Unit HydrographPrepared by : Gomez, Keesee B.Unit HydrographProbably the most popular and widely used method.The hydrograph of direct runof resulting from one unit depth (1cm) of rainfall excess occurring uniformly over the basin and at a uniform rate a specifed duration ( hours).Unit Hydrograph!t relates only the direct runoff to the rainfall excess. "ence the volume of water contained in the unit hydrograph must be e#ual to the rainfall excess. $s 1 cm depth of rainfall excess is considered the area of unit hydrograph is e#ual to a volume given by 1 cm over the catchment.Unit hydrographThe rainfall is considered to have an average intensity of excess rainfall (%&) 1' cm'h for the (h of the storm.The distribution of the storm is considered to be uniform all over the catchment.Unit hydrographUnit-Hydrograph Theory Two basic assumptions)1. Time !nvariance*. +inear &esponseUnit-Hydrograph Theory Time !nvarianceThe direct , runof response to a given efective rainfall in a catchment in time(invariant.!t implies that the &" for a given %& in a catchment is always the same irrespective of when it occurs.Unit-Hydrograph Theory+inear &esponseThe direct , runof response to a rainfall excess is assumed to be linearUnit-Hydrograph Theory%xample)-iven below are the ordinates of .(h unit hydrograph for a catchment. /alculate the ordinates of the &" due to rainfall excess of 0.1 cm occurring in . h./alculation of &" ue to 0.1 %&0.1 cm &" derived from .(h 2nit "ydrographUnit-Hydrograph Theory%xample)The two storms each of .(h duration and having rainfall excess values of 0.3 and *.3 cm respectively occur successively. The *(cm %& rain follows the 0(cm rain. The .(h unit hydrograph for the catchment is the same asgiven in previous example. /alculate the resulting &"./alculation of &" by method of 4uperposition/alculation of &" by method of 4uperpositionPrinciple of 4uperpositionApplication o Unit hydrographThe initial losses and infltration losses are estimated and deducted from the storm hyetograph to obtain the %&". The rainfall excess in each (h duration is then operated upon the unit hydrograph successively.&" due to %&"Application o Unit hydrographThe direct runof due to &1at time t is 51 6 &1 x u (t)The direct runof due to &*at the time (t() is4imilarly7 $nd Thus at any time t7 the total direct runof is4olution) !eri"ation o Unit HydrographThe area under each &" is evaluated and the volume of the direct runof obtained is divided by the catchment area to obtain the depth of %&. The ordinates of the various &"s are divided by the respective %& values to obtain the ordinates of the unit hydrograph.!eri"ation o Unit Hydrograph8lood hydrographs used in the analysis should be selected to meet the following desirable features with respect to the storms responsible for them.The storm should be isolated storms occurring individually. The rainfall should be fairly uniform during the duration and should cover the entire catchment area.!eri"ation o Unit HydrographThe duration of the rainfall should be 1'1 to 1'0 of the basin lag.The rainfall excess of the selected storm should be high. $ range of %& values of 1.3 to 9.3 cm is sometimes preferred.erivation of 2nit "ydrograph from a :ood hydrograph!eri"ation o Unit Hydrograph$ 6 beginning of &" t63;6 %nd of &" t6