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

Syllabus Introduction 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

Precipitation

Groundwater flow

Catchment

Catchment Area

One Catchment

The Other

Watershed divides the flow of water along different slopes.

Picture Shows Two Catchments

Hydrograph •What can we get from hydrograph

•a) the peak runoff flows(Qp) •b) To estimate runoff volume.

Qp

Time

Volume of runoff

The influence of catchment characteristics on hydrographs

Exercise: catchment characteristics - hydrographs

Steeper catchment

Less rough catchment

Lesser storage capacity

More connections between impervious areas

The influence of partial rain coverage

The influence of storm direction on hydrograph

Lag time

Time of concentration

Duration of excess precip.

Base flow

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

– Time to Peak, Tp: Time from the beginning of the rising limb to the occurrence of the peak discharge.

• The time to peak is largely determined by drainage characteristics such as drainage density, slope, channel roughness, and soil infiltration characteristics. Rainfall distribution in space also affects the time to peak.

– Time of Concentration, Tc: Time required for water to travel from the most hydraulically remote point in the basin to the basin outlet.

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

– 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 these equations computes the basin lag as a power function of the basin area.

– Time Base, Tb: Duration of the direct runoff hydrograph.

Description of hydrograph shape

Runoff hydrograph

Description of hydrograph

Time of Concentration Contd. •It is the time taken for the most remote area of the catchment to contribute water to the outlet.

Time of Concentration Contd.

•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

Time of Concentration Contd.

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.

Time of Concentration Contd. •From next figure, the highest runoff of a catchment (worst case) is obtained when rainfall duration (D) is equal to Tc.

•T will give lower intensity of rainfall so lower runoff while T' will give higher intensity but not all parts of the watershed are contributing to runoff since Tc has not been reached.

Rainfall Intensity Duration Curve

Rainfall Duration (D)

2 5 10 Return periods

T’ Tc T

Rainfall Intensity

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 the intensity (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

Runoff Coefficient, C

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 between time 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 kt

0eQQ tt−⋅=

∫∞

=t

-0t dteQS k

t

[ ]∞−−⋅= t0tkt

keQS

( )[ ]kt

ke0QS 0t−−−⋅=

t0t QkeQkS kt

⋅=⋅= −

Hydrograph separation: base flow recession

Linear Reservoir

kt

eQQ 1-tt

∆−⋅=

tQlnlnQ k1

1-tt ∆⋅−=

STEP 2:

Determine direct flow

Qdir

So…. The hydrograph gives information of hydrological processes in catchment

But how do we separate a hydrograph?

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

Method 1: constant slope (straight line) method

Method 2: Fixed base (concave method)

Method 3: Variable slope

Thank you for the Attention