au

g

28

-Au

g

12

-Se

p

27

-Se

p

12

-Oct

27

-Oct

11

-No

v

26

-No

v

11

-De

c

26

-De

c

wa

ter

lev

el

(m)

an

d p

um

pin

g r

ate

(m

3 /s) 0

60

120

180

240

300

rain

fall

(m

m)





27




0

10

20

30

40

50

1-J

an

16

-Ja

n

31

-Ja

n

15

-Fe

b

2-M

ar

17

-Ma

r

1-A

pr

16

-Ap

r

1-M

ay

16

-Ma

y

31

-Ma

y

15

-Ju

n

30

-Ju

n

15

-Ju

l

30

-Ju

l

14

-Au

g

29

-Au

g

13

-Se

p

28

-Se

p

13

-Oct

28

-Oct

12

-No

v

27

-No

v

12

-De

c

27

-De

c

wa

ter

lev

el

(m)

an

d p

um

pin

g r

ate

(m

3 /s) 0

60

120

180

240

300

rain

fall

(m

m)





0

10

20

30

40

50

1-J

an

16

-Ja

n

31

-Ja

n

15-F

eb

2-M

ar

17

-Ma

r

1-A

pr

16

-Ap

r

1-M

ay

16-

Ma

y

31-

Ma

y

15

-Ju

n

30

-Ju

n

15

-Jul

30

-Jul

14-A

ug

29-A

ug

13-S

ep

28-S

ep

13

-Oct

28

-Oct

12

-No

v

27

-No

v

12-D

ec

27-D

ec

wa

ter

lev

el

(m)

an

d p

um

pin

g r

ate

(m

3/s

) 0

60

120

180

240

300

rain

fall

(m

m)





0

10

20

30

40

50

1-J

an

16

-Ja

n

31

-Ja

n

15

-Fe

b

2-M

ar

17

-Ma

r

1-A

pr

16

-Ap

r

1-M

ay

16

-Ma

y

31

-Ma

y

15

-Ju

n

30

-Ju

n

15

-Ju

l

30

-Ju

l

14

-Au

g

29

-Au

g

13

-Se

p

28

-Se

p

13

-Oct

28

-Oct

12

-No

v

27

-No

v

12

-De

c

27

-De

c

wa

ter

lev

el

(m)

an

d p

um

pin

g r

ate

(m

3 /s) 0

60

120

180

240

300

rain

fall

(m

m)





28




0

10

20

30

40

501

-Ja

n

16

-Ja

n

31

-Ja

n

15

-Fe

b

2-M

ar

17

-Ma

r

1-A

pr

16

-Ap

r

1-M

ay

16

-Ma

y

31

-Ma

y

15

-Ju

n

30

-Ju

n

15

-Ju

l

30

-Ju

l

14

-Au

g

29

-Au

g

13

-Se

p

28

-Se

p

13

-Oct

28

-Oct

12

-No

v

27

-No

v

12

-De

c

27

-De

c

wa

ter

lev

el

(m)

an

d p

um

pin

g r

ate

(m

3 /s) 0

60

120

180

240

300

rain

fall

(m

m)





0

10

20

30

40

50

1-J

an

16

-Ja

n

31

-Ja

n

15

-Fe

b

2-M

ar

17

-Ma

r

1-A

pr

16

-Ap

r

1-M

ay

16

-Ma

y

31

-Ma

y

15

-Ju

n

30

-Ju

n

15

-Ju

l

30

-Ju

l

14

-Au

g

29

-Au

g

13

-Se

p

28

-Se

p

13

-Oct

28

-Oct

12

-No

v

27

-No

v

12

-De

c

27

-De

c

wa

ter

lev

el

(m)

an

d p

um

pin

g r

ate

(m

3 /s) 0

60

120

180

240

300

rain

fall

(m

m)





0

10

20

30

40

50

1-J

an

16

-Ja

n

31

-Ja

n

15

-Fe

b

1-M

ar

16

-Ma

r

31

-Ma

r

15

-Ap

r

30

-Ap

r

15

-Ma

y

30

-Ma

y

14

-Ju

n

29

-Ju

n

14

-Ju

l

29

-Ju

l

13

-Au

g

28

-Au

g

12

-Se

p

27

-Se

p

12

-Oct

27

-Oct

11

-No

v

26

-No

v

11

-De

c

26

-De

c

wa

ter

lev

el

(m)

an

d p

um

pin

g r

ate

(m

3 /s) 0

60

120

180

240

300

rain

fall

(m

m)





29




0

10

20

30

40

50

1-J

an

16

-Ja

n

31

-Ja

n

15

-Fe

b

2-M

ar

17

-Ma

r

1-A

pr

16

-Ap

r

1-M

ay

16

-Ma

y

31

-Ma

y

15

-Ju

n

30

-Ju

n

15

-Ju

l

30

-Ju

l

14

-Au

g

29

-Au

g

13

-Se

p

28

-Se

p

13

-Oct

28

-Oct

12

-No

v

27

-No

v

12

-De

c

27

-De

c

wa

ter

lev

el

(m)

an

d p

um

pin

g r

ate

(m

3 /s) 0

60

120

180

240

300

rain

fall

(m

m)





0

10

20

30

40

50

1-J

an

16

-Ja

n

31

-Ja

n

15

-Fe

b

2-M

ar

17

-Ma

r

1-A

pr

16

-Ap

r

1-M

ay

16

-Ma

y

31

-Ma

y

15

-Ju

n

30

-Ju

n

15

-Ju

l

30

-Ju

l

14

-Au

g

29

-Au

g

13

-Se

p

28

-Se

p

13

-Oct

28

-Oct

12

-No

v

27

-No

v

12

-De

c

27

-De

c

wa

ter

lev

el

(m)

an

d p

um

pin

g r

ate

(m

3 /s) 0

60

120

180

240

300

rain

fall

(m

m)





0

10

20

30

40

50

1-J

an

16

-Ja

n

31

-Ja

n

15

-Fe

b

2-M

ar

17

-Ma

r

1-A

pr

16

-Ap

r

1-M

ay

16

-Ma

y

31

-Ma

y

15

-Ju

n

30

-Ju

n

15

-Ju

l

30

-Ju

l

14

-Au

g

29

-Au

g

13

-Se

p

28

-Se

p

13

-Oct

28

-Oct

12

-No

v

27

-No

v

12

-De

c

27

-De

c

wa

ter

lev

el

(m)

an

d p

um

pin

g r

ate

(m

3 /s) 0

60

120

180

240

300

rain

fall

(m

m)





Documents

Draft - Bangladesh University of Engineering and Technologymsalamkhan.buet.ac.bd/teaching_msk_files/Hatirjheel_project_3.pdf · full stage, the retention capacities of the Gulshan