Definition, features, assumptions and limitations of a unit hydrograph

  

Date Posted: 3/23/2018 12:23:23 PM

Posted By: franco crick  Membership Level: Gold  Total Points: 2067

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A unit hydrograph is a hydrograph showing relationship of discharge of direct runoff of rainfall per unit time usually represented by one centimeters at a given drainage basin area over a given period of time distributed unformly and at a uniform period of time so as to obtain the desired storm.

FEATURES OF UNIT HYDROGRAPH ARE:
-Has a rising limb.It is also known as the increasing limb.This limb indicates an increase in discharge over time due to initial infiltration that had occurred during rainfall.the initial losses also leads to higher rates of discharge.
-The recession limb is also known as the declined limb indicating maximum water storage due decreased discharge of direct runoff.
-The crest segment which is a segment that indicates maximum discharge due to high drainage runoff.It is also referred to as the peak of the dicharge of a given storm.

ASSUMPTIONS OF A UNIT HYDROGRAPH:
a)There is time invariance as time is not exactly uniformly distributed to the hydrograph.
b)There is no valid linear response thus no lineality considered on the hydrograph.
c)The base periods of the hydrograph over effective rainfall uniformly distributed over a given period of time are not the same.
d)The effective rainfall is uniformly distributed over a given period of time.
e)The effective rainfall is also uniformly distributed through out the year.

LIMITATIONS OF A UNIT HYDROGRAPH:
i)Unit hydrograph method is not applicable to areas with a major portion of storm precipitation being on form of snow.
ii)Principle of time in-variance sometimes does not hold as the basin characteristics will chabge with man made adjustments as well as seasons.
iii)The linearity principle is strictly valid since unit hydrographs derived from small rainfall events generally has lower peaks than those derived from more larger storms.
iv)The storms that are selected should always be of short duration due to uniform rainfall intensity

over long duration.
v)The assumptions of constant base period for storms of same duration may not produce excessive error since its recession curve usually approaches zero approximately.
vi)Another limitation is that unit hydrographs can only be applied to drainage basins with small areas only and not of the larger areas.

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