TWL Model

A probabilistic simulation of spatially and temporally varying total water levels for different climate change scenarios has been developed by researchers at Oregon State University (Serafin & Ruggiero 2014). For this study, 90-year time series of the deep water components that make up the Total Water Level, TWL, for three climate change scenarios, low, medium and high, are generated. Both the erosion and flooding models depend upon TWL, which is comprised of four component such that:


             TWL Equation


where MSL is the mean sea level, nA is the deterministic astronomical tide, nNTR is the non-tidal residual and R is the wave runup. TWL is a combination of the still water level and a wave-induced component, the wave runup. The total wave runup is, in turn, a combination of the maximum static wave setup at the shoreline and swash, the time-varying oscillation about the setup, including both incident and infragravity wave motions. This vertical component of wave runup is often empirically related to the deep water wave height, wave length and the local geomorphology of the shoreline, including the beach slope. In order to increase computational efficiency, a series of lookup tables were utilized which relate the deep water wave conditions to the near shore (~30 meter depth contour) wave conditions through a radial basis function. Finally, kinetic wave theory was applied, along with LIDAR morphometric data, to calculate the location dependent wave runup.

The Grays Harbor Study area can be categorized according to two different beach types: sandy dune-backed beaches and beaches which are backed by bluffs, cliffs or barrier structures, such as revetments. Therefore, two calculations of wave runup are used, one for each beach type. Both calculations combine the wave setup and the swash using the 2% exceedance value of the swash maxima. The incident and infragravity components are combined statistically to determine the exceedance levels. Wave setup is the additional water elevation due to the effects of transferring wave-related momentum to the surf zone.

For sandy dune-backed beaches, an empirical formula dependent on the deep water wave height, the wave length and the local beach slope was used to determine the runup. (Stockdon et al. 2006).

For shoreline backed by barriers, bluffs, cliffs and cobble berms, the height and steepness of the incident wave, the shoreline and barrier slope, a reduction factor for surface roughness, a reduction factor for the influence of berms, if present, and the design characteristics of the backshore structure, including its porosity and oblique wave incidence, all contribute to the calculation of runup. This approach is referred to as the Technical Advisory Committee for Water Retaining Structures (TAW) method and is useful for both smooth and rough slopes (van der Meer et al. 2002).