For a complete list of flood plain terms, see TSARP's glossary

How are floodway determinations made?

Determining the boundaries of a regulatory floodway is difficult because, although the floodway definition is simple, the floodway may be established, manipulated and adjusted in many ways. A mixture of engineering and regulatory convenience combine for establishing the boundaries of a regulatory floodway. FEMA defines the floodway as that portion of the available flow cross section that cannot be obstructed without causing an increase of one foot in water-surface elevations resulting from a 100 yr (1%) flood. This definition, while sounding precise, actually gives a floodplain manager great freedom and broad powers in establishing the locations of floodway boundaries.

The following is how floodplain managers/modelers and their computer models work. One must be aware that floodway boundaries are not something that can be measured and checked in the field. The boundaries are a manmade computation, subject to uncertainties, human error and political whim.
  1. Floodplain topographic mapping, hydrology and hydraulics are done to determine the 100yr flood level. It is generally accepted this work is only accurate to within a range of 6-18" in making a 100 yr flood determination.
  2. Then the model "pushes" the 100 yr flood waters in from the sides until the flood level goes up one foot. Depending on topography and flow obstructions, more "pushing" is done on one side than the other.
  3. Where the "push" lines wind up is where the floodway boundaries are drawn.
  4. There are a handful of different methods/models for making the floodway determinations.
  5. The modeler will normally run more than one model for comparison purposes and make manual adjustments and interpolations on an as needed basis based on "best engineering judgment."
As much as the floodplain managers/engineers would like us average citizens to believe floodway determinations are rock solid, dead on, trust us, bullet proof rocket science we common folk must believe in, but can't comprehend, it ain't that way. A floodway determination in its pure form is an arbitrary academic and mathematical exercise that is unverifiable and fraught with scientific uncertainty. Allowing modelers, with varying degrees of expertise and experience, to manipulate the model further as they see fit and/or are instructed to, adds yet another unmeasurable factor into the equation. At best, a floodway determination should be one factor or guideline among many in making policy, but to use it as the sole basis for an arbitrary "no build" regulation is a farce.

An analogy might help here. A patient goes to the doctor for a physical. The physician doesn't like how the patient's heart sounds and orders an EKG. The EKG is abnormal. Does the doctor take the floodway Ch 19 approach and say, "You're heart is bad. You're done. Live as well as long as you can and good riddance.?" No, of course not. A doctor will order more tests before making a final diagnosis to eliminate as much uncertainty as is humanly possible and to aid in developing a treatment strategy. With Ch 19, no additional tests are permitted and there is no treatment.