RIVER DISCHARGE CONTRIBUTIONS TO HIGH TIDE FLOODS ALONG THE DELAWARE RIVER ESTUARY

Compound floods are often defined as infrequent, major floods when coastal sea level and river flow extremes occur simultaneously. While past studies have explored the mechanisms responsible for major events with return periods of decades to centuries, comparatively less is known about the nature of lower level but potentially more frequent compound high tide floods. Here, we quantify the drivers of high tide flooding along the Delaware River Estuary and determine what fraction of these floods are compound events. We consider three components of high tide floods along the Delaware River Estuary: astronomical tides, non-tidal open-ocean effects, and river discharge. Using 20-year time series of hourly water level from 9 tide gauges and daily river discharge from the stream gauge at Trenton, NJ, we identify flooding days at 8 locations along the ~240 km tidal reach of the river. We use multiple linear regression to establish the relative contributions of predicted tides, non-tidal open-ocean effects, and river discharge, to water-level variability and high tide flooding at the 8 tide gauges along the river. We find that most high tide floods along the Delaware River (~ 80%) are compound high tide floods requiring at least two components (e.g., tide and river discharge) to cross the flood threshold. While this finding is spatially consistent along the estuary, the relative contributions of open-ocean effects, discharge, and tides to flood events are location dependent. Results show that river discharge contributes to between 61 and 99% of upstream floods closest to the head of tides, 44 to 68% of floods at the urban center of Philadelphia, and between 1 and 6% of downstream floods at the estuary mouth. These results suggest that incorporating river discharge into assessments of high tide flooding is important to fully characterize flood risk along tidal rivers.