Hydrograph and its components

HYDROGRAPH  AND ITS COMPONENTS

A hydrograph of a catchment produced by a storm is a graphical representation of discharge rate of a stream with respect to the time from the commencement of storm. It is an inverted U shaped diagram.

Components of Hydrograph :
Hydrograph generally contains the following three parts.
1. Rising Limb
2. Peak (or Crest) Segment
3. Falling (or Recession) Limb

Fig : Element of flood hydrograph

1. Rising Limb : It is the ascending curved portion of the hydrograph. The rising limb rises slowly in the early stage of the flood but more rapidly toward the end portion. The shape of rising limb depends on duration and intensity distribution of rainfall. This is because in early stages the losses is more and water reaches to the stream faster.

2. Peak Segment (or Crest Segment) : Peak segment is shown by inverted U in the hydrograph. This is the part which is taken as matter of interest by hydrologists. Peak of hydrograph occurs when all parts of basins contribute at the outlet simultaneously at the maximum rate. Depending upon the rainfall-basin characterstics, the peak may be sharp, flat or may have several well defined peaks.

3. Falling Limb (or Recession Limb) : Recession Limb represents the withdrawal of water from the storage built up during the early phase of hydrograph. It extends from the point of inflection at the end of the crest to the beginning of the natural groundwater flow. The shape of recession limb depends upon basin characteristics only and independent of the storm.

Introduction to Hydrograph

Fig: Single Peaked Hydrograph

   Hydrograph or Flood hydrograph is the graphical representation of the instantaneous rate of discharge of a stream plotted with respect to time at particular point. In simple words, hydrograph are the graphical drawing used to show how the water flows in a drainage basin respond to a period of rain.

The hydrograph may have a single peak or multiple peaks, depending upon the nature of the storm . If the subsequent storm does not occur before the direct runoff of the first storm ceases, such storm is called isolated storm and it results single peak hydrograph. And if next storm(rainfall) occur before the direct runoff of the first storm ceases then such storm is called complex storm and it results multi-peaked hydrograph.

The beginning of Hydrograph starts when storm starts. As the storm(rain) is commenced, water starts flowing toward the natural stream (river) from the catchment. Slowly the discharge of the river increases (shown as rising limb in the hydrograph) as water is added to the stream from the catchment. Time come when the water from farthest point of the catchment reach the river. In this period river reaches its maximum discharge (shown as peak flow in the hydrograph). Now as the storm ceases and addition of water to the river through storm stops, the remaining water from the catchment still flows to the stream, adding less discharge to the stream than before. Hence discharge of the river slowly decreases, as only the remaining flow from the catchment of previous storm enters the river (shown as falling limb or recession limb in the hydrograph). And slowly base flow is attained and the river carries same discharge as before the storm had arrived.

*Note: The detailed parts and points of single peaked hydrograph are explained in the next post. Please keep following.