Lecture 7_Hydrograph and Base flow separation-3.pdf

8/22/2019 Lecture 7_Hydrograph and Base flow separation-3.pdf

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Lecture 7: Hydrograph and Base

flow separation

By:

Prof. Ahmed Ali A. Hassan

Dr. Peter Hany S. Riad

Ain Shams University Irrigation and Hydraulics Department

Faculty of Engineering Environmental Hydrology


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SyllabusIntroduction and review

Hydrology and Environment

Precipitation

Statistical analysis of rainfall data

Statistical analysis of rainfall data

Watershed characteristics, morphology, and time

of concentration equations.

Hydrograph component and base flow separation

Runoff Estimation (SCS method and indexes)

Unit Hydrograph and Synthetic UH (Snyder UH,

Dimensionless UH)

Changing UH duration

Hydrologic Routing

Storm Water Drainage Network and Protection

Works


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Precipitation

Groundwater flow


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Catchment

Catchment Area

One

Catchment

The

Other

Watershed divides the flow of water along

different slopes.


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Picture Shows Two Catchments


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Hydrograph

What can we get from hydrograph

a) the peak runoff flows(Qp)

b) To estimate runoff volume.

Qp

Time

Volume of runoff


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The inf luence of catchment

characteristics on hydrographs


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Exercise: catchment characteristics - hydrographs

Steeper catchment

Less rough catchment

Lesser storage capacity

More connections between

impervious areas


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The inf luence of partial rain coverage


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The influence of storm direction on hydrograph


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Lagtime

Timeofconcentration

Duration ofexcess precip.

Baseflow

Duration

Lag Time

Time of Concentration

Rising Limb

Recession Limb (falling limb)

Peak Flow

Time to Peak (rise time)

Recession Curve

Separation

Base flow

Hydrograph Components

Time Base


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Time to Peak, Tp: Time from the beginning of therising limb to the occurrence of the peak

discharge.

The time to peak is largely determined by drainagecharacteristics such as drainage density, slope, channelroughness, and soil infiltration characteristics. Rainfall

distribution in space also affects the time to peak.

Time of Concentration, Tc: Time required for waterto travel from the most hydraulically remote pointin the basin to the basin outlet.

The drainage characteristics of length and slope,together with the hydraulic characteristics of the flowpaths, determine the time of concentration.


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Lag Time, Tl: Time between the center of mass of

the effective rainfall hyetograph and the center of

mass of the direct runoff hydrograph.

The basin lag is an important concept in linear

modeling of basin response. The lag time is a

parameter that appears often in theoretical and

conceptual models of basin behavior. However, it

is sometimes difficult to measure in real world

situations. Many empirical equations have been

proposed in the literature. The simplest of theseequations computes the basin lag as a power

function of the basin area.

Time Base, Tb: Duration of the direct runoff

hydrograph.


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Description of hydrograph shape


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Runoff hydrograph


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Description of hydrograph


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Time of Concentration Contd.

It is the time taken for the most remote area of

the catchment to contribute water to the outlet.


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Tc can be related to catchment area, slope

etc. using the Kirpich equation:

Tc = 0.015 L

0.77

S

0.385

Tc is the time of concentration (min);

L is the maximum length of flow (m);

S is the watershed gradient (m/m).

Also, Tc = 1.67 TL


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L

Et

Eo

S = (Et - Eo)/L where Et is the elevation at top of the

watershed and Eo is the elevation at the outlet. Tc can

also be obtained from Table 3.1 of Hudson's Field

Engineering.


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From next figure, the highest runoff of a

catchment (worst case) is obtained when rainfallduration (D) is equal to Tc.

T will give lower intensity of rainfall so lower runoffwhile T' will give higher intensity but not all parts of

the watershed are contributing to runoff since Tc

has not been reached.


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Rainfall Intensity Duration Curve

Rainfall Duration (D)

2 5 10 Return periods

T Tc T

Rainfall

Intensity


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Runoff Prediction Methods

The Rational Formula:

It states that:

Qp = (CIA)/360

where Qp is the peak flow(m3 /s); C is dimensionless runoff coefficient; I is theintensity (mm/hr) of a storm of rainfall depth(mm) for a given return period Tc (hr). This is

the worst case of runoff.A is the area of catchment(ha).

Note: ha = 104 m2

ff ffi i


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Runoff Coefficient, C


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STEP 1 Hydrograph separation: base flow recession

Linear Reservoir S = k* Q

All groundwater in storage at a certain

time t is equal to all discharge betweentime t and infinite.

That is also equal to the groundwater

volume in the graph.

=t

tt dtQS

The amount of water in storage is:Reversed proof

k

t

0 eQQ tt

=

=t

-

0t dteQSkt

[ ]= t0t kt

keQS

kt

ke0QS 0t

=

t0t QkeQkSkt

==


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Hydrograph separation: base flow recession

Linear Reservoir

kt

eQQ 1-tt

=

tQlnlnQk1

1-tt =

STEP 2:

Determine direct flow

Qdir


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So. The hydrograph gives information of hydrological

processes in catchment

But how do we separate a hydrograph?


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Hydrograph separation

Engineering approach continued

a = constant slope method = straight line method

(sometimes horizontal line)

b = fixed base method = concave method

c = variable slope method


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Method 1: constant slope (straight line)

method


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Method 2: Fixed base (concave method)


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Method 3: Variable slope


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Thank you for the Attention

Lecture 7_Hydrograph and Base flow separation-3.pdf

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