Get Full Government Meeting Transcripts, Videos, & Alerts Forever!
Get email alerts on the Water Management topic
No spam. Unsubscribe anytime.
GSA consultants recap regional conjunctive-use work; pilot clearing relieved local flooding, Twin Spill could yield ~1,200 acre-feet
Summary
Consultants updated the GSA on a regional conjunctive-use program that pairs flood diversions and South Feather surface-water conveyance to reduce groundwater pumping. A Polarmo-area maintenance pilot cleared about 1.3 miles of ditch for roughly $100,000 and produced anecdotal flood relief; the Twin Spill project could provide about 1,200 acre-feet of groundwater benefit if implemented.
Consultants for the We Do Creek Groundwater Sustainability Agency presented a two-part recap on March 19 that tied short-term pilot maintenance to longer-term regional conjunctive-use planning.
Becky Fairbanks, the GSA project manager, summarized work funded by grants and described a Polarmo-area pilot that cleared roughly 1.3 miles of drainage from Aena Avenue to Railroad Avenue to reduce localized flooding and to help the agency learn permitting procedures. “It cost about $100,000, including the administration work, permitting and implementation,” Fairbanks said.
Larry Walker Associates’ consultant Ryan Fulton described the overall regional conjunctive-use concept: convey excess surface water from Polarmo floodwater and South Feather Water & Power Agency diversions down Wyman Ravine so downstream groundwater users can substitute surface water for some irrigation, reducing pumping during certain periods.
Consultants presented 2D hydraulic modeling (2-, 10- and 100-year storms) that identified critical overflow points at Palmo Road and Lincoln Boulevard and spill locations near Perkins Avenue and the Polarmo tributary adjacent to the school. The model relied heavily on community-reported flood observations because gauge data are sparse; teams plan to run recent storm data through the model to refine results.
Brian Thorson of GEI reviewed the Twin Spill Phase 1 concept, which would divert Polarmo-canal flows into a Wyman Ravine tributary to serve roughly 300 acres of rice and to recharge groundwater. “We expect the total benefit of about 1,200 acre feet,” Thorson said, describing an approximate split of 900 acre-feet of direct in-field recharge and about 300 acre-feet of seepage in the ravine. He said early conceptual capital costs were about $1.8 million at the 10% estimate but 30% design work brought conceptual estimates closer to $1 million, and estimated lifecycle costs near $58 per acre-foot over 50 years (with conservative assumptions of water available 7 of 10 years).
Thorson described project partners (the GSA, South Feather Water and Power Agency as water supplier/operator, and a private landowner/grower as the recharge/pumping party) and said the GSA’s role would be to track additional recharge for annual reporting and to facilitate a water-exchange or surplus-sale agreement. He noted that stream-gauge data recently installed will help refine seepage estimates.
During public comment and board Q&A, resident and former road-maintenance worker George Snodgrass argued that long-standing elevation and private-land maintenance issues require larger conveyance fixes such as buried pipe rather than occasional clearing. Consultants acknowledged that the pilot is an immediate relief step and that larger infrastructure will require additional design, easement work and permitting.
Next steps for the projects include finishing 30% designs, refining seepage and recharge estimates as new gauge data are analyzed, developing water-exchange agreements with South Feather, and pursuing grant funding for implementation.
