Clark's Approach to the Generation of Instantaneous Unit Hydrographs(IUH)
Beginners guide to Flood Routing : Todays Lecture on Clarks Method of IUH generation
An "Instantaneous Unit Hydrograph (IUH)" is a theoretical hydrograph representing the immediate runoff response of a watershed to a unit depth of "excess rainfall" applied instantaneously across the entire basin, essentially illustrating how water would flow through the watershed if all points contributed simultaneously. This concept is fundamental to the "Clark Unit Hydrograph" method, a conceptual model used to transform "excess rainfall" into a runoff hydrograph by considering the "time-area histogram" of the basin, which details the contributing area at different time intervals, and routing it through a "linear reservoir" with a "storage coefficient (R)" to simulate "attenuation" and "translation" of the water flow throughout the watershed.
Key aspects of the IUH and Clark method:
Time-Area Histogram:
A graphical representation of the cumulative contributing area of a watershed at different times, used to determine the rate at which water reaches the outlet.
Translation:
The movement of water from different parts of the watershed to the outlet with a time delay based on the basin's topography and flow paths.
Attenuation:
The reduction in peak flow due to temporary storage of water within the watershed, represented by the linear reservoir concept.
Storage Coefficient (R):
A parameter that determines the rate of attenuation in the linear reservoir, reflecting the watershed's ability to store water.
Time of Concentration (Tc):
The time it takes for water from the furthest point in a watershed to reach the outlet, a crucial factor in determining the shape of the IUH.
How it works:
1. Develop a Time-Area Histogram:
By analyzing the watershed's topography, a time-area histogram is created, showing the cumulative area contributing to flow at different time intervals.
2. Apply the IUH concept:
The time-area histogram is routed through a linear reservoir, representing the watershed's storage, to generate a hydrograph that reflects the "translation" and "attenuation" of runoff.
3. Calculate Peak Flow:
The peak of the hydrograph represents the peak discharge at the watershed outlet.
Applications:
Flood Forecasting: Estimating potential flood peaks by simulating various rainfall scenarios using the IUH concept.
Watershed Management: Evaluating the impact of land use changes on runoff patterns.
Design of Hydraulic Structures: Determining the required capacity of drainage systems based on predicted peak flows.
Limitations:
Simplified Representation:
The Clark Unit Hydrograph is a simplified model and may not accurately capture complex watershed dynamics, particularly in highly variable terrain or with significant infiltration.
Calibration Required:
The storage coefficient (R) often needs to be calibrated using observed runoff data for accurate results.
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