Two-Dimensional Modeling of Floodplains—Necessary or Excessive? Two Case Studies

WEST Consultants, Inc. was tasked to provide analysis regarding floodplain impacts and bank protection for two developments in the floodplain near Phoenix, Arizona. The developments were in areas that one-dimensional models show as only partially in the floodplain. The one-dimensional models also show either very limited or no impacts resulting from the new developments. One development, however, was in an area of very complex flows while the other development was in an area that appeared to be one dimensional. Two-dimensional models of both areas, however, showed unexpected flow patterns and impacts due to the developments. In one of the areas flood elevations along the project boundary as predicted by the two-dimensional model were as much as 2 ft above the non-encroached flood elevations obtained from the one-dimensional model. This difference in water surface elevations would have serious implications for flood protection levels if not noted and accounted for during development. One of the projects was designed such that adverse impacts from development were eliminated while the other project used a combination of drainage easements and channels to mitigate adverse impacts caused by development. This paper presents summaries of both projects and show the necessity for two-dimensional modeling in cases where complex flows or geometry exist as well as in some areas where overbank flows appear to be one-dimensional. Both projects were designed to minimize adverse impacts on neighboring property owners.