Street‐to‐Pipe Diagnosis of Compound Rain–Tailwater Flooding in a Coastal Combined Sewer System: Sewer Pre‐Filling and Outfall Throttling

Abstract Compound flooding in coastal cities occurs when heavy rainfall coincides with tide‐driven tailwater that reduces combined sewer system (CSS) discharge capacity, yet this coupling is rarely quantified from street to pipe scale. We develop a high‐resolution 1‐D/2‐D drainage–inundation model for Hoboken, New Jersey, calibrating the 1‐D drainage component with flow gauges and validating inundation extent with Sentinel‐1 SAR maps and geotagged imagery; calibration improves mean outfall NSE from 0.50 to 0.65 (validation 0.55). Across 12 rainfall–tailwater scenarios, astronomical spring‐tide tailwater pre‐fills subsurface storage (about half for the selected spring‐tide case) and submerges outfalls, reducing peak outflow by up to ∼45% and increasing flood depth, area, and surface volume; an sea‐level rise (SLR) elevated tailwater state induces nuisance flooding and further amplifies impacts. Driver decomposition shows rainfall dominance, with tide effects acting mainly through interaction and declining in fractional importance as rainfall intensity increases. Separate 3‐D Volume‐of‐Fluid simulations of an idealized outfall reach resolve density‐stratified intrusion that persists longer under lower inflow momentum but is flushed rapidly under higher momentum, providing a mechanistic explanation for the intensity dependence. Tide‐affected conditions also prolong high pump utilization and increase wastewater treatment plant (WWTP) inflow loading toward (and at times beyond) rated capacity. Duckbill check valves alone reduce depths modestly (typically m), highlighting the need for event‐aware portfolios that combine boundary controls with upstream runoff reduction/storage and adaptive operations in tidally influenced cities.