Runup & Flooding
Beach erosion and street flooding can be caused by coincident high waves and high tides. Tides are predictable years in advance, but high waves can only be forecast at most a week in advance, with sometimes large errors in wave height, period and arrival time. Furthermore, the relationship between waves offshore, seaward of the region of wave breaking, and shoreline water level is both complicated and understood poorly. Obviously, high offshore waves create high runup, but exactly how high also depends on beach face slope (steep versus shallow), and the presence or absence of offshore sand bars (Figure 1).
Figure 1. Wave runup at the shoreline
Complicated nonlinear fluid dynamics describe energy loss by breaking waves as they approach the shore. Breaking waves transfer momentum into the water column, elevating the mean water level (wave setup). Some wave energy reaches the shoreline and oscillates around the setup at the periods of incident sea-swell waves and also at longer “infragravity” periods.
We have tested and validated SWASH, a physics-based numerical model that transforms observed offshore waves (PUV in Figure 2a) to runup (observed with LiDAR, Figures 2 & 4). Overtopping (Figure 3) is difficult to model accurately because the fluid dynamics of spray and flow through riprap jetties are nasty, and field observations limited. Ongoing research aims to better understand and predict runup and overtopping. Additionally, we are exploring methods to predict runup with limited wave and bathymetry observations.
SWASH has been tested in small-to-moderate waves at Cardiff, California, with storm waves at Imperial Beach (Figure 2), and with jumbo swell (offshore height 7+ m) at Agate Beach, Oregon (Figure 4).
Figure 3. Wave overtopping at Cortez Ave., Imperial Beach.
J.W. Fiedler, A. Young, B.C. Ludka, W.C. O'Reilly, C. Henderson, M. Merrifield and R. T. Guza (2020), Predicting site-specific storm wave run-up. Nat Hazards. https://doi.org/10.1007/s11069-020-04178-3.
Fiedler, J., P. Smit, K. Brodie, J. McNinch, and R.T. Guza (2018), Numerical modeling of wave runup on steep and mildly sloping natural beaches. Coastal Engineering. 131, 106-113, doi.org/10.1016/j.coastaleng.2017.09.004.
Fiedler, J., P. Smit, K. Brodie, J. McNinch, R.T. Guza (2019). The offshore boundary condition in surf zone modeling, Coastal Engineering, 143, 12-20, ISSN 0378-3839, https://doi.org/10.1016/j.coastaleng.2018.10.014.
Fiedler, J.W., K.L. Brodie, J.E. McNinch, and R.T. Guza (2015), Observations of runup and energy flux on a low-slope beach with high-energy, long-period ocean swell, Geophys. Res. Lett., 42, 9933–9941, doi: 10.1002/2015GL066124.
Henderson, C., J.W. Fiedler, M. Merrifield, A.P. Young, R.T. Guza, Observations and modeling of wave overtopping, in prep.
Lange, A, J.W. Fiedler, J.W., M.A. Merrifield, and R.T. Guza, Estimating runup with limited bathymetry, in prep.