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Floodplain Analysis

April 13, 2020

Constructing a project within a floodplain is sometimes unavoidable and can be successful with proper planning and foresight. DGR Engineering (DGR) helped Osceola County Rural Water System, Inc. (OCRWS) navigate many challenges to develop a wellfield in a floodplain. OCRWS was scrutinizing six potential shallow well sites in a floodplain based on the results of an exploratory study of the alluvium (sand and gravel). The natural presence of groundwater within said alluvium lends itself to the need for construction within the floodplain.

DGR employed a Geographic Information System (GIS) in conjunction with the U.S. Army Corps of Engineers’ HEC-RAS program to first determine the 100-year flood elevations and delineate the floodway boundaries. This step allowed engineers to verify if the proposed wells were within the floodplain, and then determine whether they were within the floodway or the flood fringe. The aerial map at top provides an illustration of the modeling results. Of the six sites, two were determined to be outside of the floodplain, two within the flood fringe, and two within the floodway. Moving forward with three of the four wells outside of the floodway, DGR engineers set the well heights and designed flood protective measures to extend a safe distance above both the modeled flood level and known historic flood levels.

It is imperative to a safe drinking water supply that the wells are not inundated by floodwaters and potential contamination. The Iowa Department of Natural Resources (local FPA) approved permits for the wells to be constructed. The story remains to be told on the two possible well sites in the floodway. In the end, DGR’s team was able to secure permits for three new well sites, providing a vital water source to OCRW and its customers.

FLOODWAY: The stream channel and that portion of the adjacent floodplain that must remain open to permit passage of the base flood with a designated surcharge, in the water surface elevation. Flood waters generally are deepest and swiftest in the floodway, and anything in this area is in the greatest danger during a flood. The floodway must remain free from obstruction so that the 100-year flood can move downstream.

FLOOD FRINGE: The remainder of the floodplain is called the flood fringe, where water may be shallower and slower. FEMA and state regulations allow the flood fringe to be obstructed if standards (i.e., elevating and flood proofing requirements) are met.