at Thursday, 01 November 2012 11:10by Cynthia Riordan
|Engineers: Local water district projects oversized|
|Written by Town Crier Staff Report|
|Wednesday, 31 October 2012|
The Santa Clara Valley Water District’s projects related to Stevens, Adobe and Permanente creeks could be oversized for the flood protection required, by a magnitude of two and a half times what is actually needed.
After an exhaustive review of the district’s findings and calculations, engineer Richard Moll of Los Altos determined that district officials reached their conclusions on needed flood-control projects based on inaccurate data and unrealistic scenarios.
District’s assessment of water flows off mark
Moll’s conclusions arrive as residents consider their votes Tuesday on Measure B, a water district initiative that would renew the current parcel tax through 2028 to pay, in part, for flood-control projects such as the Permanente Creek Flood Protection Project. Under Measure B, each quarter-acre parcel would be assessed up to $56 annually.
The $40 million Permanente project includes detention basins at Cuesta Park Annex, McKelvey Park and Rancho San Antonio County Park. The basins are designed with the capacity to accommodate a 100-year flood, a flood that has a 1 percent chance of occurring annually. The district estimates that the project would protect more than 3,000 residents in the event of flooding.
The Los Altos School District Board of Trustees rejected the basin proposed for Blach Intermediate School last year, forcing the water district to recalculate. Revised plans would reduce the size of the Cuesta Annex basin nearly by half – from 22 feet deep to 12 feet. Because of the revisions, the district must secure a Supplemental Environmental Impact Report (SEIR) for Mountain View City Council approval before work can begin.
Moll is one of at least three Los Altos engineers who have investigated and questioned the district’s findings. Civil engineer Jerry Clements and hydraulic engineer Michael Hayden have also pointed to flaws in the water district’s premise for the Permanente Creek Flood Protection Project.
Moll concluded that the basins, among other projects, are excessive and a waste of money. He noted, for example, that the largest Adobe Creek water flow over the past 57 years occurred in 1998.
“The district’s modeling predicts that this flow happens every three years – which is obviously not correct,” Moll said, adding that district officials relied on a consultant who “manipulated data that no longer agrees with the district’s modeling data.”
In a letter to water district officials, Moll wrote:
“My concerns about the credibility of the Permanente Creek project modeling are further reinforced by the district’s mismatched predictions on Stevens Creek, where the modeling seems to predict flows that are more than twice as large as the historical real-world events recorded at Gauge 35 on Stevens Creek (Gauge 35 is located between El Camino and US 101). This is important, because Permanente Creek flows via a diversion channel into Stevens Creek, and the majority of the flooding area along Stevens Creek is near and north (toward the Bay) of El Camino. To my knowledge, this gap between modeling and reality at Stevens Creek has never been addressed or resolved by the district’s staff.”
The water district’s response
Afshin Rouhani, a water district engineer in charge of the Permanente Creek Flood Protection Project, said district staff is preparing a response to the engineers’ criticism.
“As far as the Permanente project is concerned, Mr. Moll’s comments on the Draft Subsequent Environmental Impact Report will be addressed in the final report in November,” Rouhani said.
Hayden said the premise stated in the SEIR omits key facts: “The (district) mentions the 1959 construction of the Permanente Creek Diversion, but does not describe the extent of the improvements in flood protection it provides. … It also fails to mention that it re-established the historical watershed connection from Permanente to Stevens Creek, so currently they once again function as separate watersheds. … It fails to mention that the only historical flooding which is relevant to the proposed Permanente modifications is the 1983 flood at Blach School, which caused moderate damage.”
In response to the Blach flood, the water district in 1986 implemented channel improvements but installed a flow restrictor, which reduced channel capacity near Blach after the improvements were made. The district claims that the restrictor prevents further problems along Stevens Creek.
“Removing the choke point could render the annex basin redundant, but moving the flood problem from one place to another is simply unacceptable for the district, both legally and morally,” Rouhani told the Town Crier earlier this year. “Doing so would increase the flow rate in Stevens Creek, which cannot carry its 1 percent flow rate already in the Highway 101 to El Camino Real reaches, thus inducing increased flooding there.”
Hayden countered that district engineers admit that their hydrology model, which predicts a 2,400 cfs (cubic feet per second) peak flow in the event of a 1 percent flood (or a major flood happening once every 100 years), does not correlate with data the district have collected for decades at Permanente Creek near Berry Avenue.
“They have therefore concluded that the gauge data must be inaccurate and should be massaged upwards to match their model,” Hayden wrote. “If the actual stream gauge data collected by (the district) were believed to be correct, then there would be no reason for the Rancho or Cuesta basins.”
“I believe the district’s modeling group is so strong to want to be right about what they are doing (that) they are unable to say there’s a problem,” Moll said.
Pending remaining approvals, Rouhani estimated that flood-basin construction wouldn’t begin until late 2013 “or sometime in 2014.” He said the project must be completed by 2015 under the existing parcel tax.
at Thursday, 01 November 2012 11:10by Cynthia Riordan
It would be a crime if the beauty and wildlife of the Cuesta Annex and Rancho San Antonio were destroyed simply because the water district cannot admit their flood projection errors.
at Thursday, 01 November 2012 16:05by Micahel Hayden
Unfortunately, the article incorrectly calls me a hydraulic engineer. I am actually a software engineer by trade. Also, want to clarify the point being made w.r.t. the Diversion Channel reconnecting the Permanente and Stevens Creek watersheds. Those two now function mostly as a single watershed and Hale is currently mostly a separate watershed since only 100 cfs is allowed to flow past the Diversion Channel towards Hale Creek.
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