A hydrograph is a graph that shows the discharge overtime at any specific point in time. Time is shown on the x-axis and discharge is shown on the y-axis.
Definition of Unit Hydrograph:
It is a typical hydrograph of direct runoff which gets generated from one centimeter of effective rainfall falling at a uniform rate over the entire drainage basin uniformly during a specific duration. Effective rainfall is that portion of rainfall which fully contributes towards direct runoff. Therefore, unit hydrograph can also be defined as the hydrograph of a drainage basin which gives one centimeter of direct runoff from a rain storm of specific duration.
Assumptions in Unit Hydrograph Theory:
The theory of unit hydrograph is based on certain assumptions.
The main assumptions are following:
(i) The effective rainfall is uniformly distributed over the entire drainage basin.
(ii) The effective rainfall occurs uniformly within its specifier duration.
This requirement calls for selection of storms of so small a duration which would generally produce an intense and nearly uniform effective rainfall and would produce a well defined single peak of hydrograph of short time base. Such a storm can be termed as “unit storm”.
(iii) The effective rainfalls of equal (unit) duration will produce hydrographs of direct runoff having same or constant time base.
(iv) The ordinates of the direct runoff hydrographs having same time base (i.e., hydrographs due to effective rainfalls of different intensity but equal duration) are directly proportional to the total amount of direct runoff given by each hydrograph. This important assumption is called principle of linearity or proportionality or superposition.
(v) The hydrograph of runoff from a given drainage basin resulting, from a given pattern of rainfall reflects all the combined physical characteristics of the basin. In other words the hydrograph of direct runoff resulting from a given pattern of effective rainfall will remain invariable irrespective of its time of occurrence. This assumption is called principle of time invariance.
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(i) In theory, the principle of unit hydrograph is applicable to a drainage basin of any size. In practice, however, uniformly distributed effective rainfall rarely occurs on large areas. Also on large areas effective rainfall is very rarely uniform at all locations, within its specified duration. Obviously bigger the area of the drainage basin lesser will be the chances of fulfilling the assumptions enunciated above. The limiting size of the drainage basin is considered to be 5000 km2.
When the area of the drainage basin exceeds a few thousand km2. The catchment has to be divided into sub-basins and the unit hydrographs developed for each sub-basin. The flood discharge at the basin outlet can then be estimated by combining the sub- basin floods adopting flood routing procedure.
(ii) The unit hydrograph method cannot be applied when appreciable portion of storm precipitation falls as snow because snow-melt runoff is governed mainly by temperature changes.
(iii) Also when snow covered area in the drainage basin is significant the unit hydrograph method becomes inapplicable. The reason is that the storm rainfall gets mixed up with the snow pack and may produce delayed runoff differently under different conditions of snow pack.
(iv) The physical basin characteristics change with seasons, man-made structures in the basin, conditions of flow etc. Obviously the principle of time invariance is really valid only when the time and condition of the drainage basin are specified.
(v) It is commonly seen that no two rain storms have same pattern in space and time. But it is not practicable to derive separate unit hydrograph for each possible time- intensity pattern. Therefore, in addition to limiting drainage basin area up to 5000 km2 if storms of shorter duration say 1/3 to 1/4 of peaking time are selected it is seen that the runoff patterns do not vary drastically.
(vi) The principle of linearity is also not completely valid. This is so because due to variability in proportion of surface, subsurface and groundwater runoff components during smaller and larger storms of same duration, the maximum ordinate (peak) of the unit hydrograph derived from smaller storm is smaller than the one derived from larger storm. Obviously the character and duration of recession limb which is a function of the peak flow will also be different. When appreciable non-linearity is seen to exist it is necessary to use derived unit hydrographs only for reconstructing events of similar magnitude.
(vii) The unit hydrograph can be used theoretically to construct a flood hydrograph resulting from a storm having same unit duration. Obviously it necessitates construction of several unit hydrographs to cover different durations of storms. In practice however it is seen that a tolerance of ± 25% in unit hydrograph duration is acceptable. Thus a 2 hour unit hydrograph can be applied to storms of 1.5 to 2.5 hours duration.
The limitation to the theory of unit hydrograph can be overcome to a large extent by remaining within the various ranges and restrictions indicated above.
The unit hydrograph theory has several advantages to its credit which can be summarised as below:
(i) Flood hydrograph can be calculated with the help of very short record of data.
(ii) In addition to peak flow unit hydrograph also gives total volume of runoff and its time distribution.
(iii) The unit hydrograph procedure can be computerised easily to facilitate calculations.
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