I suspect that some Alexandrians are at least aware of, and may even have personal experience with “marine clay,” a type of Coastal Plain deposit commonly found in the city. This inherently unstable material shrinks and swells in response to seasonal variations in precipitation and more dramatic events like hurricanes and other major downpours. This process destabilizes hillsides and leads to landslides and other ground failures that can damage or destroy buildings, property and infrastructure.
Examples are found throughout the city, with many concentrated along the tall escarpment that separates Duke Street from the uplands to the north.
Residents of the Seminary Ridge area are rightfully concerned by a proposal to grade, fill and squeeze four large homes into one of the last remaining natural ravines on this escarpment – in other words, disrupting a major hydrologic artery on a steep marine clay slope directly above an established neighborhood that is already experiencing storm water and slope stability issues.
Geological engineers have long known that even a minor disturbance of the natural hydrology at the top of marine clay slopes often leads to landslides, yet the city is poised to approve the “Karig Estates” development, blissfully ignoring the serious risks involved, not only to unsuspecting buyers but especially to residents living downslope. Neither the
development plan nor city staff have acknowledged the potential for off site impacts, leaving one to conclude that they naively assume geological processes such as storm water runoff and landslides respect property boundaries.
Ignoring the real risks to adjacent landowners is particularly disturbing given both the history of past landslides in the city and the volume of published information on the topic, including the delineation of landslide hazard areas in the online geologic atlas of Alexandria, a resource paid for by the city and readily available on its own website.
This, coupled with the destruction of one of the last remaining wild springs in the city along with some three acres of old age tree canopy (which has stabilized this slope for centuries), lead to the inescapable conclusion that the city’s process for managing development is dysfunctional and works at cross purposes to its own well-publicized “Eco City” and “40 percent tree canopy” aspirations, not to mention protecting existing residents and taxpayers against known hazards.
Steering development away from areas of mature tree canopy on geologically unstable hillsides above residential neighborhoods seems like a no brainer to this earth scientist.
-Tony Fleming, geologist and author of the “Online Geologic Atlas of the City of Alexandria”