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Nicholas C Kraus.

Needs assessment for water-level gauging along the Texas coast for the U.S. Army Engineer District, Galveston online

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Chapter 6 contains a summary with main conclusions and
recommendations. Appendices contain listings of detailed information
developed in this needs assessment.



Chapter 1 Introduction



2 Water Level Definition,
Measurement, and
Properties



This chapter introduces basic nomenclature related to tidal datums and
water-level measurement. Some typical characteristics of the water level
observed on the Texas coast are compared to those from tide records on
the Atlantic Ocean and Pacific Ocean coasts of the United States. The
material is intended to provide a background for further technical
discussion and to serve as motivation for establishing an appropriate
navigation datum for the inland coastal waterways of Texas.

Historical Setting

Tidal datums and their application have been an issue in the United
States since the founding of the nation. Both the need to establish a line of
delineation between the land and sea interface and the appropriate method
for determining the proper elevation were addressed early by the nation.

In 1807, the U.S. Congress assigned to the Coast and Geodetic Survey
the tasks of delineating the sea-land interface and displaying it on charts
(Shalowitz 1964). That agency, apparently having most experience on the
U.S. east coast (where the astronomical tide is very regular), concluded
that the most appropriate way of determining that interface or line would
be by measurements of the rise and fall of the tide. This procedure
entailed setting of a tide gauge, which was connected by differential
leveling to fixed points on the land that could be used by engineers to
survey the sea-land interface for charting. These procedures have



Chapter 2 Water Level Definition, Measurement, and Properties



essentially continued, with some modification, throughout the history of
the country. Eventually, a national network of permanently operating
water-level measurement stations was established to support the charting
activity.

Long-term tidal measurements from the U.S. Coast and Geodetic
Survey were the basis for establishing a boundary indicating ownership
between private upland and sovereign submerged lands (the landmark case
Borax Consolidated, Ltd. vs. Los Angeles, circa 1935; see Shalowitz
(1964)). The result of this decision by the U.S. Supreme Court was
carried over by the state courts to be the accepted method for dealing with
boundary issues. An 18.6-year water-level record, which corresponds to a
complete cycle of progression of the moon's nodes along the ecliptic
(described by Meton in the fifth century before Christ), is believed to
account for all the astronomical variations in the ranges of the tide.

Also, it was believed for many years that the tides rose and fell
periodically and the application of a mean high water was appropriate for
all coastal waters. However, as more long-term tide stations have been
operated throughout the United States, we have found that the
astronomical gravitation is not always dominant. The tidal force is
frequently exceeded by meteorological forces, acting both over the long
term (seasons) and in the short term (hours and days through strong winds
and changes in air pressure accompanying passage of weather fronts).
Meteorological forcing can be particularly significant in shallow-water
bodies, such as along the Texas coast. Observations from the TCOON
have clearly shown that tidal datums established through traditional
procedures are not well-suited within the bays and estuaries along the
Texas coast for boundary, regulatory, or navigational purposes.

Water Level and Tidal Datums

This section reviews key concepts and notation relevant to the needs
assessment. Definitions of tidal datums and associated nomenclature and
concepts can by found in the NOS "Tide and Current Glossary" (Hicks
1989), in a US ACE Special Report (Harris 1981), in the US ACE Engineer
Manual entitled "Water Levels and Wave Heights for Coastal Engineering
Design" (US ACE 1989), and in the EM entitled "Hydrographic

1 Chapter 2 Water Level Definition, Measurement, and Properties



Surveying" (US ACE 1991). A concise but authoritative overview of tidal
datums and their various uses is given in Hicks (1 985, 1986).

Basic concepts of tides and tide measurement

The tide is defined by NOS as the periodic rise and fall of the water
resulting from gravitational interactions between the sun, moon, and earth
(Hicks 1989). The term "tide" refers to the astronomically forced changes
in water level, which is deterministic or predictable. Astronomical or
daily tide should be distinguished from other contributions to changes in
water level, including meteorological forcing (seasonal changes in sea
level, changes caused by daily wind or to passage of weather fronts, etc.),
terrestrial inputs (rain runoff or river discharge), and atmospheric forcing
(change in air pressure). The distinguishing of periodicity produced by
astronomical forcing and by diurnal and longer-term wind forcing, for
example, has not been well addressed in tidal datum definition. Similarly,
the distinguishing of variation in daily water (tidal) level and in annual
water elevation does not seem to have been well addressed in the United
States with regard to definition or determination of a navigation datum. In
the following, standard NOS definitions for selected tidal datums are given
for reference in this report.

Low water is the minimum height reached by a falling tide. Low
water occurs according to the periodic tidal forces and the acting
meteorological, hydrologic, and oceanographic conditions. For tidal
datum computation, the minimum height is not considered a low water
unless it contains a tidal low water (Hicks 1989).

Mean high water (mhw) and mean low water (mlw) are,
respectively, the averages of all the high- water heights and low- water
heights observed over the NTDE. For stations with time series shorter
than 19 years, simultaneous comparisons with a control station are made
to determine the equivalent datum of the NTDE. Also, strictly speaking,
to be counted as a high, each high water must be 0.10 ft or more above,
and must occur 2 hr or more later than the adjacent low waters.
Analogous considerations hold for definition of low waters.

The mean range of tide, denoted as "Mn" by NOS, is the difference in
height between mhw and mlw. Thus, Mn corresponds to a tidal range

Chapter 2 Water Level Definition, Measurement, and Properties 1 1



defined in terms of daily highs and lows and does not include such
changes as a seasonal variation in water elevation. Note that Mn is not a
tidal datum because it is not an elevation or level, but a range. Mean Tide
Level (mtl) or "half tide level" is a tidal datum and is the arithmetic mean
of mhw and mlw. The mtl may lie above or below msl by an amount
depending on the relative amplitudes of the diurnal (having a period of
approximately 1 lunar day) components of the tide.

Mean higher high water (mhhw) and mean lower low water (mllw)
are, respectively, the averages of the higher high water heights and lower
low water heights of each tidal day (or lunar day; 24.84 solar hr) observed
over the NTDE. For example, if there are two lows in a tidal day, the
lower of the two is used to compute mllw for that day. The mllw is
presently used by NOS as a nautical chart datum.

Tide gauge and tide station

A tide gauge, or water-level gauge in a more general sense, is an
instrument for measuring the rise and fall of the tide or water level.
Presently, acoustic gauges and pressure gauges are used as automated
water-level measurement systems because of their reliability and
capability to produce an electronic signal for convenient processing.
These measurement systems also require little power and are suited for
remote sites where the power unit must be self-contained. A tide staff is a
simple form of tide gauge and consists of a vertical graduated staff from
which the height of the water can be read visually.

A tide station, or water-level station in a more general sense, is the
geographic location at which tidal or water-level observations have been
made. The defining aspect of a tide station is the presence of a system of
(tidal) benchmarks, which are fixed physical objects (typically, deep-
driven rods) that serve as a reference for a vertical datum. The tidal
benchmarks are central to water-level measurement because the relation
between elevation of the land and water can be tracked through time, the
benchmarks providing the fixed reference (in the absence of subsidence or
crustal movement in general). Water-level measurement without reference
to benchmarks cannot produce recoverable tidal datums.

1 2 Chapter 2 Water Level Definition, Measurement, and Properties



Zoning

Zoning is a procedure in which the project area of a hydrographic
survey is divided into sections or "zones" to account for differences in
tidal ranges and phases within the area. In the hydrographic survey,
adjustments or "reducers" are applied to the depth soundings, which are
taken at all stages of the tide, to reference them to a common datum. The
correction must also allow for the time differences between the survey area
and the water-level gauge used for the control. Tidal zones are
distinguished by keeping two properties, range of tide and phase of tide,
within certain limits, typically 0.2 ft and 0.2 hr, respectively (Hicks 1989).
However, as stated by Hicks (1989), "these limits are subject to change,
depending upon survey accuracy, location, and tidal characteristics." In
the case of Texas inland coastal waters, not only the astronomical tide, but
also wind forcing, must be considered in the zoning.

Character of the Tide Along the Texas Coast

Characteristics of the tide along much of the Gulf of Mexico are
different than those on the Atlantic Ocean and Pacific Ocean coasts. Even
within the Gulf of Mexico, tidal characteristics are variable from one
locality to another within short distances. Prior to the 1950s, there were
only a few locations where long-term tidal measurements had been made.
Although diurnal high and low waters occur only during the extreme north
and south declinations of the moon, the tides were classified as diurnal in
character. The mlw was therefore used as the reference datum for
hydrographic surveys.

As more systematic tidal observations were made in the coastal waters
of the Gulf of Mexico, it became evident that the classification of semi-
diurnal tides for the entire Gulf was not defensible. Although many areas
have two high and two low waters most of the time, there are nearby areas
where only one high and one low water occur. This observation led to the
creation of the Gulf Coast Low Water Datum (GCLWD) for the NOS
Nautical Charts. The GCLWD is defined as: mllw when the type of tide
is mixed, and mlw when the type of tide is diurnal. The National Tidal
Datum Convention of 1980 (Hicks 1980) established one uniform,

Chapter 2 Water Level Definition, Measurement, and Properties 1 3



continuous tidal datum system for all marine waters of the United States
and replaced the GCLWD by mllw, among several other actions.

Figures 2-7 are plots of the year-long hourly water-level records at
representative tide gauges on the Pacific, Atlantic, and Gulf coasts. These
include two inland coastal stations of Texas, the South Padre Island Coast
Guard Station located at the southern end of the Laguna Madre, and
Packery Channel located at the northern end of the Laguna Madre near its
entrance to Corpus Christi Bay. The figures are plotted to the mllw datum
of each station, and the respective values of mhw and mlw are given in the
upper right corners of the plots.

Figure 2 for Los Angeles, California, shows a clear signal of the
approximate 14-day neap and spring tide cycle corresponding to the
phases of the moon. By its definition as an average, there must be times
when the water level falls below mllw; for Los Angeles, this occurs at a
fairly regular periodic cycle corresponding to the times of spring tide,
when the moon is either new or full. Ships waiting to proceed into or
leave a port could consult tide predictions with reasonable confidence to
determine safe clearance under keel for a given draft according to load.
Overall, the water level at Los Angeles is fairly symmetric with respect to
the mtl. Little seasonal trend appears in the yearlong record.

Figures 3 and 4 show yearlong records for Sandy Hook, New Jersey,
and Fernandina, Florida. The water level is not as pronounced in spring-
neap cycle as is that of Los Angeles, in part due to the mixed character of
the tide and also to the action of storms on the gently sloping continental
shelf of the Atlantic coast as compared to the Pacific coast. Water level
falls below mllw in a somewhat less regular manner than at Los Angeles.
A slight seasonal trend of lower waters in winter is observed. For the
Atlantic- and Pacific-coast gauge records, the body of measurements stays
well and regularly within the mean range of tide defined by the mhw and
mlw lines drawn in the figures. At both locations, the deviation below
mllw is typically 20 to 25 percent of the range.

In contrast to the Atlantic- and Pacific-coast water-level records, the
record from Bob Hall Pier, Corpus Christi, Texas, on the Gulf of Mexico
(Figure 5), is irregular, with the seasonal trend change in water elevation
comparable to or exceeding the daily change in tide. Further, the deviation

14 Chapter 2 Water Level Definition, Measurement, and Properties



below mllw is typically on the order of 50 percent of the range and
sometimes equals or approaches the range in 1995. Although a spring-
neap cycle is evident, local nonastronomical tidal forcing makes the cycle
appear irregular. Similar observations hold for the South Padre Island
Coast Guard Station gauge, which is located near the Brazos-Santiago
Pass and ship channel (Figure 6).

Figure 7 displays a year-long record for Packery Channel, which is
located just off the GIWW in Corpus Christi Bay, on the bay side of Padre
Island and nearly opposite to the Bob Hall tide gauge. Strong seasonality
in the water level is apparent, and, in 1995, the water level remained below
mllw for approximately 1 month starting in mid- January. Further, the
deviation in water level below mllw was typically equal to the mean range
of tide and even reached twice the mean range below mllw. This record
indicates a need for a navigation datum that accounts for seasonal lows
and that is not based on a simple average of daily water-level lows.



Chapter 2 Water Level Definition, Measurement, and Properties 1 5




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3 Present Water-Level
Measurement Networks
Along the Texas Coast



This chapter summarizes the status of TCOON water-level
measurement and of CESWG water-level measurement as practiced by its
field offices and hydrographic survey parties. The status is current to
September 1996, with some updates. The summary introduces basic
District operational needs as an outcome, which would include the needs
of Government contractors.

TCOON Capabilities

The TCOON presently consists of approximately 40 water-level
stations. On or near the coast, eight NOS gauges serve as long-term
control stations. Table Al in Appendix A summarizes the status of the
TCOON and NOS tide stations.

Present Galveston District Field Practice and
Identified Needs

The CESWG tide staffs are presently central to its hydrographic
surveys. Examples of two staffs are given in Figures 8 and 9. It is
standard procedure to take the survey boat to a tide staff to record water
level at the start of the day's survey (typically early morning), mid-
morning, noon, mid-afternoon, and when quitting for the day. At the tide
staff, the survey boat will check its echo sounder and note the elevation of
the water on the staff and the time. In surveys of long channels, during a



22 Chapter 3 Present Networks Along the Texas Coast



typical survey day, the boat may have to visit several different tide staffs
located along the channel, depending on the rate of coverage of an area,, or
station a survey crew member at the nearest staff to regularly report water
elevation via radio. A high density of staffs in some water bodies
probably reflects as much the need to reduce transit time to and from staff
visits as recognition that the water-level elevation changes along a water
body.

Boat visits to the staffs can consume substantial time in going off
station, remaining at the staff for several minutes, and then returning on
station. An operational problem sometimes encountered is difficulty in
visually reading the tide staff in moderate waves and wind, which can
cause trouble in keeping the boat stationary at the staff.

The tide staffs, which are typically mounted on timber survey
platforms, channel markers, bulkheads, etc., are routinely replaced as the
mounting structures are destroyed by barges or other vessels. Re-
installation of the staffs at the correct elevation often requires performance
of a level survey (if the tide station is near land or a survey table) or
knowledge of the water level based on a reading made at a nearby staff.
Figure 10 shows a staff being replaced by a Southern Area Office
surveyor.

In discussions with CESWG hydrographic survey party personnel,
several operational needs were identified. Survey boats could remain on
station and continue surveying if the operators could simply phone to a
gauge and obtain the water level. The information reported by the gauge
must be in standard units (feet, local time), and a repeat option should be
available because of occasional noise or loss of attention due to other
duties on the survey boat. The water level should be reported in terms of
the CESWG navigation datum (presently mtl), or a table for converting
from the reporting datum to the navigation datum should be provided (e.g.,
mllw to mlt).



Chapter 3 Present Networks Along the Texas Coast



23




«•> - ._ ■».



Figure 8. Tide staff mounted on survey table at Channel to Victoria (see Staff 8,
Chart G, Appendix B). A dredging contractor's staff is mounted on the
third pile from left




Figure 9. Freeport Entrance Channel at the U.S. Coast Guard Station, Surfside

(see Staff 3, Chart E, Appendix B). The staff on the left is referenced to mlt,
and the staff on the right is referenced to msl



24



Chapter 3 Present Networks Along the Texas Coast




*% 1 ( l 1




Figure 10. Staff being replaced at Fulton Harbor public boat ramp, Aransas Bay
(see Staff 5, Chart G, Appendix B)



At the office, the survey parties would benefit from ready availability
of a hard copy of the water-level record during the time they were onsite
for confirmation of the voice record and for documentation. The most
convenient way would be through facsimile; however, they would have
interest in downloading via the Internet, such as from a World Wide Web
(WWW) site, if they received training and equipment to do so.

In addition, the CESWG and its field offices are routinely asked to
provide water-level data or these offices themselves need water-level data
for varied reasons, including response and analysis of boating accidents
and responses to oil spills. Convenient access to hard copy or archived
data reaching back several months would usually satisfy most such needs.

Finally, interest was expressed in certain topics in related areas. A
primer (handbook) providing explanations of water levels and their inter-
relations was thought to be useful if not essential. The water-level
information should be given in local time and to the navigation datum, not,
say, to Greenwich Time or to an arbitrary staff datum that requires further
conversion. Information on wind and current, although not typically

Chapter 3 Present Networks Along the Texas Coast



25



entering directly in routine surveying operations, was sometimes needed
for evaluating claims of bad weather by dredging contractors and for
analyzing the circumstances concerning boating accidents.

In the offshore, information on wave height, period, and direction, as
well as water level, would be valuable for confirmation of weather days
that halt dredging. Dredgers sometimes appear to have to halt due to
longer period wave motions than just short-period surface waves; a wave
gauge might be able to determine the conditions when this problem
occurs. Finally, some interest was expressed in being able to obtain
predictions of the tide, both for planning routine operations and for
emergency planning during hurricanes and storms.

Locations of Galveston District Tide Staffs

After creating preliminary maps showing general locations of tide
staffs based on information provided by the CESWG Area Offices,
13 days of field reconnaissance were conducted during September and
October 1996, of the CESWG tide staffs from Port Arthur to Brownsville.
The locations (geographic coordinates and description) of the staffs were
determined by differential global positioning system (DGPS) or global
positioning system (GPS) survey, depending on satellite coverage at the
time the particular stations were visited.

A list of the locations of 166 CESWG tide staffs identified during the
survey is contained in Appendix B. In several cases, the tide staffs were
missing at the time of the survey, and station coordinates (for staffs to be
later replaced) were obtained based on knowledge of the former staff
location as provided by the Area Office field personnel who served as
guides during the field visits. Maps showing tide station locations were
generated as shown in Charts A-J in Chapter 5 and were designed to cover
areas of the coast corresponding to areas covered by specific NOAA
charts.



26 Chapter 3 Present Networks Along the Texas Coast



MLT and NGVD Difference

The differences between the mlt datum and NGVD 29 are listed in
Table 1, as provided by personnel of CESWG. Many of these differences
were determined in the 1960s and 1970s.



Table 1

Elevation Difference Between mlt and NGVD 29


Water Body


Reach


mlt as Feet Below
NGVD 29


GIWW-Main Channel


Port Arthur - High Island


1.1


GIWW-Main Channel


High Island - Bolivar


0.5


GIWW-Main Channel


Galveston - Matagorda


1.4


GIWW-Main Channel


Matagorda - Port Mansfield


1.0


GIWW-Main Channel


Port Mansfield - Port Isabel


0.9


Sabine Neches Waterway


Sabine - Orange


0.8


Houston Ship Channel


Bolivar - Lynchburg


1.4


Houston Ship Channel


Lynchburg - Main Street


1.1


Cedar Bayou


Cedar Bayou Channel


1.4


Trinity River


Trinity River Channel


1.0 |


Ear Creek


Ear Creek Channel


1.3


Dickinson Bayou


Dickinson Bayou Channel


1.4


Galveston Bay


Galveston Harbor - Texas City Chnl.


1.4


Chocolate Bayou


Chocolate Bayou Channel


1.4


Bastrop Bayou


Bastrop Bayou Channel


1.4


Freeport Harbor


Freeport Harbor


1.4


Brazos River - Colorado River


Brazos River Floodgates - Colorado River
Locks


1.4


Palacious - La Quinta Channel


Palacious - La Quinta


1-0


Brazos Island Harbor


Brazos Island Harbor - Brownsville


0.9


Port Isabel


Port Isabel


0.9



Chapter 3 Present Networks Along the Texas Coast



27



4 Communications
Alternatives



This chapter is concerned with communications alternatives for real-
time reporting and for access to archived data in a database. New
equipment and capabilities are constantly reaching the market, so an
existing-market search would need to be done at time of implementation.

Real-Time Reporting

Table 2 compares alternatives presently available for real-time voice
reporting of water level and related information. Certain instrument
manufacturers sell proprietary turnkey measurement systems that include
radio communication, which were not investigated here.



Table 2


2 4 5 6 7

Online LibraryNicholas C KrausNeeds assessment for water-level gauging along the Texas coast for the U.S. Army Engineer District, Galveston → online text (page 2 of 7)