Atkins addresses Long Hollow Stormwater Pond capacity

The Pensacola City Council was sent this memo around noon today by City Administrator Eric Olson

To: Eric Olson, City Administrator
From: Jeff Helms, P.E. Vice President
cc: Bill Johnson, PE, Atkins Sr. Drainage Engineer
Date: November 10, 2015

Re: Long Hollow Pond & Radio Tower

This memo is intended to serve as a follow up to our completed Long Hollow drainage study dated January 2015 conducted for the City of Pensacola and to clear up possible confusion that may have been created by the recent Pensacola News Journal (PNJ) article “Tower of Questions” published on November 8, 2015. The very first sentence may be confusing in regard to the actual limits of the Long Hollow pond. The article begins with the following statement:

“From a plot of city-owned land overlooking a critical stormwater pond the size of a small lake, a soaring radio tower broadcasts Divine Radio AM 1230 across Pensacola 24 hours a day”.

This statement could lead the reader to believe that the Long Hollow stormwater pond is constructed and located south of the radio station.

Actually, the radio tower was constructed in and has always been located in the original Long Hollow pond footprint. According to the Baskerville-Donovan Long Hollow Drainage Study dated 2004, the original pond was constructed in the 1940’s to relieve generalized area flooding. In the 1970’s, it was used as a borrow pit during the construction of the I-110 corridor. The 2005 FDOT construction plans (see Attachment A) which renovated portions of the existing Long Hollow pond to add stormwater treatment, clearly shows the limits of the existing pond contours including the radio station tower within the pond footprint.

The plans also show major stormwater inflows entering the pond from Jordan Street which is located north of the existing radio tower. These inflows include a 60 inch pipe and a 5-foot by 12 foot box culvert. In addition, there is a an approximate 50 feet wide by 360 feet long meandering ditch that stores and routes stormwater from the Jordan Street inflows to deeper areas within the Long Hollow pond.

The only difference between the radio station area of the Long Hollow pond and other areas of the pond is the bottom elevation. The majority of the Long Hollow pond has a pond bottom elevation of approximately 28 feet whereas the radio tower area pond bottom elevation varies between 34 feet and 35 feet. The seasonal high water elevation is at approximate elevation 31 which means that the Long Hollow pond in the area of the radio station tower will normally be dry while the remaining areas of Long Hollow pond will be wet year round. The berm elevation for the pond (elevation that the pond would overtop) is at approximately 42 feet. During significant storm events the Long Hollow pond water elevation will stage higher and as a result the north part or radio tower area of the Long Hollow pond will be covered in water to varying depths depending on the intensity and duration of the storm event.

The radio station tower area of the Long Hollow pond is considered a dual-use pond, meaning that it functions as a stormwater management area but also functions and provides other uses, in this case a radio tower. Dual-use ponds are common in many areas of country usually in conjunction with recreational fields (i.e. soccer, football, playgrounds, etc) The third paragraph of the PNJ article may also be confusing to readers. It states the following:

“Some residents have suggested that the land upon which the tower sits should be used to expand the stormwater pond in hopes that the low-lying Long Hollow neighborhood surrounding it won’t again be devastated by flooding like it was in April 2014”. As discussed above, the existing radio tower area is already being used to attenuate stormwater from larger rain events where the pond water elevation stages above the 34 foot elevation.

In late 2014, Atkins conducted a drainage study of the Long Hollow drainage basin and simulated the April 2014 storm event using surveyed high water marks. Our analysis indicated that expanding and creating ponds to attenuate the 25 year storm event would require adding 246 acre feet of volume or approximately 36 acres of dry stormwater ponds. The April 2014 storm event was much greater than even a 100 year storm event.

As previously discussed, the prevailing limiting factor of the pond storage capacity is the high groundwater table present in the area. This greatly reduces the actual flood attenuation volume within the pond. Removing the radio tower and excavating the area of the pond down to the seasonal groundwater elevation of 31 would be insignificant in attenuating another April 2014 storm event and would only provide approximately 2.6 % (6.5 acre-feet) of the total required volume needed to attenuate a 25 year storm event.