hydrologic design and design storms
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Hydrologic Design and Design Storms. Reading: Applied Hydrology Sections 13-1, 13-2 14-1 to 14-4. Hydrologic design . Water control Peak flows, erosion, pollution, etc. Water management Domestic and industrial use, irrigation, instream flows, etc Tasks Determine design inflow - PowerPoint PPT PresentationTRANSCRIPT
Hydrologic Design and Design Storms
Reading: Applied Hydrology Sections 13-1, 13-2
14-1 to 14-4
2
Hydrologic design
• Water control– Peak flows, erosion, pollution, etc.
• Water management– Domestic and industrial use, irrigation, instream flows, etc
• Tasks– Determine design inflow– Route the design inflow– Find the output
• check if it is sufficient to meet the demands (for management)• Check if the outflow is at safe level (for control)
3
Hydrologic design scale• Hydrologic design scale – range in magnitude of the
design variable within which a value must be selected• Design considerations
– Safety – Cost
• Do not design small structures for large peak values (not cost effective)
• Do not design large structures for small peak values (unsafe)
• Balance between safety and cost.
4
Estimated Limiting Value (ELV)
• Lower limit on design value – 0• Upper limit on design value – ELV• ELV – largest magnitude possible for a hydrologic
event at a given location, based on the best available hydrologic information. – Length of record– Reliability of information– Accuracy of analysis
• Probable Maximum Precipitation (PMP) / Probable Maximum Flood (PMF)
Probable Maximum Precipitation
http://www.nws.noaa.gov/oh/hdsc/studies/pmp.html
Most recent report 1999
6
7
TxDOT RecommendationsRecommended Design Frequencies (years)
- Design Check Flood
Functional Classification and Structure Type 2 5 10 25 50 100 Freeways (main lanes): - - - - - - culverts - - - - X X
bridges - - - - X X
Principal arterials: - - - - - - culverts - - X (X) X X
small bridges - - X (X) X X
major river crossings - - - - (X) X
Minor arterials and collectors (including frontage roads): - - - - - - culverts - X (X) X - X
small bridges - - X (X) X X
major river crossings - - - X (X) X
Local roads and streets (off-system projects): - - - - - - culverts X X X - - X
small bridges X X X - - X
Storm drain systems on interstate and controlled access highways (main lanes):
- - - - - -
inlets and drain pipe - - X - - X
inlets for depressed roadways* - - - - X X
Storm drain systems on other highways and frontage: - - - - - - inlets and drain pipe X (X) - - - X
inlets for depressed roadways* - - - (X) X X
Notes. * A depressed roadway provides nowhere for water to drain even when the curb height is exceeded. ( ) Parentheses indicate desirable frequency.
8
Hydrologic design level
• Hydrologic design level – magnitude of the hydrologic event to be considered for the design or a structure or project.
• Three approaches for determining design level– Empirical/probabilistic– Risk analysis– Hydroeconomic analysis
9
Empirical/Probabilitic• P(most extreme event of last N years will be
exceeded once in next n years)
• P(largest flood of last N years will be exceeded in n=N years) = 0.5
• Drought lasting m years is worst in N year record. What is the probability that a worse drought will occur in next n years?– # sequences of length m in N years = N-m+1– # sequences of length m in n years = n-m+1
)1()1(1),,(
mnmN
mnmnNP
nNnnNP
),(
10
Example 13.2.1
• If the critical drought of the record, as determined from 40 yrs of data, lasted 5 yrs, what is the chance that a more severe drought will occur during the next 20 yrs?
• Solution: N = 40, m = 5 and n = 20
308.02522040
1520)20,5,40(
P
11
Risk Analysis• Uncertainty in hydrology
– Inherent - stochastic nature of hydrologic phenomena– Model – approximations in equations– Parameter – estimation of coefficients in equations
• Consideration of Risk– Structure may fail if event exceeds T–year design
magnitude
– R = P(event occurs at least once in n years)• Natural inherent risk of failure
nTxXPR )(11 T
xXP T1)(
n
TR
111
12
Example 13.2.2• Expected life of culvert = 10 yrs• Acceptable risk of 10 % for the culvert
capacity• Find the design return period
yrsTT
TR
n
95
11110.0
111
10
What is the chance that the culvert designed for an event of 95 yr return period will have its capacity exceeded at least once in 50 yrs?
41.095111
50
R
R
The chance that the capacity will not be exceeded during the next 50 yrs is 1-0.41 = 0.59
13
Hydroeconomic Analysis
• Probability distribution of hydrologic event and damage associated with its occurrence are known
• As the design period increases, capital cost increases, but the cost associated with expected damages decreases.
• In hydroeconomic analysis, find return period that has minimum total (capital + damage) cost.
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Design Storms
• Get Depth, Duration, Frequency Data for the required location
• Select a return period• Convert Depth-Duration data to a design
hyetograph.
Depth Duration Data to Rainfall Hyetograph
http://hdsc.nws.noaa.gov/hdsc/pfds/index.html
An example of precipitation frequency estimates for a location in California
37.4349 N120.6062 W
Results of Precip Frequency Query
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TP 40• Hershfield (1961) developed isohyetal maps of design
rainfall and published in TP 40.• TP 40 – U. S. Weather Bureau technical paper no. 40.
Also called precipitation frequency atlas maps or precipitation atlas of the United States.– 30mins to 24hr maps for T = 1 to 100
• Web resources for TP 40 and rainfall frequency maps– http://www.tucson.ars.ag.gov/agwa/rainfall_frequency.ht
ml– http://www.erh.noaa.gov/er/hq/Tp40s.htm– http://hdsc.nws.noaa.gov/hdsc/pfds/
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2yr-60min precipitation GIS map
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2yr-60min precipitation map
This map is from HYDRO 35 (another publication from NWS) which supersedes TP 40
23
Design aerial precipitation
• Point precipitation estimates are extended to develop an average precipitation depth over an area
• Depth-area-duration analysis – Prepare isohyetal maps from point precipitation
for different durations– Determine area contained within each isohyet– Plot average precipitation depth vs. area for each
duration
24
Depth-area curve
(World Meteorological Organization, 1983)
25
Depth (intensity)-duration-frequency
• DDF/IDF – graph of depth (intensity) versus duration for different frequencies– TP 40 or HYDRO 35 gives spatial distribution of
rainfall depths for a given duration and frequency– DDF/IDF curve gives depths for different durations
and frequencies at a particular location– TP 40 or HYDRO 35 can be used to develop
DDF/IDF curves
• Depth (P) = intensity (i) x duration (Td) diTP
26
IDF curve
27
Example 14.2.1
• Determine i and P for a 20-min duration storm with 5-yr return period in Chicago
From the IDF curve for Chicago,
i = 3.5 in/hr for Td = 20 min and T = 5yr
P = i x Td = 3.5 x 20/60 = 1.17 in
28
Equations for IDF curves
IDF curves can also be expressed as equations to avoid reading from graphs
fTci e
d i is precipitation intensity, Td is the duration, and c, e, f are
coefficients that vary for locations and different return periods
fTcTi ed
m
This equation includes return period (T) and has an extra coefficient
(m)
29
Example 14.2.4
Using IDF curve equation, determine 10-yr 20-min design rainfall intensities for Denver
fTci e
d
From Table 14.2.3 in the text, c = 96.6, e = 0.97, and f = 13.9
hrini /002.39.1320
6.9697.0
Similarly, i = 4.158 and 2.357 in/hr for Td = 10 and 30 min, respectively
30
IDF curves for Austin
cbtai
tscoefficien,,
stormofDurationintensityrainfalldesign
cbati
Storm Frequency a b c
2-year 106.29 16.81 0.9076
5-year 99.75 16.74 0.8327
10-year 96.84 15.88 0.7952
25-year 111.07 17.23 0.7815
50-year 119.51 17.32 0.7705
100-year 129.03 17.83 0.7625
500-year 160.57 19.64 0.7449
0
2
4
6
8
10
12
14
16
1 10 100 1000Duration (min)
Inte
nsity
(in/
hr)
2-yr5-yr10-yr25-yr50-yr100-yr500-yr
Source: City of Austin, Watershed Management Division
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Design Precipitation Hyetographs
• Most often hydrologists are interested in precipitation hyetographs and not just the peak estimates.
• Techniques for developing design precipitation hyetographs
1. SCS method2. Triangular hyetograph method3. Using IDF relationships (Alternating block method)
32
SCS MethodSCS (1973) adopted method similar to DDF to develop dimensionless rainfall temporal patterns called type curves for four different regions in the US.SCS type curves are in the form of percentage mass (cumulative) curves based on 24-hr rainfall of the desired frequency.If a single precipitation depth of desired frequency is known, the SCS type curve is rescaled (multiplied by the known number) to get the time distribution. For durations less than 24 hr, the steepest part of the type curve for required duraction is used
33
SCS type curves for Texas (II&III)
SCS 24-Hour Rainfall Distributions SCS 24-Hour Rainfall Distributions
T (hrs) Fraction of 24-hr rainfall T (hrs) Fraction of 24-hr rainfall
Type II Type III Type II Type III
0.0 0.000 0.000 11.5 0.283 0.298
1.0 0.011 0.010 11.8 0.357 0.339
2.0 0.022 0.020 12.0 0.663 0.500
3.0 0.034 0.031 12.5 0.735 0.702
4.0 0.048 0.043 13.0 0.772 0.751
5.0 0.063 0.057 13.5 0.799 0.785
6.0 0.080 0.072 14.0 0.820 0.811
7.0 0.098 0.089 15.0 0.854 0.854
8.0 0.120 0.115 16.0 0.880 0.886
8.5 0.133 0.130 17.0 0.903 0.910
9.0 0.147 0.148 18.0 0.922 0.928
9.5 0.163 0.167 19.0 0.938 0.943
9.8 0.172 0.178 20.0 0.952 0.957
10.0 0.181 0.189 21.0 0.964 0.969
10.5 0.204 0.216 22.0 0.976 0.981
11.0 0.235 0.250 23.0 0.988 0.991
24.0 1.000 1.000
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SCS Method Steps
• Given Td and frequency/T, find the design hyetograph
1. Compute P/i (from DDF/IDF curves or equations)2. Pick a SCS type curve based on the location 3. If Td = 24 hour, multiply (rescale) the type curve with P to
get the design mass curve1. If Td is less than 24 hr, pick the steepest part of the type curve
for rescaling
4. Get the incremental precipitation from the rescaled mass curve to develop the design hyetograph
35
Example – SCS Method• Find - rainfall hyetograph for a 25-year, 24-hour duration SCS
Type-III storm in Harris County using a one-hour time increment
• a = 81, b = 7.7, c = 0.724 (from Tx-DOT hydraulic manual)
• Find – Cumulative fraction - interpolate SCS table– Cumulative rainfall = product of cumulative fraction * total 24-hour
rainfall (10.01 in)– Incremental rainfall = difference between current and preceding
cumulative rainfall
hrin
btai c /417.0
7.760*2481
724.0
inhrhrinTiP d 01.1024*/417.0*
TxDOT hydraulic manual is available at: http://manuals.dot.state.tx.us/docs/colbridg/forms/hyd.pdf
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SCS – Example (Cont.)Time Cumulative
Fraction Cumulative Precipitation
Incremental Precipitation
(hours) Pt/P24 Pt (in) (in) 0 0.000 0.00 0.00 1 0.010 0.10 0.10 2 0.020 0.20 0.10 3 0.032 0.32 0.12 4 0.043 0.43 0.12 5 0.058 0.58 0.15 6 0.072 0.72 0.15 7 0.089 0.89 0.17 8 0.115 1.15 0.26 9 0.148 1.48 0.33
10 0.189 1.89 0.41 11 0.250 2.50 0.61 12 0.500 5.01 2.50 13 0.751 7.52 2.51 14 0.811 8.12 0.60 15 0.849 8.49 0.38 16 0.886 8.87 0.38 17 0.904 9.05 0.18 18 0.922 9.22 0.18 19 0.939 9.40 0.18 20 0.957 9.58 0.18 21 0.968 9.69 0.11 22 0.979 9.79 0.11 23 0.989 9.90 0.11 24 1.000 10.01 0.11
0.00
0.50
1.00
1.50
2.00
2.50
3.00
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24Time (hours)
Prec
ipita
tion
(in)
If a hyetograph for less than 24 needs to be prepared, pick time intervals that include the steepest part of the type curve (to capture peak rainfall). For 3-hr pick 11 to 13, 6-hr pick 9 to 14 and so on.
37
Triangular Hyetograph Method
• Given Td and frequency/T, find the design hyetograph1. Compute P/i (from DDF/IDF curves or equations)2. Use above equations to get ta, tb, Td and h (r is available for
various locations)
Time
Rain
fall
inte
nsity
, i
h
ta tb
d
a
Ttr
Td
Td: hyetograph base length = precipitation duration
ta: time before the peak
r: storm advancement coefficient = ta/Td
tb: recession time = Td – ta = (1-r)Td
d
d
TPh
hTP
221
38
Triangular hyetograph - example
• Find - rainfall hyetograph for a 25-year, 6-hour duration in Harris County. Use storm advancement coefficient of 0.5.
• a = 81, b = 7.7, c = 0.724 (from Tx-DOT hydraulic manual)
hrin
btai c /12.1
7.760*681
724.0
inhrhriniP 72.66*/12.16*
hrtTt
hrrTt
adb
da
336
365.0
Time
Rain
fall
inte
nsity
, in/
hr
2.24
3 hr 3 hr
6 hr
hrinTPhd
/24.2644.13
672.622
39
Alternating block method• Given Td and T/frequency, develop a hyetograph in
Dt increments1. Using T, find i for Dt, 2Dt, 3Dt,…nDt using the IDF curve for
the specified location2. Using i compute P for Dt, 2Dt, 3Dt,…nDt. This gives
cumulative P.3. Compute incremental precipitation from cumulative P.4. Pick the highest incremental precipitation (maximum
block) and place it in the middle of the hyetograph. Pick the second highest block and place it to the right of the maximum block, pick the third highest block and place it to the left of the maximum block, pick the fourth highest block and place it to the right of the maximum block (after second block), and so on until the last block.
40
Cumulative Incremental Duration Intensity Depth Depth Time Precip (min) (in/hr) (in) (in) (min) (in) 10 4.158 0.693 0.693 0-10 0.024 20 3.002 1.001 0.308 10-20 0.033 30 2.357 1.178 0.178 20-30 0.050 40 1.943 1.296 0.117 30-40 0.084 50 1.655 1.379 0.084 40-50 0.178 60 1.443 1.443 0.063 50-60 0.693 70 1.279 1.492 0.050 60-70 0.308 80 1.149 1.533 0.040 70-80 0.117 90 1.044 1.566 0.033 80-90 0.063 100 0.956 1.594 0.028 90-100 0.040 110 0.883 1.618 0.024 100-110 0.028 120 0.820 1.639 0.021 110-120 0.021
Example: Alternating Block Method
90.136.96
97.0
de
d TfTci
tscoefficien,,stormofDuration
intensityrainfalldesign
fecTi
d
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0-10 10-20 20-30 30-40 40-50 50-60 60-70 70-80 80-90 90-100
100-110
110-120
Time (min)
Prec
ipita
tion
(in)
Find: Design precipitation hyetograph for a 2-hour storm (in 10 minute increments) in Denver with a 10-year return period 10-minute