SHREE S’AD VIDHYA MANDAL INSTITUTE OF TECHNLOGY

Faculty:- Payal shahSUBJECT:- Hydrology & Water resources engineering

SR NO . NAME ENROLLMENT NO .

1 Patel Margi M. 130450106035

2 Shah Ishani M. 130450106044

Hyetograph :

A hyetograph can be defined as a plot of intensity of rainfall against the time interval, represented as a bar chart.

Time (hours)

Rainfall intensity (cm/hr)

The intensity of rain is expressed in cm/hr . This time intensity curve is generally assumed to be

made up of straight lines as shown in figure and is called hyetograph .

The hyetograph can be prepared either from the mass curve of rainfall, or directly from the data obtained from automatic raingauges .

The area under a hyetograph represents the total rainfall received in that period .

This chart is very usefull in presenting the characteristics of a storm, and is particularly important in developing the design to predict extreme floods .

Hyetograph from mass curve of rainfall :

The mass curve of rainfall is a plot of accumulated rainfall (cumulative rainfall) against time .

The hyetograph is derived from the mass curve and is usually represent as a bar chart . Time (hours)

1st storm

2nd storm

3rd storm

A

D

B C

E

F

i = ∆𝑝

∆𝑡 ∆𝑝

∆𝑡

Cumulative rainfall (cm)

The steepness of the curve indicates the intensity of rainfall.

The horizontal portion of the curve indicates that there was no rainfall during that period .

The mass curve of rainfall is rising curve . The intensity of rainfall during any period is given by

intensity of rainfall = i = ∆𝑝

∆𝑡

The mass curve of rainfall gives very useful information regarding duration and magnitude .

HYDROGRAPH

A hydrograph is a graph showing variation of

discharge (Q) with time (t) , at a particular point of

stream or river,

River discharge = Cross sectional area x rivers mean

velocity

Discharge is usually expressed in cumec or hectare

meter per day and time is expressed in hours or days.

Since the hydrograph is generated from runoff due to

precipitation resulting from an isolated storm or a series

of consecutive storms, it is also known as a storm

hydrograph.

Components of a single peaked hydrograph

D

B

CA

Rising limb recession

peak

Direct runoff

Ground water

Flow due to channel storage

EPoint of inflection

N DAYS

Discharge

time

The rising limb is the ascending portion of the hydrograph corresponding to increase of discharge due to gradual accumulation of storage in the channels existing in the area.

The peak segment includes the part of the hydrograph from the inflection point on rising limb to an inflection point on recession line.

The recession limb extends from the inflection point to the point which represents the recommencement of the natural base flow or ground water flow.

Separation of base flow from direct run off1) Straight line method :

Figure shows the two points A and C which mark the beginning and end of the direct run off.

2)Two tangent method:This is the most widely used method in whichthe base flow is separated from direct runoffby two lines.

The first line AB is obtained by extending the base flow curve existing prior to the commencement of the surface runoff, till it intersects a vertical line, drawn from the peak.

Point B and C are joined by straight line. Empirical equation to locate point C:

N = 0.83 daysWhere

N = time interval in days from peakA = area of drainage basin

A2.0

UNIT HYDROGRAPH A unit hydrograph is a hydrograph representing 1

cm of runoff from a rainfall of some unit duration.

ASSUMPTIONS :1) The effective rainfall is uniformly distributed

within its duration or specified period of time.2) The base or time duration of hydrograph of direct

runoff due to effective rainfall of unit duration is constant

3) The ordinates of all direct runoff hydrographs are proportional to the total amount of direct runoff .

4) The effective rainfall is uniformly distributed over the entire area of drainage basin.

DERIVATION OF UNIT HYDROGRAPH : The unit hydrograph method is used for estimation

of the maximum flood discharge of stream as well as for developing a flood hydrograph corresponding to anticipated rainfall.

For the application of this method unit hydrograph of suitable unit duration is derived from an observed hydrograph of drainage basin.

In order to derive unit hydrograph from storm hydrograph of same unit duration following steps are followed.

1) From the past records select hydrograph resulting from an isolated, intense, short duration rainfall of uniform duration over the drainage basin.

Construction of unit hydrograph

Direct runoff

Base flow

N DAYS

Discharge

time

Flood hydrograph

Unit Hydrograph

2) Separate the base flow from direct runoff.3) Subtracting the ordinates of base flow from the total

ordinates, find the ordinates of direct runoff.4) Compute the depth of direct runoff .

Depth of direct runoff =

where ,= sum of direct runoff ordinates in cumec.= time interval in hours between successive ordinates.

A = area of drainage basin in square kilometers.

5) by dividing each of direct runoff by depth of direct runoff , the ordinates of unit hydrograph are obtained.

LIMITATION :

1) The unit hydrograph theory can not be applied to catchment areas greater than 500square km.

2) The unit hydrograph theory cannot be applied to very small catchments with area less than 2 km square.

3) This theory cannot be applied when the major portion of the storm is in form of snow.

4) This theory is not accurate.

USES:

1) It can be used for flood forecasting and flood warning based on the observed rainfall in the basin.

2) To determine runoff from a storm of unit duration.3) To draw maximum flood hydrograph for the design of

spillways.

SUMMATION HYDROGRAPH(s –hydrograph)

S- hydrograph is hydrograph that is produced by a continuous effective rainfall at a constant rate for indefinite period.

It is continuous rising curve , in the form of letter s , till equilibrium is reached.

SYNTHETIC UNIT HYDROGRAPH

The unit hydrograph determined for the ungauged catchment is known as synthetic unit hydrograph.

Several techniques are adopted for establishing relationship between the basin characteristics and the unit hydrograph characteristics for unit rainfall , but most commonly method is Snyder's method.

INSTANTANEOUS UNITHYDROGRAPH

If the duration of rainfall excess becomes infinitesimallysmall ( T 0 ) the resulting unit hydrograph is calledinstantaneous unit hydrograph.

This is only a theoretical concept , because 1 cm ofrainfall cannot occur in a zero time on any catchment ,but it is useful because , such a unit hydrographrepresents watershed’s response to a rainfall withoutreference t the duration of rainfall.

A comparatively easier method for developing an IUHhas been evolved by clark and is known as clark’s method or time area histogram method.