FEMA Levee Certification – A Case Study with Emphasis on Vegetation Evaluation

AMEC Earth & Environmental, Inc. (AMEC) and Schnabel Engineering (Schnabel) are the Prime A/E and lead geotechnical subconsultant, respectively, on a Team working on an IDC contract with the Pennsylvania Department of Environmental Protection (PADEP) to assist non-federal sponsors of levee systems. The Teams’ role is to provide PADEP the engineering expertise required to evaluate existing flood protection systems (FPS) in the Commonwealth and subsequently obtain levee certifications through FEMA’s 44 CFR 65.10 and/or provide Letter of Map Revision (LOMR) for formerly protected areas.
The Team’s approach consisted of following FEMA and USACE technical criteria for evaluating existing levees using a phased approach. Phase 1 evaluation typically consists of a desktop file review, levee inspection, and preliminary freeboard evaluation to preliminarily assess the system’s compliance with 44 CFR 65.10. Phase 2 consists of a more detailed engineering evaluation, should the Phase 1 evaluation indicate that certification is likely.
One of the larger, more complex FPS evaluated was for the Borough of Danville, Pennsylvania, in central Pennsylvania along the Susquehanna River. The Danville FPS consists of about 2.5 miles of earthen levees, concrete floodwalls, and several types of closure structures. Vegetation problems are endemic to a majority of levee systems, and Danville was no exception. During the Phase 1 evaluation, tree encroachments were identified. However, just as troubling were systems of large diameter woody roots (4 to 6 inches in diameter) that were observed extending from trees that were outside of the “vegetative free zone”.
This paper describes a field study undertaken in collaboration with a consulting forester, to evaluate the characteristics of the root system and how it could impact levee under- and through-seepage. The trees were mostly silver maples, a species common along rivers and streams throughout Pennsylvania. A sensitivity analysis was performed using the SEEP/W numerical modeling package to evaluate the potential impact of the root mass on seepage. By understanding the potential impact of the root system, a rational, systematic approach to vegetation removal was developed specific to the Danville levee system and its dominant species of woody vegetation. 14 pp. 9 references.