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8/3/2019 5551Urban Hydrology and Hydro Logic Design
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Urban Hydrology and
An-Najah National University
College of Graduate Studies
Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed1
Hydrologic Design
Dr. Sameer Shadeed
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Introduction
The hydrology of urban areas is dominated by twodistinct characteristics:
1. The prevalence of impervious surfaces (e.g.,
pavement, roofs)
2. The presence of man-made or hydraulically
mprove ra nage sys em e.g., a sewer sys em
Thus the response of an urban catchment to rainfall
is much faster than that of a rural catchment of
equivalent area, slope, and soils
In addition, the runoff volume from an urbancatchment is larger because there is less pervious
area available
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Urbanization
Urbanization changes the hydrology of a
drainage basin. Roads and artificial surfaces cut
down infiltration and storage while storm sewers
.
It is suggested that urbanization increases the risk
of flooding as rivers respond much more violently
to a storm event.
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Effect of Urbanization on Urban
Runoff Hydrograph
Urban Hydrology and Hydrologic Design4 Dr. Sameer Shadeed
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Effect of Urbanization on Mean
Annual Flood
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Urban Drainage System
Drainage systems in urban areas may rely on naturalchannels, but most cities have a sewer network for removal
of storm water
If the system is exclusively for stormwater removal, it is
called a stormstorm sewersewer
,a combinedcombined sewersewer
Storm and combined sewers are installed to remove
stormwater from the land surface, thus preventing flooding
and permitting normal transportation on highways and a like
As such, they are usually designed to handle a peak flowcorresponding to a given return period according to local
regulations (2-10 years for suburban drainage and 10-50
years for major highways is typical)
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Urban Drainage System
Urban Hydrology and Hydrologic Design7 Dr. Sameer Shadeed
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The Engineering Problem in
Urban Hydrology The engineering problem in urban hydrology usually consists of
the need to control peak flows and maximum depths throughout
the drainage system
If the hydraulic grade line is too high, sewers may surcharge; that
is, the water level may rise above the top of the sewer conduit,
leading occasionally to basement flooding or discharge to streets
,and existing drainage systems must often be modified to correct
for them
Exceeding the capacity of an existing system is a problem that
often occurs in newly developed areas that are served by an old
sewer system
The water quality of urban runoff may also be poor, and special
measures may be required simply to improve the quality of runoff
prior to discharge into receiving waters, particularly for combined
sewerUrban Hydrology and Hydrologic Design8 Dr. Sameer Shadeed
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Alternatives for Control of
Urban Runoff Quantity
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Design Objectives
The engineering objectives when dealing with
urban hydrology are:
1. Controlling peak flows and maximum depths at
2. Minimizing runoff volumes as well as basement
flooding
3. Controlling water quality and simultaneously
protecting the environment
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Rainfall-Runoff
Conversion ofrainfall into runoff in urban areas is usuallysomewhat simplified because of the relative high
imperviousness of such areas, although in residential
and open-land districts the calculation of infiltration into
pervious surfaces may still represent a critical factor in
the analysis
When hydrographs are to be computed, special effort is
required to obtain adequate rainfall data
This is because urban areas respond quickly to rainfall
transients, in contrast to natural catchments, which
dampen out the short-term fluctuations Thus rainfall data should be available at 5-min
increments or shorter to predict the runoff hydrograph
adequatelyUrban Hydrology and Hydrologic Design11 Dr. Sameer Shadeed
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Urban Catchment Description
The urbanurban catchmentcatchmentis characterized by its area, shape,
slope, land use, imperviousness, roughness, and storage
The area and imperviousness are the two most important
parameters fora good prediction of hydrograph volume
Although it is a seemingly straightforward parameter,
estimation of the percent imperviousness can be restrained
In particular, it is usually necessary to distinguish between
directlydirectly imperviousimpervious areasareas (areas that are drain directly
into drainage system, such as a street surface with curbs
and gutters that directs the runoff into a storm sewer inlet)
and NondirectlyNondirectly connectedconnected imperviousimpervious areasareas (rooftops
or driveways that drain onto pervious areas). Runoff from
such areas does not enter the storm drainage system
unless the pervious area become saturated
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Estimation of Imperviousness
Estimation of imperviousness can be made by measuring
such areas on aerial photographs or by considering land
use
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Estimation of Imperviousness
For large urban areas, imperviousness can be estimated onthe basis of population density
)017.0573.0(6.9 PDInPDI
Where
I = percent imperviousness
PD = population density (persons/acre)
The above equation is based on a regression analysis of567 communities in New Jersey, so it should be used with
caution elsewhere
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Rainfall Data Required for Urban
Hydrology
Two types of rainfall data are commonly required in urban
hydrology (hydrologic design):
1. Point rainfall data (actual hyetographs)
2. Processed data (Intensity-Duration-Frequency, IDF curves)
Urban Hydrology and Hydrologic Design15 Dr. Sameer Shadeed
Tipping-bucket rain gages are
commonly used to provide an
adequate resolution of high
frequencies
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Sample of Tipping Bucket Rainfall
Measurements
Rainfall depth
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Hydrologic Design
17
What rainfall event should we use?What rainfall event should we use?
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Time Series of Nablus Daily Rainfall
If we would like to consider the daily rainfall forhydrologic design (a drainage system), then whichvalue to pick?
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Design Storm
Design storm: rainfall pattern defined for
use in the design of hydrologic system
Serves as an input to the hydrologic system
19
Can by defined by:
1. Hyetograph (time distribution of rainfall)
2. Isohyetal map (spatial distribution ofrainfall)
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Extreme Rainfall Events
Most extreme rainfall events from historic record
sometimes used as design value.
Extreme rainfall events are very severe, rare and
intense and determined b their
20
Temporal scale
Spatial scale
Economic and social losses due to extreme events
have increased in the last decades
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Design Point Rainfall
Historic data of rainfall is converted to differentdurations (next table)
Annual maximum rainfall for a given duration is
selected for each ear
21
Frequency analysis is performed to derive
design rainfall depths for different return
periods
The depths are converted to intensities by
dividing by rainfall durations
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Computation of Rainfall Depth and
Intensity at a Point
Apparently,
with increasing
the duration,
Maximum
rainfall
22
intensitybecomes less
This is
somehow a
general trend
but not a linear
one
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Intensity Duration Frequency
Relationships
One of the first steps in many hydrologic designprojects is the determination of the rainfall event or
events to be used in the design
The most common approach is to use a design
storm or event that involves a relationship
between rainfall intensity (or depth), duration, and
the frequency appropriate for the facility and site
location
As such, the IDFIDF curves can be used by
hydrologists
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Intensity Duration Frequency
Relationships
IDFIDF curves enables the hydrologists to develophydrologic systems that consider worst-case
scenarios of rainfall intensity and duration during a
given interval of time
The idea here is that high intensity rainfall in
consequences
For instance, in urban catchments, flooding may
occur such that large volumes of water may not be
handled by the storm water system
Thus, appropriate values of rainfall intensitiesand frequencies should be considered in the
design of the hydrologic systems
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Intensity versus Depth of Rainfall
Intensity is expressed as:
Pi
25
where P is the rainfall depth (mm) and Td is
the duration (hr)
d
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Rainfall Intensity and
Corresponding Depth
In general, we may have different rainfall intensities butwith the same depth
Apparently, rainfall duration plays an important role indetermining rainfall depth
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Recorded Total depth
0.46
0.48
duration
Rainfall Intensity Versus Duration
27
0.33
0.7115-min
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Rainfall Intensity versus Duration
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Interpretation of IDF Curves
For example, in anytime duration of 90
minutes, a location
could experience a
peak 2 in/hrstorm
29
The 20-yr 90-min
design storm for the
location would havea depth ofP = 3 in
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Interpretation of IDF Curves
A 20-yr 30-min designstorm would have anintensity of4.6 in/hrbutwith a depth of only 2.3in
30
storm produces lessdepth, its high intensitycould be the governingfactor in determiningthe size of drainage
works. The probabilityof occurrence of bothstorms would be thesame
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Development of IDF Curves
Select a specific rainfall duration
For each year and for the selected duration find the
maximum rainfall
31
order
The return period equals T = (n + 1) / m where m is
the rank and n is the total number of years
Repeat the above procedure but for different rainfall
durations
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Example 1
Given themaximum
rainfall intensity
for the years
from 1949 to
32 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
1972 fordifferent rainfall
durations,
compute the IDF
curves
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Example 1 (Solution)
1. Rank for each duration
2. Compute the return period
(frequency)
3. The highlighted lines
represent frequencies of
interest
33 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
. probability
5. Compute intensities that
correspond to the different
durations then select for
specific frequencies
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Example 1 (Solution)
34
Intensity Duration Frequency Curves
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Example 1 (Solution)
35
Depth-Duration-Frequency-Curves
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IDF Curves for Nablus
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Formulas for IDF Curves
Regression analysis can be used to fit IDFIDF curvescurves, andthe constants can be interpreted as regional
characteristics
Many formulas have been used to fit these curves, but
most of them are in a form of intensity (i) inversely
ro ortional to duration t
37
Meyer, 1928 approximated IDFIDF curvescurves by the following
function:
Where the constants a and b are regression coefficientswhich serve as characteristic feature of both the rainfall
region and the frequency of occurrence in each area
Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
tb
ai
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Example 2
Fit the following data to determine the 10-year IDFcurve
t =duration (min) 5 10 15 30 60 120
38 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
i = intensity
(mm/hr)17 15 12 10 6 4
1/i 0.059 0.067 0.083 0.1 0.167 0.25
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Example 2 (Solution)
1. A model of the form ii == a/(ba/(b ++ t)t) can be expressed in linear
form as 11//ii == t/at/a ++ b/ab/a
2. The regression of1/i versus t yeilds 11//ii == 00..001001 tt ++ 00..053053, from
which a = 1000 and b = 53
3. Thus the rainfall intensity formula is ii == 10001000/(/(5353 ++ t)t).. The
correlation coefficient RR22 == 00..9999
39 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
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Formulas for IDF Curves
For the IDFIDF curvesillustrated in the
Figure, the following
intensity formula can
be used
b
40 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
eC dT
Where i = intensity (in./hr), and the e, b, and d coefficients are
given in the following table
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Design Rainfall Hyetographs
Most often hydrologists are interested in
precipitation hyetographs and not just the peak
estimates
Techniques for developing design precipitation
41 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
hyetographs
1. SCS method
2. Triangular hyetograph method
3. Using IDF relationships (Alternating block
method)
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SCS Method
SCSSCS (1973) adopted method to develop synthetic
storm hyetograph (dimensionless rainfall temporal
patterns called type curves) forfour different regions in
the US for storms of 6 and 24 hours duration
SCSSCS type curves are in the form of percentage mass
42 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
cumu a ve curves ase on - r ra n a o edesired frequency
If a single rainfall depth of desired frequency is known,
the SCSSCS 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 duration is used
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SCS Method
43 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
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SCS Method Steps
Given return period (Tr) and rainfall duration (Td), findthe design rainfall hyetograph
1. Compute the total rainfall intensity (i) (from IDFIDF
curves or equations)
2. Compute the total rainfall depth by multiplying the
44 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
3. Pick a SCS type curve based on the location
4. If Td = 24 hour, multiply (rescale) the type curve
with precipitation (P) to get the design mass curve
5. If Td is less than 24 hr, pick the steepest part of the
type curve for rescaling
6. Get the incremental rainfall from the rescaled mass
curve to develop the design hyetograph
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Find rainfall hyetograph for a 25-year, 24-hour
duration SCS Type-III storm in Harris County
Example 3
45 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
us ng a one- our me ncremen . rom
curves, it was found that i = 0.417 in/hr for a 25-
year return period
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Find
Total 24-hourrainfall = 0.417*24=10.01 in
Cumulativefraction -interpolate SCS
Example 3 (Solution)
46 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
Cumulative rainfall= product ofcumulative fraction* total 24-hourrainfall (10.01 in)
Incremental
rainfall =differencebetween currentand precedingcumulative rainfall
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Example 3 (Solution)
47 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
If a hyetograph for less than 24 needs to be prepared, picktime 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.
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Triangular Hyetograph Method
fallintensity,
i
h
ta tb
d
a
T
tr
A triangle is a simple shape for adesign hyetograph because once
the design rainfall R and duration Tdare known, the base length (Td) and
height of the triangle are determined
48 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
Time
Rain
Td
ta: time before the peak
r: storm advancement coefficient (r is available for variouslocations)
tb: recession time = Td ta = (1-r)Td
d
d
T
Rhwhichfrom
hTR
2
2
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Determine the triangle rainfall hyetograph for the design
of un urban storm sewer Harris County. The design
return period is 25 years, and the design rainstorm
Example 4
49 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
duration has been set at 6 hours. The storm
advancement coefficient is r = 0.5. From IDF curves, it
was found that i = 1.12 in/hr for a 25-year return period
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Find
Total 6-hour rainfall, R = 1.12*6 = 6.72 in
h = 2R/Td = 2(6.72)/6 = 2.24 in/hr
ta = rTd = 0.5(6) = 3 hrs
Example 4 (Solution)
50 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
tb = Td - ta = 6 - 3 = 3 hrs
Time
Rainfallintensity,
in/hr
2.24
3 hr 3 hr
6 hr
From the obtained triangle, values
of rainfall intensity at regular
intervals can be calculated and
converted to rainfall depth forrainfall-runoff analysis for the storm
sewer
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Alternating Block Method
Given return period (Tr
) and duration (Td
), develop a
hyetograph in Dt increments
1. Using Tr, find i forDt, 2Dt, 3Dt,nDt using the IDF curve
for the specified location
2. Using i compute R for Dt, 2Dt, 3Dt,nDt. This gives
cumulative R.
51 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
3. Compute incremental rainfall from cumulative R.
4. Pick the highest incremental rainfall (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 highestblock and place it to the right of the maximum block
(after second block), and so on until the last block.
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Determine, in 10 minute increments, the
design rainfall hyetograph for a 2-hour storm
Example 5
52 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
w a -year re urn per o . rom curves,
it was found that the values of rainfall intensity
for durations at intervals of 10 minutes are
shown in column 2 of the next table
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In column 6 the rainfall depths, are ordered so that the
maximum block (0.693 in) falls at 50=60 min; the next largest
block (0.308 in) is placed to the right of the maximum block, at 60-
70 min, the third largest block (0.178 in) is placed to the left of the
maximum block (40-50 min), and so on (see figure in the next
slide)
Example 5 (Solution)
1 2 3 4 5 6
53 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
Duration(min)
Intensity(in/hr)
CumulativeDepth
IncrementalDepth
Time(min)
Rainfall(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
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Example 5 (Solution)
54 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
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Design Aerial Rainfall
Point rainfall estimates are extended to develop an
average rainfall depth over an area
Depth-area-duration analysis
55 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
Prepare isohyetal maps from point rainfall for
different durations
Determine area contained within each isohyet
Plot average rainfall depth vs. area for each
duration
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Depth-Area Curve
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Probable Maximum Precipitation (PMP)
PMP Greatest depth of precipitation for a given
duration that is physically possible and
reasonably characteristic over a particular
geographic region at a certain time of year
57 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
Not completely reliable; probability of
occurrence is unknown
Variety of methods to estimate PMP
1. Application of storm models
2. Maximization of actual storms
3. Generalized PMP charts
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Probable Maximum Flood (PMF)
PMF greatest flood to be expected assuming
complete coincidence of all factors that would produce
the heaviest rainfall (PMP) and maximum runoff
Flood of unknown frequency
From the economic viewpoint, it is usually prohibitive
58 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
,spillways whose failure could lead to excessive
damage and loss of life
Most structures are designed for greatest floods that
may be reasonably expected for local conditions
(meteorology, topography, and hydrology)The design flood is commonly called standard
project flood derived from standard project storm
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Methods For Quantifying Analysis
There are several possible parameters to be
determined in an urban hydrologic analysis,
59 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
flowflow, runoffrunoff volumevolume, or the complete runoffrunoff
hydrographhydrograph
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Peak Flows by Rational Method
The rational method is based on the idea that the rate of
runofffor any storm depends on:
The average intensity of the storm
The size of the drainage area
60 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed
The type of drainage area surface
It is based on the theory that for a rainfall of average
intensity, I, falling over an impervious area of size A, the
maximum rate of runoff at the outlet of the drainage
area, Q, occurs when the whole area is contributing to
the runoff at the same time
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Q = C I A (english) orQ = 0.0028 C I A (metric)
Where
Q = peak flow (ft3/sec) or (m3/sec)
A = drainage area (acres orhectares)
C = runoff coefficient that represents the fraction of
Peak Flows by Rational Method
61
runo o ra n a w c epen s on: Soil type
Shape of drainage area
Previous moisture conditions
Slope of catchment
Amount of impervious soil
Land use
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II = intensity of rainfall (in/hour or mm/hour) usuallyobtained from IDF curves for a specific return period
under the assumption that the duration (tr) equals the
time of concentration (tc).
This is physically realistic because the time of
Peak Flows by Rational Method
62
concen ra on a so s e me o equ r um, a w ctime the whole catchment contribute to flow at the
output.
Thus, ifttrr > ttcc, then equilibrium would have been reached
earlier and higher intensity should be used
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Peak Flows by Rational Method
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Peak Flows by Rational Method
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A 175-acre rural drainage area consists of threedifferent watershed areas as follows:
Steep grassed areas = 50%
Forested areas = 30%
Example 6
65
Cultivated fields = 20%
For a storm intensity of 2.2 in/hr, what would be
the runoff rate?
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The weighted runoff coefficient should first bedetermined for the whole drainage area. From theprevious table, midpoint values for the differentsurface types are:
Steep grassed areas = 0.6
Example 6 (Solution)
66
= . Cultivated fields = 0.3
Cw = 0.50.6+0.30.2+0.20.3 = 0.42
The storm intensity is 2.2 in/hr
Thus, Q = 0.422.2175 = 162 ft3/sec
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Estimate the peak flow for a 25-year storm for an area of
20 acres. The overland flow distance is 125 ft and the
average land slope is 2.5%. The land use for the drainage
basin is 75% residential, multiple units, detached, and
Example 7
25% lawns, sandy soil with an overall average slope of
about 2.7%. A channel leading to the outlet is 1,550 ft long
with a slope of 0.016 ft/ft. Mannings n value for the
channel is 0.030. The channel is trapezoidal with a bottomwidth of 3 ft and side slopes of 2 ft vertical to 1 ft horizontal
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C = (75% 0.5 + 25% 0.15)/100% = 0.41
The time of concentration = time of overland flow+
time of the main channel
Example 7 (Solution)
For the overland flow: tc = 1.8(1.1 C)L0.5/S0.333
where tc is the time of overland flow in min, C = the
rational coefficient, L = overland flow length in ft,
and S surface slope in %
tc = 1.8(1.1 0.41)1250.5/2.50.333 = 10.23 min
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For the main channel and assuming a depth of 2 ft,
R is calculated to be 1.071 ft
Using Mannings equation: V = (1.49/n)R2/3S1/2
Example 7 (Solution)
V = (1.49/0.03)(1.071)0.666(0.016)0.5 = 6.57 ft/s
tf= L/V = 1,550/(6.57 60) = 3.93 min
Total time of concentration is 10.23 + 3.93 = 14.16min
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From the IDF curve and using a time of
concentration of 14 min for 25-year return
period, the rainfall intensity is found to be 6.4
Example 7 (Solution)
in/hr
Q = 0.41 6.4 20 = 52.48 cfs
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When measured rainfall and runoff data are
available, it is common to regress the runoff against
the rainfall
If a linear equation is fit to the data, it will have the
form
Regression of Runoff Vs. Rainfall
71
== --Where
R = runoff depth
P = rainfall depth
CR = slope of the fitted line (approximate runoffcoefficient)
DS = depression storage (depth)
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For an urban area, rainfall and runoff depths for ten
monitored storms are listed in the following table. Use
linear regression to fit the given data.
Example 8
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Example 8 (Solution)
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Although the depression storage value of about 0.06
in. indicates that at least that much rain must fall
before runoff is expected, the parameter is not
Example 8 (Solution)
This is typical of urban areas in which impervious
land cover tend to generate some runoff even for
small rainfall totals
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Detention storage involves determining or solving
runoff, as in a reservoir, and then releasing it,
typically over a period of from 24 to 72 hours
In retention storage, runoff is not released
downstream and is usually removed from the storage
Detention/Retention Storage
75
evaporation
Both types of storage are very common, although
designed retention becomes less practical as the
size of the drainage area increases
The required retention basin volume should bebased on an analysis of storm event volumes
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Given that the 5-yr storm event rainfall depth is
approximately 8.18 in. Using the regression
relationship developed in example 8 [runoffrunoff ==
--
Example 9
.. ..
detention basin required to hold the runoff from a 5-yr
storm for an urban area of 2230-ac
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runoffrunoff == 00..308308(rainfall(rainfall--00..059059))
== 00..308308((88..1818--00..059059)) == 22..55 inin.. == 00..2121 ftft
Example 9 (Solution)
The required volume is the depth times the
catchment area:
volume = 0.21 ft 2230 ac 43560 ft3/ac = 2.04
107
ft3