FloodingIssuesAtCapePointCampground

Larry Martin
NPS-WRD
April 2010
Flooding Issues at Cape Point Campground
Cape Hatteras National Seashore
Introduction: One of the NPS management issues in the Cape Point area is dealing with the
frequent flooding of the campground and interdunal roads. The flooding is a natural process
resulting from ponding of surface water and high water table conditions following large rainfall
events or wet periods. In the past, the water would be drained from the area through a network of
ditches to discharge to the ocean, thereby reducing the duration of flooding. That is no longer an
acceptable practice. Drainage of water would cause adverse impacts to wetlands and would be
contrary to North Carolina state law and NPS management policies.
In a previous memo to the park (December 23, 2005), I provided a summary of the issue and the
hydrology of the Cape Point area. I also presented alternatives to alleviate the impact of flooding
on beach access via interdunal roads. Those same alternatives apply to reducing the impact of
flooding at the Cape Point campground. Briefly, those alternatives are;
1. Reactivate the system of drainage ditches and discharge flood water to the ocean.
2. Raise the elevation of the campground.
3. Relocate the campground to higher ground.
None of these alternatives seem very reasonable. The only remaining alternative is "no action".
Simply accept the fact that the campground will sometimes be flooded and unusable.
In this report, I will demonstrate that the flooding is primarily a result of high water table
conditions following wet periods. I will also show that the frequency and duration of flooding
will continue to increase as sea level rises.
Hydrogeolog_y and conceptual model of groundwater flow
Dr. William Anderson conducted a detailed investigation of the hydrology of Hatteras Island for
his Ph.D. dissertation (Anderson, 1999). Anderson's conceptual model of the hydrogeologic
framework of the area underlying the Cape Point area is an upper and lower surficial aquifer each
composed primarily of medium -to -coarse sand, separated by a semi -confining layer composed of
fine sand and silt. The surficial aquifers are underlain by a low -permeability confining unit that is
40 feet thick and effectively isolates the surficial aquifers from deeper aquifers. The
hydrogeologic framework is illustrated in Figure 1.
Dr. Anderson continued investigating the hydrology of Hatteras Island after obtaining his Ph.D.
He monitored water levels from July 2001 to March 2004 at a series of wells across the island.
These data were used to calibrate the computer model described in this report.

RN
Upper Buxton Woods
Surficial Aquifer
medium -to -coarse sand
10`1 Semi -confining unit, fine sand & silt
30 1,
Lower Buxton Woods
Surficial Aquifer
medium -to -coarse sand
Confining Layer
X161" silty -to -clayey sand
Larry Martin
NPS-WRD
April 2010
Computer model parameter values
KX, Ky = 125 ft/d
KZ = 25 ft/d
Sy = 0.15, n = 0.2
KX, Ky = 75 ft/d, KZ = 25 ft/d
Sy = 0. 15, n = 0.2
KX, Ky = 125 ft/d
KZ = 25 ft/d
Sy = 0. 15, n = 0.2
Not modeled
Figure 1. Hydrogeologic framework of the Cape Point area (from Anderson, 1999).
KX, Ky, K, j, the permeability (hydraulic conductivity) in the x, y, and z directions,
Sy is the specific yield, n is the porosity.
The conceptual model of the groundwater flow system has recharge over the entire island from
infiltration of precipitation. The water table rises during wet periods and lowers during dry
periods. Infiltration of rainfall creates a mound of fresh groundwater under the island with the
high part of the mound in the middle of the island. Groundwater flows from the interior parts of
the island toward the margins where it discharges to the ocean or sound. Water table elevations at
the shoreline are controlled by the elevation of mean sea level.
There is some outflow of water from the interior parts of the island as surface water. This
outflow occurs primarily via a system of drainage ditches in the northern part of the island that
discharge to Pamlico Sound. There is very little natural surface -water flow. Most of the water
that accumulates in the interior part of the island infiltrates and leaves as groundwater flow with
discharge at the shoreline and near -shore environment. This process of discharge via
groundwater flow is a slow process and causes the water table to remain elevated for several
months following wet periods. High water table conditions cause some of the low-lying areas to
remain flooded for extended periods.
K

Larry Martin
NPS-WRD
April 2010
A simplified computer model of the groundwater flow system was developed to illustrate the
relationship between precipitation (and recharge to the aquifer) and water table elevation. The
model represents a north -south cross-section across the island (Figure 2). The model has three
layers corresponding to the hydrogeologic framework as shown in Figure 1. Each layer has the
same properties of permeability and porosity throughout the layer.
The first and last columns of the model are constant head cells; with the head equal to zero feet
mean sea level. That is, the water table is forced to be 0.0 feet at the shoreline of the ocean and
sound. The model computes the water table elevation at all other cells within the model.
Recharge to the model area was varied as a function of monthly rainfall. The recharge rate was
assumed to be 50% of the total precipitation. The recharge rate was entered as a variable, with a
different value for each month from January 2000 to February 2010. Precipitation data were
retrieved from the National Weather Service database; iniat mcsti.edq/cronosl.
Figure 2. Representation of the grid for a computer model of a strip across Hatteras Island
and location of two monitoring wells.
Model Calibration
Model calibration was evaluated by comparing predicted water levels with measured water levels
at observation wells in 2001-04. Values for hydraulic conductivity (K) (aka permeability) of the
aquifer were adjusted in the computer model until the predicted water levels approximated the
measured water levels. If the model permeability was too low, there would be too much
resistance to flow and the water table would build up too high. If the model permeability was too
high, groundwater would flow toward the shore too easily and the predicted water table would be
too low. The "calibrated" values of hydraulic conductivity for the model fall within the range of
expected values for these types of sediments (Fetter, 2001).
3

Larry Martin
NPS-WRD
April 2010
Comparison of water levels calculated by the computer model with measured water levels is
shown in Figure 3. Water level measurements at the monitoring wells were recorded twice daily.
The computer model calculates water levels at a monthly time step. Thus, the measured water
levels show much more detail and variability than the water levels calculated by the computer
model. Also, the computer model is a very simple representation of the hydrogeologic system. It
is used to show only the general patterns and relationship of water table elevation to recharge
from infiltration of precipitation and the recession of the water table during dry or normal
periods. A more detailed computer model would require several years of detailed water table
monitoring and documentation of depth and duration of flooding at areas of interest. Additional
geologic and hydrogeologic data would also be needed to identify the thickness and extent of
various model layers and type of sediments associated with each layer of the model.
The calculated water levels show general agreement with the measured water levels and the
computer model is therefore assumed to be capable of providing a general representation of the
water table elevation throughout the model period (2000-2010). Monitoring well RU -8 is near
the middle of the island and is compared to the calculated water level at a hypothetical
observation well at approximately the same distance from the shoreline in the model grid.
Monitoring well RU -10 is about 2500 feet west of the campground and is in a similar hydrologic
setting as the Cape Point campground.
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12
0
7/1/01 9/30/01 12/30/01 3/31/02 711/02 9/30/02 12/30/02 4/1/03 7/1/03 @/30103 12/31/03 V0144
e RUi 10 ......„,.,FRU S ...._ ^Mid-Bsland Camp0rownd
Figure 3. Comparison of computed and measured water levels.
L9

Larry Martin
NPS-WRD
April 2O 10
Model Results
Figure 4 shows a graph of the computer model calculated water table elevations at the Cape Point
campground for the period from 2000 to early -2010. The water table elevation closely mimics the
rainfall pattern, as it should because that's how the model was set up; precipitation is the only
variable input to the model. Rainfall is plotted on Figure 4 as the 3 -month moving average to
smooth the curve and because water table elevations change slowly compared to the flashy nature
of rainfall events.
If the land surface elevation at the campground is 6 feet above sea level, we could look at the
graph in Figure 4 and identify periods of time when the water table was predicted to be above
land surface. Those would be periods when the campground was flooded.
a
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CL
2
1
0
6 j
c
4
2
0
111100 12131/00 12131/01 12131/02 1/1/04 12/31 /04 12131/05 12131106 111108 12131108 12131109
Figure 4. Calculated water table elevations at the Cape Point campground and rainfall.
We could develop a better model if we had more site-specific data regarding: the geologic
material underlying the campground and surrounding area, historical record of water table
elevations, and documentation of dates and periods when the campground was flooded.
However, a better model is hardly necessary. We can monitor rainfall and know that during wet
periods it is more likely that the campground will be flooded. Flooding might occur after several
months with above average rainfall, or it might occur after one big rainfall event such as might
occur with a hurricane. Likewise, the length of time required for a flood to recede will be
dependent on the nature of the precipitation pattern that caused the flooding to occur and rainfall
during the recession period.
I

Larry Martin
NPS-WRD
April 2010
Effects of Sea Level Rise
Mean sea level has been rising for several decades. It is expected to continue rising for the near
future. Riggs and Ames (2003) present data showing that mean sea level has risen about a foot
from the 1920s to 2000 (Figure 5). As sea level rises, the base level for fresh groundwater
discharge from the surficial aquifer also rises, with the resultant effect that the water table
elevation across the island rises.
Figure 5. Tide gauge data from Hampton, VA and Charleston SC showing sea level from the
1920s to 2000 (Figure 6-3-2 from Riggs and Ames, 2003).
Figure 6 shows a cross-section of a barrier island at the ocean shoreline. It shows a lens of
freshwater floating on the underlying saltwater. Fresh groundwater is recharged by infiltration in
the interior of the island and flows toward the shoreline where it discharges to the ocean. As sea
level rises (or falls), the elevation of the discharge zone for the fresh groundwater will also rise
(or fall), causing water table elevation back from the shoreline to rise (or fall).

Larry Martin
NPS-WRD
April 2010
Figure 6. Relationship of freshwater and saltwater at the ocean shoreline.
Figure 7 shows model results for a scenario with one foot of sea level rise. In this case the
constant head boundary of the model corresponding to the ocean shoreline is specified to be 1.0
feet. Under this scenario, the frequency and duration of flooding is expected to increase. We
can't say how much because we don't have a calibrated model, only a very general, conceptual
model.
Sea level has risen over the past several decades. So if it seems that the campground is flooded
more frequently and for longer duration than it was 50 years ago, it probably is.
10
4
The upper (red) line is the predicted water
table elevation for sea level = 1.0 feet. The
1 lower (blue) line is for sea level + = 0.0 feet.
0
111100 12111 MO 12!31101 1 M V02 111104 12/31104 12131105 12/31106 1/1108 12/31/00 12131109
Figure 7. Comparison of predicted water table elevation change at the Cape Point campground
for one foot of sea level rise.
0

Larry Martin
NPS-WRD
April 2010
Hydrograph for Monitoring Well RU -10
The hydrograph of Anderson's Monitoring Well RU -10 (Figure 8) provides some insights into
the relationship between precipitation and water table elevation and the effects of the drainage
ditches in the vicinity of the campground.
The water level in the early part of the graph is lower than normal. Rainfall was far below
average from October 2000 through 2001. Total rainfall in 2001 was about half of normal; 29
inches instead of the normal 58 inches.
Rainfall and water table elevation in 2002 are more representative of average conditions. Total
rainfall in 2002 was 69 inches, 11 inches greater than average. Large water table rises are
associated with large rainstorms; 6'/z inches on June 20-22, 4 inches on September 10, and 4
inches on November 16-17. The natural rate of water table decline is observed in April -June and
July -August.
A large amount of rain associated with Hurricane Gustav in mid-September 2002 caused
flooding in the area. NPS opened the headgate west of the campground to drain water flooding
the campground. The graph shows a corresponding steep rate of water table decline in late
September. The headgate was closed on September 30. The water level recovered to a stable
elevation in equilibrium with hydrologic conditions. Another storm on November 16-17
produced 4 inches of rainfall, causing more flooding and causing NPS to reopen the headgate and
drain floodwaters for a week. The headgate was closed on November 25.
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711 MI W30101 1212(m01 3131(02 7114MM2 'WOElp2fM0102
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Figure 8. Water table elevation in a well west of the campground and rainfall at Cape Hatteras.

Larry Martin
NPS-WRD
April 2010
We might surmise that flooding at the Cape Point campground occurs when the water table in the
area rises to higher than about 51/2 feet MSL from the data on Figure 8 and NPS response of
opening and closing the headgate on the drainage ditch system.
The End of Draining Floodwaters to the Ocean
Rainfall in 2003 was far above normal. Total rainfall for the year was 84 inches; average is 58
inches. The headgate was opened on March 3, 2003 to drain floodwaters from the campground.
Hurricane Isabel passed through the area in September 2003, dropping an additional 4 inches of
rain. The drainage ditch west of the campground continued discharging to the ocean on the south -
facing beach. In April of 2004, officials from the North Carolina Division of Water Quality and
the U.S. Army Corps of Engineers visited the area and reviewed the drainage ditch system with
park staff. The park was subsequently issued a Notice of Violation for draining of wetlands. Park
staff closed the headgate on April 28, 2004 and placed sandbags against the headgate to limit the
flow of water from the drainage ditches to the interdunal area and the ocean. The headgate has
remained closed since that time.
Summary
Flooding at the Cape Point campground is the result of natural hydrologic processes. The water
table rises during wet periods. When the water table is higher than land surface, the area will be
flooded. The frequency and duration of flooding can be expected to increase as sea level rises.
There are no practical mitigation processes to reduce flooding of the campground area.
References
Anderson, Jr., William P., 1999,The Hydrology of Hatteras Island, North Carolina, Ph.D.
dissertation, unpublished, North Carolina State University, Raleigh, NC, 293 pp.
Fetter, C.W., 2001, Applied Hydrogeology, 4th ed., Prentice -Hall publisher, 598 pp.
Riggs, Stanley R., and Dorthea V. Ames, 2003, Drowning the North Carolina Coast: Sea Level
Rise and Estuarine Dynamics, NC Sea Grant Program, Raleigh, NC, Pub. No. UNC -SG -
03 -04, 152 p.
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