-1 DR. S & S. S GHANDHY GOVT. ENGINEERING COLLEGE, SURAT. SUB : HYDROLOGY & WATER RESOURCES ENGINEERING ( ) TOPIC : HYETOGRAPH & HYDROGRAPH ANALYSIS GUIDED BY: S.I. WAIKHOM

PREPARED BY : CHAUHAN CHIRAG A. ( ) CHAUHAN MANISH J. ( ) CHAUHAN PINKAL H. ( ) CHAVDA JAYPALSINH R. ( ) GANDHI DEEP R. ( ) DHIMMAR SHIVANI C. ( ) GADHIYA KISHAN J. ( ) GAJIWALA NIRALI D. ( ) -2-2

HYETOGRAPH : A Hyetograph can be defined as a plot of rainfall against the time interval, represented as a bar chart. The intensity of a rain is expressed in cm/hr. Sometime the rainfall rate will greatly exceeds 5 cm/hr and sometime it may be less than 5 cm/hr.

RUNOFF : Runoff is defined as the portion of the precipitation which is not evaporated. It is one of the important component of the hydrological cycle. It is thus represented the output from the catchment in a given unit of time. There are three types of runoff. a) Surface runoff b) Sub-surface runoff c) Ground water flow. Precipitation = Evaporation + Runoff P = R + E

FACTOR AFFECTING RUNOFF : The main factors affecting runoff are: 1) Precipitation characteristics. 2) Shape and size of the catchment. 3) Topography 4) Meteorological characteristics. 5) Storage characteristics of the catchment.

1 ) PRECIPITATION CHARCTERISTICS : Intensity of rainfall Duration of rainfall Form of precipitation Frequency Runoff increases with the intensity of rainfall. The more the rainfall, more will be the runoff. The precipitation may occur either in the form of rain or snow. 2) SHAPE AND SIZE OF THE CATCHMENT : A drainage basin may be either fan shaped or fern –leaf shaped.

In case of fan shaped catchment, all the tributaries are approximately of the same size. In case of fern-leaf shaped catchment the tributaries are generally different lengths and main stream at the regular intervals.in such a narrow catchment, the peak flood intensity is reduced.

TOPOGRAPHY : A steeply sloping drainage basin produces greater runoff, because a steep slope helps quicker drainage, no dispersion storage and less evaporation losses. If the catchment is mountainous the intensity of rainfall will be more and hence runoff will also be more. 4 ) STORAGE CHARACTERISTIC OF THE CATCHMENT a) Depressions b) Pools and ponds c) Stream d) Check dams

HYDROGRAPH : A Hydrograph is a graph showing variation of discharge ( Q) with time (t). Discharge is usually expressed in cumec or hectare meter per day and time is expressed in hours or days. Discharge is plotted on y-axis and the corresponding time is plotted on x- axis.

-Hydrograph Record of River Discharge over a period of time -River Discharge = cross sectional area rivers mean (average) velocity X (at a particular point in its course) - Storm Hydrographs -Show the change in discharge caused by a period of rainfall

UNIT HYDROGRAPH : A unit hydrograph is a hydrograph representing 1 cm of runoff from a rainfall of some unit duration. For example a 3- hour unit hydrograph indicates the hydrograph which gives 1 cm depth of direct runoff when a storm of 3-hour duration occurs uniformly over the catchment.

ASSUMPTION OF UNIT HYDROGRAPH The effective rainfall is uniformly distributed within its duration. The effective rainfall is uniformly distributed over the entire area of the drainage basin. The time duration of the hydrograph of direct runoff due to an effective rainfall of unit duration is constant. The ordinates of all the direct runoff hydrographs of a common base period are directly proportional to the total amount of direct runoff.

LIMITATION OF UNIT HYDROGRAPH : The UH theory cannot be applied to catchment area greater than 5000 km 2. The UH theory cannot be applied to very small catchments with area less than 2 km 2. This theory cannot be applied when the major portion of the storm is in the form of snow. This theory is not very accurate. The accuracy obtained is ± 10 %

USES OF UH : To draw maximum flood hydrograph for the design of spillways. It can be used for flood forecasting and flood warning based on the observed rainfall in the basin. To determine runoff a storm of unit duration.

Example The hydrograph of Example 7.1 was produced by a storm of 12-hour duration considered to have uniform intensity over the basin. determine the unit hydrograph.

Example