Board asks for sensitivity analysis as dewatering, creek crossings could swing costs

A subcommittee member said the next step should be a sensitivity analysis to clarify how assumptions change cost estimates; Wallace Group confirmed it can supply that analysis and said it has budget to do additional scenario testing. Carrie listed example variables—bridge attachment method, dewatering needs, depth increases and whether equalization storage is required—that could move costs significantly.

Board members repeatedly raised dewatering as a serious risk in the southern district where groundwater is shallow and shoring and dewatering needs would increase construction and contingency costs. “When you dig in trenches, you have a lot of coming into it, then you have to shore everything up more probably,” one board member said, urging the team to quantify that exposure. Carrie acknowledged prior experience in nearby Los Osos where dewatering had been a large cost and said the draft deliberately included contingencies for dewatering but that further refinement is needed.

The subcommittee directed staff and Wallace Group to identify the assumptions that most materially change the life‑cycle cost and to return with a sensitivity‑analysis scope for the 60% design phase. The work will also look at whether equalization storage is required and whether that need could be mitigated by tying into a different point of connection to Solvang's collection system.