West Lake CAG NRRB Submission REVISED ADDENDUM January 4 2018

Below is a table from a document prepared by Maggie Weng, a student at Washington University,
under guidance of her professor showing the percentage of the landfill that will be flooded under
various scenarios (see full study including data sources and analysis techniques in Appendix F).
Recurrence Interval
(years)
Stage of flood, including
height above NAVD88 (m)
Area in map below
flood elevation (km2)
17.4 136.966 21.96 44.02
100 139.02 231.769 51.63
300 140.459 234.84 61.95
500 141.1267 236.17 63.58
% landfill below
flood elevation
The 2008 ROD says, “Flood control measures at the Site should meet or exceed design standards
for a 500-year storm event under the assumption that the existing levee system is breached.” The
EPA accepted the PRPs’ claim that a 500 year flooding event would be easy and gradual. The PRPs
do not consider various flooding scenarios, including a breach in the levee that leads to torrent and
violent water movement due to a levee failure. The PRPs only assume what would happen if a levee
is overtopped. The FEMA flood maps are not applicable to a scenario that involved a breach in the
levee that sends torrent waters toward the West Lake Landfill, therefore this risk scenario is
incomplete. Unlike the below picture that was just water impacting farmland, a breach and massive
torrent of water moving toward the West Lake Landfill would likely include large debris objects
considering the surrounding area has been commercially developed.
Eroded farmland following the breach of the Birds Point-New Madrid Floodway
Photo credit: Laura Simon from the Southeast Missourian
Note: there is also a concern that the installation of a non-permeable cap over a significant acreage
at the site may contribute to flooding of Cowmire Creek and therefore, will add to the already
frequently occurring flooding of local roads and businesses in this area. Specifically, Cowmire
Creek is already handling additional runoff being directed from the Superfund Site’s Outfall #4,
which due to the removal of Outfall #6, now drains approximately 72.5 acres into the creek. Any
Remedy that includes a non- or semi-permeable cap on this Superfund Site needs to include a
detailed analysis of the impact the additional drainage will have on Cowmire Creek and the Railway
Embankment and surrounding areas, which are not built to handle this type of drainage. See
Appendix E for a detailed discussion of the flooding history and future risk of flooding.
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