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Definition of HydrographComponents of Hydrograph

Effluent and Influent Streams

Separation of Surface and Groundwater Runoff

Estimating the Concentration Time of a CatchmentValley Storage

Example

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HYDROGRAPH

Hydrograph is a graph showingvariations of discharge with time, at aparticular point of a stream.

It shows the time distribution of totalrun-off at the point of measurement.

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Components or Parts of Storm Hydrograph

Single Peaked Hydrograph Resulting from an Isolated Storm

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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 streamBefore stormFlow from groundwater to the stream

High flow in streamAfter storm

Flow from the stream to the groundwater

Flood level in the stream recedes

Groundwater again starts contributing to the stream

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How to Separate Direct Runoff and GroundwaterRunoff?

1.Extend the recession of the flow existing prior to the storm

to a point under the peak (or crest) of the hydrograph (lineAB).

2.Select a point C on the recession limb of the hydrograph Ndays 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 = A10.2 (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 flowbd Surface runoff + Influent stream

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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.