Two factors determine riverine flooding; the quantity of water that must pass a given point and the configuration of the channel and adjacent overbank area that must carry that water. TWO engineering procedures, hydrology and hydraulics, assist in determining the factors. Hydrology, a science dealing with the distribution and circulation of water in the atmosphere, on land surfaces, and underground is used to determine flood flow frequencies. Hydraulics, a science that deals with fluids in motion, is used to determine how a quantity of water will flow through a channel or floodplain.

The components of a hydrologic analysis are the flood discharge and the frequency. Discharge is generally measured in cubic feet per second by gauging mechanisms on streams which are established by the U.S. Geological Survey. Flood flow frequency is estimated by statistical analysis of gauge records where available. Where gauged records do not exist or where the length of record is too short to be useful, discharges may be estimated by statistical comparisons with other watersheds where records do exist or by rainfall-runoff models of various types. Examples of rainfall-runoff models are the Corps of Engineers' HEC-1 and the Soil Conservation Service's TR-20 and TR55. Flood discharges are associated with average recurrence intervals measured in years. The intervals used in the Flood Insurance Study are 10, 50, 100 and 500 years. The "100 year flood" is the expected discharge to be equalled or exceeded once on the average every 100 years, or with a one percent chance of occurring in a given year.

For the hydraulic analysis, the geometric configuration of the stream channel and adjacent overbank areas are obtained by ground or aerial survey. This information provides a "cross section", or profile, of the floodplain. The measurements of culverts, bridges, flood control structures and other encroachments in the floodplain are included with this cross section information. Hydraulic analysis by backwater modeling is used to determine how much of the floodplain is required to pass the given flood discharge. The Corps of Engineers' HEC-2 & HEC RAS, the Soil Conservation & rvice's WSP-2 , the U.S. Geological Survey's E-431 and the USGS/FHWA HY8 "WSPRO" models are examples of water surface profile programs used in a Flood Insurance Study.

Under certain circumstances, an engineering analysis is required before an encroachment is allowed in a SFHA. Examples are: an unnumbered A gone, where no flood elevation data are available; areas where base flood data are available but no floodway has been computed; and subdivision proposals of greater than 50 lots or 5 acres, where no base flood elevations are available. In most cases where no Flood Insurance Study is available, neither hydrologic no hydraulic data are available and the developer's engineer must develop the data using acceptable engineering methodologies such as those noted above. Generally, it is required that the hydraulic analysis be made both with and without the encroachment included, to show the effect of the encroachment.

If a Flood Insurance Study has been performed for the community and base flood elevations are shown on the FIRM covering the encroachment site, data from the hydraulic analysis are available from FEMA. The developer's engineer should obtain these data and use it to show the effect of the encroachment on the base flood elevations.