Hydrograph

·         Hydrograph represent the relation between instantaneous discharge and time at a given point on the stream.

·         It shows integrated effect of physiographic and meteorological condition on runoff (discharge) from a catchment.

           Components of Hydrograph

·     

·         Uses of Hydrograph

§  In flood damage mitigation by structural and non-structural measures.

§  In flood forecasting.

§  In design of hydraulic structures like dam, spillway, water convey structure etc.

 Hydrograph depends:

·         Meteorological factors :-

§  Rainfall intensity and pattern

§  Areal distribution or rainfall over the basin

§  Size and duration of the storm event

·         Physiographic or watershed factors :-

§  Size and shape of the drainage area

§  Slope of the land surface and channel

§  Channel morphology and drainage type

§  Soil types and distribution

§  Storage detention in the watershed

§  land use and land cover

Factor affecting shape of hydrograph

·         The Drainage Basin

·         Shape of the drainage basin

·         It influences the time taken by water to reach the outlet from farthest point and hence affect the time after which peak discharge attained.

·         Basin A will yield peak discharge early because most of the point are near to the basin outlet.

·         Basin B will have late peak discharge, because point are far away from outlet of basin.

·         Basin C will have two peak discharge, also called complex hydrograph.

·         For a circular drainage basin, the river’s hydrograph have a steep rising limb, symmetrical and high peak discharge.

·         Reason- in this type of basin all point in drainage basin are at equal distance from river or stream so water can take equal travel time to reach the river.

·         Some other factor based on shape are as follows:

1.    Form factor: it is the ratio of average width of the basin to the axial length.

2.    Compactness Coefficient: it is the ratio of perimeter of the basin to the perimeter of a circle, whose area is same as that of the basin.

                               Compactness Coefficient:             P/(2*pie) 

                                 For circular basin value =1

·         Stream density

·         This is defined as the no. of streams per unit area of the catchment area. If value of stream density is more peak discharge will happen early.

·         Drainage density

·         This is defined as the total length of stream per unit area of the catchment. If value of stream density is more peak discharge will happen early.

·         The size of the drainage basin

·         Large size basin catch more water compare to small size basin so hydrograph resulting from large size basin have high peak compare to small size basin.

·         Hydrograph resulting from small size basin have shorter leg time compare to large size basin.

Reason- in small size basin water can take less time to reach river or basin.

Q_P= k. Aⁿ 

K= constant,     A= Area of catchment, Q_P= Peak discharge

·         Basins slopes

·         Basin having Steep slope give high peak discharge and a short lag time hydrograph.

Reason- water take less travel time on steep steep slope.

·         Soil & Rock Type

·         Non-porous and impervious rocks responsible for high peak discharge and shorter lag time.

·         Porous and permeable rocks/ soil results in low peak discharge and greater lag period.

·         Rainfall intensity

·         More rainwater results in more water in the river or in other word higher discharge. So higher intensity storm or rainfall responsible for higher peak discharge and shorter lag time

·         Effect of wind

·         Direction of wind’

·         Soil moisture condition

·         Land use and land cover