The US Geological Survey has many examples discussing flood frequency by physiographic area, state and region. In a smaller country, some of these examples might give you some expectations on the numbers of long term data needed in stream gauging of various size hydrologic units needed to develop guidance to others. NOAA and National Weather Bureau have developed maps in USA of rainfall frequency and durations. Seldom is 10 years of station data sufficient, and 25 years or more are preferred to make moderate projections. Some geomorphologists have been able to identify and estimate extent of past flooding through sediment dating and paleoflood Landforms. You might want to review some of Dave Rosgen's papers on stream classifications relative to entrenchment, sediment loading and flooding (www.wildlandhydrology.com). Stream type can affect flooding, as gullies are so entrenched, they may need a 500 year storm to flood. Braided streams have excess sediment loading, and may flood several to many times each year. Balanced streams have a bankfull event approximately every 1.5 years, where flooding begins to expand. Land use and channel activities can also influence runoff rates, channel stability and/or flooding. For best results, seek out experienced hydrologists and meteorologists for advice, help, data bases, statistical programs or models applicable, and any existing publications on area of interest. The USGS collects much data on a field preliminary level, but due to the importance and difficulty in analyzing and interpreting huge data sets with potential to affect design and cost of structures, property and life, they used highly experienced, certified or licensed professionals to review and certify work before final publishing. My advice would be to try to collaborate with other professionals and carefully qualify results so reader can decide how to consider and use.

You can try this book

https://books.google.com.bd/books?id=xxKSZ5yzV98C&printsec=frontcover&dq=measurement+of+flood+frequency&hl=en&sa=X&ved=0ahUKEwigwL7gzLDbAhVMb30KHWg4DWwQ6AEIKTAB#v=onepage&q=measurement%20of%20flood%20frequency&f=false

Flood frequency measuring can be obtained once a number of flow-meters are fixed along rivers and streams of the entire region. It would be simple measure over time series and then elaborate the flood frequency on the national level.