To Mike Reese:

You are absolutely right. There are too many factors involved in hydrology and floodplain hydraulics (and in meteorology) to consider using any single factor to apply to several different watersheds for consistently determining peak discharges (and especially flood elevations) with any degree of accuracy. The most simplistic methods in hydrology include the Rational Method and Regression equations to determine peak discharges, but even then, the accuracy is very limited, eventhough these approximating methods may include more than one factor. Also, there are definite limits to the accuracy of such methods due to circumstances within a watershed that may not be taken into account in the method used (for example, a flood detention basin, channelization, losses in floodplain valley storage, or a gate-operated reservoir). Also, wet moisture soil conditions versus dry moisture conditions, and type of soils, and degree and type of vegetative cover (and seasonal changes) can produce a wide range of peak discharges and flood elevations. To derive a single factor (such as a drainage area ratio, for example) all watersheds considered would need to be extremely similar in all aspects of hydrologic and hydraulic parameters in order to hope for any appreciable degree of accuracy. Even the use (and continued misuse) of the Rational Method and Regression Equations have led to poor design, property damages, loss of life, and lawsuits. In my opinion, misconceived notions like these (i.e., that hydrologic and hydraulic processes can be "boiled down" one factor) is not realistic and not prudent engineering. It will only lead to problems.

There are several texts available on the subject of hydrologic processes, including several "Handbook of Hydrology", by such authors as Ven Te Chow and by David Maidment, to name a few, plus numerous other texts on water resources engineering and hydraulics.

Dwayne Stubblefield, P.E.
dwayne@nwrservices.com