Hydrologic analyses are studies of the amount of water flowing in a stream during flood events.

Generally, FISs are concerned with the peak rates of flow or discharges in streams for the 10-percent (10 year), 4-percent (25 year), 2-percent (50 year), 1-percent (100 year), and 0.2-percent (500 year) annual-chance exceedance flood events. Peak discharges are typically measured in cubic feet per second (cfs).

The 4-percent annual chance (25-year) flood event may not be calculated in older Flood Insurance Studies.

In newer Flood Insurance Studies a 1-percent-plus annual chance profile may also be included to communicate uncertainty in estimated discharge due to factors such as saturated ground conditions. It can also reflect uncertainty for other parameters or future hydrologic conditions.

Example: The 1-percent annual chance flow is assumed to be under typical conditions, however the soil in the watershed may be saturated because of higher than average rainfall. The 1-percent-plus profile reflects the potential higher base flood elevation under those conditions. It can also provide more information for communities to predict future flooding conditions in areas under heavy development or when considering higher standards like freeboard.

The major items addressed in this section are:

• Sources of Data
• Methods of Analysis (Riverine)
• Summary of Discharges Table