22 September 2007

According to the NRCS report, when the NRCS inspected the Lake Oglethorpe dam in 2005, the Lake Oglethorpe Association requested that the NRCS model the results of a dam failure. The NRCS may being kind to themselves and the Association or just covering their butt. It is likely that in the then 35 years since the beginning of its construction the dam had never been inspected and an Inundation Zone (IZ) never calculated for the failure of the dam. The IZ is supposed to be a public document that at least alerts, if not regulates, development downstream. The apparent first-time inspection has been said to have been requested by the Lake Oglethorpe Association (LOA) Board of Directors, or an Association member, but as a datum regarding a controversial new use of the secondary spillway, not to fulfill what may have been a neglected legal requirement. It is unclear if the LOA knew about a Dam Failure Analysis and its calculated IZ until during the inspection.

In their report back, the NRCS provided a five page summary of the dam and its normal and flood pools, the assumptions about a failure, a summary of methods, and brief conclusions. Attached to the report were two Figures showing the Inundation Zone. Although a major event, they found no residences in danger of inundation, based on their Figure 2 which I found to be unsupported by their methods. My placement of their IZ on a map that we all can use, a Parcel Map, indicates that several are within the zone or the zone plus their stated uncertainty. I do not know if the assumptions on which their calculations are based are reasonable or how to estimate the probability of a major failure of the dam.

Read the report and my comments about it and you may not need to look at Figures 1 and 2. If in a rush, my comments include what I think are the most important assumptions, methods, and conclusions of the report.

The main challange was to accurately place the Topo-defined Inundation Zone on a Parcel Map so one could easily understand where the water would reach. The problem requires an accurate placement of streams on the Parcel Map. The first refinement of those positions, used here and elsewhere, used an aerial photograph for the registration. Although probably sufficient for placing the IZ on the Parcel Map, a second refinement to the Parcel Map will use survey maps for those parcels thought to contain streams and lakes. There is more work to be done on the IZ itself; the IZ was calculated for the watershed almost to the Oconee River. I have so far only worked with that portion above Wolfskin Road.

I will be glad to add your comments or any other materials to this archive. I suggest, in particular, that the following be provided so I can mount them here: knowledge of any requirements for inspections of the dam; knowledge of a Dam Failure Analysis and its IZ prior to the inspection; and knowledge if the dam was or now is in compliance with either possible requirement or if no requirements are applicable.


26 September 2007

In what was intended to be an unrelated study of the characteristics of the different pools that the lake can maintain, Lake Oglethorpe and Flood Control, the volume of the Flood Pool is calculated to be 1.4-fold larger than the volume used by the NRCS to calculate the Inundation Zone if the dam should fail when it is holding a Flood Pool. I do not know how to scale their result if the larger volume is assumed for the Flood Pool, not knowing if the Inundation Zone is limited more by the appature in the dam or by the volume of the pool behind it. Something else to discover.

Glenn Galau