Runup & Flooding

Research

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.

Figure 2. January 18, 2019, ~ 5:00 AM Cortes Ave, Imperial Beach, CA. (a) aerial view of the beachfront. Sand elevation has been measured regularly along a transect (dashed line). PUV in ~8 m depth (including tide) observed incident waves. (b) LiDAR observations, and SWASH-modeled water elevation versus cross-shore location. Model bathymetries (solid black curves) combine historical and LiDAR bathymetry observations(Henderson et al., in prep).

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.

Figure 4. Wave breaking and runup were measured at Agate Beach, Oregon with in situ sensors (marked with poles) and with a scanning LiDAR. LiDAR measured sea surface elevation along a cross-shore transect, several times per second. Pressure sensors (P06, P11, P13) measured water level at a point. About 7 minutes of wave record are shown in one image, and time progresses. Offshore waves were pumping (T=15 sec, H = 7.5 m) and runup excursions on the beach face reached 100 m. Four right panels shows the similarity of sea surface elevation measured with LiDAR and with pressure sensors. One LiDAR returns information equivalent to a transect of pressure sensors. However, LiDAR observations can be degraded by fog, rain, snow, ice, and foam.

Publications

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.