Question

Pick two of the five assumptions that are required to apply the unit hydrograph to a watershed. For each assumption, state what it is, describe it, and give an example of how that assumption might be violated in the real world.

Answer 1

A hydrograph of watershed is defined as a graph showing the rate of flow of water inthe watershed versus time period of the respective storm past a specific point in a river, channel, or conduit carrying flow.

A unit hydrograph of an watershed is defined as the hydrograph of direct runoff resulting from one unit depth (usually we consider 1cm) of rainfall excess occurring uniformly over the basin and at a uniform rate for a specified duration

Assumptions in developing Unit Hydrograph

(i) The effective rainfall is uniformly distributed over the entire drainage basin.

For the easier analysis and developing the mathematical model of the water recieved by the storm, we assume that the rainfall that cause the flow occur uniformly through out the basin. This assumption is very much important because the calculation of the potential flow of water due to storm would be easily determined mathematically.

Violation :- But this assumption is an ideal case. Rainfall could not occur uniformly through out the basin. When the size of the basin is very large then the accumulation of the error would occur. Moreover the rain is effected by the wind which cause non uniform rainfall.

(ii) The effective rainfall occurs uniformly within its specifier duration.

The storm is assumed to occur the uniforn rainfall at any point on the basin over the period of time within its specified duration. The variation of the intensity of the rain with respect to the time period is neglected. Generally storm with the smaller time duration would occur more or less uniform and intense rain with the well defined single peak hydrograph of small timebase.

Violation:- As mentioned above the small duration storm will produce uniform and intense rainfall over the specified time. But this does not holds good for the storm with the longer period duration. The storm with longer duration is most likely to vary the intensity of the precipitation over the time.

(iii) The effective rainfalls of equal duration will produce hydrographs of direct runoff having same or constant time base.

The storms having the same intensity and equal duration is assumed to produce the hydrographs with the similar pattern and the constant time base. To make it more clear we assume that the flow rate, peak flow and the time base of a unit hydrograph is assumed to be independent on the external factors and the properties of the basin.

Violation:- The properties of an hydrographs are considerably effected by the properties of the soil (like grain size, cohesive property, infiltration rate of the soil, presence of moisture mineral composition of the soil etc), properties of the basin (like slopes and gradients of the ground, presence of the obstruction to flow like vegetation etc) and external features like temperature and pressure.

(iv) The ordinates of the direct runoff hydrographs of a storm having 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.

Violation :- As explained in the (iii) ordinates of an hydrograph is affected by the external features along with the properties of the storm,

(v) The hydrograph of the runoff resulting from given pattern of rainfall, from a given drainage basin, 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.

Violation:- Even though the time has the minimum effect on the unit hydrograph, there occur a variation in the runoff due to water loss effects(like infiltration and evaporation) which are the functions of the time.

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