Application of digital bathymetry data in an analysis of flushing times of two large estuaries

Abstract

The objectives of this study were: (1) define the best method of using digital bathymetric data to compute estuarine flushing time using the fraction of freshwater method and (2) use this method to compare flushing times of two neighboring estuaries of different trophic state. We examined the sensitivity of the fraction of freshwater method to various methods of calculating estuarine volume using digital bathymetric data. Raster and vector bathymetry data are available from the National Geophysical Data Center (NGDC), and can be used to calculate estuarine volume using a geographic information system (GIS). The vector data was of higher spatial resolution than the raster data (NGDC Coastal Relief Model) and produced a higher estuarine volume, but did not produce significantly different flushing times than the raster data. Water column salinity data can be used to quantify segmented vertical freshwater volumes for integration along the estuary, thereby providing a two-dimensional freshwater distribution profile of the estuary. The vertical representation of water column salinity did not produce flushing times significantly different from a vertically averaged salinity method. Processing and analysis of the Coastal Relief Model raster data is faster and less complex than processing the vector data available from the NGDC. We conclude that the Coastal Relief Model raster data is the preferred bathymetric data source, and that representation of vertical salinity distribution is unnecessary for the analysis of estuaries with morphology similar to the Cape Fear River's. After using the Cape Fear River estuary as a test site for the above comparisons, we applied the preferred method to the New River estuary. In addition to having a direct connection with the ocean, the Cape Fear River has much higher freshwater inflow than the New River, and therefore has a much faster mean flushing time. The Cape Fear River estuary flushing time ranged from 1 to 22 days, while the New River estuary ranged from 8 to 187 days. Similar seasonal patterns were observed in both estuaries: short flushing times occurred during the high-flow winter months and long flushing times occurred during the low-flow summer months.