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Gated storm surge barriers are being studied by the United States Army Corps of Engineers (USACE) for coastal storm risk management for the New York City metropolitan area. Surge barrier gates are only closed when storm tides exceeding a specific “trigger” water level might occur in a storm. Gate closure frequency and duration both strongly influence the physical and environmental effects on enclosed estuaries. In this paper, we use historical observations to represent future storm tide hazard, and we superimpose local relative sea-level rise (SLR) to study the potential future changes to closure frequency and duration. We account for the effects of forecast uncertainty on closures, using a relationship between past storm surge and forecast uncertainty from an operational ensemble forecast system. A concern during a storm surge is that closed gates will trap river streamflow and could cause a new problem with trapped river water flooding. Similarly, we evaluate this possibility using historical data to represent river flood hazard, complemented by hydrodynamic model simulations to capture how waters rise when a hypothetical barrier is closed. The results show that SLR causes an exponential increase of the gate closure frequency, a lengthening of the closure duration, and a rising probability of trapped river water flooding. The USACE has proposed to prevent these SLR-driven increases by periodically raising the trigger water level (e.g., to match a prescribed storm return period). However, this alternative management approach for dealing with SLR requires waterfront seawalls to be raised at a high, and ongoing, additional future expense. For seawalls, costs and benefits will likely need to be weighed on a neighborhood-by-neighborhood basis, and in some cases retreat or other non-structural options may be preferable.

Recent hurricanes have demonstrated the need for real-time flood forecasting at street scale in coastal urban areas. Here, we describe the high-impact high-resolution (HIHR) system that operationally forecasts flooding at very high resolution in the New York–New Jersey metropolitan region. HIHR is the latest upgrade of the Stevens Flood Advisory System (SFAS), a highly detailed operational coastal ocean modeling system. SFAS, based on the Hydrologic–Hydraulic–Hydrodynamic Ensemble (H3E) modeling framework, consists of four sets of nested coastal and inland flood models that provide ensemble flood forecasts with a horizon of at least 96 h from regional to street scales based on forcing from 100 different meteorological output fields. HIHR includes nine model domains with horizontal resolution ranging from 3 to 10 m around critical infrastructure sites in the region. HIHR models are based on an advanced hydrodynamic code [the Stevens Estuarine and Coastal Ocean Model (sECOM), a derivative of the Princeton Ocean Model] and nested into the H3E models. HIHR was retrospectively evaluated by forecasting the coastal flooding caused by Superstorm Sandy in 2012 using water-level sensors, high-water marks, and flood maps. The forecasts for the 95th percentile show a good agreement with these observations even three days before the peak flood, while the 50th percentile is negatively biased because of the lack of resolution on the meteorological forcing. Forecasts became more accurate and less uncertain as the forecasts were issued closer to the peak flooding.

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