Bethel Sinkhole Won’t Be a Quick Fix

Correction 3:15 p.m. Jan. 3: Alan Biggs of WestSound Engineering called my attention to some inaccuracies in the story that ran in today’s Kitsap Sun. The Web version of the story will be corrected by the end of the business day. Note Biggs’ comments that the volume of water generated by the Dec. 2 & 3 rains was “on the order of a 500-yr recurrence interval.”

Here’s the correction we will run in the print edition, and I have amended the blog entry to reflect the correct information:
Wrong Reference: An article in Wednesday’s Kitsap Sun on the Bethel Avenue sinkhole indicated incorrectly that the diameter of a failing pipe is 24 inches. The pipe is trapezoidal and its diameter is not easily referenced. Also the amount of water cited was an estimate of what would flow through the pipe during a 100-year storm, not during the Dec. 3 storm. A local expert estimates the amount of flow during the storm was actually greater than 86 cubic feet per second.

Here’s what Biggs had to say about the volume of water generated by the Dec. 3 storm: “Readily available data for the larger recurrence interval storms are hard to come by, so we informally extrapolated a “best fit” curve from the information available in the Stormwater Manual and estimated that the storm was more on the order of a 500-yr recurrence interval-perhaps even exceeding the 24-hr, 1,000-yr storm, depending on how you fit the curve to the data.”

Here’s my amended entry:

It’s time now to take a trip below the streets of Port Orchard. Visualize, if you will, a concrete channel, buried 20 feet underground beneath the blacktop on Bethel Avenue.

The trapezoidal channel is more than 50 years old and covered with a concrete lid that is crumbling in places, barely able to handle the typical winter rain.

Then on Dec. 3, more than half the rainfall in the second soggiest December ever falls in a 24-hour period.

Visualize 86 cubic feet of water per second trying to make its way through that pipe – that’s a 2-by-1-foot column of water the length of 2 pickup trucks. That’s the amount Alan Biggs of WestSound Engineering of Port Orchard estimates would fall during a “100-year recurrence interval storm.” Biggs said the amount of water coursing downhill on its way to the sound at the height of the flood that day was actually greater. How much greater? Here’s what he had to say:

“Readily available data for the larger recurrence interval storms are hard to come by, so we informally extrapolated a “best fit” curve from the information available in the Stormwater Manual and estimated that the storm was more on the order of a 500-yr recurrence interval-perhaps even exceeding the 24-hr, 1,000-yr storm, depending on how you fit the curve to the data.”

No wonder Bethel Avenue now has a gaping 50-by-20-by-30-foot cavity.

The 86 cubic feet per inch is the amount a new pipe will need to be able to handle in the event of another “100-year recurrence interval storm,” Biggs said in a report to the city. The cost to fix it, at least $400,000, will be handled by the Federal Emergency Management Agency, said City Engineer Maher Abed.

Read more about the city’s plans to fix the Bethel sinkhole and to renovate its entire storm water system here.

In the meantime, maybe the city could make some money giving underground tours.

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