26, 27 September 2007

A few comments have been made that Lake Oglethorpe may be intended for flood control, even though it might not be managed for that purpose and the need for flood control has not been demonstrated. From my understanding of the concept, the structures of overflow devices, and the normal pool level at which the lake is kept, I had thought there just is no ability of the lake to hold incoming flood water.

Testing this notion was an opportunity to get a better sense of the relationship between water level and the surface area and volume of its pools. This seemed necessary to better understand the consequences of withdrawing water as well as how the lake might be managed to retain flood water if that need were shown. The name of this Subject Page has been changed from Flood Control to Pool Areas to reflect the relative importance of its conclusions.

The short summary: I think the analysis is accurate enough. A pool level overflowing the secondary spillway probably floods some lake-side homes and a pool level overflowing the dam proper probably floods most of them. There are two ways to manage the lake level for flood control in the watershed. Both require active management of a valve, and one requires a retrofit, but neither requires new exit devices. Both are limited to crudely regulating five-foot increments in pool height. Each management plan greatly affects the shoreline: The first option keeps the lake too low in nonflood conditions and the second option too high in flood conditions. In the absence of a perceived need, each would correctly be rejected by the community.

As regards the analysis, I have reviewed the Topo Maps of the stream bed before the dam was constructed and had modest success in adding the contours to the Parcel Map. The County Parcel Maps report the size of Lake Oglethorpe to be slightly smaller than does NRCS, so I adjusted the boundaries of its Normal Pool on the Parcel Map to match a Topo Elevation of 630 feet, which resulted in reasonable agreement with its reported area (63 Acres). Then, using the relative heights of the outflow and exit devices, the Topo contours were used to make predictions of the perimeter, area, and volume of the pool at four heights above the bottom of the lake (at 600 feet) at the southern end of the lake, if the pool was maintained by: the 18-inch pipe (at 625 feet) or the riser (at 630 feet) in the primary spillway; the secondary spillway (at 635 feet); or the dam (at 640 feet). These were compared with the characteristics of the Normal Pool and Flood Pool that are reported in the Dam Failure Analysis. The process was somewhat iterative (somewhat circular) in that the volume of the Normal Pool was that that was reported by NRCS and it was used to help scale the relative volumes of the other three pools. The second document above, the Pool Volumes and Perimeters, contains Figures showing a summary of the results.

The level of water is now passively kept at the level of the nonadustable opening at 630 feet, called the riser, in the primary spillway (called the standpipe). Lowering water below the level of the riser requires opening the valve on an 18-inch pipe that penetrates the standpipe about five feet below the riser. Excess water automatically flows out of the lake through the riser opening. If the riser is functioning and has a large capacity, then flood water cannot accumulate in the lake to any level above this opening. If the riser in the primary spillway is blocked, and the secondary spillway at 635 feet (the ramped bank on which the community dock is sited) is blocked, then flood water would rise to its Flood Pool level, the level of the top of the dam proper at 640 feet, before spilling over the top of the dam. In their calculation of the Inundation Zone of a failure of the Lake Oglethorpe dam, it was assumed by the NRCS to holding holding a Flood Pool when the dam fails.

My analysis confirms that a Flood Pool at 640 feet in Lake Oglethorpe (red perimeter) would flood many of the lake-side residents if their homes are close to the lake and are not elevated above the level of the land when it was surveyed for the Topo maps before the lake was constructed. I guessed that would be the case. But I am surprised that some houses might be also be in danger of flood at a pool level that is spilling over the secondary spillway at 635 feet, what I call in the bottom of the Figure the Secondary Spillway Level (purple perimeter). I do not know where houses are sited in the parcels, and I may have overestimated the actual heights of the secondary spillway and the dam or underestimated the extent of elevating of houses.

Is this all just wrong data or calculations, correct conclusions but already well known to the residents, or possibly unwelcome news? Was there any mention by NRCS of hazard to the residents of the lake during the dam inspection in November 2005 and the discussion at that time of doing a Dam Failure Analysis? If the sorts of systematic errors in the Topo data and my use of it are small and my conclusions generally correct, then the Association has a powerful incentive not to allow the Pool level much above that of the riser in the primary spillway at 630 feet and certainly to be sure that both the riser and the secondary spillways remain unblocked. And my proposed second method of how the lake could be used for flood control (see below) should be rejected outright as hazardous to the those living on the lake.

Surprisingly, my calculated area for the Flood Pool is 97 Acres compared with the NRCS claim of 90 Acres and my calculated volume of the Flood Pool is about 1.4-fold larger than that used by the NRCS in its Dam Failure Analysis. I do not know the reason for these differences. I think they are outside the limits of the uncertainties in the overlay of the Topo contours on the Parcel Map and if so it suggests the Inundation Zone is larger than calculated and drawn by the NRCS and redrawn by myself. But as I have mentioned elswhere, I do not know how to predict how much larger.

So if the areas and volumes of the four pools are reasonably correct, there are two ways in which Association could store flood water for a later slow release. Using the valved 18-inch opening in the primary spillway at 625 feet, five feet below the riser, the Association could manually regulate the pool level between the 18-inch opening and the riser. The 18-inch escape at the minus five-foot level would always be left open (gold perimeter) but would immediately be closed if flood waters enter the lake. This would maintain the level of a Flood-Control Pool five feet below its present Normal Pool level at 630 feet (blue perimeter) in order for the lake to hold up to 330 Acre-Feet of flood water, about a third of it Normal Pool volume. Keeping the lake at five feet below its Normal Pool level would be unacceptable to the Association. For instance, it recently rejected a request to temporarily lower the Normal Pool level by one foot.

I think a major retrofit of the riser in the primary spillway might provide an alternative five-foot window of flood control, but this one is above the Normal Pool level. The riser would have to be reconstructed so that it can be regulated rather than passively remain open. The riser would normally be closed and flood waters would rise to the level of the secondary spillway which they would overtop. The riser would later be opened to release the stored water. This method would provide temporary storage of about 410 Acre-Feet of flood water, equivalent to about two-thirds of the Normal Pool volume, but as noted above would be a temporary hazard to many of the homes on the lake.

So neither method of flood control would be accepted by the Association. The first would have a normal pool too low in the lake and the second a flood-control pool too high and hazardous. Furthermore, neither method easily allows adjustments to regulate downstream flow in nonflood conditions, about which there is still some discussion and an expectation of an eventual agreement.

If the lake is believed to provide flood control today, or if my other conclusions are wrong, that should be easily documented and I will mount that documentation here. It should include a blueprint or equivalent that shows or lists the relative heights and capacities of the three exits from the primary spillway and similar data for the secondary spillway and the dam proper. In particular, a management plan should be keyed to rainfall. This information would be useful to have here in any case, especially if I have misunderstood the heights of the overflow devices of the dam.

Glenn Galau