HEC-RAS FLOODPLAIN MODELING AND FLOOD MAP REVISION
Modern Landfill, Inc.
Lewiston, New York
Modern Landfill, Inc. owns and operates a Municipal Solid Waste landfill and retained DE to prepare a Solid Waste Management Facility Permit Modification application to reconfigure the landfill footprint. This reconfiguration required filling a portion of Twelvemile Creek’s floodplain, designated as Zone A on the Federal Emergency Management Agency’s (FEMA’s) Flood Insurance Rate Map (FIRM).
For the reconfiguration to be approved by the Town, DE first completed the hydrologic and hydraulic modeling to establish the floodway and the base flood elevations for the existing conditions. Then, the three-dimensional characteristics of the modified landfill final conditions were incorporated into the surface model to predict the change in base flood elevations associated with the 100-year flood.
The modeled reach drains a total of 4.0 square miles and consists of three watersheds with a split flow condition. Hydrologic analysis relied on a relationship between drainage area and peak discharge rates developed for a detailed Flood Insurance Study published by FEMA for a nearby portion of the Creek.
Backwater surface profiles were computed using the U.S. Army Corps of Engineers’ Hydrologic Engineering Center-River Analysis System (HEC-RAS) software. Model inputs included steady state flow boundary conditions, cross-sectional geometry of the channel and “overbank” floodplain areas, site-specific Manning’s roughness coefficients for both the channel and overbank areas representative of field conditions, junction geometry, and necessary measurements and calculations pertaining to the hydraulic properties of all bridges, culverts, and similar structures.
By removing some obsolete culverts to restore open flow conditions, rerouting a portion of the creek, adding a compensating flood storage area, and constructing a small containment berm in a flatter upstream area, DE successfully employed HEC-RAS modeling to show that the expected change in 100-year flood elevation attributed to the proposed development was within the 1.0 foot maximum allowed by FEMA and no adverse effect was expected for any adjoining property. In turn, this supported Modern’s request for a Conditional Letter of Map Revision (CLOMR) from FEMA and the NYSDEC to approve and permit the reconfigured landfill.