# Unit Hydrograph | Assumptions, Limitation and Uses of Unit hydrograph

### 1. Introduction

Hydrography resulting from one unit depth of excess rainfall (1 cm) generated uniformly over the basin for an effective duration (D hour) is called unit hydrograph.

– Rainfall excess= 1 cm

– Total Depth of rainfall excess = total depth of direct runoff.

– Runoff volume = Δ x 1 cm

Rainfall intensity = 1/D cm/hr

### 2. Assumptions of Unit Hydrograph

a. The excess rainfall has a constant intensity of 1/D cm/hr with an effective storm duration of D hours.

b. Principle of time invariance holds.

c. Unchainging basin characteristics.

d. Uniform distribution of excess rainfall over the basin.

e. Constant base time of DRH for excess rainfall of a given duration.

f. Linear Model: Principle of superposition and proportionality holds.

### 3. Uses of Unit Hydrograph

a. Unit hydrograph can be used to develop the flood hydrographs for extreme rainfall magnitude for the design of the hydraulic structure.

b. It is used for watershed simulation models.

c. It is used for flood forecasting and warning system.

d. Extension of flood flow records based on rainfall records.

e. Once a unit hydrograph is prepared for a duration D hour of a basin, the storm hydrograph for that basin for any other storm of different intensities but the same duration can be developed.

### 4. Limitations of Unit Hydrograph

a. It can’t be applied to very large(A>5000 Km2) and very small (A<2 Km2) catchment areas.

b. Not suitable for the long basin.

c. Applicable for a short duration.

d. Precipitation must be from rainfall only.

e. The UH theory is only ± 10% only.

f. The base period is not exactly the same for all storms.

g. The principle of linearity is not strictly correct.

h. Catchment area shouldn’t have large storage such as tanks, ponds, etc.

i. Not applicable for basins having a high variation of rainfall intensity.

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