CITY OF TALLAHASSEE
CITY COMMISSION AGENDA ITEM
ACTION REQUESTED ON: November 10, 2009
SUBJECT/TITLE: Corn Hydro Spillway Modification Presentation
TARGET ISSUE:

STATEMENT OF ISSUE

The City leases and operates the Corn Hydro Electric Generating facility (Corn) on Lake Talquin. Corn is operated in accordance with a license issued by the Federal Energy Regulatory Commission (FERC) and a lease between the City and the State. Under the FERC license, the facility undergoes periodic independent safety inspections (Part 12 inspection) every five years. The last Part 12 inspection occurred in July 2003. During the course of this inspection, FERC raised questions concerning the design of the emergency spillway and fuse plug at the facility. These questions were the result of a failure of a similarly designed emergency spillway and fuse plug at Silver Lake Dam located in Michigan. The City completed the analysis requested by FERC and identified that the existing emergency spillway and fuse plug were subject to erosion similar to that that occurred in Silver Lake Michigan. As a result, FERC ordered the City to develop a dam safety remediation plan to address these concerns. This work has been completed, and FERC has approved the proposed modifications. This agenda item provides a briefing on the project and downstream resident’s concerns.

RECOMMENDED ACTION

Option 1: Accept status report and provide feedback to staff

FISCAL IMPACT

Option 1: None at this time

Kevin G. Wailes, General Manager – Electric Utility

Anita Favors Thompson, City Manager

For information, please contact: Rob McGarrah, Manager Power Production, x 5534

SUPPLEMENTAL MATERIAL/ISSUE ANALYSIS
HISTORY/FACTS & ISSUES

1. The City leases and operates the Corn Hydro Electric Generating facility (Corn) on Lake Talquin. Corn is operated in accordance with a license issued by the Federal Energy Regulatory Commission (FERC) and a lease with the State of Florida. Under the terms of the FERC Operating License, the facility is operated as an instantaneous run-of-river. This means the City is to release the same amount of water from the lake as is entering the lake. Under the terms of the lease, the lake is, and is to remain, a recreational lake. The lease also sets a normal lake level of 68.5 feet and allows the City to fluctuate the lake: (i) +/- 1 foot during normal operations; (ii) up to two feet below normal level is allowable to prevent the injury or loss of life or property during emergency situations and states any further emergency drawdown shall be specifically coordinated with the Sublessor in advance, conditions permitting.

2. The Corn hydro facility is comprised of:

a. Lake Talquin

b. A powerhouse with three generating units

c. A concrete spillway with seven flood gates

d. A 3,600 foot earthen dam (elevation 77 feet)

e. An emergency spillway (elevation 72.3 feet) and a fuse plug (74.3 feet)

3. The emergency spillway and fuse plug are designed to allow for release of water from Lake Talquin in those flood events in which: (i) there is insufficient capacity to release the water through the existing floodgates and generating units; and (ii) the lake level reaches a level of 72.3 feet or above. The emergency spillway was designed to increase the total spillway capacity of the project by allowing overtopping flow without erosion. The fuse plug portion of the emergency spillway was originally designed to allow for erosion to occur, during periods where there is water flow over the crest, such that the fuse plug spillway crest would drop to 68.3 feet. There is a concrete sill located at 68.3 feet that is supposed to stop the erosion when the fuse plug operates as designed.

4. Under the FERC license, the facility undergoes periodic independent safety inspections (Part 12). The last Part 12 inspection occurred in July 2003. During the course of this inspection, FERC raised questions concerning the design of the existing Corn hydro emergency spillway and fuse plug. These questions were the result of a failure of a similarly designed emergency spillway and fuse plug at Silver Lake Dam located in Michigan. In that failure, the fuse plug spillway eroded farther than designed which resulted in the uncontrolled release of water. As a result of that failure, the FERC requested the City perform geotechnical studies to determine if the soils located in our emergency and fuse plug spillways were similar in nature.

5. Based on the geotechnical study performed by the City, our engineers identified that the soils in the Corn facility emergency and fuse plug spillways were such that, if water were allowed to flow over these structures, there would be a high probability that both the emergency spillway and the fuse plug spillway would NOT operate as designed, resulting in significant erosion and an uncontrolled release of water from Lake Talquin. Based on the City’s engineering modeling for this project, should this failure occur, the downstream river would see a significant rise (>2 feet) in a very short period of time (<30 minutes).

6. As a result of the engineering analysis, the City has taken two actions, one short term and one long term, as a result of this safety concern.

a. The short-term action has been to reinforce the facility emergency operations plans to maintain the lake level well below the elevation of the existing emergency spillway and fuse plug. This eliminates the potential for the failure of these earthen structures.

b. The long-term action was to develop, with review and oversight by FERC, modifications to the facility to address this issue from a long-term perspective. The proposed modifications include: installation of a new concrete ogee emergency spillway, with a crest height of 69.5 feet (one foot over the normal lake level and slightly over one foot above the current fuse plug concrete sill crest height), adjacent to the existing floodgate spillway and closure of the existing earthen emergency spillway and fuse plug. This proposal was accepted by FERC in May 2009.

7. The proposed modifications were designed to meet the following design criteria:

a. Meet the flow capacity required

b. Comply with the applicable provisions of the FERC Engineering Guidelines

c. Minimize the impacts to downstream concerns

d. Minimize impacts to the environment

e. Able to be constructed during non-hurricane season for safety reasons

f. Cost

8. One of the design criteria is to be able to construct the facility during the non-hurricane season. This is due to the need to uncover the existing earthen dam and the risk of erosion during a tropical rain event. This means the construction must occur between November and June.

9. As a result of an open house to inform “downstream” residents of the project, some citizens have raised the question about the potential for additional flooding downstream due to the elevation of the new ogee spillway crest. Based on the facility operations, we would not expect the lake to reach a level of 69.5 feet and water flowing over the new ogee spillway until: (i) the existing floodgates would be fully open; and (ii) the downstream river would be above 50 feet (msl), which is almost four feet above the published floodstage (46.69 feet MSL). Based on our engineer’s modeling, this would not occur except for flood events that are at the 100-year intensity or greater. At the point at which water begins to flow over the new ogee spillway, there is the potential for a slight increase (0.22 feet or 2.69 inches) in downstream river levels above that of the existing design (presuming the existing spillway and fuse plug operate as originally designed), but this rise would occur slowly. This eliminates the potential for the significant short-term rise that would occur with the failure of the existing emergency spillway and fuse plug.

10. Since the facility has been under the control of the City (middle 1980’s), the City has experienced three significant flood events. These events occurred in October 1994, Fall of 2008 (TS Fay), and Spring of 2009. The table below depicts the: maximum outflows of water in cubic feet per second (cfs), the maximum downstream river level (msl) and the maximum lake level (feet)*.

*Unless noted otherwise, the data in the table is based on USGS real time data downloaded at the time of the event.
**The downstream river level can be expressed in one of two ways. The table expresses the river elevation level in feet above mean sea level (msl). The river level can also be expressed in gage height, which represents the depth of the water in the river above the gage elevation. The difference between the two values is the elevation of the river gage. For the downstream river gage at the Corn facility, the gage elevation is 24.69 feet. Flood stage at the downstream river gage (Bloxham) is 46.69 feet (msl) or 22 feet gage height.
***The October 1994 data is based on USGS historical data. In the case of the lake level, it is the mean daily lake level
****Based on USGS recorded gage height and current stream flow charts

11. In these three events, the highest the lake level reached in these three events was 69.52 feet during the 1994 event. This means that only during the 1994 event, would there have been any water flowing over the new ogee spillway, and in that event, it would have just barely flowed over the spillway.

12. At the current time, the project has been approved by FERC, and the construction is on hold pending the issuance of several state permits and one federal permit. The original construction schedule called for construction to commence on or about October 1, 2009 and be completed by June 1, 2010. At this time, staff has directed the construction contractor, PCL Civil Constructors (PCL), to demobilize. PCL is moving forward with the pre-construction interface issues with the City’s engineer as well as the procurement of the sheet pile material that is required for the job. The City will take delivery and pay for the sheet pile material once it is ready for shipment. It will be stored at the manufacturer’s facility until construction is ready to commence.

13. Staff identified that this project was being pursued at the following public forums:

a. April 22, 2009: Tri County flooding meeting in Wakulla County hosted by Franklin County. There were representatives in attendance from Franklin, Wakulla, and Liberty Counties as well as “downstream” (Crooked Road) residents. Leon County was invited to attend the meeting but had no representatives present.

b. May 18, 2009: Friends Of Lake Talquin meeting held at the Ft. Braden Community Center.

c. September 22, 2009: City of Tallahassee Corn Spillway Open House held at the Ft. Braden Community Center. At this meeting, the City had its project team available to address any questions raised by those attending. There were approximately 40 individuals who attended, including a number of Crooked Road Residents.

d. September 24, 2009: Apalachee Regional Planning Council. At this meeting, the City presented information concerning the March 2009 flooding event and the spillway modification. Commissioner Jane Sauls was in attendance at that meeting.

CHARITABLE CONTRIBUTIONS

Not applicable

OPTIONS

Option 1: Accept status report and provide feedback to staff

FISCAL IMPACT

Option 1: None at this time.

ATTACHMENTS/REFERENCES

Corn Hydro Spillway Project Presentation