Engineering Hydrology Chapter 2...
Transcript of Engineering Hydrology Chapter 2...
Engineering Hydrology
Chapter 2 Precipitation
Eng. Naeem Kaheil
2016 - 2017
Adequacy of Rain gauge
Stations 2
vC
N PC m
V1100
1
)(1
2
1m
PPm
i
m
m
iPm
P1
1
N = optimal number of stations
ε = allowable degree of error in the estimate of the mean
rainfall
Cv = coefficient of variation of the rainfall values at the
existing m stations (in percent) = standard deviation , = mean precipitation
Pi = precipitation magnitude in the i4th station 1m P
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Question_1 • A catchment area has seven rain gauge
stations. In a year the annual rainfall recorded by the gauge are as follows:
• For a 5% error in the estimation of the mean rainfall, calculate the minimum number of additional stations required to be established in the catchment ?
V U T S R Q P Station
146.9 102.1 165.2 108.5 118.2 142.1 130 Rainfall
(cm)
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Question_1 • Solution
o 𝑷 = 𝟏𝟑𝟎+𝟏𝟒𝟐.𝟏+𝟏𝟏𝟖.𝟐+𝟏𝟎𝟖.𝟐+𝟏𝟔𝟓.𝟐+𝟏𝟎𝟐.𝟏+𝟏𝟒𝟔.𝟗
𝟕 = 130.42 cm
o 𝝈 = (𝑷𝒊−𝑷 )𝟐𝒎𝟏
𝒎−𝟏 =
(𝑷𝒊−𝟏𝟎𝟑.𝟒𝟐)𝟐𝟕𝟏
𝟕−𝟏 = 22.5
o 𝑪𝒗 = 𝟏𝟎𝟎∗𝝈
𝑷 =
𝟏𝟎𝟎∗𝟐𝟐.𝟓
𝟏𝟑𝟎.𝟒𝟐= 𝟏𝟕. 𝟐𝟖𝟔
o 𝑵 = (𝑪𝒗
𝜺)𝟐 = (
𝟏𝟕.𝟐𝟖𝟔
𝟓)𝟐 = 𝟏𝟐
o 5 additional stations are required
V U T S R Q P Station
146.9 102.1 165.2 108.5 118.2 142.1 130 Rainfall
(cm)
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Question_2
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Estimation of Missing Data
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Estimation of Missing Data
• P1 , P2 … Pm : القراءات للمحطات بعد الخطأ
• N1 , N2 .. Nm القراءات للمحطات قبل الخطأ
• M : عدد المحطات بدون المحطة الخطأ
• Nx : الخطأقيمة المطر عند المحطة التي حدث فيها
• Px : قيمة المطر المفقودة عند المحطة التي حدث فيها الخطأ
% error = {(Nx – Navg)/Nx} * 100%
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Question_3 • The normal annual precipitation of five rain
gauge stations P,Q,R,S and T are
respectively 125, 102, 76, 113 and 137 cm.
During a particular storm the precipitation
recorded by stations P,Q,R and S are 13.2,
9.2, 6.8 and 10.2 cm respectively. The
instrument at station T was inoperative
during that storm. Estimate the rainfall at station T during that storm?
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Question_3
• Solution
• Navg = (125 + 102 + 76 + 113 + 137) / 5 = 110.6
• % error = {(137 – 110.6)/137}*100% = 19.27% > 10%
cmPx 84.12113
2.10
76
8.6
102
2.9
125
2.13
4
137
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Test for Consistency of Record
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Test for Consistency of Record
• Solution steps 1. Arrange in reverse order the data of: ( للقديم الجديد من )
1. The annual rainfall of station X (𝑷𝒙)
2. The average rainfall of the group of base stations (𝑷𝒂𝒗𝒈)
2. Calculate the accumulated precipitation of station X ( 𝑷𝒙)
and the accumulated values of average of the selected base group ( 𝑷𝒂𝒗𝒈).
3. Plot 𝑷𝒙 againest 𝑷𝒂𝒗𝒈 for various consecutive time periods
{ Double mass curve }
4. Locate the break in the slope of the resulting plot (which is the
change in precipitation regime of station X)
5. Correct the precipitation values at station X beyond the
period of regime change by:
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Question_4
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year station A
)cm(
cum. ann. rainfall
at station A
station 8
average
)cm(
cum. average
ann. rainfall for
stations 8
1967 163 163 161 161
1966 130 293 146 307
1965 137 430 130 437
1964 130 560 143 580
1963 140 700 135 715
1962 142 842 163 878
1961 148 990 135 1013
1960 95 1085 115 1128
1959 132 1217 143 1271
1958 145 1362 155 1426
1957 158 1520 164 1590
1956 141 1661 156 1746
1955 196 1857 193 1939
1954 160 2017 128 2067
1953 144 2161 117 2184
1952 196 2357 152 2336
1951 168 2525 155 2491
1950 194 2719 161 2652
1949 162 2881 147 2799
1948 178 3059 146 2945
1947 144 3203 132 3077
1946 177 3380 143 3220
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67 66
65 64
63 62
61 60
59 58
57 56
55 54
53 52
51 50
49 48
47 46
0
0.5
1
1.5
2
2.5
3
3.5
4
0 0.5 1 1.5 2 2.5 3 3.5
Acc
um
ula
ted
an
nu
al r
ain
fall
at
X
in u
nit
s o
f 1
03 cm
accumulated annual rainfall of 8 station mean
in units of 103 cm
Mc= 0.9496
Ma = 1.172
= 0.6 / 0.75 = 0.8 or = 0.9496/1.172= 0.81
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2.2-2.8
0.6=
2.2-2.95 0.75 =
corrected value = original value * (Mc/Ma)
year original value( cm) corrected value
(cm)
1954 160 129.6
1953 144 116.64
1952 196 158.76
1951 168 136.08
1950 194 157.14
1949 162 131.22
1948 178 144.18
1947 144 116.64
1946 177 143.37
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Rainfall Representation
1) Mass curve
2) Hyetograph
3) IDF curve
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Mass curve
storm -1نهاية ال
ال
يوجد
أمطار
ال
يوجد
أمطار
Slope of the curve = intensity
i= 2.4/8=0.3cm/
hr
i= 4.4-2.4/8=0.25c
m/hr
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Hyetograph
area under hyetograph = total preci. in that period
Every storm has its own Hyetograph
Area = 0.3*8 = 2.8 cm
Sum Area of Every Boxes = 10 cm
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Question_5
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Question_5 • Solution
Time since start
of storm (min)rainfall (cm)
Intensity
(cm/min)Cumulative ( cm )
30 1.75 0.0583 1.75
60 2.25 0.075 4
90 6 0.2 10
120 4.5 0.15 14.5
150 2.5 0.083 17
180 1.5 0.05 18.5
210 0.75 0.025 19.25
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Question_5 • A. Hyetograph representation
0
0.05
0.1
0.15
0.2
0.25
30 60 90 120 150 180 210
Inte
nsi
ty (
cm
/min
)
Time (min)
Hyetograph of Storm
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Question_5
• b. Mass curve representation
0
5
10
15
20
25
0 50 100 150 200 250
Cu
mu
lati
ve
rain
fall
(cm
)
Time (min)
Mass curve of Storm
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Question_6
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Question_6 • Solution
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Question_7
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Mean Precipitation over an Area
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Mean Precipitation over an Area
:هذه الطريقة تعتمد على حساب المساحة التي تمثلها كل محطة ولحساب المساحة
.A وصل المحطات ببعض ليكونوا مثلثاتيتم
.Bإلى أن تلتقي األعمدة المنصفة( منصف عمودي)يتم تنصيف أضالع المثلثات
.C يتم حساب المساحات الخاصة بكل محطة مع استبعاد المساحة التي تكوت خارج
Catch.Area
.D ثم حسابP بالقانون التالي:
Thiessen-Mean Method
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Mean Precipitation over an Area
: لحساب المساحة بهذه الطريقة
.A ارسم خطوط كنتورية، بحيث يمر كل خط في النقاط التي لها نفس قيمة Rainfall .B احسب المساحة بين كل خطين(Isohyetal ) مع استثناء المساحة التي تقع خارج
Catch.Area .C احسب متوسطRainfall بين كل خطين
.D ثم احسبP من القانون التالي:
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Question_8
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Question_9 • The following figure shows a catchment area with six rain
gauge stations. The rainfall recorded by each of these stations
are indicated in the figure. Calculate 1) The mean precipitation using Thiessen-mean method.
2) The Total precipitation over the area in m3
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Question_10
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Frequency of Point Rainfall
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Frequency of Point Rainfall
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Frequency of Point Rainfall
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Question_11
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Question_11
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Question_11
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Question_12
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Question_12
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Question_13
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Plotting Position
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Plotting Position
• P = m/N+1
• T = 1/P
• ترتيب تنازلي من األعلى لألصغر
• اذا كان هناك تكرار لقيمة معينة في أكثر من سنة نحذف
التكرار الموجود
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Question_14
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Question_14 m
N = 20
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Question_14
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Intensity Duration Frequency (IDF) Curve
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IDF Curve
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Rainfall Intensity and Corresponding Depth
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Rainfall Intensity versus
Duration
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Interpretation of IDF curve
P= 2in/hr * 90/60 = 3in
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Interpretation of IDF curve
= 4.6in/hr * 30/60 = 2.3 in
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Development of IDF curve
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Development of IDF curve
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Development of IDF curve
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Development of IDF curve
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Development of IDF curve
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Depth-Duration Frequency curve (DDF)
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Intensity versus Return Period
for Deferent Durations
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Question_15 • The mass curve of rainfall in a storm of a total
duration of 270 minutes is given in the table:
• 1- Draw the Hyetograph of the storm at 30
minutes time step
• 2- Plot the Maximum Intensity-Duration curve
• 3- Plot the Maximum Depth-Duration curve
Time 0 30 60 90 120 150 180 210 240 270
Cumulative Rainfall
0 6 18 21 36 43 49 52 53 45
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Question_15 • Solution
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Question_15
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Question_15
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Question_15
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Question_16
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Question_17
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Question_17
Retern
Period /
Duration
1 10
1 50 60
2 62 72
5 80 92
10 97 105
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Question_17
0
50
100
150
200
250
1 2 5 10
Rai
nfa
ll D
epth
Duration
10-Year
1-Year
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Question_17
Return period /
Duration 1 2 5 10
Depth at 1-Yr 50 62 80 97
Depth at 10-Yr 60 72 92 105
intensity at 1-Yr 50 31 16 9.7
intensity at 10-Yr 60 36 18.4 10.5 0
10
20
30
40
50
60
70
1 2 5 10
Inte
nsi
ty
Duration
Intensity at 1-Yr
Intesity at 10-Yr
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Homework
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