Study identifies flood trouble spots in Ross ValleyRichard HalsteadMarin Independent Journal Article Launched:02/08/2007 12:51:57 AM PST Two sites in Ross and another in San Anselmo create the worst bottlenecks in Corte Madera Creek, topping a list of seven prime culprits that cause flooding in the Ross Valley, engineers say. The blockages, where flooding would occur first during heavy rainfall, were identified in a new hydraulic computer model developed by engineering consultants hired by the county. Marin County Supervisor Hal Brown said the information will be used to decide how to spend taxpayer money if voters approve a new storm drainage user fee on a proposed April ballot. Most homeowners would be asked to pay $125 or less per year, but the maximum charged homeowners will be $180 a year - for 20 years. Fees will vary according to the size of parcels and the amount of impervious surfaces on the property. Brown is leading an ambitious new effort to reduce the risk of flooding in Marin's Ross Valley. A storm that hit Marin early in the morning of Dec. 31, 2005, damaged about 1,200 homes and businesses. Most were located along Corte Madera Creek. Brown said the plan is to ask voters to approve the user fee in April. "We don't want to put this off again," he said. Two of the three worst blockages are in Ross - where a U.S. Army Corps of Engineers flood control project has been stalled since 1971. The Ross choke points are at the Lagunitas Road Bridge and a wooden fish ladder about 600 feet downstream from the bridge. Equally troublesome is the Madrone Avenue Bridge in San Anselmo, said Jack Curley, who is overseeing the county's participation in the flood control effort. Using a computer model of the creek, engineers found that water would spill over the creek's banks at those three sites first, Curley said. When the model simulated additional rainfall, flooding occurred on the creek where it is spanned by a commercial building at 636 San Anselmo Ave. in downtown San Anselmo, and along a 550-foot-long culvert that runs under downtown Fairfax. Two other San Anselmo bridges, at Nokomis and Sycamore avenues, would flood next if even heavier rainfall occurred, the model indicated. The results came as no great surprise. "These are all the prime suspects that everybody in San Anselmo will tell you about," Curley said. Nevertheless, Curley said he didn't expect the fish ladder to rank at the top. "I knew it was a constriction. I just didn't know it was such a limiting constriction," he said. The Army Corps of Engineers is waiting for Friends of Corte Madera Creek Watershed, an environmental group, to submit recommendations for replacing the fish ladder. The fish ladder is included in the Army Corps' plan to widen an 800-foot stretch of the creek near the Ross Post Office and to erect flood walls along the creek. The plan has been in the works for decades. Last week, the effort suffered its latest setback when the House of Representatives voted to strip "earmarks" - legislators' pet projects - from the fiscal 2007 federal budget. Marin County had hoped to get $700,000 this year to move the project through the planning process. Jim Miller, who manages the project for the Army Corps, said there is still a chance that some of the federal money will be made available. "We don't know yet," Miller said. Curley said that the computer model indicated that even if all bottlenecks are removed from Corte Madera Creek, flooding would still occur in the Ross Valley in case of a storm like the one that hit Dec. 31, 2005. But storms of that magnitude occur only about once every 100 years, Curley said. The model showed that the construction of catch basins upstream could further reduce flood risk. Basins capable of holding 1,300 acre-feet of water would need to be built and all the bottlenecks removed to prevent flooding during another 100-year event, Curley said. "The community may decide in the long run that 50-year protection is all they're willing to pay for," Curley said. --- MEETING ON FLOOD PROGRAM PLANNED Supervisor Hal Brown and county staff will review data on Corte Madera Creek bottlenecks and share information about the flood protection program's progress at 9:30 a.m. Saturday at Sir Francis Drake High School, 1327 Sir Francis Drake Blvd., San Anselmo. Contact Richard Halstead via e-mail at rhalstead@marinij.com |
 Jack Curley, assistant engineer with the Marin County Flood Control and Water Conservation District, walks toward the Lagunitas Road bridge. The bridge and a wooden fish ladder downstream have been identified as major constriction points on Corte Madera Creek in Ross. (IJ photo/Frankie Frost)  The Lagunitas Road bridge and the wooden fish ladder downstream from the bridge have been identified as constriction points on Corte Madera Creek in Ross. Jack Curley, assistant engineer with the Marin County Flood Control and Water Conservation District, steps across the ladder. (IJ photo/Frankie Frost) |