5551Urban Hydrology and Hydro Logic Design

8/3/2019 5551Urban Hydrology and Hydro Logic Design

1/77

Urban Hydrology and

An-Najah National University

College of Graduate Studies

Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed1

Hydrologic Design

Dr. Sameer Shadeed


2/77

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

Urban Hydrology and Hydrologic Design2 Dr. Sameer Shadeed


3/77

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.



4/77

Effect of Urbanization on Urban

Runoff Hydrograph



5/77

Effect of Urbanization on Mean

Annual Flood



6/77

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)



7/77

Urban Drainage System



8/77

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


9/77

Alternatives for Control of

Urban Runoff Quantity



10/77

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



11/77

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


12/77

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



13/77

Estimation of Imperviousness

Estimation of imperviousness can be made by measuring

such areas on aerial photographs or by considering land

use



14/77

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



15/77

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)


Tipping-bucket rain gages are

commonly used to provide an

adequate resolution of high

frequencies


16/77

Sample of Tipping Bucket Rainfall

Measurements

Rainfall depth

16 Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed


17/77

Hydrologic Design

17

What rainfall event should we use?What rainfall event should we use?

Urban Hydrology and Hydrologic Design Dr. Sameer Shadeed


18/77

Time Series of Nablus Daily Rainfall

If we would like to consider the daily rainfall forhydrologic design (a drainage system), then whichvalue to pick?



19/77

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)



20/77

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



21/77

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



22/77

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



23/77

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



24/77

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



25/77

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



26/77

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



27/77

Recorded Total depth

0.46

0.48

duration

Rainfall Intensity Versus Duration

27

0.33

0.7115-min



28/77

Rainfall Intensity versus Duration



29/77

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



30/77

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



31/77

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



32/77

Example 1

Given themaximum

rainfall intensity

for the years

from 1949 to


1972 fordifferent rainfall

durations,

compute the IDF

curves


33/77

Example 1 (Solution)

1. Rank for each duration

2. Compute the return period

(frequency)

3. The highlighted lines

represent frequencies of

interest


. probability

5. Compute intensities that

correspond to the different

durations then select for

specific frequencies


34/77


34

Intensity Duration Frequency Curves



35/77


35

Depth-Duration-Frequency-Curves



36/77

IDF Curves for Nablus



37/77

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


tb

ai


38/77

Example 2

Fit the following data to determine the 10-year IDFcurve

t =duration (min) 5 10 15 30 60 120


i = intensity

(mm/hr)17 15 12 10 6 4

1/i 0.059 0.067 0.083 0.1 0.167 0.25


39/77


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



40/77

Formulas for IDF Curves

For the IDFIDF curvesillustrated in the

Figure, the following

intensity formula can

be used

b


eC dT

Where i = intensity (in./hr), and the e, b, and d coefficients are

given in the following table


41/77

Design Rainfall Hyetographs

Most often hydrologists are interested in

precipitation hyetographs and not just the peak

estimates

Techniques for developing design precipitation


hyetographs

1. SCS method

2. Triangular hyetograph method

3. Using IDF relationships (Alternating block

method)


42/77

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


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


43/77

SCS Method



44/77

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


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


45/77

Find rainfall hyetograph for a 25-year, 24-hour

duration SCS Type-III storm in Harris County

Example 3


us ng a one- our me ncremen . rom

curves, it was found that i = 0.417 in/hr for a 25-

year return period


46/77

Find

Total 24-hourrainfall = 0.417*24=10.01 in

Cumulativefraction -interpolate SCS



Cumulative rainfall= product ofcumulative fraction* total 24-hourrainfall (10.01 in)

Incremental

rainfall =differencebetween currentand precedingcumulative rainfall


47/77



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.


48/77

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


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


49/77

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


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


50/77

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



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


51/77

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.


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.


52/77

Determine, in 10 minute increments, the

design rainfall hyetograph for a 2-hour storm

Example 5


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


53/77

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)


1 2 3 4 5 6


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


54/77




55/77

Design Aerial Rainfall

Point rainfall estimates are extended to develop an

average rainfall depth over an area

Depth-area-duration analysis


Prepare isohyetal maps from point rainfall for

different durations

Determine area contained within each isohyet

Plot average rainfall depth vs. area for each

duration


56/77

Depth-Area Curve



57/77

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


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


58/77

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


,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


59/77

Methods For Quantifying Analysis

There are several possible parameters to be

determined in an urban hydrologic analysis,


flowflow, runoffrunoff volumevolume, or the complete runoffrunoff

hydrographhydrograph


60/77

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


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


61/77

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


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



62/77

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


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



63/77




64/77




65/77

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?



66/77

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


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



67/77

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



68/77

C = (75% 0.5 + 25% 0.15)/100% = 0.41

The time of concentration = time of overland flow+

time of the main channel


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



69/77

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


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



70/77

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


in/hr

Q = 0.41 6.4 20 = 52.48 cfs



71/77

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)



72/77

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



73/77




74/77

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


This is typical of urban areas in which impervious

land cover tend to generate some runoff even for

small rainfall totals



75/77

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



76/77

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



77/77

runoffrunoff == 00..308308(rainfall(rainfall--00..059059))

== 00..308308((88..1818--00..059059)) == 22..55 inin.. == 00..2121 ftft


The required volume is the depth times the

catchment area:

volume = 0.21 ft 2230 ac 43560 ft3/ac = 2.04

107

ft3

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5551Urban Hydrology and Hydro Logic Design

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