Philomath Wastewater Treatment Plant Expansion CLOMR/LOMR (OR)
The City of Philomath, OR was looking to expand their existing wastewater treatment lagoons which are located within the Mary’s River Floodplain in rural Benton County, OR. As part of the lagoon expansion design, a hydraulic analysis was conducted to understand the potential impact on the regulatory flood elevations from various lagoon layout alternatives. Each alternative was modeled in HEC-RAS to help select the appropriate design that had the least impact on flooding.

The hydraulic model results indicated that although the proposed project would create a small rise in the 100-year flood elevations compared to existing conditions, the revised elevations would still be lower than the effective FEMA floodplain maps indicated. This was the result of inaccuracies found in the effective HEC-2 hydraulic model as well as the availability of newer and more accurate topography used for the updated modeling.

The selected alternative was presented to the Benton County Planning Commission at a public hearing. Benton County requested that the project be submitted to FEMA as a Conditional Letter of Map Revision (CLOMR) to help insure the project was in compliance with the FEMA program.

As part of the CLOMR submission, new floodplain maps and flood profiles were developed. The CLOMR was approved and construction of the project is nearly complete. A Letter of Map Revision (LOMR) will be developed and submitted to FEMA upon completion of the lagoon expansion project.

A challenging element of the modeling was that Bellfountian Road, which is elevated above the surrounding ground and is located near the downstream boundary of the model, overflows during the 100-year flood. It also contains two bridges (one over the main stem and one over an overflow path) and several culverts. This required the use of the multiple opening option available in HEC-RAS.