Hydrology: Unit Hydrograph Derivation and Storm Complexity

In the realm of hydrology and flood analysis, the derivation of unit hydrographs is a crucial aspect that aids in understanding and predicting runoff patterns. Unit hydrographs provide insights into the response of a watershed to a unit of rainfall excess. This process involves several key steps to ensure accuracy and reliability in the analysis.

Isolated Storm Hydrographs

The foundation of deriving unit hydrographs lies in the selection of isolated storm hydrographs. These hydrographs, resulting from short spells of rainfall excess, should have a consistent duration (typically 0.9 to 1.1 times the basin lag). The first step involves separating the base flow from each surface runoff hydrograph.

Quantifying Direct Runoff

To proceed, the area under each Direct Runoff Hydrograph (DRH) is evaluated. The volume of direct runoff is then determined by dividing this area by the catchment area. This process yields the depth of Effective Rainfall (ER). The ER values are essential for normalizing the subsequent unit hydrographs.

Unit Hydrograph Derivation

The ordinates of the isolated DRHs are divided by their corresponding ER values, resulting in the ordinates of the unit hydrograph. This procedure is repeated for numerous storm events, and the unit hydrographs are then plotted. Due to variations in rainfall and deviations from theoretical assumptions, these unit hydrographs may not be identical.

Averaging for Consistency

To address variations, the mean of the derived unit hydrographs is often adopted as the representative curve for the given duration. The average of peak flows and time to peaks is computed, and a mean curve is drawn to approximate the base length. Any departure from unity is corrected by adjusting the peak value.

Unit Hydrograph Duration Considerations

The ideal duration for a unit hydrograph is assumed to be one in which small fluctuations in rainfall intensity do not significantly affect runoff. It should not exceed 1/5 to 1/3 of the basin lag. For larger catchments (above 250 sq. km.), a 6-hour duration is generally satisfactory.

Unit Hydrograph from Complex Storms

In situations where simple isolated storms are unavailable, data from complex storms of longer duration may be used. This involves decomposing a measured composite flood hydrograph into its component DRHs and base flow. Assumptions about a common unit hydrograph of appropriate duration are made, constituting an inverse problem compared to deriving the flood hydrograph.

Complex Storm Considerations

Consider a composite rainfall excess with consecutive durations and ER values. After base flow separation, the composite DRH is obtained. The unit hydrograph ordinates at various time intervals are compared with those of the composite DRH.

![Unit hydrograph from a complex storm](Figure: Unit hydrograph from a complex storm)


While deriving unit hydrographs is a valuable tool in hydrological analysis, it’s essential to acknowledge the potential disadvantages. Errors can propagate and increase as computations proceed, especially when dealing with complex storm scenarios. However, by adhering to best practices and considering the specific characteristics of the catchment, the derived unit hydrograph remains a powerful tool for flood prediction and water resource management.

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