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Falls Church planning to buy 0.5 MGD capacity from Fairfax, council briefed on peak‑flow risks and $10M detention option
Summary
City staff presented a draft agreement to buy an additional 0.5 million gallons per day of sewer capacity from Fairfax County and described new hourly peak and daily volume limits in the draft that would require corrective action or infrastructure if repeatedly exceeded.
City staff presented a draft capacity purchase and related mitigation plan Monday to address long‑standing inflow‑and‑infiltration (I&I) and peak‑flow issues in sewers that convey Falls Church flows through Fairfax County to the Alexandria Renew (AlexRenew) treatment plant.
The draft agreement would increase Falls Church’s annual average daily allocation to about 1.5 million gallons per day (MGD) by purchasing an additional 0.5 MGD from Fairfax County. Staff said the draft purchase price is roughly $8.8 million (figures in the staff presentation indicated a purchase price in that range) and that availability fees collected from developers so far have been used to fund earlier conveyance upgrades and the capacity purchase.
Fairfax County inserted additional protections into the draft terms that are new to the city’s agreement: specific limits on hourly conveyance peak flows and a maximum total daily volume that, if exceeded with particular frequency, would trigger corrective requirements. Staff said the draft sets thresholds that — if breached repeatedly within a 12‑month period — could require the city to develop and implement plans to reduce flows or, in extreme cases, be subject to restrictions on new connections until the situation is remedied.
Engineering consultant Dustin Dvorak (Bramble Group/Humble Engineering Group) described the three technical measures embedded in the draft: the annual average allocation (1.5 MGD), an hourly conveyance‑peak limit (an averaged hourly value derived from 15‑minute meter data), and a maximum daily volume limit. Historical metered data show the city’s flow approaches the existing 1.0‑MGD allocation in very wet years; staff said the additional 0.5‑MGD purchase is driven by future entitlements and development that would otherwise exhaust capacity.
Staff briefed council on measures the city is pursuing to reduce I&I, including a multi‑year rehabilitation program that has relined pipes and manholes (staff said roughly 65% of the sewer network has been rehabilitated to date), targeted manhole and pipe repairs in problem areas such as the Cedar Lane subshed, and a flow‑monitoring program. A city capital planning slide assumed additional I&I reduction work will continue through the CIP and staff said the program has been a multidecade effort.
The most significant operational option staff presented to protect downstream customers was an equalization (detention) basin sized to store wet‑weather peaks so flows to Fairfax would not exceed conveyance thresholds. Staff and consultants said preliminary sizing work remained incomplete but discussed rough order‑of‑magnitude options; staff repeatedly referenced a conceptual detention‑basin cost in the order of $10 million. The basin would allow the city to store storm‑related inflow temporarily and release it gradually to avoid surcharging Fairfax pipes.
Councilors pressed for clarity on several matters: how many past exceedances predate the proposed thresholds; whether the detention basin would be needed even absent new development; how costs would be allocated; and the degree to which private‑side I&I (sump pumps, downspouts illegally tied to sanitary sewers) versus public‑side defects contribute to peaks. Staff said developer availability fees have funded the average‑flow purchase and pipe upgrades to date, and that debt service on basin financing could be modeled to be covered by projected future sewer revenues rather than a one‑time steep rate increase.
Why it matters: the issue affects development approvals, sewer connection availability and potential capital investments. Under the draft terms, the city must plan corrective actions if thresholds are exceeded repeatedly, and the council will need to balance development objectives, ratepayer impacts and capital spending.
Next steps: staff expect to finalize negotiations with Fairfax and return a completed agreement to council (staff said they aim for an April 7 agenda); staff also will continue preliminary engineering on the equalization basin, complete the Cedar Lane repairs and present financing scenarios showing how any detention‑basin debt could be serviced.
