|Scour Technology | Bridge Hydraulics | Culvert Hydraulics | Highway Drainage | Hydrology | Environmental Hydraulics|
|FHWA > Engineering > Hydraulics > Hydrology > HEC 25|
HEC 25 - Tidal Hydrology, Hydraulics, and Scour at Bridges
Chapter 6 Data Considerations and Sources
There are many data sources for use in developing models for tidal hydraulics and scour computations. The primary data effort begins with collection of bathymetry. Proper attention must be given to the datum for the soundings or mapping. As mentioned previously, there are many different datum references (datum) that can vary significantly. In areas of low tide, MLLW can be mistaken for NAVD. The second category of data is boundary conditions. These include appropriate upstream hydrographs, rainfall, and ocean boundary conditions. The latter includes both astronomical tides as well as storm surges. Table 6.1 provides a listing of internet sources of data which can be used in a typical tidal project. Additional searching on the internet will provide other useful data in these categories.
6.2 Tide Gages
Tide gage data should be obtained for model boundary condition development and for use in model calibration. NOAA websites can provide predicted tides and, in many locations, real time tide measurements. In addition to NOAA data, Table 6.1 includes various university and proprietary sites and software resources for predicting tides. In addition to these predicted and observed tide sources, it is usually necessary to install tide gages at several locations in the project waterway. If there is a NOAA real time gage, this can serve as one of the project gages. Project tide gages should also be installed at or near the downstream boundary, at the bridge crossing, and one or several locations well up the estuary or at various locations within a bay. The project tide gages should be run for a minimum of one week and, if possible, for several weeks to a month. The time period should include spring tide conditions if possible. The data will not be sufficient to independently develop tide constituents. The data can be compared with other long-term gages in the area that have well defined tide heights to develop the equivalent of a subordinate station time and height differences as discussed in Chapter 2. The tide gages must be surveyed to accurately establish the tide elevation in a fixed project datum.
6.3 Tidal Benchmarks and Vertical Datums
A vertical datum is a reference level for elevations. The elevations used on most topographic maps, bridge plans, floodplain maps, etc. are referenced to a fixed vertical datum. A fixed datum is a reference level surface that has a constant elevation over a large geographical area. Hydraulic analysis always requires reference to a fixed vertical datum.
In the United States, the most commonly used fixed datums are those established by the National Geodetic Survey (NGS). In some locations, however, local fixed datums are used. Local datums are established by the state, city or county and are independent of the standard NGS datums.
A tidal datum is a reference level that applies at one geographic point in a tidal water body. Much of the data available for the analysis of tidal waterways refers to tidal datums rather than fixed datums. Most bathymetric data, navigation charts and tide level observations, for instance, present elevations above tidal datums. Tidal datums are defined on the basis of mean tide fluctuations.
6.3.1 Tidal Datums
Several terms related to tide levels are defined in Section 2.2. For the purposes of explaining tidal datums, some of these terms are illustrated again in Figure 6.1. Usually the reference level for a tidal datum is mean low water (MLW) or mean lower low water (MLLW). Occasionally mean sea level (MSL) is used as a reference datum.
The relationship between a tidal datum and a fixed datum can vary widely within a single estuary or bay system because the mean tide range is variable. Moreover, a tidal datum at a given point changes with time. Mean tide levels are established for a complete tidal epoch, a period of approximately 19 years (see Section 2.2). Because of ocean level rise the values usually change from one tidal epoch to the next.
Confusion often arises when MSL is referenced as a vertical datum. Mean tide level (MTL) is the midway point between MLW and MHW. MSL is not necessarily synonymous with MTL. Often, when a bridge drawing or other document refers to MSL as its vertical datum, it is actually referring to a fixed datum. When the datum is named as MSL, the user of the data must clarify whether the reference is equivalent to MTL for the current tidal epoch. If it is found that MSL actually refers to a fixed datum, the user must determine the relationship between that fixed datum and the datum used in the study at hand.
6.3.2 Fixed Datums
The fixed datums used most commonly in the United States are the National Geodetic Vertical Datum of 1929 (NGVD29) and the North American Vertical Datum of 1988 (NAVD88). These reference surfaces were established by the NGS. High-accuracy benchmarks referenced to these datums have been set by the NGS throughout the country.
The National Geodetic Vertical Datum of 1929 (NGVD29) was called the Sea Level Datum of 1929 until the name was changed in 1973. Many older plan sets, maps, and documents refer to a datum of Sea Level or Mean Sea Level that is actually equivalent to NGVD29. One must not assume this equivalency, however, without verification. NGS accomplished the establishment of NGVD29 by connecting the major vertical benchmark networks in the country to 26 tidal benchmarks along the Atlantic, Gulf, and Pacific Coasts.
The North American Vertical Datum of 1988 (NAVD88) was established by the NGS in 1991. It is considered to be a significant technical improvement over NGVD29 in terms of accuracy and applicability over very large areas. All new NGS benchmarks are established in NAVD88. The difference between NGVD29 and NAVD88 is variable, depending on region. For a particular tidal study, the region is usually small enough that difference should be consistent. This is not the case, however, for tidal datums. Tidal datums can vary significantly within a bay or estuary.
6.3.3 Datum Conversions
Hydraulic modeling and analysis must be performed in a fixed-datum. The model should be developed with reference to the same fixed datum as the bridge plans.
In past years, the conversion of elevations from a tidal datum to a fixed datum was usually a difficult process. Now, however, the required information is readily available on the Internet. Table 6.1 presents the current Internet location of several data sources, including the NOAA tidal benchmarks web site and the NGS benchmark data sheet web site. The tidal benchmark web site lists official tidal benchmarks in a closely-spaced network throughout the coastal regions of the United States. The description of each benchmark includes the height of the benchmark above MLLW at that location. The MHHW, MHW, MTL, and MLW levels above MLLW are also provided.
In most cases a direct Internet link is provided to the NGS data sheet for the particular benchmark. The NGS data sheet provides the NAVD88 elevation and the NGVD29 elevation of the benchmark. At this web page there is also a link to a graphical plot comparing the tide levels to both NAVD88 and NGVD29 (Figure 6.2).
Once this comparison is obtained, the conversion from the tidal datum to the fixed datum is straight forward. The tidal benchmark represented on Figure 6.2 is located near the mouth of the Wando River in Charleston County, South Carolina. Elevation 0.0 in NAVD88 is 3.51 feet above MLLW at this location. To determine the NAVD88 elevation of any point in the subject data, one would subtract 3.51 feet from the height of the point above MLLW. For example, a point fifteen feet below MLLW has an elevation of -18.51 feet NAVD88 at this location. A point one foot above MLLW has an elevation of -2.51 NAVD88. Although the arithmetic is extremely simple, it is common to make the adjustment in the wrong direction by adding when subtraction is called for, or vice versa.
When data covering a significant portion of an estuary or bay are to be converted, it is usually necessary to break the data into zones by proximity to different tidal benchmarks. Each tidal benchmark requires its own conversion from a tidal datum to a fixed datum. If there is a significant difference in the datum adjustment for two adjacent tidal benchmarks, the user should consider using transition areas between the zones.
The relationship between NGVD29 and NAVD88 varies geographically. If a conversion between these two fixed datums is necessary, the locally appropriate adjustment should be verified. The USACE CorpsCon program is useful for converting large data sets from one fixed vertical datum to another. It includes a database of conversion constants for the entire country. CorpsCon also converts horizontal coordinates between various systems, such as Latitude-Longitude to Universal Transverse Mercator (UTM) or state-plane, etc.
6.4 Hurricane Surge Data
As discussed in Section 2.3, there are numerous sources that should be investigated for hurricane surge data. Hurricane surge heights can be estimated for various return periods or categories based on the ADCIRC modeling results in Appendix E. Other sources for surge height are FEMA, NOAA, U.S. Army Corps of Engineers, and individual state studies. These studies can be compared with long-term tidal gage records. If several different studies are available for an area of coastline, then these studies should be compared before selecting a surge height for design purposes. When comparing surge data from various sources, it is important to exclude surface waves. Surface waves are not simulated in the hydrodynamic models because they do not contribute to the pressure head term of the momentum equation. Therefore, the FEMA still water elevation should be used. If a recorded or estimated surge includes wave height then this should be excluded in the boundary condition development.
If a hurricane is active along the U.S. coastline, then the NOAA tides online website located at http://tidesonline.nos.noaa.gov/ (Search terms: NOAA tides online) will include tide gages that are operating in storm mode. These gages record real-time measurements of the tide and surge heights. Figure 2.18 is an example of data obtained from gages operating in storm mode.
6.5 Wind Data
Wind data are generally more difficult to obtain. NOAA tide stations at the tides online website that are operating in storm mode may also include wind speed and direction measurements. NOAA also includes wind data at the National Data Buoy Center website located at http://www.ndbc.noaa.gov/ (Search terms: NOAA wind data). These data may be supplemented with local National Weather Service station records. If winds are to be included in modeling or in hurricane surge simulation, it is important that the wind values include adjustments for duration and terrain as discussed in Section 2.4. The 1-minute wind speed used to define hurricane category is not an appropriate wind speed for wave height computations or for simulating wind stress over a body of water.
6.6 Bathymetric and Topographic Data
Accurate bathymetric and topographic data are required if realistic results are expected from tidal hydrodynamic modeling. In many cases, this requires project specific hydrographic survey, topographic mapping and bridge structure ground surveys. Hydrographic survey is generally conducted with boat mounted survey grade GPS and fathometers recording location and water depth and converted into the project horizontal and vertical datums. In many cases, the significant effort required for the survey is avoided by using other sources of data. One source is the National Geophysical Data Center GEODAS hydrographic survey data located at http://www.ngdc.noaa.gov/mgg/bathymetry/hydro.html (Search terms: NOAA GEODAS bathymetry). The GEODAS data includes data from several sources, but is primarily from the NOS hydrographic data base. Since these data are from as early as the 1930s, the bathymetry may have changed significantly and the data would no longer be useful for modeling. Even more recent data should be checked for accuracy. In addition to the GEODAS data, other sources of bathymetric data include bridge plans and inspection records for crossing up- and downstream of the project site and even navigation charts. Regardless of the source, the data must be converted into a consistent horizontal and vertical project datum.
Similar care should be used in establishing the topography of areas inundated by surge conditions. USGS quad sheets and Digital Elevation Models (DEMs) should only be used if more accurate mapping is not available.
6.7 Aerial Photography and Mapping
An extremely useful resource for aerial photography and USGS quad maps is the Terraserver website located at http://terraserver.microsoft.com/ (Search term: terraserver). This site contains recent aerial photographs that have been georeferenced. The NOAA National Ocean Service data explorer website (http://oceanservice.noaa.gov/dataexplorer/ or search "NOAA data explorer") with access to aerial photographs, navigational charts and coastal survey maps. Historic mapping can be obtained from various government agencies, including the USGS EROS Data Center and USDA Farm Service Agency.