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Understanding Water Level Observations and Forecasting Methods
Is Water Level the Same Thing as Tide?
No. Tides are defined as the alternating rise and fall of sea level with respect to land, as influenced by the gravitational attraction of the moon and sun. In certain locations where water elevation is predominately dictated by astronomical factors water level and tide are often used interchangeably. In other regions like south Texas where the average tidal range is minimal (< 1 ft) other natural forces such as wind, shoreline orientation, topography, and bathymetry dominate over the astronomical forces. In these cases, the observed water level is not the same as the predicted tide influenced water level. The combination of the tide and other influences can be referred to as the total water level fluctuation.
Why Not Simply look at the NOAA Tides Information?
National Oceanic and Atmospheric Administration tide forecasts are predicted based on the periodicity of astronomical factors (the respective motion of the moon and the sun). NOAA has stated that "presently published predictions do not meet working standards" when assessing the performance of current tidal predictions, tides being closely related to water level predictions, for regular weather conditions in Aransas Pass and Corpus Christi Bay NOAA 1991, NOAA 1994. Water level forecasting is complicated by meteorological influences (in particular wind forcing) and is often further complicated by the unique shoreline orientation and shallow water depth associated with the bays and lagoon system that exists along the Texas Gulf Coast. For example, strong winds have been observed to drive water out of the shallow lower Laguna Madre resulting in a lower than predicted water level or in contrast pile water up along the shore resulting in an increased observed water level [1]?. Differences in predicted verses observed water levels could be on the order of 1 to 2 feet along Texas shorelines. Such dramatic differences are typically observed under the action of two opposing wind regimes; a) winds directed out of the southeast - dominating greater than 50% of the time or b) winds directed out of the north - strong pulses of energy associated with frontal passage.
Why Are The Present Forecasting Methods Inadequate For the Texas Coast?
Water levels are typically predicted using tide tables computed based on the gravitational influence of celestial bodies (Harmonic Analysis). While this method of prediction is adequate for most coastlines, along the Gulf of Mexico meteorological effects can significantly influence the observed water level and make the tide tables ineffective. Although astronomical forcing is very predictable meteorological influences are not always periodic and are difficult to predict for periods longer than 1 or 2 days. The influence of meteorological effects, however, is frequently stronger than the influence from the celestial bodies. For example, the Corpus Christi, Texas, airport is ranked by the National Weather Service as the third windiest in the U.S. based on a multiannual average wind speed of 23.5 kph (Smith 1978).
Until recently it was not possible to make accurate water level predictions for the Texas coast. This is because the present models do not include both meteorological and astronomical effects. For example, a fourteen-day comparison between measured water levels and tidal forecasts for the Port Aransas station is shown below. The tide station is located in the Corpus Christi ship channel near Port Aransas, Texas. The difference between tidal forecasts and actual water level can be larger than 1 foot (~31 cm) for several consecutive days. A predicted difference of 1 foot is particularly significant because of the small tidal range (< 1 ft) observed along the Texas Coast.
Predicted (red) vs. observed (blue) tides at Port Aransas, Texas
How Does the Artificial Neural Network (ANN) Produce a Better Water Level Forecast?
What are the Consequences of Inaccurate Forecasts?
The inability of present models to accurately predict water level fluctuations can result in severe consequences, such as ship groundings. In an effort to improve information required for safe navigation NOAA established the Physical Oceanographic Real-Time System (PORTS). Although the PORTS system is beneficial to navigators in the Galveston area it is not available at other Texas ports, thus the ANN water level forecasts will fill this gap for navigators. In addition to navigation hazards present forecasting inaccuracies impair the management of roadways and low-lying coastal regions during presently unanticipated increases in water levels.
Access DNR Harmonic Forecasts
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