Uncertainty in estuarine extreme water level predictions due to surge-tide interaction

Abstract

Storm surge is often the greatest threat to life and critical infrastructures during hurricanes and violent storms. Millions of people living in low-lying coastal zones and critical infrastructure within this zone rely on accurate storm surge forecast for disaster prevention and flood hazard mitigation. However, variability in residual sea level up-estuary, defined here as observed sea level minus predicted tide, can enhance total water levels; variability in the surge thus needs to be captured accurately to reduce uncertainty in site specific hazard assessment. Delft3D-FLOW is used to investigate surge variability, and the influence of storm surge timing on barotropic tide-surge propagation in a tide-dominant estuary using the Severn Estuary, south-west England, as an example. Model results show maximum surge elevation increases exponentially up-estuary and, for a range of surge timings consistently occurs on the flood tide. In the Severn Estuary, over a distance of 40 km from the most upstream tide gauge at Oldbury, the maximum surge elevation increases by 255%. Up-estuary locations experience short duration, high magnitude surge elevations and greater variability due to shallow-water effects and channel convergence. The results show that surge predictions from forecasting systems at tide gauge locations could under-predict the magnitude and duration of surge contribution to up-estuary water levels. Due to the large tidal range and dynamic nature of hyper-tidal estuaries, local forecasting systems should consider changes in surge elevation and shape with distance up-estuary from nearby tide gauge sites to minimize uncertainties in flood hazard assessment.

Fig 1. Severn Estuary model domain extending from Ilfracombe (51°12.668'N, 4°6.743'W) and the Mumbles (51°34.203'N, 3°58.534'W) in the west, to Gloucester (52° 89.3020’N, -2°2. 6361’W) in the east.

The bathymetry is relative to chart datum (CD).

Fig 2

Model output validation for realistic timing of total water level and tide only model runs compared to observational data at a) Hinkley Point and b) Sharpness tide gauge, Severn Estuary, southwest England.

Fig 3

Modeled tidal time series (black); modeled surge elevation for the realistic surge timing (red line); range of surge elevations for time shifted configurations shaded (blue band); observed filtered surge (orange line) at a) Hinkley; b) Newport; c) Portbury; d) Oldbury; e) Sharpness for the 3^rd January 2014 event.

Fig 4

a) Tidal range; b) Surge elevation range for observed event timing; c) Variability in surge values; d) Variability in skew surge values for time shift configurations along thalweg of the Severn Estuary.