Tsunami Detection and Forecasting - Tsunami Warning and Preparedness - page 102

linear effects hamper the determination of open-ocean tsunami wave parameters (e.g., Titov et
al., 2005) without eliminating the stations' utility for the TWCs (e.g., Whitmore, 2003).
Tide stations were typically conigured to measure sea level height in a stilling well, a
vertical pipe that is secured to a piling, pier, wharf, or other shore-side structure. These pipes
have a small oriice(s) to allow water to enter relatively slowly thus iltering out the short period
(3-30 seconds) wind waves, and even tsunamis, so that the hourly recorded sea level values
from within the pipe are not aliased by the short period variability. This technology works
well for measuring tides and other long period phenomena, but even if the sampling rate is
increased from hourly to minutes the true tsunami signal may not be well observed given
these iltering effects. Furthermore, a large tsunami can overtop a well and render it useless
in extreme events. Consequently, sea level observations intended for tsunami detection are
now often accomplished inside a tsunami-hardened station equipped with a rapid-sampling
pressure, acoustic, or microwave sensor with an oriice set apart from the structure (National
Tsunami Hazard Mitigation Program, 2008).
The most important roles for coastal sea level data in the tsunami forecasting and warning
process are currently the initial detection of a tsunami, scaling the tsunami forecast models in
near-real time, and post-tsunami validation of tsunami models (see Weinstein, 2008; Whitemore
et al., 2008). These roles require accurate, rapidly sampled sea level observations delivered in
near-real time via an appropriate telemetry system. In practice, these requirements translate
into a need for sea level averages at least as often as every minute that are made available
in near-real time (U.S. Indian Ocean Tsunami Warning System Program, 2007), and a need for
assiduous maintenance of the sea level gauges so that near-real-time data can be trusted and
will be available most of the time. Furthermore, subsequent to collection, the data need to be
carefully processed through a set of rigorous quality control procedures to maximize the value
for model validation after the fact (U.S. Indian Ocean Tsunami Warning System Program, 2007).
As an example of the importance of high temporal data resolution, Figure 4.2 shows how sea
level data sampled every six minutes completely missed the largest component (the third crest
and trough) of the Kuril Islands tsunami of November 15, 2006 (the modeled wave heights of
which are shown in Figure 4.3).
Coastal sea level data used by the TWCs originate from a number of different networks
(PTWC, WC/ATWC, National Ocean Service (NOS), and University of Hawaii Sea Level Center
(UHSLC)), which are maintained by various national and international organizations (Figure 4.4).
Ideally, these stations are maintained to the standards listed in the Tsunami Warning
Center Reference Guide (U.S. Indian Ocean Tsunami Warning System Program, 2007) for sea
level stations that are intended to provide data for tsunami warning. For coastal tide gauge sta-
tions, the requirements are:
• independent power and communications, for example, solar and satellite;
• fault-tolerant redundant sensors (multiple sensors for tsunami, tides, and climate);
• local logging and readout of data (local back-up of data);
• warning center event trigger (ramping up of sampling rate and transmission upon
detection of a tsunami);