Erosion leaves Lake Manatee dam in 'severely distressed state'

jdeleon@bradenton.comFebruary 14, 2014 

MANATEE -- Manatee County government is taking "corrective actions" after engineering consultants found that the Lake Manatee dam is in a "severely distressed state" because of erosion, officials said Friday.

Heavy rains over a four- or five-day period "could compromise parts" of the Lake Manatee dam, Manatee County government said in a news release Friday afternoon.

Officials held a news conference late Friday afternoon. They said nearby residents have been advised of the problem.

"Lake Manatee Dam is in severely distressed state," wrote engineers with Carollo Engineers Inc. and AMEC Environment & Infrastructure Inc. in an assessment for the county. "Without immediate intervention there is a high risk of an uncontrolled release of reservoir, most likely following a large rainfall event and opening of spillway."

Specifically, engineers said they found:

-- Large voids known as "piping" with fast-moving water under the stilling basin.

-- Voids extending under the downstream training walls.

-- Voids extending under downstream concrete apron.

-- Large zones of internal erosion behind training walls.

According to the engineers, piping is "the process of the formation of a large continuous void resulting from soil erosion due to high seepage gradients that begins downstream and works its way upstream, threatening an uncontrolled release of the reservoir."

The engineers recommended a series of short-term and longer-term measures. In the short term -- that is, before the start of hurricane season, engineers recommended:

-- Considering lowering the reservoir.

-- Reestablishing seepage control function of dam core.

-- Working with specialty contractors to collect necessary information and develop cost estimates.

Longer term, the engineers recommended:

-- Filling voids under stilling basin and training walls.

-- Densifying backfill soils behind approach and training walls.

-- Reestablishing seepage control for spillway structure and embankment.

Repairing the dam -- which also would extend its life by another 50 years -- could cost $3 million to $6 million, officials said.

"After in-depth testing and a review of cumulative data from the 50-year-old dam, a private consultant has determined that some erosion has occurred and a four- or five-day rainfall could compromise parts of the dam. The consultant notified Manatee County Utilities officials of their assessment (Thursday) and recommended corrective actions that are now being initiated by the county," the county said in a news release Friday afternoon.

The 50-year-old dam is the primary source of raw water for the Manatee County Utility System. The lake's watershed covers 82,240 acres, and its total storage volume is 7.5 billion gallons.

Ground for the dam was broken in October 1963, and construction was completed in December 1967.

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