FY 2015

A Tsunami Forecast Model for Point Reyes, California

Spillane, M.C.

NOAA OAR Special Report, PMEL Tsunami Forecast Series: Vol. 6, 176 pp, doi: 10.7289/V5W9573D (2014)

Operational tsunami forecasting by NOAA’s Tsunami Warning Centers relies on the detection of tsunami wave trains in the open ocean, inversion of these data (telemetered via satellite) to quantify their source characteristics, and real-time modeling of the impact on threatened coastal communities. The latter phase of the process involves, for each such community, a pre-tested forecast model capable of predicting the impact, in terms of inundation and dangerous inshore currents, with sufficient resolution and within the time constraints appropriate to an emergency response. To achieve this goal, considerable advance effort is required to tune each forecast model to the specific bathymetry and topography, both natural and manmade, of the impact area, and to validate the model’s performance with a broad set of tsunami sources. Where possible, the validation runs should replicate observed responses to historical events, but the sparse instrumental record of these rare but occasionally devastating occurrences dictates that comprehensive testing also include a suite of scenarios that represent potential future events.

During the forecast model design phase, and in research mode outside the pressures of an emergency situation, more detailed and slower-running models can be investigated. These models, referred to as reference models, represent the most credible numerical representation of tsunami response for a study region, using the most detailed bathymetry available and without the run-time constraint of operational use. Once a reference model has been developed, the process of forecast model design is to determine where efficiencies can be gained by reducing the grid resolution and increasing the model time step, while still adequately representing the salient features of the full solution.

This report documents the reference and forecast model development for Point Reyes, California, and its vicinity, comprising much of western Marin County. The Point Reyes headland juts out into the Pacific Ocean and its lighthouse is a prominent navigation landmark northwest of the entrance to San Francisco Bay. A tide gauge within Drakes Bay, in the lee of the headland, provides observations for model validation from numerous historical tsunamis. While much of the study region lies within a National Seashore area, limiting the population and waterfront infrastructure, there are a number of nearby communities exposed to tsunami impact. Beaches and other natural amenities and the mild climate foster extensive recreational use, and there is a clear need for emergency preparedness. This report addresses the tsunami aspects of the natural hazard spectrum.