Spatial Analysis of High-Resolution Radar Rainfall and Citizen-Reported Flash Flood Data in Ultra-Urban New York City

Abstract

New York City (NYC) is an ultra-urban region, with over 50% impervious cover and buried stream channels. Traditional flood studies rely on the presence of stream gages to detect flood stage and discharge, but these methods cannot be used in ultra-urban areas. Here we create a high-resolution radar rainfall dataset for NYC and utilize citizen and expert reports of flooding throughout the city to study flash flooding in NYC. Results indicate that interactions between the urban area and land-sea boundary have an important impact on the spatial variability of both heavy rainfall and flooding, sometimes in contrast to results obtained for other cities. Top days of daily and hourly rainfall exhibit a rainfall maximum over the city center and an extended region of higher rainfall downwind of the city. The mechanism for flooding appears to vary across the city, with high groundwater tables influencing more coastal areas and high rain rates or large rain volumes influencing more inland areas. There is also a strong relationship between sewer type and flood frequency, with fewer floods observed in combined sewer areas. Flooding is driven by maximum one-hour to one-day rainfall, which is often substantially less rain than observed for the city-wide daily maximum.

Exploring the application of topographic indices in urban areas as indicators of pluvial flooding locations

“…This indicates that differences in storm systems throughout the year did not exert a strong influence on occurrence and relative topographic location of flooding, which differs from other similar work that has found seasonal differences (Smith & Rodriguez, 2017). This also suggests that burst pipes in winter and clogged drains in fall may also have not been issues in these study watersheds but are still important considerations when evaluating the explanatory power of TIs for predicting areas prone to nuisance flooding.…”

“…Island study watersheds were less developed (though still very developed) and exhibited more surface drainage compared with the others (Table 1; Smith & Rodriguez, 2017). Notably, the Staten Island sewer system includes some land drained by combined sewers, but most of the two study watersheds are drained by MS4s.…”

“…Datasets derived from community member reporting have also been combined with land cover datasets (e.g. Bouwens, ten Veldhuis, Schleiss, Tian, & Schepers, 2018), high-resolution rainfall datasets (Smith & Rodriguez, 2017), and field hydrological data (Sadler, Goodall, Morsy, & Spencer, 2018) to infer important predictors and patterns of urban nuisance flooding.…”

“…Even with this large number of reports, the timing of these reports shown alongside daily rainfall indicates that there were many rain events where no nuisance flooding reports were made (Figure S1; note that this co-occurrence between flooding and rainfall is more clearly explored inSmith & Rodriguez, 2017). Comparisons between watershed distributions and TWI and sink depth at nuisance flooding reports were also tabulated with respect to watershed-wide thresholds.…”

Changes in event‐based streamflow magnitude and timing after suburban development with infiltration‐based stormwater management

“…Our inability to capture spatial variability in precipitation patterns may also explain why precipitation depth was a better predictor of streamflow response than precipitation intensity. Precipitation intensity was tested as it was important in other urban watersheds (Smith, Smith, Baeck, Villarini, & Wright, 2013) and would be expected to indicate infiltration-excess overland flow and lead to flash and pluvial flooding (Rosenzweig et al, 2018;Smith & Rodriguez, 2017). However, precipitation depth may be more reliably measured with a single rain gage across neighbouring watersheds than precipitation intensity, leading to the observed importance of precipitation depth in determining streamflow response.…”