Consultant Brian urges targeted flow monitoring after terracotta-pipe rehab reduced peak flows

Brian, the consultant presenting to the Marin City Council on April 22, told members that long-range water and sewer planning should guide the capital improvement program and prioritize projects to reduce inflow and infiltration (I&I). He described the system’s history — a plant built in the 1950s with biological nutrient upgrades in 2000 and later enhanced nutrient removal work — and argued that much of the system’s compliance risk stems from wet-weather flows entering the collection system.

Why it matters: Load-based permit limits for total nitrogen and phosphorus mean that reducing storm-driven flows has direct regulatory and cost benefits for the town. Brian and operations staff said the city’s ARPA-funded projects (Broad Street, Roberts Mill, Meadowbrook and similar efforts) appear to have substantially reduced peak wet-weather flows and helped the plant meet permit conditions in recent years.

Staff described terracotta (clay) mains in older downtown neighborhoods as a primary source of infiltration. Brian said mapping shows completed work in Meadowbrook and near the elementary school but flagged Old Town (Middle Street, George Street, Cloverberry) as a remaining, technically challenging priority.

Operations staff (Kevin) gave concrete examples of reduced peaks: where a 1.5-inch rain once drove plant influent from ~500,000 gallons per day to over 2,000,000–2,500,000 gpd, post-rehab events now tend to remain well under 1,000,000 gpd in comparable conditions. “We have cut a lot of water out of that system, but there is still a lot to go,” Kevin said.

Councilors pressed for evidence of program effectiveness. Brian and staff recommended a targeted flow-monitoring deployment (meters plus a rain gauge) to collect apples-to-apples before/after comparisons and to verify which projects produce the greatest reductions. Staff recalled a 2020 deployment of seven meters and one rain gauge that cost $32,560 and said an updated deployment today would likely cost between $45,000 and $70,000 depending on scope and inflation.

The council asked staff to prepare a memo with a recommended scope, goals and a vendor cost proposal in time for the May budget discussions. Brian said the monitoring would not prove a single project’s exact gallons saved but would provide a clearer picture of trends and better inform project prioritization.

The meeting proceeded to capital planning and budget items after the technical discussion; council agreed to consider whether to fund the proposed meters in the FY27 budget.