Time Area Method 2003
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2.2.1.1 Wo rked Example 2.7- A pplying Ti me-Area Method to a Catchment in Ipoh Calculate the runo hydrograph or the de!ign o a ma"or !torm#ater !y!tem near the outlet o a catchment #ith medium den!ity re!idential de$elopment in Ipoh a! !ho#n in %igure 2.7. The Tc i! &' min. The de!ign A(I or the ma"or !y!tem i! 1'' year! )a!ed on Ta)le *.1. Step 1- Delineate Subcatchment Using Isochrones The #hole !tudy area i! di$ided into !u)catchment area! )y plotting + minute! i!ochrone! #hich repre!ent line! o e,ual lo# time to the outlet a! !ho#n in %igure 2.7. The time inter$al i! determined u!ing the (ational Method approach. The !u)catchment area! )et#een each !ucce!!i$e pair o i!ochrone! are mea!ured and recorded a! !ho#n in Column % o Ta)le 2.+. Step 2- Calculate Design Effective Rainfall %or Ipoh the e!ign (ainall Inten!ity or A(I o 1'' yr i! 17/.+ mm0hr. ence Tot al (ainall epth 17/.+3'.+ 44.2+ mm Column 5 !ummari!e! the temporal pattern ratio! or thi! particular location. The (ainall epth in Column C i! c alculated )y multiplying Column 5 )y Total (ainall epth. 6i!ted in Column are the lo!!e! )a!ed on Ta )le 1*.*. 2*+1+7&+.doc %ree !ot#are at http800### .m!mam. com 1
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Transcript of Time Area Method 2003
ADVANCED MSMA COURSE
2.2.1.1 Worked Example 2.7- Applying Time-Area Method to a Catchment in Ipoh
Calculate the runoff hydrograph for the design of a major stormwater system near the outlet of a catchment with medium density residential development in Ipoh as shown in Figure 2.7. The Tc is 30 min.
The design ARI for the major system is 100 years based on Table 4.1.
Step 1- Delineate Subcatchment Using Isochrones
The whole study area is divided into subcatchment areas by plotting 5 minutes isochrones which represent lines of equal flow time to the outlet as shown in Figure 2.7.
The time interval is determined using the Rational Method approach.
The subcatchment areas between each successive pair of isochrones are measured and recorded as shown in Column F of Table 2.5.
Step 2- Calculate Design Effective Rainfall
For Ipoh, the Design Rainfall Intensity for ARI of 100 yr is 176.5 mm/hr.
Hence Total Rainfall Depth= 176.5*0.5= 88.25 mm
Column B summarises the temporal pattern ratios for this particular location.
The Rainfall Depth in Column C is calculated by multiplying Column B by Total Rainfall Depth.
Listed in Column D are the losses based on Table 14.4.
Column E shows the Effective Rainfall which is equal to Rainfall Depth (Column C) minus the Losses (Column D).
Step 3- Calculate Discharge Hydrograph
The ordinate of the discharge hydrograph is calculated as follows:
First, multiply the subcatchment area (Column F) with the Effective Rainfall (Column E) based on Equation 2.11 as follows:
These are tabulated in Columns G to L.
Next, sum Columns G to L to get the total hydrograph ordinate in Column M.
The peak discharges are computed as shown in Table 2.5 and the hydrograph plotted in the table.
The above computation can be easily done using a spreadsheet.
Equation 2.11 can be expanded for all subcatchment areas and all the ordinates of the Effective Rainfall Depth as follows:
The above equations can be repeated until q12.Note that I7A1 is zero.
TABLE 2.5 TIME-AREA METHOD CALCULATION
FIGURE 2.7 STUDY CATCHMENT FOR TIME-AREA METHOD
2.2.1.2 How to Create the Spreadsheet
Following are the steps for applying the Time-Area Method using a spreadsheet (Excel Filename: DrQuekTimeArea1a.zip):
1. Open the spreadsheet DrQuekTimeArea1a.zip.2. Enter the Areas between successive isochrones into Column F.
3. Enter the design rainfall intensity (mm/hr) for ARI of 100 years- Cell B4.
4. Convert the above into rainfall depth in mm- in Cell D4.
5. Enter the rainfall temporal pattern- Column B.
6. Calculate the Rainfall Depth in Column C by multiplying Column B by Total Rainfall Depth (Cell D4).
7. Enter the losses in Column D based on Table 14.4.
8. Calculate the Effective Rainfall (Column E) which is equal to Rainfall Depth (Column C) minus the Losses (Column D).
9. Copy Cells (F15..F20) values to Cells (G13..L13)- notice cells in blue! This is done to simplify the use of formulas in spreadsheet.
10. The ordinate of the discharge hydrograph is calculated by multiplying the subcatchment area (Column F) with the Effective Rainfall (Column E) based on Equation 2.11 as follows:
11. Important! Notice the conversation factor: I (mm) x A (m2)/(1000 x 5 min x 60 s)= q (m3/s).
12. These are tabulated in Columns G to L.
13. Next, sum Columns G to L to get the total hydrograph ordinate in Column M.
14. The rainfall hyetograph and flow hydrograph are plotted as shown.
Two questions to test your understanding of the Time-Area Method:
1. Question 1: Why are the cells in yellow blank? Answer:_______________________________________________________________________________________________________________________
2. Question 2: Why are the cells in the lower diagonal all zeros? Answer:_______________________________________________________________________________________________________________________
2.2.1.3 Worked Example 2.8- Applying Time-Area Method to A Catchment in Penang
Following are relevant data:
1. Design based on 100 year storm for Penang.
2. Enter the Areas between 5 successive 5-minute isochrones: 90,000, 107,000, 115,000, 120,000, 99,000.
3. Assume the following rainfall temporal pattern: 0.1, 0.2, 0.39, 0.21, 0.1.
4. Enter the losses in Column C based on Table 14.4- for pervious areas (Initial loss-proportional loss).
5. What is the Qp of the hydrograph? Answer:____________________________
2.2.1.4 Worked Example 2.9- Applying Time-Area Method to A Catchment in Kuching, Sarawak
Following are relevant data:
1. Design based on 5 year storm for Kuching, Sarawak.
2. Areas between 7 successive 5-minute isochrones: 20,000, 90,000, 120,000, 80,000, 70,000, 65,000 and 23,000.3. Enter the losses in Column C based on Table 14.4- for impervious area.
4. What is the Qp of the hydrograph? Answer:____________________________
E
EE
EEE
EOutlet
ET30 (30 min)
T2 (10 min)
T0 (0 min)
T5 (25 min)
T4 (20 min)
T1 (5 min)
T3 (15 min)
EEEE
1NewApproachMSMAM11.doc Free software at http://www.msmam.com
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_1322381974.xlsDrQuek
TIME-AREA CALCULATION
Design ARI (yr)=100
Tc (min)30
Design I (mm/hr)=176.5Design P (mm)=88.25
Pattern Type=3
Time Interval (min)=5
Duration (min)30
Location=Ipoh
88.25123456
ABCDEFGHIJKLM
TimeRainfall RatioRainfallLossesERTime-Area CurveRunoff Generated By the Effective Rainfall (mm)Hydrograph
(min)(mm)(mm)(mm)(m2)12.6222.0629.127.949.715.30(m3/s)
00.0000.000.000.0000.000.00
50.16014.121.5012.62120,0005.050.005.051
100.25022.060.0022.06100,0004.218.830.0013.032
150.33029.120.0029.12300,00012.627.3511.650.0031.623
200.0907.940.007.94880,00037.0222.069.713.180.0071.974
250.1109.710.009.71678,00028.5264.7229.122.653.880.00128.895
300.0605.300.005.30200,0008.4149.8685.437.943.242.12157.006
350.0014.7165.8223.309.711.77115.307
400.000.0019.4217.9528.485.3071.148
450.000.000.005.3021.9415.5342.779
500.000.000.000.006.4711.9718.4410
550.000.000.000.000.003.533.5311
600.000.000.000.000.000.000.0012
IMPORTANT NOTE:
This is a free evaluation program developed by DR. QUEK & ASSOCIATES
for students who attended the "Workshop Series On Compliance With MSMAM".
You may distribute this version Free-of-Charge!
To download many more FREE programs on MSMAM go to:
Website: http://www.msmam.com
Email: [email protected]
DrQuek
TIME (MIN)
FLOW (M3/S)
INFLOW HYDROGRAPH
TIME PERIODS @ 5 MIN INTERVALS
EFFECTIVE RAINFALL (MM)
RAINFALL EXCESS HYETOGRAPH
TIME PERIODS @ 5 MIN INTERVALS
RAINFALL (MM)
RAINFALL HYETOGRAPH
IMPORTANT NOTE:This is a free evaluation program developed by DR. QUEK & ASSOCIATES for students who attended the "Workshop Series On Compliance With MSMAM".You may distribute this version Free-of-Charge!To download many more FREE programs on MSMAM go to: Website: http://www.msmam.com Email: [email protected]
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