This document discusses hydrograph concepts including: - Defining a hydrograph as a graph showing variations in stream discharge over time. - Components of a single peaked hydrograph from an isolated storm. - Separating surface runoff, interflow, and groundwater flow. - Estimating the concentration time of a catchment using the Izzard and Kirpich formulas. - Defining valley storage as water temporarily stored in stream channels. - Working through an example problem to calculate time of concentration and peak runoff rate.

2. HYDROLOGY
Second Term, Final Year
B.E. Civil Engineering

3. •Definition of Hydrograph
•Components of Hydrograph
•Effluent and Influent Streams
•Separation of Surface and Groundwater Runoff
•Estimating the Concentration Time of a
Catchment
•Valley Storage
•Example

4. HYDROGRAPH
“Hydrograph is a graph showing variations of
discharge with time, at a particular point of a
stream”.
It shows the time distribution of total run-off at
the point of measurement.

5. Components or Parts of Storm Hydrograph
Single Peaked Hydrograph Resulting from an Isolated Storm

6. Effluent and Influent Streams
Rain water (Excluding Basin Recharge) may follow three paths to a stream:
(1) Overland flow (surface run-off)
(2) Interflow (influent stream)
(3) Ground water flow (effluent stream)
Overland flow + Interflow = Direct Runoff
Low water flow of streams is derived
from groundwater
Low flow in stream
Before storm
Flow from groundwater to the stream
High flow in stream
After storm
Flow from the stream to the groundwater
Flood level in the stream recedes
Groundwater again starts contributing to the stream

7. How to Separate Direct Runoff and Groundwater Runoff?
1.Extend the recession of the flow existing prior to the storm to a point under
the peak (or crest) of the hydrograph (line AB).
2.Select a point C on the recession limb of the hydrograph N days after the
peak. Join point B and C.
A rough selection of N (in days) is given by
N = 0.84 A0..2
(A = Drainage Area in sq. km.)
N = A1
0.2
(A1 = Drainage Area in sq. miles)
ABC Groundwater Divide Line
Area ABCDA Volume of Direct Runoff
At any time instant t,
ab Ground water storage or base flow
bd Surface runoff + Influent stream

8. Estimating the Concentration Time of a Catchment
Izzard’s Formula:
For small plots having no defined flow channels from which run off occurs as
laminar overland flow
Where
K = Runoff coefficient
So = Slope of the surface
Lo= Length of overland flow (m)
p = Rainfall intensity (cm/hr)
Cr = Retardance coefficient (Refer Table)
To = Over land Flow Time / Inlet Time (min)
These equations are applicable only when, p.Lo < 387.
( )
minutes
..111
3/2
3/1
Kp
Lb
T o
o
=
3/1
000275.0
tcoefficienab
o
r
S
Cp +
==
Types of Surface Value of Cr
Smooth asphalt surface 0.007
Concrete pavement 0.012
Tar and gravel pavement 0.017
Closely clipped soil 0.046
Dense blue grass turf 0.060

10. Kirpich Formula:
For design of hydraulic structures:
Where,
To = Overland flow time or Inlet Time (hour)
Lo = Distance from the farthest (i.e. critical) point to outlet at hydraulic
structure, such as culvert, etc (km)
H = Total fall in level from the critical point to the outlet, such
as culvert, etc(m)
Lo and H can be found from the survey plan of the area, and To can be easily
calculated.
For small drainage basin having flow channels in it
Tc = To + Tf
Where
Tf = channel flow time which exists anywhere in the channel
= length of the longest channel / average flow velocity in the channel
at about bankful stage.
385.03
885.0 





=
H
L
T o
o

11. Valley Storage
“Water which is temporarily stored within the channel of the basin is called
Channel storage or valley storage”.
Pre-requisite for Valley storage:
Increase in
Rate of overland flow => Rate of channel flow => Depth of flow
Equilibrium Condition will occur only when a sufficient amount of rain has
occurred to supply this valley storage.

12. Example:
An area of 5 hectares in a single family residence district
has an average length of overland flow of 40 meters,
average slope of the plots of 0.003, and the design
rainfall is given by
, where pc is in cm/hr and To is in minutes.
Ignoring the intensity term in the retardance coefficient
equation and using Cr = 0.05; find the time of
concentration for the overland flow from this area.
Assuming gutter flow time to add 10 min., find the peak
rate of runoff to be expected.
21
64
oT
p =

13. Solution:
We have the relationship for over land flow time,
where
Ingnoring p term and substitution of values yields,
Substitution of values in the equation for over land flow time, we have
which yields To = 75 minutes
Here,
Time of concentration with channel flow time, To = 75 min
Gutter flow time, Tf = 10 min
Hence,
( )
minutes
..111
3/2
3/1
Kp
Lb
T o
o
= 31
0002750
b /
o
r
S
Cp. +
=
3470b 31
.
S
C
/
o
r
==
3221
31
)]64(3.0[
)40).(347.0(111
o
o
T
T =

14. Therfore,
and,
cm/hr6.92856464 2150
=== .
tpC
( )( )( ) cumec32880592630
36
1
36
1
...A.p.KQ cmax
===