Stillwater
Table of Contents
	1	Introduction
		1.1	Category and Topics
		1.2	Stillwater Focused Study Group
2	Critical Topics
	2.1	Topic 53: Atlantic/Gulf Storm Surge
		2.1.1  Description of the Topic and Suggested Improvement
		2.1.2  Description of Procedures in the Existing Guidelines
		2.1.3  Alternatives for Improvement
		2.1.4  Recommendations	
		2.1.5  Related Available and Important Topics	
	2.2	Topics 54 and 55: Pacific Storm Surge (including Non-Open Coast)
		2.2.1  Description of the Topic and Suggested Improvement
		2.2.2  Description of Procedures in the Existing Guidelines	
		2.2.3  Alternatives for Improvement	
		2.2.4  Recommendations	
		2.2.5  Related Available and Important Topics
3	Available Topics	
	3.1	Topic 52: Stillwater Non-Stationary Processes
		3.1.1  Description of the Topic and Suggested Improvement
		3.1.2  Confirm Availability	
		3.1.3  AvailabilityOther Factors	
		3.1.4  Related Available and Important Topics	
4	Important Topics
5	Additional Observations	
6	Summary	
7	References	
Tables
1	Summary of Findings and Recommendations
2	Preliminary Time Estimate for Guideline Improvement Preparation
Figures
1	Illustration of the primary aspects of surge modeling.	
2	Sample comparison of predicted and recorded tides during a severe storm 
at San Diego.
3	Sample comparison of predicted and recorded tides during a severe storm at 
San Francisco.	
4	Flow Chart of Florida Department of Environmental Protection	
5	Example of correlation between one-dimensional and two-dimensional 
numerical surge models.	
6	Observed sea level trends along U.S. coastlines.
Acronyms
1-D	one-dimensional
2-D	two-dimensional
ADCIRC	Advanced Circulation Model for Coastal Ocean Hydrodynamics
BFEs	Base Flood Elevations
CO-OPS	Center for Operational Oceanographic Products and Services
FEMA	Federal Emergency Management Agency
FL-DEP	Center for Operational Oceanographic Products and Services
G&S	Guidelines and Specifications
GROW	Global Re-analysis of Ocean Waves
LIDAR	Airborne Light Detection and Ranging
NFIP	National Flood Insurance Program
NOAA	National Oceanic and Atmospheric Administration
PWA	Philip Williams & Associates
USACE	U.S. Army Corps of Engineers
USGS	U.S. Geological Survey
WES	Waterways Experiment Station
1	INTRODUCTION
1.1	CATEGORY AND TOPICS
This report describes a proposed approach for the development of new FEMA Guidelines for the 
determination of stillwater levels in coastal areas.  Stillwater means the flood level not including 
the effects of waves (wave amplitude and wave setup; setup is addressed in a separate Focused 
Study Group report) or tsunamis, but including storm surge and astronomic tide. The particular 
topics addressed in this report were determined during Workshop 1 of the project and are 
identified below.
Topic 52 is judged to be relatively straightforward, amounting to identification of available 
information on such non-stationary factors as sea level rise and land subsidence that might affect 
a coastal study.
Topics 5355 are construed to address development of general guidelines for storm surge 
evaluation on both the Atlantic/Gulf (Topic 53) and Pacific (Topics 54 and 55) Coasts, including 
Pacific bays and estuaries (sheltered water areas). Furthermore, the necessary storm surge 
guidance is considered to be of two types: 1) guidance regarding storm surge hydrodynamic 
modeling, which will apply to both the Atlantic/Gulf and Pacific Coast insofar as general tools 
and principles are involved (addressing both Topics 53 and 54), and 2) guidance regarding other 
methods to estimate storm surge on the Pacific Coast and in Pacific bays, such as analysis of tide 
gage records (addressing both Topics 54 and 55). Note that additional guidance is provided in an 
accompanying Focused Study prepared by the TWG on Sheltered Waters, which addresses 
many of the coastal flood issues found in non-open coastal areas.
1.2	STILLWATER FOCUSED STUDY GROUP 
The Stillwater Focused Study Group is made up of Robert Battalio, Ian Collins, Robert Dean, 
Darryl Hatheway, Norm Scheffner, and David Divoky who served as Team Leader.  
2	CRITICAL TOPICS
2.1	TOPIC 53: ATLANTIC/GULF STORM SURGE
2.1.1	Description of the Topic and Suggested Improvement
This topic includes not only the identification of data sets and methods for verifying and testing 
surge models, but also development of general guidelines regarding storm surge modeling.  The 
general modeling guidelines developed under this topic will apply equally to modeling on the 
Pacific Coast (Topics 54 and 55).
2.1.2	Description of Procedures in the Existing Guidelines
Existing guidelines found in Appendix D are relatively brief, consisting primarily of checklists of 
requirements for data submission and documentation during a study. The material concerned 
with general surge modeling is contained in Section D.1.2.4, Hydrodynamic Storm Surge 
Model, which, in full, is as follows:
	* Report the unique model characteristics used for the study, including a discussion of the 
	specific grid system and sub-grid systems employed, the grid used for bottom topography 
	and shoreline, small-scale features such as harbors and barrier islands, and the location 
	and conditions applied for the open boundaries to the grid. 
	* Describe and document the adjustment to land features to account for erosion. 
	
	* Describe and document the method used to determine average ground elevations and 
	water depths within the cells of the grid system. This discussion is to be augmented by 
	diagrams that show the grid systems as computer listings of the grid data used in the 
	actual model calculations. 
	
	* Describe the method used to relate windspeed and surface drag coefficient.
	
	* Discuss the Mannings n values used in the calculation of bottom and overland friction 
	and provide values in tabular form. This information will include a discussion of any 
	sensitivity tests used to estimate these values in nearshore water. Nearshore bottom and 
	overland friction is an important part of the overall analysis and, therefore, shall be 
	described with care and sufficient detail. 
	
	* Provide a graphical depiction of the model cells and grid system as an overlay to the 
	bathymetric charts and topographic maps covering the study area, annotated with the 
	individual cell inputs for the grid system. 
	
	* Discuss the method by which barriers, inlets, and rivers have been treated. 
	
	* Explain the procedures used to determine inland flooding, including parameterization of 
	local features and selection of the friction factors used for the terrain. 
Additional storm surge guidance is contained in Section D1.2.5, Storm Surge Model Calibration 
and Verification, which consists of two paragraphs commenting on verification procedures and 
required backup documentation; Section D1.4.1, [Intermediate Data Submission] Before Storm 
Surge Model Calibration Runs, consisting of a list of eight items to be submitted for review 
before proceeding with model runs; and Section D1.4.2, Before Operational Storm Surge 
Runs, consisting of a checklist of seven items to be submitted for review before performing the 
main set of statistical simulation runs. Additional general material is provided in Section D2.2, 
Data Requirements.
These guidelines are generally based on the use of the FEMA storm surge model, although brief 
mention is made of the Stone and Webster (1978) northeaster model and the possible 
determination of stillwater elevations using statistical analysis of available tide gage records, 
provided those records include 20 or more years of data.  Section D.2.2 also states that synthetic 
computer models for storm surge assessments shall be used where tide gage data is limited and 
complex shorelines are present which cause appreciable variation in flood elevations for a 
community.  
2.1.3	Alternatives for Improvement 
Storm Surge Modeling Guidelines
A numerical storm surge model simulates the effects of a hurricane, tropical storm, northeaster, 
or other storm type passing over a given study area.  Two basic types of data must be provided to 
the model. First, the model implementation must include an accurate description of the physical 
characteristics of the study area, including:
	* Offshore bathymetry and onshore topography; 
	
	* Roughness characteristics of the ocean bed and landcover that may affect the flow of 
	water;
	
	* The nature of barriers and structures that may impede or divert the overland flow of 
	the flood; 
	
	* The extent of elements (especially tall vegetation) that may partially shield the water 
	surface from wind stress. 
Second, the model must include a realistic representation of the storm being simulated; in 
particular, the time- and space-varying wind and pressure fields of the storm must be reflected in 
the model through use of an appropriate storm submodel. Note that sheltered waters may pose 
special requirements for both basin and storm description, to account for the sheltering effects of 
terrain, complex flow resistance through developed areas, and changes in storm properties 
associated with the on-land weakening known as filling. Further details regarding Sheltered 
Waters are provided herein in a separate Focused Study Report on Sheltered Waters.
In addition to these factors describing the basin and the forcing disturbance, the model must 
solve a set of equations capable of capturing the essential features of the process, including the 
effects of wind, pressure, friction, overland flow (wetting and drying of land areas), and tidal 
forcing and tidal potential terms. This also requires the selection of a large number of empirical 
factors and functional expressions to describe, for example, bottom friction and wind stress.
Figure 1 (adapted from an unpublished diagram by Professor Robert Reid (Texas A&M) 
illustrates the primary aspects of surge modeling, including the determination of the types of 
waves that produce wave setup. 
Figure 1.  Illustration of the primary aspects of surge modeling.
The static component of setup is, strictly speaking, a stillwater component generated by radiation 
stress rather than wind stress. The development and application of methods that unify the setup 
and surge computation remain tasks for the future, however, and are discussed in the separate 
Focused Study Group report on Wave Setup.
It is proposed that new guidelines should include general guidance regarding these factors.  
Several candidate storm surge models are in current use or development that might be accepted 
by FEMA for future storm surge studies. It would not be the intention of the proposed work to 
evaluate specific models or to attempt to describe the details of use of any of those models at this 
time, because model documentation and users manuals are not available at the present time.
Instead, the proposed new guidelines would involve the development of more general, high-level 
guidance incorporating explanatory discussions of modeling factors that should be understood 
and considered by a study contractor or a FEMA project officer. Among these factors (in italics) 
are the following:
	
	* The governing equations of the model, typically the nonlinear long wave equations 
	accounting for conservation of mass and momentum, with surface wind and barometric 
	pressure terms representing the influence of the storm
	
	* The numerical scheme used by the model, whether finite differences computed on a grid 
	of rectangular cells (commonly of fixed size) or in curvilinear coordinates, or finite 
	elements represented by triangular or quadrilateral cells (of varying sizes); the numerical 
	scheme may also be explicit or implicit, affecting time step constraints
	
	* The flooding/drying treatment of cells as the flood advances onto land and then recedes
	
	* The storm representation, such as a planetary boundary layer model (for a hurricane) or a 
	simpler empirical/parametric description, including both wind and pressure; the storm 
	representation will be quite different for hurricanes, northeasters, and Pacific storms, 
	although the modeling principles remain the same in each case; on-land filling will be 
	significant for sheltered waters
	
	* The wind stress coefficient, which relates the wind speed at the surface to the stress felt 
	by the fluid
	
	* The sheltering treatment, adjusting the effective wind stress to account for partial 
	reduction by tall vegetation, terrain, and structures (especially significant for sheltered 
	waters)
	
	* The offshore bottom friction treatment over the relatively smooth ocean or bay bottom, 
	which retards the flow
	
	* The onshore flow resistance treatment accounting for bottom friction and resistance 
	offered by tall vegetation and structures (critical for sheltered waters)
	
	* The source and quality of bathymetric data, defining the varying depths at the site
	
	* The source and quality of topographic data, such as traditional quad sheets or newer 
	LIDAR data
	
	* The manner in which normal storm erosion alters the topography used in the model 
	
	* The manner in which catastrophic erosion might affect the modeling assumptions, in the 
	event of loss of a major barrier to inland flooding
	
	* The representation of the bathymetry and topography in the model grid system, which 
	depends on the numerical scheme
	
	* The faithfulness of the grid to the irregular bathymetry and terrain, including 
	conformance to boundary shapes and inclusion of small sub-grid barriers 
	
	* The resolution of the grid, whether fixed or varying through the study area
	
	* The boundary conditions, which impose approximate rules along the edges of the model 
	area, both offshore and onshore, permitting termination of the calculations at the expense 
	of accuracy
	
	* The treatment of astronomic tide, which might be handled as part of the simulation 
	through the boundary conditions or treated as an added effect separate from the surge 
	simulations; if the computational domain is large, tidal potential terms need to be 
	accounted for in a simulation
	
	* The types and limits of calibration that might be done, including small-amplitude 
	astronomic tide reproduction, for which calibration data are reliable
	
	* The role of verification hindcasts to confirm the apparent reasonableness of the final 
	model when compared with historical surge records
	
	* The role of wave setup (a separate topic in this guideline development project)
	
	* The general manner in which surge statistics are generated from multiple surge 
	simulations (the subject of Topic 50 of the separate Storm Meteorology effort)
These guidelines will be developed through review of the storm surge literature and consultation 
with developers and users of major storm surge models.  Although hurricanes are usually the 
focus of this discussion, northeasters are also to be included in the guidelines. Numerical 
hydrodynamic modeling, and the use of tide gage analysis, will be addressed as envisioned 
below in Topics 54 and 55 for the Pacific Coast.
 
No new methodology development is proposed, with one possible exception. The problem of 
catastrophic erosion of a coastal dune should be considered as a special modeling problem. 
Consider an embayment and an inland region protected from surge by a high dune ridge. In 
previous FEMA surge modeling, such dunes may have been overtopped in the course of a 
simulation, but were treated as being simply submerged. However, as happened near Hatteras 
during Hurricane Isabel, overtopping can lead to washout of a considerable portion of the dune, 
creating a new inlet and permitting a sudden large increase in flood penetration not envisioned in 
the model. For Pamlico Sound, this may or may not have been significant for the overall 
determination of surge, since the sound is so large that the additional flow occurring during the 
few hours of high storm tide may not have appreciably affected sound-side water levels.
 
However, a similar circumstance could make a significant difference in a region with a high 
barrier protecting low, developed areas. (It is noted that Scheffner, in a study for Fire Island to 
Montauk Bay, included erosion and breaching of the barrier island as part of a surge simulation 
and found a significant effect in Great South Bay.) This is not a deterministic process, although it 
is a frequent event during very large storms. It is proposed that its importance to storm surge 
modeling and stillwater determination be assessed and that, if it is found to be significant, then 
suggestions for future study beyond the present scope should be developed.
The style of the proposed guidelines will be consistent with the general approach of the existing 
guidelines, although more descriptive than prescriptive. Topic discussions may be illustrated by 
examples drawn from past surge studies performed with both finite difference and finite element 
models (perhaps both the FEMA surge model and the newer ADCIRC model).
The existing guidelines described above are primarily concerned with documentation and interim 
review of the storm surge modeling effort. That material, added to the guidelines in 20022003, 
was a significant improvement over the original 1995 draft, which was essentially mute on surge 
modeling.  The proposed guidelines would preserve and refine the 20022003 documentation 
and review sections of the most recent existing guidelines.
Extremal Analysis of Tide Gage Data 
Although the discussion above assumes only two-dimensional (2-D) hydrodynamic modeling 
would be used to determine storm surge levels, the direct use of tide data is another approach that 
must be considered. As will be discussed below for surge estimates on the Pacific Coast, it is 
possible to extract stillwater data from tide gage records by subtracting the known astronomical 
component. The residual data represents the contribution of all other low-frequency (i.e., 
stillwater) processes, including wave setup, although it should be noted that owing to large 
spatial variability, the setup captured at the gage may not be representative of setup in even 
relatively nearby areas.
With a sufficient period of record (the existing guidelines mention 20 years), an extremal 
analysis of the residual record after tide removal can be used to estimate the 100-year stillwater 
level at the gage site. Consideration of this approach will be included in the work outlined below 
for Topics 54 and 55. As discussed there, limiting factors include the quality and duration of the 
available data and the possibility of significant spatial variation with increasing distance from the 
gage site. The new work will include reconsideration of the required period of record as it affects 
confidence levels. The general approach to this task is not unlike extremal analysis in other 
hydrologic applications, including the problems of selecting an appropriate idealized probability 
distribution function, such as an extreme value distribution, and a method (e.g., moments, 
maximum likelihood) of determining the parameters of that distribution based on the local data 
sample (which could be the annual series of peak events). Many approaches are possible, with a 
great variety of choices of specific procedures. The proposed work will evaluate these 
alternatives and specify recommended procedures.
Evaluating the Accuracy of Storm Surge Estimates
A perceived need in the present coastal flood study program is a way to determine whether or not 
an existing study gives a reliable 100-year estimate, or whether a restudy that uses newer 
assumptions or tools is warranted.  This is a difficult question, especially on the Atlantic and 
Gulf coasts where hurricanes are the dominant flood contributors, because hurricanes are 
extremely sporadic and variable, and because mapped flood levels cannot be identified with any 
particular storm. Many agencies have different purposes and numerical modeling approaches for 
evaluating hypothetical storm effects which may also confuse this issue. For example the 
National Oceanic and Atmospheric Administration (NOAA) prepares hurricane evacuation 
maps, which depict the inundation of particular hypothetical storms defined by storm track and a 
Saffir/Simpson rating. The purpose of these maps are different than those needed for FIS studies.
In performing these surge studies, there is little opportunity for so-called model calibration. 
Beyond minimal calibration of ordinary small-amplitude conditions based on the simulated 
behavior of astronomic tide, for example, storm surge models are relatively closed-box affairs, 
assumed to be pre-wired with all the essential physics of the flood processes.  In any case, the 
basic requirements for calibration are rarely well satisfied.  To calibrate, one needs accurate 
knowledge of both the forcing disturbance (the storm) and the basin response (the resulting high 
water); neither of these are abundant for hurricane surge, although data are available from long-
term National Ocean Survey (NOS) stations, publications such as Characteristics of the 
Hurricane Storm Surge (Harris, 1963), and in a variety of reports from the U.S. Army Corps of 
Engineers (USACE).  Storm details are not known with any great accuracy because storms can 
fluctuate rapidly in size and intensity, and may appear chaotic when compared with the idealized 
representations used in models. Similarly, the basin response is seldom known with accuracy at 
more than a very small number of points inside surviving structures and at tide gages; highwater 
marks obtained in open areas may be contaminated with an undetermined amount of runup and 
setup. Gages commonly fail during the most significant events; for example, the gage at Duck 
Pier, North Carolina, failed just as the surge from last years Hurricane Isabel began to rise.
In view of these twin deficiencies, robust calibration of a storm surge model is not a common 
option in a FEMA study. To calibrate a model against typical storm and high water data (for 
example, by adjusting the wind stress coefficient) would be to build a systematic error into the 
model that cancels the unknown random errors in the storm description and flood observations. 
This systematic error would then be imposed on all subsequent simulations made during the 
development of the surge statistics. In lieu of calibration, modelers perform model validation 
tests by hindcasting historical storms to ensure that the model produces results that are in 
qualitative, if not quantitative, agreement with observations. With the luxury of several storms, 
the modeler might simply hope to be high in some cases and low in others. Still, without a real 
calibration, it is reasonable to question whether the basic hydrodynamic model might contain a 
systematic bias, either high or low, affecting all simulations that contribute to the 100-year 
determination. 
After a study has been completed and mapped, new storms will eventually occur at the site and 
will inevitably be compared with the study.  If a storm produces elevations less than those 
mapped, the conclusion might be reached that it simply was not a 100-year storm because 
weaker storms occur all the time and so are not surprising. Of course, the entire past history at 
the site can also be compared with the mapped levels.  If the record contains no severe events, 
then the temptation might be to assume that the study was biased to the high side. Conversely, if 
a new storm creates levels above those mapped, then it is very likely that the accuracy of the 
study will be questioned. Worse, if two or more such strong storms occur within a few years 
after the study, or if the record at the site contains several such events, then it may seem natural 
to conclude that the study was biased to the low side, is understating the hazard, and should be 
redone. 
This reasoning is not decisive, however, and (when clarified) suggests a way to test the accuracy 
of the existing 100-year coastal flood levels, and perhaps to help perform a global calibration, 
where a local calibration had been impossible. The key observation is that random events do not 
occur more or less uniformly over their domain, but instead must exhibit predictable 
irregularities of occurrence. In the case of floods observed at a large number of sites, some sites 
must be found that have gone for extremely long periods without experiencing a severe event, 
whereas other areas must have experienced multiple severe events. There must be good luck 
and bad luck communities.  If the mapping were to be fine-tuned so that experience and 
mapping were highly consistent throughout, then the mapping would be flawed.
This suggests the possibility of a statistical test of the reliability of the existing 100-year values, 
which might proceed along the following conceptual lines. Imagine that the coastline were 
divided into a series of zones, each large enough so that floods within them could be considered 
statistically independenti.e., large enough that a particular storm tends to affect only one such 
zone, yet small enough that occurrence of a 100-year event affects the majority of the zone. 
Considering floods of 100-year magnitude, the zone size might be on the order of the radius of 
maximum winds typical of an areaperhaps just a few tens of miles.  This would suggest on the 
order of 100 zones covering the entire area of the Gulf and Atlantic Coasts.
Next, imagine that, for all zones, there are N years of historical flood data (high water marks). In 
any given zone, there is a certain probability of having experienced no event exceeding the 100-
year level during those N years, another probability of having experienced one such event, or 
two, or three, and so forth. From these considerations, one can estimate how many of the 
conceptual coastal zones should have experienced 0, 1, 2, ... floods exceeding the 100-year level 
in the N years of record. These expected numbers can then be compared with observation. If it 
were found that the count of observed exceedances was significantly greater than expected, then 
one would suspect that the mapping systematically understates the flood hazard. Conversely, if 
the count of exceedances was substantially less than expected, the mapping might be suspected 
to overstate the hazard.
Had all studies been performed in a systematic way using exactly the same surge modeling 
techniques, one could imagine performing a global calibration of the model to raise or lower the 
general levels of the mapping, in order to achieve a reasonable fit between the observed and 
expected rates of extreme occurrences. In reality, the existing flood studies were not all 
performed in a systematic way, even when the same surge model was useddifferent Study 
Contractors undoubtedly made differing assumptions that would affect the homogeneity of the 
data used in this conceptual approach.  However, a statistical review (such as that recommended 
above) might help reveal such anomalous local studies, which would be identified as zones of 
inconsistency with adjacent zones.
This section discusses how an approach might be developed. There are difficulties with the zone 
idea (presented as a conceptual aid), especially in the definition of such zones (large enough to 
ensure independence, yet small enough to respond as a unit to the 100-year flood). Consider, for 
example, a strong alongshore storm that could affect a long stretch of coast, and so violate the 
independence assumption.  For the present, we propose only to investigate (in consultation with a 
statistician such as Professor Borgman [University of Wyoming]) whether such an approach 
could prove fruitful and, if so, to outline specific methods for future work. A substantial portion 
of the effort required in this task would be the identification of suitable data sources. The 
immediate effort described above remains in the critical category; if successful, the follow-on 
effort would be categorized as important, requiring a longer performance period than is presently 
available.
Regional Modeling
In early FEMA storm surge studies, it was common to perform a separate study for each county. 
One major reason for this was limited computer capacity, which severely restricted the grid sizes 
that could be accommodated in even the largest machines at the time. For example, even the 
vaunted CDC 7600 supercomputer had only 64K words of small-core memory and 512K words 
of large-core memory, with comparably limited disk storage capacity, and a 36 MHz clock speed 
(1% of the speed and capacity typical of desktop personal computers today). Use of the CDC 
7600 typically cost on the order of $1 per second. Because each study area was restricted in size, 
many separate studies were required; because computing costs were high, the original coastal 
flood studies were extremely expensive (typically involving computer charges of about $100,000 
per county).
With tremendous recent advances in computational power, as measured by both speed and 
capacity, many of those early modeling constraints have been eliminated, and direct machine 
charges are now negligible (although proprietary modeling software may be a substantial cost). 
This suggests that it might be preferable to plan future surge modeling efforts on a regional, 
rather than a community, basis.
It is proposed, therefore, to provide general guidance on factors that should be considered in 
scoping a regional modeling effort.  In particular, surge modeling is significantly challenged with 
the problem of boundary conditions. Performing a regional study encompassing many counties 
would not only reduce costs, but also enhance modeling accuracy by greatly reducing the 
number of problematic boundaries.  Furthermore, through judicious placement of the regional 
study boundaries, difficult open-water boundaries may be traded for more tractable land 
boundaries. Recent work of a regional nature includes studies of the coast of South Carolina and 
Texas from Sabine to San Luis Pass (Scheffner et al., 2001, and in prep.)
2.1.4	Recommendations
It is recommended that four distinct tasks be undertaken in response to Topic 53. The first is a 
general review of storm surge modeling requirements from the perspective of FEMA and coastal 
flood insurance studies, leading to the development of a set of broad guidelines for conducting 
storm surge studies. This will require an assessment of many factors that go into conducting a 
surge study, ranging from the inherent abilities and limitations of numerical surge models to 
practical considerations of model selection and implementation in particular cases. The 
guidelines should include illustrative materials drawn from past studies and an annotated 
bibliography as a resource for more detailed study. It is beyond the scope and intent of the 
proposed work to evaluate the merits of particular models; that effort will remain separate as part 
of FEMAs accepted models review process, although the material developed in this study will 
help to provide a framework for that determination.
The second recommendation is for an outline of procedures to extract stillwater data from tide 
gage records. This overlaps with Topics 54 and 55 for the Pacific Coast, including Non-Open 
Coast regions, and is discussed in the following section.
Third, the Focus Study Group recommends an effort to develop a global method to assess the 
accuracy of FEMAs coastal storm surge studies. The random and sporadic nature of local surge 
history makes it difficult to determine whether coastal maps are appropriate. Recent catastrophic 
events may be given more weight than they deserve, since it is to be expected that several events 
exceeding local determinations must occur at some locations over an interval, while a lack of 
extreme events should characterize other areas. By considering the global history over the entire 
length of the U.S. coastline, it may be possible to determine whether the established coastal 
elevations are exceeded more or less frequently than expected for the assumed case of accurate 
maps and random local experience. 
The fourth recommendation is for development of guidance regarding study planningin 
particular, how studies might be grouped regionally to minimize costs while at the same time 
improving accuracy. Whereas existing FEMA studies were typically performed on a county-by-
county basis, the enormous advances in modeling technology over the past 20 years now permit 
much greater flexibility in model design. Multi-county and statewide (or larger) efforts are 
entirely feasible, and may also result in improved accuracy of results.
2.1.5	Related Available and Important Topics
Table 3 at the end of this report presents estimates of times required to accomplish the tasks in 
this topic.
2.2	TOPICS 54 AND 55: PACIFIC STORM SURGE (INCLUDING NON-OPEN COAST)
2.2.1	Description of the Topic and Suggested Improvement
Storm surge is of smaller magnitude on the Pacific Coast than on the Atlantic and Gulf coasts 
and so may not commonly require a detailed numerical model to obtain reasonable estimates.  
Instead, it may be possible to derive estimates of storm surge from tide gage records or 
simplified computations. When a 2-D hydrodynamic modeling effort is required, the proposed 
guidelines discussed above for Topic 53 will be appropriate, provided that the selected surge 
model has the capability to represent the wind and pressure fields appropriate to the Pacific 
Coast.
2.2.2	Description of Procedures in the Existing Guidelines
No specific guidelines have been identified for Pacific Coast storm surge, although the Atlantic 
and Gulf Coast guidelines discussed above are generally applicable. That is, basic numerical 
modeling considerations will be the same, although site-specific differences including especially 
the wind and pressure model must be accounted for.
2.2.3	Alternatives for Improvement
Tide Gage Analysis
Instead of the storm surge modeling discussed above in Topic 53, an alternate approach is to 
derive the 100-year stillwater estimate from an analysis of historical data. For this purpose, a 
wealth of tide gage data are available for coastal stations on both the Pacific and Atlantic/Gulf 
Coasts. The NOAA CO-OPS data archive (http://co-ops.nos.noaa.gov/data_res.html), for 
example, includes 117 coastal gages with 25 or more years of data. These data, by their nature, 
include all stillwater components but do not include the higher frequency wave effects, which are 
not appropriate to use in a stillwater determination.  The stillwater components captured in the 
gage data include storm surge (i.e., wind setup and pressure effects), wave setup, tsunamis, 
astronomic tide, and possibly a freshwater contribution from stream discharges. Most gages are 
located in protected. Sheltered Waters areas in bays and harbors and areas on the open coast 
without gage data will be discussed later. Owing to the spatial variability of wave setup, it is 
noted that although the local setup is captured in gage data, it may not be representative of other, 
relatively nearby areas.
The portion of the record attributable to astronomic tide is considered to be reasonably well 
known for each gage site by previous determination of the local tidal constituents. This fact 
makes it possible to compute the expected tide contribution at any time and then to subtract it 
from the record, leaving as the difference the sum of all other stillwater contributors. In this 
approach, wave setup is automatically included with the storm surge component, unlike present 
surge modeling practice, in which surge and setup are computed separately and appropriately 
added. In fact, all long-period processes, including tsunamis, are automatically included.
After subtracting the predicted tide from the gage records, an extremal analysis can be performed 
on the residual data to estimate the local 100-year level. The quality of this estimate will depend 
on both the reliability of the data and the duration of the record. Examples of the available 
NOAA CO-OPS data for two storms are shown in Figures 2 and 3.
Figure 2 shows data recorded during a January 1988 storm in Southern California, which is 
thought to approximate the 100-year (or greater) event; despite the severity of the storm, the 
storm tide component is seen to be quite small. Figure 3 shows data recorded at San Francisco 
during a 1998 storm. In this case, the water level was elevated above the expected tide by about 
two feet at the Presidio tide gage. Part of the anomaly (residual) was attributable to the El Nio 
climatic condition, which was strong in winter 199798. Water levels in the vicinity were 
elevated an average of one foot for the entire winter. It is interesting to note that Sausalito is 
within 5 miles of the Presidio tide gage but experienced noticeably higher stillwater levels. The 
additional elevation was probably caused by local wind setup induced by strong southeasterly 
winds, and by rainfall runoff entering San Francisco Bay from upstream drainage basins, 
including the Sacramento and San Joaquin Rivers (Philip Williams and Associates Ltd. [PWA], 
2002). This demonstrates that local variability may be substantial in large sheltered-water 
embayments, so that direct use of gage data may be limited to the near vicinity of the gage. It is 
noted, however, that in large sheltered waters where gage data is not comprehensive, and where 
the simplified one-dimensional (1-D) storm surge model discussed below is not appropriate or 
adequate, the full capability of one of FEMAs approved 2-D surge hydrodynamic models can be 
used to determine surge behavior and statistics.
The proposed task is threefold: to identify candidate sources of appropriate tide data, to examine 
a sample set to determine the extent to which the candidate sources can be used for flood 
insurance studies, and to estimate the reliability of the derived 100-year flood elevations.  As 
discussed above, the methods of data analysis are similar to the analysis of other stochastic 
hydrologic data, including selection of an appropriate probability distribution function, 
determination of distribution parameters from the site sample, and so forth.  Reliability 
considerations will include not only sample error associated with the duration of the record, but 
also the potential significance of variability near the site. This is particularly important in 
sheltered waters where tidal hydrology can vary substantially with location. Recent FEMA flood 
studies in Puget Sound/Strait of Georgia provide examples of tide gage data analysis approaches 
(PWA, 2002). Previous baywide studies have also addressed the distribution of high waters using 
tide gage data (U.S. Army Corps of Engineers [USACE], 1984).  The importance of variability is 
not limited to sheltered waters, however. For example, the contribution from wave setup can 
vary rapidly from place to place, even along the open coast.  The suggested effort will also 
provide case study examples for inclusion in the proposed guidelines.
 
Figure 2.  Sample comparison of predicted and recorded tides 
during a severe storm at San Diego.
 
Figure 3.  Sample comparison of predicted and recorded tides 
during a severe storm at San Francisco.
Simplified Surge Modeling
Where adequate records are not available, such as on the Open Coast in areas without gage sites, 
more traditional efforts such as numerical simulation of surge, wave hindcasts, tsunamis, and 
combined probability studies may be necessary. However, because the surge component is 
expected to be relatively small, it may also be possible in many cases to derive estimates of 
sufficient accuracy from simplified computations. This might be done, for example, following 
the approach used by the Florida Department of Environmental Protection (FL-DEP) for 
determining coastal construction control lines.
The FL-DEP applies a storm surge calculation approach that uses both 1-D and 2-D storm surge 
models (see, for example, Dean et al., 1992). A primary benefit of this approach is the fact that a 
very large number of simulations (including an appropriate representation of astronomical tide) 
can be made at minimal cost, from which the 100-year surge levels can be derived. The 2-D 
model is applied for verification of historical storms and for calibration of the one-dimensional 
model. Once calibrated, the 1-D model is used for the numerous production runs.
A flow chart of the procedure, taken from a FL-DEP study, is presented in Figure 4. Any valid 
2-D model, such as the FEMA Surge Model or ADCIRC, could be used, although the FL-DEP 
uses a variable-grid explicit-implicit model that allows for overland flooding. The 2-D model is 
first applied for comparison with historical storm data (although the chart specifically mentions 
hurricanes and factors specific to the source study, the procedure would be modified to use 
Pacific storms for West Coast applications). Generally, no adjustments are made to the 2-D 
model, which is used at this stage primarily for validation and/or to estimate the degree to which 
it agrees with the historical data. 
Following the verification stage, the 2-D and 1-D models are run for a common set of storms 
with ranges of storm parameters bracketing those anticipated to produce the 100-year surge. For 
various classes of storms, correlations are developed between the 2-D and 1-D generated 
maximum surges in the linear form: 
       								(1)
An example result is shown in Figure 5 for landfalling hurricanes on a particular transect 
(profile) in Palm Beach County, Florida.
It should be noted that the average difference between the 1-D and 2-D simulations in this 
example is only 7%, and that this is the level of difference found in the FL-DEP study for Palm 
Beach County, Florida.  For the hurricane surge conditions in Florida, a difference of this 
magnitude approaches 1 foot and so is significant. However, the situation is quite different for 
the Pacific Coast.
If the expected 100-year surge at a Pacific Coast site is on the order of 2 feet, then a 7% 
difference would amount to less than two inches. In other words, the added effort and cost of 2-D 
simulations might not generally be needed on the Pacific Coast, unless the 1-D estimate was 
calculated to be more than, for example, 7 feet, corresponding to a 0.5-foot error.  Tests would 
need to be done for a few typical Pacific Coast conditions (bathymetry and wind fields) to verify 
the degree of 1-D model accuracy and to provide guidance about when additional 2-D 
simulations would be needed.
 
An attractive feature of the FL-DEP approach is that using such an efficient and economical 1-D 
surge model makes it possible to handle the combination of surge and tide in an extremely 
natural way. The procedure, discussed under Topic 51 of the Storm Meteorology Focused Study 
Report, is to randomly choose a different tide history (drawn from the peak storm surge season) 
to be used as the seaward boundary condition for each 1-D simulation. That is, to determine the 
100-year surge, one simulates a large number of storms with different combinations of 
characteristics drawn from the local storm population. For each of these storms, a starting time is 
chosen at random from the appropriate storm season. Then the nearshore tide variation is 
determined, starting at that time and continuing for the duration of the surge simulation. By 
taking this time-varying random tide segment as the boundary condition, the influence of that 
tide is accounted for. By repeating this for many hundreds or thousands of storm simulations 
(fast and inexpensive with a 1-D model), all likely tide amplitudes and phases are reflected 
properly in the results.
2.2.4	Recommendations
The Focused Study Groups recommendations consist of two major tasks. The first is to establish 
procedures for extracting the required surge data from tide gage records and prepare 
corresponding guidelines for Study Contractors. Recent flood studies in Puget Sound/Strait of 
Georgia (Region X) can be used as examples of analysis methods (PWA, 2002). This does not 
require the development of any fundamentally new methodology. However, it will be useful to 
clearly lay out the procedures for Study Contractors and it will be necessary to identify data 
sources and perform test studies to verify the suggested procedures and assess limitations of the 
approach. Discussions of limitations will include statistical limits inherent in the varying lengths 
of available data records.  Separate discussions and guidance should be developed regarding the 
physical limitations and temporal and spatial variation often found within large bays and 
sheltered waters. The guidelines to be developed should include illustrative examples drawn 
from the test studies.
The second major task will be to develop procedures for surge estimation in areas for which an 
adequate tide gage record does not exist, including most Open Coast areas. Procedures for 
defining the modeling domain and selecting an appropriate model will be presented. When 
warranted, the detailed numerical modeling methods used for hurricane studies on the 
Atlantic/Gulf Coasts would also serve for the Pacific Coast, as long as the adopted numerical 
models are able to properly simulate Pacific Coast wind and pressure fields.  However, because 
surge is much smaller on the Pacific Coast than on the Atlantic/Gulf Coasts, simplified methods 
may suffice. In particular, the use of a 1-D surge model may be adequate for most cases, 
minimizing the costs of model implementation and simulation. An assessment of storm 
meteorology and data sources would be necessary to determine the best manner for specifying 
winds and pressures and their associated frequencies. Test studies should be performed at 
selected sites to verify the feasibility of the recommended approach. New guidelines 
summarizing the procedures would be developed, including illustrative examples.
2.2.5	Related Available and Important Topics
Table 2 at the end of this report presents estimates of times required to accomplish the tasks for 
these topics.
3	AVAILABLE TOPICS
3.1	TOPIC 52: STILLWATER NON-STATIONARY PROCESSES
3.1.1	Description of the Topic and Suggested Improvement
The task identified under Topic 52 is a straightforward effort to provide guidance alerting a 
Study Contractor to the possible importance of non-stationary (or non-steady) processes in a 
study.  The guidance might include, relative sea level rise, tectonic uplifting, land subsidence, or 
a combination of these processes (effective elevation change).  These might need to be accounted 
for in the interpretation of historical data, whereas ongoing subsidence would need to be 
considered for its immediate impact on a new study and discussed with the FEMA project 
officer.  The effort suggested here is primarily one of providing guidance alerting the user to 
these possibilities and advising on the availability of suitable data. In addition to relative sea 
level changes, changes in winds and waves and other climatic features should be addressed. 
These aspects have been summarized in several books and papers by Komar, including the 
individual processes of sea level rise, uplift, and subsidence and the effects of combining these, 
including data and statistics for areas on the Pacific Coast (Komar, 1998, 1988, and 1997).
3.1.2	Confirm Availability
Both sea level rise and land elevation changes (uplift and subsidence) contribute to relative sea 
level changes; a great deal of data and data summaries exist for both of these processes. For 
example, the Philadelphia District of the U.S. Army Corps of Engineers maintains a web page 
(www.nap.usace.army.mil/cenap-en/slr_links.htm) with links to numerous government data 
sources for sea level change, including the NOAA CO-OPS Sea Levels Online site. NOAA has 
determined the rate of mean sea level rise/fall for 117 long term water level stations and, from 
these, has determined trends, seasonal cycles, and interannual variations caused by fluctuations 
in ocean conditions, including El Nio effects. Figure 6 indicates the distribution of those study 
sites and the approximate magnitudes of the long-term trends that have been determined. 
 
Figure 6. Observed sea level trends along U.S. coastlines.
The estimated trends in many regions along the Pacific Coast are seen to be small and may have 
little importance for flood insurance studies; however, as noted below, it may still be valuable to 
document the changes and indicate their significance as part of a flood insurance study. 
Land subsidence may be more significant than area-wide sea level change for many study sites. 
The U.S. Geological Survey (USGS) and other sources have documented land subsidence 
throughout the United States, although subsidence is frequently a very local result of 
groundwater extraction or oil and gas extraction.  Along the Pacific Coast, however, significant 
tectonic uplifting occurs as a result of regional geologic processes and active plate tectonics. 
Consequently, despite the great quantity of large-scale data, it will still be necessary for Study 
Contractors to explore local data sources to identify local problems and determine whether such 
effects merit discussion with the responsible FEMA program manager. Such sources of 
information would include discussions with and information from community officials, resource 
agencies, and local surveyors. New guidelines should identify the major national and regional 
data sources and provide general advice regarding ways to locate local data. (For example, see 
San Francisco Bay Conservation and Development Commission, 1987). 
3.1.3	AvailabilityOther Factors
There are indications within the literature that weather patterns are changing, and these could 
have an impact on the interpretation of flood studies and study data.  For example, recently 
revised historical wind patterns that were undertaken for GROW (Global Re-analysis of Ocean 
Waves; see, for example, Cox and Swail, 2001) appear to show increasing winds and wave 
heights in the North Atlantic.  However, the real increases in winds may be at least partially the 
result of the fact that measurement instruments and techniques have changed with time (for 
example, anemometers on modern ships are at a greater elevation, above the standard 10 meter 
elevation, than was the case on older vessels).  Efforts have been made to account for such 
effects, but it has not always been possible to determine the actual measurement conditions. 
Another confounding factor is that the wind measured at most offshore data buoys is at elevation 
5 meters rather than the traditional standard of 10 meters.
Additional factors, such as variations in solar (sun spot) activity and El Nio cycles, can also be 
considered as potentially significant non-stationary factors.  However, a database of 2025 years 
(the minimum desired to estimate the 100-year event with confidence for FEMA studies) for a 
process that might be affected should already include the net effects of such phenomena.  Study 
Contractors should be aware of these factors and avoid confusing such cyclic non-stationary 
influences with other hydrometeorologic processes.
Although standard FEMA practice is to address current conditions only, it could also be 
appropriate to identify and discuss periodic seasonal changes (such as significant El Nio 
oceanic conditions) and future changes arising from other significant non-stationary 
contributions.  In a 1991 FEMA report titled Projected Impact of Sea Level Rise on the National 
Flood Insurance Program, for example, the potential impact of rising sea levels was 
investigated.  It was concluded, at that time, that a relative sea level rise of up to 1 foot could be 
tolerated without major impact, but that a longer term rise of 3 feet would have severe financial 
consequences. Such background discussion might be appropriately included in the guidelines, 
even if not deemed essential to performance of a study. 
More directly pertinent to a study would be an effort to document the expected magnitudes of 
non-stationary effects, even though small, and to estimate their projected impact over time; if 
nothing else, this might allay concerns and questions.  If a linear trend were assumed for sea 
level rise, say, one could easily prepare a table for a given study site showing how the BFEs 
would change were the trend to continue. With time, the 100-year level would rise in 
approximately the same way as sea level (as long as the change is small), so that the 100-year 
level as determined by the study would be a more frequent event at any future date. Were the 
projected rate of rise to be 2 feet per century, for example, then after ten years (well within the 
life of a typical flood insurance study) the true BFE would have risen 0.2 foot and the mapped 
flood would have declined from the 100-year level to, say, the 90-year level.  These magnitudes 
may not be critical in most areas, yet their documentation as part of a study might be useful to 
both FEMA and the communities.
3.1.4	Related Available and Important Topics
Table 3 at the end of this report presents estimates of times required to accomplish the tasks in 
this topic.
4	IMPORTANT TOPICS
None identified.
5	ADDITIONAL OBSERVATIONS
None.
6	SUMMARY
The Stillwater Focused Study addressed two broad topics: non-stationary processes such as 
effective sea level rise, and storm surge issues.  Non-stationary processes (Topic 52) are 
categorized as Available; the primary effort will be to identify data sources, provide a discussion 
of ways in which non-stationary processes relate to flood insurance studies, and provide 
guidance to Study Contractors regarding their possible significance in a study and what material 
should be presented to FEMA for consideration.
The storm surge issues are divided into modeling factors for the Atlantic/Gulf Coasts (Topic 53) 
and alternate and/or simplified methods for the Pacific Coast (Topics 54 and 55), where surge is 
of less consequence. The primary effort recommended for the Atlantic/Gulf Coasts is to write 
detailed guidelines regarding storm surge and storm surge modeling, including discussions and 
recommendations for the numerous factors that affect a modeling effort. A secondary effort will 
be to review existing and planned coastal studies to suggest how regional study efforts might 
prove more economical and more accurate than county-by-county studies, as has been the usual 
practice. A final recommendation is to investigate ways to assess the accuracy of existing and 
future coastal studies, including a global statistical review and comparison of mapped BFEs with 
the historical record.
Table 1 summarizes the Stillwater Focused Study topics and recommendations. Table 2 presents 
a preliminary estimate of time necessary to complete recommended tasks.
7	REFERENCES
Cox and Swail.  2001.  A Global Wave Hindcast over the Period 1958-1977: Validation and 
Assessment.  Journal of Geophysical Research Vol 106 
Dean, R.G., T.Y. Chiu, and S.Y. Yang.  1992.  Combined Total Storm Tide Frequency Analysis 
for Palm Beach County, Florida. Florida Department of Natural Resources.
Federal Emergency Management Agency (FEMA).  1991.  Projected Impact of Sea Level Rise 
on the National Flood Insurance Program.
Harris, D. Lee.  1963.  Characteristics of the Hurricane Storm Surge. (Technical Paper 48.) U.S. 
Weather Bureau.
Komar, P.D.  1988.  Beach Processes and Sedimentation, 2nd edition.  Prentice-Hall.
.  1997.  The Pacific Northwest Coast: Living with the Shores of Oregon and Washington.  
Duke University Press, Durham, NC.
.  1998.  The 1997-98 El Nio and Erosion on the Oregon Coast.  Shore & Beach, 
66(3):3334.
Philip Williams and Associates Ltd. [PWA].  2002 (November 6).  Sausalito Flood Study.  
Prepared for the City of Sausalito, California.
San Francisco Bay Conservation and Development Commission (BCDC). 1987. Future Sea 
Level Rise: Predictions and Implications for San Francisco Bay. Prepared by Moffatt & 
Nichol Engineers, Wetlands Research Associates, and BCDC, San Francisco, CA.
Scheffner, N.W., and F.C. Carson.  2001.  Coast of South Carolina Storm Surge Study.  
(Technical Report CHL TR-01-11.)  U.S. Army Corps of Engineers, Waterways 
Experiment Station.
Scheffner et al.  In preparation.  Coastal Erosion Study for the Open Coast from Sabine to San 
Luis Pass, Texas Tropical Storm Surge Frequency Analysis. (Technical Report 
ERDC/CHL.) U.S. Army Corps of Engineers, Waterways Experiment Station.
Stone and Webster Engineering Corporation.  1981.  Manual for Wave Runup Analysis, Coastal 
Flood Insurance Studies.  Boston, MA.
U.S. Army Corps of Engineers.  1984 (October).  San Francisco Bay Tidal Stage vs. Frequency 
Study.  San Francisco District.
U.S. Army Corps of Engineers.  2004.  Sea Level Rise Links.  <www.nap.usace.army.mil/cenap-
en/slr_links.htm>.