Field data of waves in the surfzone and runup on ocean beaches in extreme conditions are rare; empirical prediction formulas of runup are therefore largely based on mild and moderate conditions. Here, the effects of varied offshore conditions on surfzone and runup, including wave heights greater than 7 m, are explored with two unique data sets and numerical modeling. Field studies of infragravity (IG, 0.004-0.04 Hz) waves and wave runup were conducted at Cardiff Beach, California (Winter 2012--2013) and Agate Beach, Oregon (Fall 2013). The beaches were instrumented with a cross-shore transect of pressure sensors, current meters and a scanning lidar at the shoreline measuring wave runup.

Runup saturation in the infragravity band is not observed at the low-sloping Agate beach despite considerable IG energy losses in the surfzone. That is, runup continues to increase as offshore wave height increases. The numerical model 1D SWASH accurately predicts the observed bulk runup properties on both beaches without model tuning, even though the assumption of 1D bound long waves at the offshore boundary significantly amplifies the IG energy seaward of the surfzone. Despite agreement in the bulk wave properties, the bound wave boundary condition can propagate errors in nonlinear statistics of the various sea swell and infragravity band interactions well into the domain, depending on offshore conditions. These errors are addressed via inclusion of an offshore boundary condition derived from co-located current and pressure sensor measurements, which accounts for reflection and includes nonlinear phase-coupling. Boundary conditions that can be implemented without infragravity observations (e.g. bound waves) do not accurately simulate infragravity waves across the surfzone, and could corrupt predictions of morphologic change. Infragravity wave properties near the shoreline, however, are predicted to be largely independent of IG offshore boundary conditions, and dominated by local generation and dissipation.