NOAA Technical Report NOS CO-OPS 041

NORTH CAROLINA BATHYMETRY/TOPOGRAPHY SEA LEVEL

RISE PROJECT: DETERMINATION OF SEA LEVEL TRENDS

Silver Spring, Maryland

May 2004

noaa National Oceanic and Atmospheric Administration

U.S. DEPARTMENT OF COMMERCE

National Ocean Service

Center for Operational Oceanographic Products and Services
Center for Operational Oceanographic Products and Services

National Ocean Service

National Oceanic and Atmospheric Administration

U.S. Department of Commerce

The National Ocean Service (NOS) Center for Operational Oceanographic Products and

Services (CO-OPS) collects and distributes observations and predictions of water levels

and currents to ensure safe, efficient and environmentally sound maritime commerce.

The Center provides the set of water level and coastal current products required to

support NOS' Strategic Plan mission requirements, and assists in providing operational

oceanographic data/products required by NOAA's other Strategic Plan themes. For

example, CO-OPS provides data and products required bythe National Weather Service

to meet its flood and tsunami warning responsibilities. The Center manages the

National Water Level Observation Network (NWLON), and a national network of

Physical Oceanographic Real-Time Systems (PORTS) in major U.S. harbors. The

Center establishes standards for the collection and processing of water level and current

data; collects and documents user requirements which serve as the foundation for all

resulting program activities; designs new and/or improved oceanographic observing

systems; designs software to improve CO-OPS' data processing capabilities; maintains

and operates oceanographic observing systems; performs operational data

analysis/quality control; and produces/disseminates oceanographic products.
NOAA Technical Report NOS CO-OPS 041

NORTH CAROLINA BATHYMETRY/TOPOGRAPHY SEA LEVEL

RISE PROJECT: DETERMINATION OF SEA LEVEL TRENDS

Chris Zervas

May 2004

noaa National Oceanic and Atmospheric Administration

U.S. DEPARTMENT National Oceanic and National Ocean Service

OF COMMERCE Atmospheric Administration Richard W. Spinrad 

Donald Evans Secretary Vice Admiral Conrad C. Lautenbacher Assistant Administrator

Under Secretary

Center for Operational Oceanographic Products and Services

Director, Mike Szabados
ii

NOTICE

Mention of a commercial company or product does not constitute an

endorsement by NOAA. Use for publicity or advertising purposes of

information from this publication concerning proprietary products or the

tests of such products is not authorized.
iii

TABLE OF CONTENTS

Introduction ............................................................. 1

Mean Sea Level Trends .................................................... 5

Average Seasonal Cycle .................................................... 9

Variability of 50-Year Mean Sea Level Trends at Wilmington . . . . . . . . . . . . . . . . . . . . . . 15

Changes in Tidal Range .................................................... 17

Changes in the M2 Tidal Constituent ......................................... 21

Tidal Datum Trends ....................................................... 23

Summary ............................................................... 27

Acknowledgments ........................................................ 29

References .............................................................. 31
iv

LIST OF FIGURES

Figure 1. Location of eight CO-OPS water level stations in North Carolina . . . . . . . . . . . . 3

Figure 2. Monthly mean sea level for Wilmington, NC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Figure 3. Mean sea level trends and 95% confidence intervals for North Carolina water

level stations...................................................... 5

Figure 4. Mean sea level trends and 95% confidence intervals for North Carolina water

level stations with trends for Oregon Inlet Marina, Atlantic Beach, and Yaupon

Beach based on water level differences with Beaufort . . . . . . . . . . . . . . . . . . . . . 7

Figure 5. Average seasonal cycle of mean sea level for Duck . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 6. Average seasonal cycle of mean sea level for Oregon Inlet Marina . . . . . . . . . . . 10

Figure 7. Average seasonal cycle of mean sea level for Cape Hatteras . . . . . . . . . . . . . . . . 10

Figure 8. Average seasonal cycle of mean sea level for Beaufort . . . . . . . . . . . . . . . . . . . . . 11

Figure 9. Average seasonal cycle of mean sea level for Atlantic Beach . . . . . . . . . . . . . . . . 11

Figure 10. Average seasonal cycle of mean sea level for Wilmington . . . . . . . . . . . . . . . . . . 12

Figure 11. Average seasonal cycle of mean sea level for Southport . . . . . . . . . . . . . . . . . . . . 12

Figure 12. Average seasonal cycle of mean sea level for Yaupon Beach . . . . . . . . . . . . . . . . 13

Figure 13. Mean sea level trends for Wilmington based on the entire data set and for 

selected 50-year periods ............................................. 15

Figure 14. Monthly mean range of tide and great diurnal range of tide for Wilmington, NC 17

Figure 15. Trends in mean range of tide for North Carolina water level stations . . . . . . . . . . 18

Figure 16. Trends in great diurnal range of tide for North Carolina water level stations . . . . 19

Figure 17. Comparison of trends for tidal datums at Oregon Inlet Marina . . . . . . . . . . . . . . . 24

Figure 18. Comparison of trends for tidal datums at Beaufort . . . . . . . . . . . . . . . . . . . . . . . . 24

Figure 19. Comparison of trends for tidal datums at Wilmington . . . . . . . . . . . . . . . . . . . . . 25
v

LIST OF TABLES

Table 1. CO-OPS water level stations in North Carolina . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Table 2. Mean sea level trends for North Carolina water level stations . . . . . . . . . . . . . . . . . 6

Table 3. Changes in amplitude and range of the M2 tidal constituent . . . . . . . . . . . . . . . . . . 21

Table 4. Tidal datum trends for North Carolina water level stations . . . . . . . . . . . . . . . . . . . 23

1

INTRODUCTION

NOAA’s Center for Sponsored Coastal Ocean Research (CSCOR) recently initiated a program

entitled “Ecological Impacts of Sea Level Rise”. This program is intended to evaluate the potential

impact of long-term sea level rise on a specific coastal region, 25 to 50 years in the future. An initial

goal of the project is to generate a digital elevation model (DEM) for the bathymetry and topography

of the North Carolina coastal region, with all elevations tied to a common datum. Present-day sea

level trends will be extended into the future, raising the tidal datums and thereby shifting the position

of the shoreline. For North Carolina, the shoreline is defined as the mean high water (MHW) tidal

datum (NOS, 2001). Maps will be generated indicating which low-lying areas of the coastal region

will be submerged. A prerequisite for this project is the accurate determination of sea level trends

using historic data at as many coastal locations as possible.

NOAA’s Center for Operational Oceanographic Products and Services (CO-OPS) holds historic

North Carolina water level data tied to established tidal bench marks. These records extend as far

back as 1933 for Southport and 1935 for Wilmington. Eight stations have data spanning a period

of at least 20 years and therefore are good candidates for the determination of mean sea level (MSL)

trends with reasonable confidence intervals (Table 1). 

The stations at Duck, Cape Hatteras, Atlantic Beach, and Yaupon Beach are located directly on the

Atlantic Ocean (Figure 1). The stations at Oregon Inlet Marina, Beaufort, and Southport are located

behind barrier islands a short distance from the ocean. The station at Southport is at the entrance to

the Cape Fear River. The station at Wilmington is located on the Cape Fear River approximately

40 km from the ocean.

CO-OPS has recently adopted a new National Tidal Datum Epoch of 1983-2001 (NOAA, 2003).

New tidal datums are now available which supercede the 1960-1978 tidal datums. The level of the

new MSL datum relative to the North American Vertical Datum of 1988 (NAVD 88) is given in

Table 1 for the five stations that are connected to NAVD 88.

The monthly mean sea levels at Wilmington for the period 1935-2002 are shown in Figure 2 relative

to the recently established 1983-2001 MSL datum. The stations at Duck, Cape Hatteras, Beaufort,

and Wilmington do not have any long periods without data (Table 1). Oregon Inlet Marina, Atlantic

Beach, Southport, and Yaupon Beach have large gaps in their data records. Confidence intervals for

the trends at the latter stations may be narrowed by difference comparisons with stations having more

complete data records.

A recent study of tidal ranges at CO-OPS coastal water level stations with over 25 years of data

(Zervas, 2003) indicated that only four out of 116 stations had large and statistically significant

increases in tidal range. Two of these stations, Wilmington and Beaufort, were in North Carolina.

The other two were Philadelphia, PA, and Anchorage, AK. The increasing tidal ranges at

Wilmington and Beaufort may be attributed to the continual deepening of the navigational channels

to these ports by the U.S. Army Corps of Engineers. Because of these increases in tidal range, trends

calculated for the tidal datums of mean higher high water (MHHW), mean high water (MHW), mean

low water (MLW), and mean lower low water (MLLW) were significantly different from each other
2

and/or from the MSL trend. In this report, trends in the mean range of tide (Mn) and the great

diurnal range of tide (Gt) will be calculated for all of the eight North Carolina stations. For any

station showing a large increase in tidal range, trends will be obtained for each of the tidal datums

(i.e., MHHW, MHW, MLW, and MLLW). 

Table 1. CO-OPS water level stations in North Carolina with data spanning over 20 years

Station

Number Station Name Periods of Data 1983-2001 MSL relativeto NAVD 88 (meters)

8651370 Duck 1978-2002 -0.128

8652587 Oregon Inlet Marina 1977-1980, 1994-2002 -0.026

8654400 Cape Hatteras 1978-2002 -0.135

8656483 Beaufort 1973-2002 N/A

8656590 Atlantic Beach 1977-1983, 1998-2000 N/A

8658120 Wilmington 1935-2002 0.01

8659084 Southport 1933-1954, 1976-1988 -0.141

8659182 Yaupon Beach 1977-1978, 1996-1997 N/A
3

Figure 1. Location of eight CO-OPS water level stations in North Carolina with long term data

sets.

Figure 2. Monthly mean sea level for Wilmington, NC relative to the 1983-2001 mean sea level

datum.

5

MEAN SEA LEVEL TRENDS

The trend for a monthly water level time series is obtained by means of a linear regression with an

autoregressive coefficient and a seasonal cycle as described in Zervas (2001). The seasonal cycle

is accounted for by twelve parameters, one for each month. The autoregressive coefficient is

included to account for the serial correlation of the residual time series. It has little effect on the

value of the trend, but results in a more accurate estimate of the uncertainty of the trend. All results

will be displayed with a 95% confidence interval which is +/- 1.96 times the standard error.

Mean sea level trends calculated for the eight North Carolina stations are displayed in Figure 3 and

Table 2. The stations with the longest data intervals (68 years at Wilmington and 56 years at

Southport) have the narrowest confidence intervals. The trends range from 2.04 mm/yr at Southport

to 4.27 mm/yr at Duck. The average for all eight North Carolina stations is 2.88 mm/yr. There

appears to be a regional gradient with the trends increasing from south to north. This implies that

the land is sinking more rapidly along the northern portion of the coastline.

In an effort to obtain better-constrained MSL trends for Oregon Inlet Marina, Atlantic Beach,

Southport, and Yaupon Beach, the monthly water level differences between these stations and the

stations at Beaufort and Wilmington were analyzed. The seasonal and interannual water level

variations are highly correlated along the Atlantic coastline (Zervas, 2001). Therefore, water level

differences between two stations have much less variability than the water level time series at any

Figure 3. Mean sea level trends and 95% confidence intervals for North Carolina water level

stations from north (Duck) to south (Yaupon Beach).
6

one station. A determination of the trend in a water level difference time series should produce a

well-constrained estimate of the difference in trend between the two stations. Then the difference

in trend can be added to the trend determined from the longer data record to get the trend at the

station with the shorter data record (Mitchell et al., 1994). The water level difference trends are

determined by the same method described for determining water level trends (i.e., linear regression

with an autoregressive coefficient and a seasonal cycle). The trend standard error at the station with

the shorter data record is the square root of the sum of the squares of the trend standard errors at the

station with the longer record and the water level difference. 

Table 2. Mean sea level trends for North Carolina water level stations in mm/yr

Station

Number

Station 

Name

Based on water level Based on water level differences

MSL

Trend

Standard

Error

Differenced

from

MSL

Trend

Standard

Error

8651370 Duck 4.27 0.74

8652587 Oregon Inlet Marina 2.55 1.21 Beaufort 3.11 0.91

8654400 Cape Hatteras 3.46 0.75

8656483 Beaufort 3.2 0.54

8656590 Atlantic Beach 2.48 1.99 Beaufort 1.93 1.29

8658120 Wilmington 2.12 0.23

8659084 Southport 2.04 0.25 Wilmington 2.21 0.3

8659182 Yaupon Beach 2.92 0.77 Beaufort 1.79 0.76

The better-constrained MSL trends from water level difference analysis for the stations with shorter

data records were compared with the MSL trends for the stations with longer data records (Table 2

and Figure 4). The best estimates for the trends at Oregon Inlet Marina, Atlantic Beach, and Yaupon

Beach were from difference comparisons with Beaufort. The older data at Southport (1933-1954)

could only be differenced with Wilmington; the resulting trend standard error was greater than that

obtained with the Southport data alone. The trends range from 1.79 mm/yr at Yaupon Beach to 4.27

mm/yr at Duck. The average for all eight North Carolina stations is 2.74 mm/yr. These trends are

similar to those obtained at other stations on the U.S. east coast (Zervas, 2001), but are less than the

rapidly rising MSLs observed in Louisiana (9 to 10 mm/yr) and eastern Texas (5 to 8 mm/yr).
7

Figure 4. Mean sea level trends and 95% confidence intervals for North Carolina water level

stations with trends for Oregon Inlet Marina, Atlantic Beach, and Yaupon Beach based on water

level differences with Beaufort. 

9

AVERAGE SEASONAL CYCLE

In obtaining the mean sea level trends discussed in the previous section, the average seasonal cycle

is also produced. The average monthly MSLs for the North Carolina water level stations are

displayed in Figures 5-12. For most stations, the data used were relative to the 1983-2001 MSL

datum. However, for Atlantic Beach and Yaupon Beach, the 1983-2001 MSL datums were not

available and the seasonal cycles are presented relative to the arbitrary station datums. 

It can be seen that the seasonal cycle is very similar at all stations with the highest monthly water

levels of the year occurring in September and October and the lowest monthly water levels of the

year occurring in January and February (December and January for Wilmington). Atlantic Beach

and Yaupon Beach have large confidence intervals because of limited data. The seasonal cycle for

Yaupon Beach is based on only 26 months of data.

Figure 5. Average seasonal cycle of mean sea level for Duck in meters.
10

Figure 6. Average seasonal cycle of mean sea level for Oregon Inlet Marina in meters.

Figure 7. Average seasonal cycle of mean sea level for Cape Hatteras in meters.
11

Figure 8. Average seasonal cycle of mean sea level for Beaufort in meters.

Figure 9. Average seasonal cycle of mean sea level for Atlantic Beach in meters.
12

Figure 10. Average seasonal cycle of mean sea level for Wilmington in meters.

Figure 11. Average seasonal cycle of mean sea level for Southport in meters.
13

Figure 12. Average seasonal cycle of mean sea level for Yaupon Beach in meters.

15

VARIABILITY OF 50-YEAR MEAN SEA LEVEL TRENDS AT WILMINGTON

Since the data record at Wilmington is 68 years long, the 95% confidence interval for the mean sea

level trend is small (less than 1 mm/yr). In order to determine whether any decadal variations in

trend may have occurred over that period, mean sea level trends were calculated for overlapping 50-

year subsets of the data. These results are shown in Figure 13. Although there is some variability

in the trends obtained, due to several years of anomalously high water levels in the late 1940s, the

95% confidence intervals indicate that the differences are not statistically significant.

Figure 13. Mean sea level trends for Wilmington based on the entire data set (1935-2002) and

for selected 50-year periods.

17

CHANGES IN TIDAL RANGE

In most cases, it can be assumed that there will be no significant differences between the trend of the

mean sea level and the trends of all the tidal datums such as MHHW, MHW, MLW, MLLW.

However, a recent study (Zervas, 2003), shows that both Wilmington and Beaufort have undergone

significant increases in tidal range since the stations were installed. Both stations are at some

distance from the ocean, and the navigational channels connecting these ports to the ocean have been

widened and deepened over the years by the U.S. Army Corps of Engineers. At this time, the Cape

Fear River, connecting Wilmington to the ocean, is being deepened from 38 feet to 42 feet and the

ocean bar and entrance channel is being deepened from 40 feet to 44 feet (Hackney, 2002). The

mean range of tide (Mn = MHW - MLW) and the great diurnal range of tide (Gt = MHHW - MLLW)

will be analyzed for the eight North Carolina CO-OPS stations to determine whether any of the other

stations are also being affected by increasing tidal ranges. 

The Mn and Gt data have a prominent 18.6-year astronomical tidal cycle related to the obliquity of

the moon’s node (Figure 14). Any calculated trend for tidal range should be based on data spanning

at least one nodal cycle. Fortunately, all of the stations except Southport have data from the late

1970s and the late 1990s when the positive effect ofthe nodal cycle on semidiurnal tidal constituents

was the greatest. Southport has Mn data available from 1933-1954 spanning an entire nodal cycle.

The trends for the tidal ranges are obtained by linear regression with an autoregressive coefficient,

a 12-month seasonal cycle, and the 18.6-year astronomical cycle.

Figure 15 shows the Mn trends. The three stations with large and statistically significant increases

in the mean range of tide are Oregon Inlet Marina, Beaufort, and Wilmington (greater than 3 mm/yr).

The stations directly on the Atlantic Ocean (Duck, Cape Hatteras, Atlantic Beach, and Yaupon

Beach) do not have large changes in the mean range of tide. Southport, just inside the entrance to

the Cape Fear River, has had only a small increase in Mn tidal range.

Wilmington

Figure 14. Monthly mean range of tide (lower line) and great diurnal range of tide (upper line)

for Wilmington, NC. 
18

Figure 16 shows the Gt trends. The only large and statistically significant trends are at Oregon Inlet

Marina and Beaufort. Duck, Cape Hatteras, Atlantic Beach, and Yaupon Beach, all directly on the

Atlantic Ocean, show no significant change in Gt. Southport is not shown because Gt values (or

MHHW and MLLW values) were not tabulated for U.S. east coast stations before the 1970s and the

data ends in 1988. For Wilmington, the Gt values only go back to 1973. The calculated Gt trend

for Wilmington is small and barely significant. This is explained by the Mn record for Wilmington

(Figure 14), which shows a non-linearly increasing tidal range with most of the increase taking place

before the 1970s, followed by an apparent leveling off in the trend since the 1970s. The ongoing

dredging project in the Cape Fear River is expected to lead to further increases in tidal range

(Hackney, 2002).

Figure 15. Trends (mm/yr) in mean range of tide for North Carolina water level stations.
19

Figure 16. Trends (mm/yr) in great diurnal range of tide for North Carolina water level stations.

21

CHANGES IN THE M2 TIDAL CONSTITUENT

The M2 semidiurnal tidal constituent, with a period of 12.42 hours, is the largest amplitude tidal

constituent at U.S. east coast water level stations. It can be obtained by the harmonic analysis of

hourly water level data. The resulting tidal amplitudes are adjusted for the timing of the analyzed

data within the 18.6-year astronomical tidal cycle. Harmonic analyses were carried out for hourly

data near the installation dates of seven of the stations and for the most recent data to verify that the

large increases in Mn and/or Gt at Oregon Inlet Marina, Beaufort, and Wilmington are caused by

increased tidal amplitudes. No hourly water level data were available for Southport from 1933-1954.

The increases in the amplitude of M2 are shown in Table 3. An estimate of the rate of change per

year in the M2 tidal range (which is twice the amplitude) is also derived. These values are

comparable to the rates of increase in Mn since the stations were installed (Figure 15).

Table 3. Changes in amplitude and range of the M2 tidal constituent

Station Harmonic

Analysis Period

M2 Amplitude

(m)

Change in M2

Range (m)

Rate of Change

(mm/yr)

Duck

6/1/78 - 5/31/79 0.477

0 0

1/1/02 - 12/31/02 0.477

Oregon Inlet Marina

5/7/79 - 1/31/80 0.071

0.12 5.22

1/1/02 - 12/31/02 0.131

Cape Hatteras

1/1/79 - 12/31/79 0.435

-0.004 -0.17

1/1/02 - 12/4/02 0.433

Beaufort

1/1/78 - 12/31/78 0.418

0.092 3.83

1/1/02 - 12/31/02 0.464

Atlantic Beach

2/16/78 - 2/15/79 0.547

0.002 0.1

1/1/99 - 12/31/99 0.548

Wilmington

1/1/36 - 12/31/36 0.441

0.366 5.55

1/1/02 - 12/31/02 0.624

Yaupon Beach

1/7/78 - 7/6/78 0.694

0.002 0.11

5/16/96 - 1/31/97 0.695
22

These results indicate that increasing tidal range can be accurately estimated by performing harmonic

analyses on limited segments of hourly water level data separated by long data gaps. Numerous

locations on the North Carolina coastline have hourly water level data available from the 1970s.

Collection of several months of new water level data at these sites could further delineate regions

where tidal ranges have changed significantly.
23

TIDAL DATUM TRENDS

Since there are large and significant increases in tidal range at Oregon Inlet Marina, Beaufort, and

Wilmington, trends were obtained for the MHHW, MHW, MLW, and MLLW tidal datums at these

stations (Table 4). The trends were calculated by linear regression with an autoregressive coefficient,

a 12-month seasonal cycle, and the 18.6 year astronomical tidal cycle described previously. The

trends and the 95% confidence intervals are shown in Figures 17-19. 

For Oregon Inlet Marina, the high water trends are over 6 mm/yr while the low water trends are

below 3 mm/yr. There is a small overlap of the confidence intervals. A large data gap between 1980

and 1994 results in the large trend uncertainties. For Beaufort, the high water trends are above 4.5

mm/yr while the low water trends are below 2.5 mm/yr. The high water trends are statistically

different than the low water trends, although some of the confidence intervals overlap with the MSL

trend confidence interval. 

The monthly MHHW and MLLW were not tabulated for U.S. east coast stations until the 1970s.

Therefore, for Wilmington, the MHW and MLW trends were calculated for the entire 1935-2002

period and for the 1973-2002 period for comparison with the MHHW and MLLW trends. Figure

14 suggests that the tidal range at Wilmington increased steadily until the 1970s and then appeared

to level off. The trends based only on data from 1973-2002 have high uncertainties and are not

statistically different than the MSL trend (Figure 19). For the entire 1935-2002 period, however, the

MHW trend is above 4 mm/yr while the MLW trend is negative and both are significantly different

than the MSL trend. 

Table 4. Tidal datum trends for North Carolina water level stations (in mm/yr)

Station

Number

Station

Name

MHHW MHW MLW MLLW

Trend Std.

Error Trend Std.Error Trend Std.Error Trend Std.Error

8652587 Oregon

Inlet Marina 6.28 1.04 6.71 1.01 2.62 1.26 2.86 1.23

8656483 Beaufort 5.92 0.59 4.91 0.54 1.5 0.56 2.47 0.61

8658120 Wilmington

(1935-2002) 4.18 0.21 -0.96 0.28

8658120 Wilmington

(1973-2002) 2.43 0.69 2.38 0.65 1.68 0.86 1.53 0.9
24

Figure 18. Comparison of trends for tidal datums at Beaufort. MHW, MSL, and MLW trends are

based on data from 1973-2002. MHHW and MLLW trends are based on data from 1976-2002.

Figure 17. Comparison of trends for tidal datums at Oregon Inlet Marina based on data from

1977-2002 with a large gap from 1980 to 1994.
25

Figure 19. Comparison of trends for tidal datums at Wilmington showing MHW, MLW, and

MSL trends based on data from 1935-2002 and MHHW, MHW, MLW, and MLLW trends based

on data from 1973-2002.

27

SUMMARY

This analysis of historical North Carolina water level data was carried out in preparation for the

North Carolina Bathymetry/Topography Sea Level Rise Project. MSL and tidal datum trends will

be applied to a coastal DEM for North Carolina in order to predict shoreline movements and to map

submerged zones, 25 to 50 years in the future. MSL trends were obtained from monthly data from

eight CO-OPS water level stations (Duck, Oregon Inlet Marina, Cape Hatteras, Beaufort, Atlantic

Beach, Wilmington, Southport, and Yaupon Beach). Water level difference comparisons for the

stations with large data gaps were made with the stations with more complete records to narrow the

trend confidence intervals. The calculated MSL trends increase from south (1.79 mm/yr at Yaupon

Beach) to north (4.27 mm/yr at Duck), with an overall average of 2.74 mm/yr. Average seasonal

cycles were similar for all stations, with the highest monthly water levels in early fall and the lowest

monthly water levels in winter. The longest time series (68 years at Wilmington) shows no

significant changes in 50-year trends. 

The mean ranges (Mn) and great diurnal ranges (Gt) were analyzed to determine any long term

changes in tidal range. Oregon Inlet Marina, Beaufort, and Wilmington have large and statistically

significant increases in Mn and/or Gt (greater than 3 mm/yr). Stations located directly on the

Atlantic Ocean had no such increase in tidal range. Harmonic analyses were carried out for a period

near the installation date of each station and for the most recent data. The observed increases in the

amplitude of the largest tidal constituent (M2) agree with the calculated increases in tidal range. For

the stations at Oregon Inlet Marina, Beaufort, and Wilmington, the MHHW, MHW, MLW, and

MLLW trends were obtained. Continual dredging of the channels connecting these stations to the

ocean has resulted in variation in the trends of different tidal datums. In some locations, the high

water datums have been rising at a rate significantly faster than the MSL trend. Whether or not this

difference in trend continues, depends on future dredging activity and on the opening or closing of

inlets in North Carolina’s chain of barrier islands.

29

ACKNOWLEDGMENTS

The author wishes to thank William Stoney, Len Hickman, Kurt Hess, Carol Auer, Jim Hubbard,

Steve Gill, Laura Rear, Lori Fenstermacher, and Brenda Via whose reviews and suggestions have

resulted in substantial improvements to this document.

31

REFERENCES

Hackney, Courtney T., 2002. Monitoring effects of a potential increased tidal range in the Cape Fear

River ecosystem due to deepening Wilmington Harbor, North Carolina, Year 1: August 1, 2000 -

July 31, 2001, Dept. of Biological Sciences, Univ. Of North Carolina at Wilmington,

(http://people.uncw.edu/hackney/capefearriver.htm).

Mitchell, Clifton E., P. Vincent, R. J. Weldon II, and M. A. Richards, 1994. Present-day vertical

deformation of the Cascadia margin, Pacific Northwest, United States, Journal of Geophysical

Research, Vol. 99, No. B6, 12257-12277.

NOAA, 2003. Notice of change to the nation’s tidal datums with the adoption of a new National

Tidal Datum Epoch period of 1983 through 2001, Federal Register/Notices, Vol. 68, No. 102,

Wednesday, May 28, 2003.

NOS, 2001. Tidal datums and their applications, NOAA Special Publication, NOS CO-OPS 1,

National Ocean Service, NOAA, 111 pp.

Zervas, Chris E., 2001. Sea level variations of the United States 1854-1999, NOAA Tech. Rep. NOS

CO-OPS 36, National Ocean Service, NOAA, 186 pp.

Zervas, Chris E., 2003. Long term changes in tidal response associated with the deepening of

navigational channels, Proceedings, Coastal Zone 03: Coastal zone management through time,

Baltimore, MD, July 13-17, 2003, NOAA Coastal Services Center.