What data are available or must be collected. What methods of analysis are available including the relative strengths and weaknesses in terms of cost and accuracy. 811.3 Peak Discharge Peak discharge is the maximum rate of flow of water passing a given point during or after a rainfall event. Peak discharge is different for every storm and it is the highway engineer's responsibility to size drainage facilities and structures for the magnitude of the design storm and flood severity. The selection of design storm frequency and flood probability are more fully discussed under Topic 818, Flood Probability and Frequency.
Modern concepts tend to define a flood in terms of probability.
Probability of exceedance, the statistical odds or chance of a flood of given magnitude being exceeded in any year, is generally expressed as a percentage. Frequency of recurrence is expressed in years, on the average, that a flood of given magnitude would be predicted. Refer to Topic 818 for further discussion of flood probability and frequency. The highway engineer should become familiar with the many factors or characteristics that affect runoff before making a hydrologic analysis. The effects of many of the factors known to influence surface runoff only exist in empirical form.