Get AI Briefings, Transcripts & Alerts on Local & National Government Meetings — Forever.
Stormwater models show outfall consolidation raises local ponding; green infrastructure may help
Loading...
Summary
Hazen’s stormwater modeling showed consolidating two outfalls into a single Francis Street outfall increases short-term street ponding during high-tide rainfall; consultants recommended evaluating rain gardens, tide gates, and possible small pumps as mitigation.
Hazen engineers presented stormwater modeling that evaluated rainfall-driven flooding during high-tide conditions and tested whether consolidating two outfalls would increase or reduce street ponding on Washington Street.
The firm modeled design storms (SCS Type III distribution) of about 1 inch (roughly a 6‑month event), 3 inches (about once per year), and 5 inches (about once in 10 years) while aligning peak rainfall with a high-tide tailwater time series provided by the coastal team. Ben of Hazen said the modeling found that eliminating the Meador Street outfall and routing flow only to the Francis Street outfall can increase localized flooding on Washington Street because the Francis Street outfall and pipe configuration lack the head differential and capacity to discharge all runoff at peak tide.
“Consolidating the outfalls and taking away the outfall on Meador Street does increase the potential for flooding if it’s raining at or close to high tide,” Ben said. The model showed modest ponding in common events and larger ponding in the 3-inch and 5-inch scenarios; when both outfalls are overwhelmed in extreme events, consolidation made less difference.
Mitigation options and findings: Hazen tested tide gates and found they do not improve the modeled short-term rainfall-driven street flooding in the scenarios presented (model results showed little change versus the baseline for the three rainfall scenarios), though consultants noted tide gates can reduce future storm-surge inflow and have value for longer-term sea-level rise conditions. The team also proposed pursuing green infrastructure — rain gardens and bioretention areas in identified orange plan areas — to capture and slow runoff. Ben said the rain‑garden option is worth further modeling to quantify how much street flooding would be reduced.
The consultants also discussed operational options: increasing pipe capacity, adding a modest pump to provide additional head during peak tide, or extending an outfall to an area with lower tailwater could reduce ponding; operational maintenance is a parallel concern because local outfalls are prone to sand or debris clogging. The group flagged maintenance frequency as a real cost if tide gates or other devices are used and suggested an operations-and-maintenance plan would be necessary.
Context and next steps: Hazen will run additional scenarios that redirect modeled street runoff into proposed rain gardens in the Salt Marsh parking area and test whether green infrastructure and modest drainage improvements together can reduce Washington Street ponding. Staff also raised that future coordination with larger roadway projects (raising Washington Street or conduit/utility work) could create an opportunity to integrate drainage upgrades.
Ending: The stormwater team recommended further modeling of rain gardens and targeted drainage options and advised the workgroup to consider maintenance responsibilities and potential pump/pipe solutions as the design advances.
