Regional Flood Frequency Analysis (RFFA) is one of methods which commonly used to determine design floods in rural areas. The example of utilising this method is for designing water-related structures such bridges, regulators, and dams.
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| Probability Distribution of Annual Maximum Streamflow |
Two major steps in creating RFFA are: (1) determine the homogeneity regions (2) produce a frequency curve. Both of these steps need statistical analysis specifically probability distributions.
Among many method of RFFA, Index Flood Method initiated by Darlymple (1960) is the famous one. He proposed that homogeneity region is based on recurrence interval of 10 years and probability distribution using EV 1 with method of moment (MOM). United States of Geological Survey (USGS) is also adopting this method in their standard procedures.
To do this analysis, the first step is collecting the data. The importance data needed are as follows:
1. Annual Peak Flow
Annual peak flow or Maximum Annual Flood (AMAX) usually can be collected from water authority. This is a peak daily discharge within a year. The data was derived from rating curves of water levels and flow measurement. It is important therefore to check the raw data and plot them together so we can get the sense of the quality of the data.
Annual peak flow or Maximum Annual Flood (AMAX) usually can be collected from water authority. This is a peak daily discharge within a year. The data was derived from rating curves of water levels and flow measurement. It is important therefore to check the raw data and plot them together so we can get the sense of the quality of the data.
The data must be random and independent. If the peak discharges are getting higher and higher over the years. This is an indication that we cannot use this data. Probably due to rapid development or control structures have been built in the catchment area.
2. Annual Rainfall
Annual rainfall is a cumulative of rainfall within a year. Similar to flow data, the rainfall data should have not a trend over the years. If the data higher from time to time or vice verse then it cannot be used as it was not independent anymore.
Annual rainfall is a cumulative of rainfall within a year. Similar to flow data, the rainfall data should have not a trend over the years. If the data higher from time to time or vice verse then it cannot be used as it was not independent anymore.
3. Catchment Area
Catchment area is the most significant parameter in determining the peak flow. Other factors such as slope, altitute, may also contribute to rising flow significantly. However, adding more parameters which make formula much complicated and the results are significantly improved, it is better to take the simple one but still represent the reality.
Catchment area is the most significant parameter in determining the peak flow. Other factors such as slope, altitute, may also contribute to rising flow significantly. However, adding more parameters which make formula much complicated and the results are significantly improved, it is better to take the simple one but still represent the reality.
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