All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
However, the document contains useful guidance to support implementation of the new standards.
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
D.2.8 Wave Runup and Overtopping
This subsection provides guidance for calculating wave runup and overtopping on barriers.
Special cases where runup occurs on steep slopes and where runup exceeds barrier or bluff crests
are discussed. Guidance for mapping flood hazards based on runup and overtopping values is
given.
D.2.8.1 Wave Runup
D.2.8.1.1 Overview
Wave runup is the uprush of water from wave action on a shore barrier intercepting stillwater
level. The extent of runup can vary greatly from wave to wave in storm conditions, so that a wide
distribution of wave runup elevations provides the precise description for a specific situation.
The water wedge generally thins and slows during its excursion up the barrier, as residual
forward momentum in wave motion near the shore is fully dissipated or reflected. The notable
characteristic of this process for the present purposes is the wave runup elevation, the vertical
height above the stillwater level6, ultimately attained by the extremity of the uprushing water.
Wave runup at a shore barrier can provide flood hazards above and beyond those from stillwater
inundation and incident wave geometry, as illustrated in Figure D.2.81.
Limit of Wave Runup
Breaker Depth
Stillwater
Elevation
Hypothetical Slope
Source: FEMA, 2003
Figure D.2.81. Wave Runup Sketch.
6 The Mapping Partner must be aware of the relationship between stillwater level, wave setup, and wave runup.
Outputs from some runup and overtopping calculation procedures include wave setup effects; thus, an accurate
specification of input water level for the procedures is necessary to avoid doublecounting wave setup. The Mapping
Partner must also know whether waterlevel outputs from modern hydrodynamic and stormsurge models  which
will be used as inputs to transectbased wave height, wave runup, and wave overtopping analyses  include or
exclude wave setup. If wave setup is not included in the model waterlevel output, the runup procedures described
here can be applied directly. If wave setup is included in the model waterlevel output, the Mapping Partner must
estimate and subtract out wave setup from the stillwater level before using the runup procedures described here.
D.2.81 Section D.2.8
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
Current policy for the NFIP is to define the wave runup elevation as the value exceeded by
7
2 percent of the runup events . The 2percent value was chosen during the development of the
Pacific Coast Guidelines and Specifications for Flood Hazard Mapping Partners (see Appendix
D, Subsection D.4). This runup elevation is a shortterm statistic associated with a group of
waves or a particular storm. It is a standard definition of runup, commonly denoted as R 2%. This
2 percent is different from the 1percentannualchance condition that is associated with
longterm extreme value statistics. The 1percent condition has a 1percent annual probability of
occurrence, which corresponds approximately to the 100year condition, while the runup statistic
corresponds to a 2percent exceedance occurance in several hours of waves. To avoid confusion,
the 2percent runup is referred to as the “total runup” or just the “runup ” and is denoted as R2%.
Unless otherwise indicated, the runup referred to in all subsections of D.2 is the 2percent runup.
Incident wave runup on natural beaches or barriers is usually expressed in a form originally due
to Hunt (1959) in terms of the socalled Iribarren number,., as follows:
.=
HLm
(D.2.81)
in which m is a representative profile slope and is defined, depending on the application, as the
beach slope or the slope of a barrier that could be either a dune or a constructed element such as
a breakwater or revetment. H and L are wave height and length, respectively. The wave
characteristics in the Iribarren number can be expressed in terms of breaking or deepwater
characteristics. For these purposes, two wave characteristics in the Iribarren number are used,
including that based on the significant deepwater wave height (Ho) and peak or other wave
period (T) of the deepwater spectrum, and that based on the significant wave height at the toe of
a barrier. The first definition for a sandy beach is as follows:
.=m
o HL
oo
(D.2.82)
where Lo is the deepwater wave length:
g 2
Lo =T2p
(D.2.83)
and g is the gravitational constant. The beach profile slope is the average slope out to the
breaking depth associated with the significant wave height.
7 Walton (1992) concluded that both theory and laboratory experiments show that the 2percent runup height above
the stillwater level is approximately 2.2 times the mean runup height. Past NFIP policy was to define the runup
elevation based on the mean runup.
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
D.2.82 Section D.2.8
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
The 2percent incident wave runup on natural beaches (Rinc) is expressed in terms of the Iribarren
number as:
Rinc =0.6 m HoHL
oo
(D.2.84)
For the case of runup on a barrier, the Iribarren number is formulated using the significant wave
height at the toe of the barrier (see Subsection D.2.8.1.5).
The following subsections discuss runup on beaches and barriers in more detail, using RUNUP
2.0, ACES, and other methods. Special runup cases are also discussed.
D.2.8.1.2 FEMA Wave Runup Model Description (RUNUP 2.0)
NOTE: The result obtained from FEMA’s RUNUP 2.0 model is the mean runup value. Since
current NFIP policy is to use the 2percent runup, if RUNUP 2.0 is used in an FIS, interim
guidance calls for the mean runup height, obtained with the RUNUP 2.0 model, to be
multiplied by 2.2 to obtain the 2percent runup height. This value is then added to the
1percentannualchance stillwater level without wave setup to obtain the total wave runup
elevation for an FIS.
The current version of the FEMA Wave Runup Model is RUNUP 2.0. This model requires the
following inputs: the stillwater flood level (without wave setup), the shore profile and
roughness, and incident deepwater wave conditions. The program computes, by iteration, a mean
wave runup elevation fully consistent with the guidance available (Stoa, 1978). This
determination includes an analysis separating the profile into an approach segment next to the
steeper shore barrier, and interpolation between runup guidance for simple configurations
bracketing the specified situation.
Additional description of the workings of RUNUP 2.0 can assist informed preparation of input
and interpretation of output. The incorporated guidance gives runup elevation, as a function of
wave condition and barrier slope, for eight basic shore configurations distinguished by water
depth at the barrier toe, along with the approach geometry. W here those basic geometries do not
appropriately match the specified profile, r eliance is placed on the composite slope method
(Saville, 1958); this assumes that the input shore profile (composite slope) is equivalent to a
hypothetical uniform slope, as shown in Figure D.2.81. The runup elevations are derived from
laboratory measurements in uniform wave action, rather than the irregular storm waves usually
accompanying a flood event. Runup guidance for uniform waves, however, also pertains to the
mean runup elevation from irregular wave action with identical mean wave height and mean
wave period. Figure D.2.82 presents an overview of the basic computation procedure in
RUNUP 2.0.
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
D.2.83 Section D.2.8
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
Figure D.2.82 Overview of Computation Procedure Implemented in FEMA Wave Runup
Model (RUNUP 2.0)
D.2.8.1.2.1 RUNUP 2.0 Input Preparation
The input to the Wave Runup Model is done by transects along the study area shorelines, as was
done for overland wave propagation calculations. Because the runup results are very sensitive to
shore slope or steepness, it is important to have at least one transect for each distinct type of
shore geometry. Often, areas with similar shore slopes are located throughout a community, and
the results of one transect can be applied to all similar areas. This is especially typical of New
England communities with rocky bluffs. When the Wave Runup Model is being applied to dune
remnants where eroded slopes are fairly uniform, transect location is governed by the upland
landcover characteristics, which are major considerations in the WHAFIS model.
D.2.84 Section D.2.8
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
The ground profile for the transect is plotted from the topography and bathymetry referenced to a
common vertical datum. The profile must extend from an elevation below the breaker depth to an
elevation above the limit of runup, or to the maximum ground elevation. An adequate vertical
extent for the transect description will usually be 1.5 times the wave height above and below the
SWEL. If the landward profile does not extend above the computed runup, it will be assumed
that the last positive slope segment continues indefinitely. This is very common with low
barriers. The Mapping Partner shall select the last slope carefully, so it is representative. To
complete the description, each slope segment of the profile will need a roughness coefficient.
Common values are presented in Table D.2.81. The roughness coefficient must be between zero
(maximum roughness) and one (hydraulically smooth), and values for slope segments above the
SWEL control the estimated runup. The roughness coefficient (r) is used as a multiplier for
runup magnitude (R), defined on a smooth barrier to estimate wave runup with a rough barrier.
Table D.2.81. Values for Roughness Coefficient in Wave Runup Computations
Transects are approximated by the minim um ad equate number of linear segments, up to a limit
of 20. Segments may be horizo ntal, or higher at the landward end; portions with the opposite
inclination should be represented as horizontal when developing the transect approximation. The
use of many linear segments to represent a transect may be a wasted effort, because the Wave
Runup Model may combine adjacent segm ents in defining the appropriate approach and barrier
extents. With the runup computation procedure, the Mapping Partner shall apply engineering
judgment in transect representation to obtain the most valid estimate of wave runup elevation.
The input transect must reflect waveinduced modifications expected during the base flood
event, including erosion on sandy shores with dunes. The Mapping Partner shall represent only
coastal structures expected to remain intact throughout the base flood event on a specific
transect. Besides the transect specification, other required input data for the Wave Runup Model
are the base flood SWEL and the incident mean wave condition in deepwater. The specified
SWEL should exclude any contributions from windwave effects. If available elevations include
wave setup, the Mapping Partner shall remove that component before using this model so that
the calculated runup elevations do not indicate a doubled wave setup. Basic empirical guidance
relates runup at a barrier to the water level in the absence of wave action and thus includes the
wave setup component.
The mean wave condition to be specified for valid results with the Wave Runup Model may be
derived from other common wave descriptions by simple relationships. Wave heights in
D.2.85 Section D.2.8
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
deepwater generally conform to a Rayleigh probability distribution, so that the mean wave height
equals 0.626 times either the significant height based on the highest onethird of waves, or the
zeromoment height derived from the wave energy spectrum. No exact correspondence between
period measures exists; but the mean wave period can usually be approximated as 0.85 times the
significant wave period or the period of peak energy in the wave spectrum.
Table D.2.82 lists a series of wave height and period combinations, of which one should be
fairly suitable for runup computations at fully exposed coastal sites (depending on the local
storm climate). These mean wave conditions have wave steepness values typical of U.S.
hurricanes or within 30 percent of a fully arisen sea for extratropical storms. Commonly, the
Mapping Partner may have some difficulty in specifying a precise wave condition as
accompanying the base flood. In that case, the Mapping Partner shall also consider wave heights
and periods 5 percent higher and 5 percent lower than those selected (or whatever percentages
suit the level of uncertainty) and shall run the model with all nine combinations of those values.
The average of computed runup values then provides a suitable estimate for mean runup
elevation8. A wide range in computed runups signals the need for a more detailed analysis of
expected wave conditions or for reconsideration of the transect representation.
Table D.2.82 Appropriate Wave Conditions for Runup Computations Pertaining to 1Percent
AnnualChance Event in Coastal Flood Map Projects
Mean Deepwater Wave Height (Feet)
HURRICANES
8 12
9 15.5
10 19
11 23
12 27.5
EXTRATROPICAL STORMS
11 18
12 21.5
13 25
14 29
15 33.5
8 The resulting averaged mean runup can then be multiplied by 2.2 to obtain the 2percent wave runup value for an
FIS.
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
D.2.86 Section D.2.8
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
D.2.8.1.2.2 RUNUP 2.0 Operation
The input to the FEMA Wave Runup Model consists of several separate lines, specifying an
individual transect and the hydrodynamic conditions of interest within particular columns. All
input information is echoed in an output file, w hich also includes computed results on wave
breaking and wave runup.
The input format is outlined in Table D.2.83. The first two lines of the input give the Nam e and
Job Description, which must be included for each transect. The next line of input is the Last
Slope, which contains the cotangent of the shore profile continuing from the most landward point
provided. This is followed by the profile points, whi ch define the nearshore profile in
consecutive order from the m st seaward point. Each line gives the elevation and station of a
o
profile point and the roughness coefficient for the segment between that point and the following
point. The roughness coefficient on the last profile line is for the continuation defined in the Last
Slope line. The number of profile points cannot exceed 20. The final input is the series of
hydrodynamic conditions of interest. Each line he re contains the SWEL, a mean wave height in
deepwater, and a mean wave period.
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
D.2.87 Section D.2.8
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
Table D.2.83 Description of Five Types of Input Lines for Wave Runup Model
Name Line
This lin e is required and must be the first input line.
Columns Contents
12 Blank
328 Client's Name
2960 Blank
6170 Engineer's Name
7180 Job Number
Job Description Line
Columns Contents
12
376
7780
Blank
Project description or run identification
Run Number
Last Slope Line
This line is required and defines the slope immediately la ndward of the profile actually specified in
deta il.
Columns
Contents
14
Slope (horizontal over vertical or cotangent) of profile
continuation
580
Blank
Profile Lines
These lines must appear in consecutive order from the most seaward point landwa rd. Each line has the elevation and
station of a p rofile point and the roughness coefficient for the sect ion between that point and the following point.
The roughness coefficient on the last profile line is for the continuation defined in the Last Slope Line. The M apping
Partner shall ensure that at least one profile point with a ground elevation greater than the SWEL is specified. The
number of Profile Lines cannot exceed 20.
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
D.2.88
Section D.2.8
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
Columns Contents
1
Last point flag. The most landward point on the profile is indicated by a
1. If not the last point, leave blank.
2 Blank
37 Elevation in feet
8 Blank
914 Horizontal distance. It is common to assign the shoreline (elevation
0.0) as point 0, with seaward distances being negative and landward
distances positive.
15 Blank
1620 Roughness coefficient in decimal form between 0.00 (most rough) and
1.00 (smooth)
2180 Blank
Water Level and Wave Parameter Lines
These lines specify the hydrodynamic conditions for runup calculations on each profile; namely, the base flood
SWEL and mean wave height and period for deepwater. Typically, the SWEL remains constant for a given profile,
while the selected wave conditions closely bracket that expected to accompany the base flood. A maximum of 50 of
these lines can be input for each profile.
Columns
Contents
1
Last line, new transect flag. A 1 indicates the last line for a given
transect 1 and notifies that another transect is following. If not the last
line, or if the last line of the last transect, leave blank.
26 SWEL in feet.
7 Blank
'
812
Incident mean wave height described in deepwater, Ho , in feet,
greater than 1 foot
13 Blank
1418 Mean wave period, Ts , in seconds
1980 Blank
The o tput as shown in Table D.2.84 has two parts. The first page is a printout of the tran u sect
listed as a numbered set of profile points, the cotangents (slopes) of the segments, and the
roughness coefficient for each segment. The second page is the output table of computed re sults
for each set of conditions, including runup elevation and breaker depth values, each with respec t
to the specified SWEL, along with an identif ication of the segment numbers giving the seaward
limit to wave breaking and the landward limit to mean wave runup.
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
D.2.89
Section D.2.8
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
Table D.2.84. Output Example for the FEMA Wave Runup Model
D.2.8.1.2.3 RUNUP 2.0 Output Messages
Several output messages alert the user to specific problems encountered in running the program.
All but the last three indicate that the program has stopped without completing the runup
calculations.
•
“NEGATIVE RUN PARAMETER, PROGRAM STOPS” An input value of wave height
or wave period is read as negative or zero. Check that the input has been entered in the
correct columns.
•
“MORE THAN 20 POINTS IN PROFILE, PROGRAM STOPS” The program accepts a
maximum input of 20 points de fining the nearshore profile. This encourages a profile
approximat ion that is not overly detailed, because each transect is to represent an
extensive area.
•
“**** Ho/Lo LESS THAN 0.002 ****” or “**** Ho/Lo GREATER THAN 0.07 ****”
These limits on wave steepness pertain to the extent of incorporated guidance on breaker
location. They should be adequate to include appropriate mean wave conditions for
D.2.810
Section D.2.8
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
extreme events and also conform to the usual limits in detailed guidance on wave runup
elevations.
•
“DATA EXCEEDED TABLE” An entry into subroutine LOOK of the program is not
within the parameter bounds of the data table from which a value is sought.
•
“SOLUTION DOES NOT CONVERGE” After 10 iterations, the current and previous
estimates of runup elevation continue to differ by more than 0.15 foot, and both values
are provided in the output table. The calculation is usually oscillating between these two
runup estimates when this occurs.
•
“COMPOSITE SLOPE USED BUT WAVE MAY REFLECT, NOT BREAK” The
output runup elevation relies to some extent on a compositeslope treatment, but the
overall slope is steep enough that the specified wave may reflect from the nearshore
barrier. Thus, the application of a calculated breaker depth in determining overall slope
and runup elevation is questionable.
•
“WARNING; COMPOSITE SLOPE USED, BUT INPUT PROFILE DOES NOT
EXTEND TO BREAKER DEPTH” If the input profile does not extend seaward of the
breaker depth, an incorrect breaker depth may be computed, and the associated runup
elevation will also be incorrect. The input profile should include bathymetry to 30 or 40
feet in depth.
D.2.8.1.3 Wave Runup using ACES
FEMA also permits use of the Automated Coastal Engineering System (ACES, USACE, 1992)
for runup and overtopping calculations against vertical and sloping s tructures. (Note that ACES
v. 1.07 is on the FEMA list of accepted models for coastal wave effects, which can be found at
http://www.fema.gov/plan/prevent/fhm/en_coast.shtm). It should also be noted that ACES uses
more uptodate methods than those contained in the Shore Protection Manual (USACE, 1984)
or those used in RUNUP 2.0.
ACES v. 1.07 has three wave runup programs: Irregular Wave Runup on Beaches, Irregular
Wave Runup on Riprap, and Wave Runup and Overtopping on Impermeable Structures. Wave
setup contributions are included in each of the runup calculations.
The Irregular Wave Runup on Beaches module calculates several values of runup (Rmax, R2%,
R10%, R33%, and R ) based on laboratory experiments of runup on smooth, impermeable slopes.
The calculations are made given the deepwater significant wave height, peak wave period, and
foreshore slope (which yield the surf similarity parameter, . = tan . / (Ho/Lo)1/2 ), and using the
general relationship
Rx%b
= a.
H0
(D.2.85)
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
D.2.811
Section D.2.8
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
where a and b are constants that depend on the statistic (x%) desired, from Mase (1989).
The Irregular Wave Runup on Riprap calculation is part of the Rubblemound Revetment Design
module. This method calculates the expected maximum runup elevation and provides a
conservative estimate of the maximum runup elevation, based on thesmallscale laboratory tests
of Ahrens and Heimbaugh (1988). The calculations are made given the deepwater significant
wave height, peak wave period, and foreshore slope (which yield the surf similarity parameter),
and using the general relationship
Rmax
=
a.
(1+
b.
)
H
0 (D.2.86)
where a and b are constants given by Aherns and Heimbaugh (1989).
The Wave Runup and Overtopping on Impermeable Structures module calculates the runup
elevation associated with incident uniform waves at the structure toe (described by Hi = Hs)
acting on smooth or rough structures. Other inputs are the peak wave period, nearshore slope,
structure slope, and roughness coefficients. The pertinent relationships are:
R
=c.
(1+
d .
)
Hi
(D.2.87)
for rough slopes,
and
R
=C
Hi
(D.2.88)
for smooth slopes,
where c and d are the armor unit coefficients given by Ahrens and McCartney (1975), and
coefficient C varies with the surf similarity parameter . , based on the work of Ahrens and Titus
(1985).
The ACES runup modules represent improved guidance over that contained in the SPM
(USACE, 1984). ACES guidance may be preferable to RUNUP 2.0 in some instances. The
Irregular Wave Runup on Beaches calculation is maintained in the CEM. The Irregular Wave
Runup on Riprap calculation is reported to be beneficial because it works well for both shallow
water and deepwater at the toe of the revetment.
D.2.8.1.4 Runup on Vertical Structures
Basic empirical guidanc e incorporated within the RUNUP 2.0 computer model generally does
not extend to vertical or nearly vertical flood barriers. For such configurations, RUNUP 2.0 will
usually provide a runup elevation, but the result may be misleading because reliance on the
compositeslope method can yield an underestimate of actual wave runup with the abrupt barrier.
Where a vertical wall exists on a transect, the Mapping Partner shall develop a runup estimate
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
D.2.812 Section D.2.8
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
using the specific guidance in Figure D.2.83, taken from the Shore Protection Manual (USACE,
1984). As within RUNUP 2.0, these empirical results for uniform waves should be used by
specifying the mean wave height and mean wave period for entry and taking the indicated runup
as a mean value in storm wave action.
Figure D.2.83 Wave Runup Guidance from Vertical Wall, From Shore Protection Manual
(USACE, 1984)
D.2.8.1.5 Methodology for Calculating W ave Runup on Barriers
In this subsection, “barriers” include steep dune features and coastal armoring structures, such as
revetments. Runup elevations on barriers depend not only on the height and steepness of the
incident wave (and its interaction with the preceding wave), but also on the geometry (and
construction) of the structure. Runup on structures can also be affected by antecedent conditions
resulting from the previous waves and structur e composition. Because of thes e complexities,
runup on structures is best calculated using equations developed with tests on similar structures
with similar w ave characteristics, with coefficients developed from laboratory or field
experiments.
The recommended a pproach to calculating wave runup on structures is based on t he Iribarren
number (.) and reduction factors developed by Battjes (1974), van der Meer (1988), de Waal and
van der Meer (1992), and described in the CEM (USACE, 2003). This approach, referred to as
the Technical Advisory Committee for Water Retaining Structures (TAW) method, is clearly
D.2.813 Section D.2.8
D.2.813 Section D.2.8
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
articulated in van d r Meer (2002) and includes reduction factors for surface roughness, the
e
influence of a berm, structure porosity, and oblique wave incidence. The TAW method is useful,
as it covers a wide range of wave conditions for calculating wave runup on both smooth and
rough slopes. In addition to being well documented, the TAW method agrees well with both
small and largesc ale experiments.
It is important to note that other runup methods and equations for structures of sim ilar form may
provide more ac curate results for a particular structure. The Mapping Partner shall carefully
evaluate the applicability of any runup method to verify its appropriateness. Figure D.2.84
shows a general cross section of a coastal structure, a conceptual diagram of wave runup on a
structure, and definitions of parameters.
Total Runup
Still Water Level (SWEL)
Wave Runup Level
Armor Layer
Wave Setup
Figure D.2.84. Runup on Coastal Structures, Definition Sketch
Most of the wave runup research and literature shows a clear relationship between the vertical
runup elevation and the Iribarren number. Figure D.2.85 shows the relative runup (R/Hmo)
plotted against the Iribarren number for two different methods: van der Meer (2002) and Hedges
and Mase (2004). In Figure D.2.85, both runup equations are derived from laboratory
experimental data and are plotted within their respective domains of applicability for the
Iribarren number. Each equation shows a consistent linear relationship between the relative
runup and .om for values of .om below approximately 2. For values of .om above approximately 2,
only the van der Meer method is applicable. Moreover, due to its long period of availability and
wide international acceptance, the van der Meer relationship (also referred to as the TAW runup
methodology) is recommended here. The Mapping Partner shall characterize the wave conditions
in terms of .om and be aware of the runup predictions provided by the various methods available
in the general literature.
D.2.814 Section D.2.8
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
0
3
1
2
NonDimensional Total
Hedgesand
Runup, R/Hmo
ase(2004)
TAW(vanderMeer,2002)
M
01234
Iribarren Number, .
om
Figure D.2.85. Nondimensional Total Runup vs. Iribarren Number
The general form of the wave runup equation recommended for use (modified from
van der Meer, 2002) is:
.1.77... . . 0.5 =.. < 1.8.
rb ßPom bom
.
...
..
RH= ..
mo . 1.6 .
.... . 4.3 
. 1.8 =..
. rb ßP
b om .
.
.
.
. om . ..
..
(D.2.89)
where:
• R is the 2percent runup = 2s2
• Hmo= spectral significant wave height at the structure toe
• .r= reduction factor for influence of surface roughness
• .b= reduction factor for influence of berm
• .ß = reduction factor for influence of angled wave attack
• .P = reduction factor for influence of structure permeability
Equations for quantifying the . parameters are presented in Table D.2.85. The reference water
level at the toe of the barrier for runup calculations is DWL2%. Additionally, because some
D.2.815 Section D.2.8
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
wave setup influence is present in the laboratory tests that led to Equation D.2.89, the following
adjustments are made to the calculation procedure for cases of runup on barriers.
Table D.2.85. Summary of . Runup Reduction Factors
Value of . for Runup
Roughness
Reduction Factor,
r.
Smooth Concrete,
Asphalt, and Smooth
Block Revetment
r. = 1.0
(D.2.810)
1 Layer of Rock With
Diameter, D.
= 1 to 3./sHD
r. = 0.55 to 0.60
2 or More Layers of Rock.
= 1.5 to 6./sHD
r. = 0.5 to 0.55
Quadratic Blocks r. = 0.70 to 0.95. See Table V5
3 in CEM for greater detail
Berm Section in
Breakwater,
b.
, B = Berm
Width,
hd
x
.
p
.
..
in radians
.
.
Berm Present in Structure
Cross section. See Figure
D.4.58 for Definitions of
B, Lberm and Other
Parameters
1 1 cos , 0.6 1.0
2
h
b B
berm
dB
L x
p.
..
..
.=
+
<
<.
..
..
..
.
0
2 0
h
mo mo
h
mo
mo
dRRif
H H
x
dHif
H
.
2
=
=..
=
.
.
=
=
..
(D.2.811)
Minimum and maximum values of
b. = 0.6 and 1.0, respectively
Wave Direction
Factor, .
ß
,
ß
is in degrees
o for
ally incident
and = 0
norm
waves
LongCrested Waves
.
ß
1.0, 0 10
cos( 10 ),10 63
0.63, 63
o
o o
o
ß
ß
ß
ß
.
<
<
.
..
=

<
<.
.
>..
o
( D.2.812)
ShortCrested Waves
1 0.0022 , 80
1 0.0022 80 , 80
o
o
ßß
ß

=

= (D.2.813)
Porosity Factor,
P.Permeable Structure Core P. = 1.0, om.
< 3.3; P. =
0.46
2.0
1.17( )om.
, om.
>
3.3 and porosity = 0.5. for smaller porosities,
proportion P.
according to porosity .
See Figure D.2.87 for definition of porosity
(D.2.814)
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
D.2.816 Section D.2.8
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
dh
Lberm
Hmo
Hmo
Figure D.2.86. Berm Parameters for Wave Runup Calculations
Figure D.2.87. Structure Porosity Definition
For a smooth, impermeable structure of uniform slope with normally incident waves, each of the
.
runup reduction factors is 1.0.
In calculating the Iribarren number to apply in Equation D.2.89, the Mapping Partner shall use
Equation D.2.82 and replace Ho with Hmo and replace T with Tm1.0 (the spectral wave period).
Hmo and Tm1.0are calculated as:
H mo =
4.0mo
(D.2.815)
T
T =
p
m1.0
1.1 (D.2.816)
where Hmo is the spectral significant wave height at the toe of the structure and Tp is the peak
wave period. In deepwater, Hmo is approximately the same as Hs, but in shallow water, Hmo is 10
to15percent smaller than the Hs obtained by zero up crossings (van der Meer, 2002). In many
cases, waves are depth limited at the toe of the structure, and Hb can be substituted for Hmo, with
D.2.817 Section D.2.8
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
Hb calculated using a breaker index of 0.78 unless the Mapping Partner can justify a different
value. The breaker index can be calculated based on the bottom slope and wave steepness by
several methods, as discussed in the CEM (USACE, 2003). In terms of the Iribarren number, the
TAW method is valid in the range of 0.5 < .om < 810, and in terms of structure slope, the TAW
method is valid between values of 1:8 to 1:1. The Iribarren number as described above is denoted
.om, as indicated in Equation D.2.89.
Runup on structures is very dependent on the characteristics of the nearshore and structure
geometries. Hence, better runup estimates may be possible with other runup equations for
particular conditions. The Mapping Partner may use other runup methods, based on an
assessment that the selected equations are derived from data that better represent the actual
profile geometry or wave conditions. See the CEM (USACE, 2003) for a list of presently
available methods and their ranges of applicability.
D.2.8.1.6 Runup from Smaller Waves
In some cases, neither of the previously described methods for computing runup on beaches or
barriers is applicable. These special cases include steep slopes in the nearshore, with large
Iribarren numbers or conditions otherwise outside the range of data used to develop the total
runup for natural beach methods. Also, use of the TAW method is questionable where the toe of
a structure, or a naturally steep profile such as a rocky bluff, is high relative to the water levels,
limiting the local wave height and calculated runups to small values. In these cases, it is
necessary to calculate runup with equations in the form of Equation D.2.89, to avoid double
inclusion of the setup, and to carry out the calculations at several locations across the surf zone
using the average slope in the Iribarren number. With this approach, it is possible that
calculations with the largest waves in a given sea condition may not produce the highest runup,
but that the highest runup will be the result of waves breaking at an intermediate location within
the breaking zone.
The recommended procedure is to consider a range of (smaller) wave heights inside the surf zone
in runup calculations. The concept of a range of calculated runup values is depicted
schematically in Figure D.2.88, where an example transect and setup watersurface profile are
shown. Figure D.2.88 also shows the corresponding range of depthlimited breaking wave
heights calculated on the basis of a breaker index and plotted by breaker location on the shore
transect. The Iribarren number was also calculated and plotted by breaker location in Figure
D.2.88. The calculation of .
at each location uses the deshoaled deepwater wave height
corresponding to the breaker height, the deepwater wave length and the average slope calculated
from the breaker point to the approximate runup limit. Note that this average slope, also called
composite slope as defined in the CEM (USACE, 2003) and SPM (USACE, 1984), increases
with smaller waves because the breaker location approaches the steeper part of the transect near
the shoreline. This increases the numerator in the . equation. Also, the wave height decreases
with shallower depths, reducing the wave steepness in the denominator of the . equation. Hence,
as plotted in Figure D.2.88, . increases as smaller waves closer to shore are examined,
increasing the relative runup (R/H). However, because the wave height decreases, the runup
value (R) reaches a maximum and then decreases.
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
D.2.818 Section D.2.8
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
The following specific steps are used to determine the highest wave runup caused by a range of
wave heights in the surf zone:
1.
Calculate the runup using the methods described earlier for runup on a barrier. This
requires iteration for this location to determine the average slope based on the
differences between the runup elevation and the profile elevation at the location and
the associated crossshore locations. Iterate until the runup converges for this location.
2.
Repeat the runup calculations at different crossshore locations until a maximum
runup is determined.
Wave Runup at Shoreline Resulting From
Breaking at Crossshore Location, y
DepthLimited Wave HeightVariables as a Functionof Breaker LocationIribarren Number
Crossshore Distance, y
Breaker Location Causing
y
Maximum Runup
2% Setup Level
Total Maximum
Potential Runup
Figure D.2.88. Example Plot Showing the Variation of Surf Zone Parameters
D.2.819
Section D.2.8
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
D.2.8.1.7 Wave Runup in Special Situations
To interpret and apply the calculated results properly, the Mapping Partner shall examine the
output of RUNUP 2.0 carefully for each situation. One important consideration is that a mean
runup elevation below the crest of a given barrier does not necessarily imply that the barrier will
not occasionally be overtopped by floodwaters (see Subsection D.2.8.2). Other cases may yield
results of more immediate concern, in that RUNUP 2.0 may calculate a runup elevation
exceeding the maximum barrier elevation; this outcome can occur because the program assumes
the last positive slope to continue indefinitely. For bluffs or eroded dunes with negative landward
slopes, a general rule has been used that limits the wave runup elevation to 3 feet above the
maximum ground elevation, even when the potential runup along the imaginary slope extension
exceeds 3 feet. When the runup overtops a barrier, such as a partially eroded bluff or a structure,
the floodwater percolates into the bed and/or runs along the back slope until it reaches another
flooding source or a ponding area. The runoff areas are usually designated as Zone AO, with a
depth of 1, 2, or 3 feet. Ponding areas are designated as Zone AH (depth of 3 feet or less), with
BFEs shown. Procedures for the treatment of sizable runoff and ponding are discussed in Section
D.2.8.2.4. .
A fairly typical situation on the Atlantic and Gulf coasts is that wave runup exceeds the barrier
top and flows to another flooding source, such as a bay, river, or backwater. It may not be
necessary in this situation to compute overtopping rates and ponding elevations; only the flood
hazard from the runoff must be determined. Simplified procedures have been used to determine
an approximate depth of flooding in the runoff area (Williams, 1983). These procedures are
illustrated in Figure D.2.89 and discussed below.
When the potential runup is at least 3.0 feet above the barrier crest, a VE Zone is delineated
landward of the barrier, as shown in Figure D.2.89. The BFE for that VE Zone is capped at
3 feet above the crest of the barrier. When the runup depth in excess of the barrier crest is 0.1 to
1.5 feet, the VE Zone BFE is the runup elevation (rounded to the nearest whole foot), and an AO
Zone with a depth of 1 foot should be mapped landward until another flooding source is
encountered (Zone AE) or the floodplain limit is reached (Zone X). Similarly, for a runup depth
of 1.5 to 2.9 feet above the barrier crest, the VE Zone BFE is the runup elevation (rounded to the
nearest whole foot). In this case, however, an AO Zone with depth of 2 feet should be mapped,
then transitioned landward into an AO Zone with a depth of 1 foot and then into subsequent
flood insurance risk zones, if any.
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
D.2.820 Section D.2.8
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
a. Potential runup at least 3
feet above crest Imaginary Slope Extension
Potential Runup
> 3.0 ft
SWEL
VE
VE
AO/AE
Runup Splash Zones
Zone Zone
b. Potential runup less than
3 feet above crest
Imaginary Slope Extension
Potential Runup
< 3.0 ft
SWEL
VE
AO2' and/or
AE
Runup AO1' Zones Zone
Zone
Figure D.2.89. Simplified Runoff Procedures (Zone AO)
D.2.821 Section D.2.8
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
A distinct type of overflow situation can occur at low bluffs or banks backed by a nearly level
plateau, where calculated wave runup may appreciably exceed the top elevation of the steep
barrier. A memorandum entitled Special Computation Procedure Developed for Wave Runup
Analysis for Casco Bay, FIS  Maine, 9700153 provides a simple procedure to determine
realistic runup elevations for such situations, as illustrated in Figure D.2.810 (French, 1982). An
extension to the bluff face slope permits the computation of a hypothetical runup elevation for
the barrier, with the imaginary portion given by the excess height R' = (RC) between the
calculated runup and the bluff crest. Using that height (R') and the plateau slope (m), Figure
D.2.811 defines the inland limit to a wave runup (X) corresponding to the runup above the bluff
crest (mX) or an adjusted runup elev ation of Ra = (C + mX). This procedure is based on a
Manning's “n” value of 0.04, with some simplifications in the energy grade line, and is meant for
application only with positive slopes landward of the bluff crest. A different treatment of wave
overflow onto a level plateau, for possible Flood Map Project use, is provided in Overland Bore
Propagation Due to an Overtopping Wave (Cox and Machemehl, 1986).
Figure D.2.810. Treatment of Runup onto Plateau above Low Bluff
D.2.822 Section D.2.8
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
Figure D.2.811. Curves for Computation of Runup Inland of Low Bluffs
These runup assessment procedures are given for general guidance, but they may not be entirely
applicable in certain situations. For example, runup elevations need to be fully consistent with
the wave setup and wave overtopping assessments described in the subsubsections that follow. In
problematic cases, the Mapping Partner shall use good judgment and rely on the historical data to
reach a solution for the realistic flood hazards associated with a shore barrier. Subsection D.2.11
considers the integration of separately calculated wave effects into coherent hazard zones for the
base flood. When a unique situation is encountered, the Mapping Partner shall prepare a Special
Problem Report and discuss it with the FEMA Project Representative.
D.2.8.1.8 Advanced Wave Models
Wave models are becoming more sophisticated and able to account for the complexities of water
waves. A rapidly developing class of these models is the Boussinesq group, which is both
commercially and publicly available. The commercial models are generally more user friendly.
In addition to wave setup, Boussinesq models can calculate wave runup. In conjunction with the
development of these Guidelines and Specifications, 1D Boussinesq models have been applied
to calculate total wave runup, and the average and oscillating components were calculated
separately. The comments below are b ased on an assessment of these Boussinesq results.
Compared to other methods , Boussinesq models yield generally realistic results. The main
concern with Boussinesq modeling is the “learning curve” required to carry out these types of
computations with confidence. Additionally, it was difficult to carry out calculations for
deepwater waves with a small directional dependency. The reason for this difficulty lies in the
associated substantial longshore wave lengths and the need for them to be represented by a 2D
D.2.823 Section D.2.8
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
model. One possible FEMA application that would avoid the repeated learning curve
requirement would be to carry out computations on a regional basis using Boussinesq models.
The rate of the improvement/development of Boussinesq models is moderate at present;
however, it is likely that this type of model will be much more capable in 10 to 20 years than at
present. Thus, at this stage, a Mapping Partner may elect to apply Boussinesq models; however,
for application on a regional basis, it is preferable to wait for further developments and
improvements. If a Boussinesq model is applied, the Mapping Partner shall obtain FEMA’s
approval, and it is suggested that calculations also be carried out using the DIM methodology for
comparison of results.
With these more advanced wave models, the wave setup component is combined with the storm
surge modeling, resulting in SWELs that include both storm surge and wave setup. Care must be
taken not to double count wave setup when calculating wave runup with one of the methods
presented here. The wave runup methods are based on scale laboratory tests that are thought to
include wave setup, so that calculated runup values shall be added to the storm surge, excluding
wave setup.
D.2.8.1.9 Documentation
The Mapping Partner shall document the procedures and values of parameters employed to
establish the 1percentannualchance total wave runup on the various transects on natural
beaches and barriers, which could include steep dunes and structures. In particular, the basis for
establishing the runup reduction factors and their values shall be documented. The
documentation shall be especially detailed if the methodology deviates from that described
herein and/or in the recommendations of the supporting documentation. Any measurements
and/or observations shall be recorded, as well as documented or anecdotal information regarding
previous major storminduced runup. Any notable difficulties encountered and the approaches to
addressing them shall be described clearly. Additional information on required documentation
criteria can be found in Subsection D.2.12.
D.2.8.2 Overtopping (Open Coast and Sheltered Waters)
D.2.8.2.1 Overview.
Wave overtopping occurs when a barrier crest height is lower than the potential wave runup
level, as shown in Figure D.2.812. Waves will flow or splash over the barrier crest, typically to
an elevation less than the potential runup elevation (R'). The exact overtopping water surface and
overtopping rate will depend on the incident water level and wave conditions and on the barrier
geometry and roughness characteristics. Moreover, overtopping rates can vary over several
orders of magnitude, with only subtle changes in hydraulic and barrier characteristics, and are
difficult to predict precisely.
The assessment of potential wave overtopping for flood hazard mapping purposes must rely on
readily available empirical guidance, historical effects, and engineering judgment. Except for
very heavy overtopping, useful guidance is typically derived from laboratory tests with irregular
waves, because the intermittently large overtopping discharges in storm situations are difficult to
reproduce in the laboratory. Recent numerical modeling and field experiments are advancing the
state of the art in overtopping predictions, but applying those methods in routine flood hazard
mapping purposes is still problematic. Therefore, the Mapping Partner shall estimate only the
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
D.2.824 Section D.2.8
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
order of magnitude of mean overtopping rates, because there are clearly documented thresholds
below which wave overtopping may be classified as negligible. While this approach does not
account explicitly for highly variable peak overtopping rates and does not offer a complete
specification of overtopping hazards, its use is recommended until overtopping rate calculation
guidance is improved significantly.
Overtopping Water Surface
SWEL
Potential Runup
Barrier
'R ' cz
Figure D.2.812. Definition Sketch for Wave Overtopping
If a preliminary estimate indicates severe overtopping that threatens the stability of a
given structure, that structure might be removed from the transect for analyses of the base flood,
and further overtopping consideration may not be required. Two publications, Design of
Seawalls Allowing for Wave Overtopping (Owen, 1980) and Random Seas and Design of
Maritime Structures (Goda, 1985), appear to provide trustworthy and wideranging summari es of
mean overtopping rates with storm waves. The former publication addresses smoothplane or
bermed slopes, and the la tter publication considers vertical walls with or without a frontin g
rubble mound. Before surveying those primary sources of overtopping guidance, however, some
introductory considerations can help to determine wh ether a detailed wave overtopping
assessment is needed for base flood conditions at a specific shore barrier.
The initial consideration is an interpretation of the mean runup elevation already calculated (.R),
in terms of likely extreme elevations according to the Rayleigh probability distribution usually
appropriate for wave runups. To parallel the extreme wave height addressed in coastal studies
(NAS, 1977), a controlling (base flood) runup magnitude may be defined as 1.6 times the
significant runup, or 2.5 times the mean runup, according to the Rayleigh distribution.
The first overtopping calculation the Mapping Partner should make is a comparison o f the
potential mean runup (.R) to the freeboard (F) offered by the barrier. If the elevation of the
barrier crest above the base flood total stillwater (MWL) elevation (freeboard) equals or exceeds
D.2.825 Section D.2.8
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
2.5.R, then the landward area is considered not subject to wave overtopping discharges during
the base flood. If F = (2.0.R), then wave overtopping can be appreciable during the base flood,
and the Mapping Partner shall assess overtopping rates and potential ponding behind the barrier.
The extreme runups introduced here (2.0.R and 2.5.R) bracket the elevation exceeded by the
extreme 2 percent of wave runups, which is a value commonly considered in structure design9.
D.2.8.2.2 Mean Overtopping Rates
Once the need for quantitative overtopping assessment is established, wave overtopping
estimates for a specified situation must generally be based on measurements in a similar
configuration. Before considering some implications of quantitative guidance for idealized cases,
an overview of overtopping magnitudes gives a useful introduction (Goda, 1985; Gadd et al.,
1984).
Wave overtopping is often specified as a mean discharge: water volume per unit time and per
unit alongshore length of the barrier, commonly in cubic feet per second per foot (cfs/ft). By
interpreting or visualizing a given mean overtopping rate, the Mapping Partner may take into
account actual discharges that are generally intermittent and isolated, being confined to some
portion of occasional wave crests at scattered locations.
Distinct regimes of wave overtopping may be described as spray, splash, runup wedge, and
waveform transmission, in order of increasing intensity. Flood discharges corresponding to those
regimes naturally depend on the incident wave size, but certain overtopping rates have been
associated with various characteristics (Goda, 1985). The right axis of Figure D.2.813 shows
this association.
The mean overtopping rate of 0.01 cfs/ft seems to correspond to a value that generally should be
considered appreciable, and a 1cfs/ft mean overtopping rate appears to define an approximate
threshold where the structural stability of even wellconstructed shore barriers becomes
threatened by severe overtopping. The 1cfs/ft mean overtopping rate also appears to be well
within the range where buildings exposed to overtopping are damaged.
Figure D.2.813 summarizes some empirical overtopping guidance for storm waves, in a
schematic form meant to help Mapping Partners determine the likely significance of flooding
behind a coastal structure. Variables describing the basic situation are cotangent of the front
slope for a smooth structure with ideally simple geometry, and freeboard of the structure crest
above total stillwater (mean water) level, as normalized by incident significant wave height
(F/Hs). The mean overtopping rate ( Q ) is provided in dimensionless form as
3)0.5
Q* = Q /(gH s (D.2.817)
with test results shown for structure slopes of 1:1, 1:2, and 1:4 (Owen, 1980), and for a smooth
vertical wall (Goda, 1985). These results pertain to significant wave steepness of approximately
2pHs/gT2
p = 0.035, fairly appropriate for extreme extratropical storms or hurricanes; water depth near the
9 According to Walton (1992), the Rayleigh distribution would result in a 2percent wave runup height that is
approximately 2.2 times the mean runup and a maximum wave runup height (for levee analyses) that is
approximately 2.9 times the mean wave runup.
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
D.2.826 Section D.2.8
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
structure toe of approximately dt = 2Hs, so that incident waves are not appreciably attenuated;
and moderate approach slopes of 1:30 for a vertical wall or 1:20 for other structures. The major
feature of interpolated curves is fixed as a maximum in overtopping rate for a structure slope of
1:2, corresponding to the gentlest incline producing (at this wave steepness) total reflection
rather than breaking, and thus peak waveform elevations (Nagai and Takada, 1972).
These measured results for smooth and simple geometries clearly show severe or “green water”
overtopping even at relatively high structures (F=Hs) for a wide range of common inclinations
(cotangents between 0 and 4). Also, for freeboards considered here, a vertical wall (cotangent 0)
permits less overtopping than common sloping structures with cotangent less than approximately
3.5. Gentler barriers are uncommon, because the construction volume increases with the
cotangent squared, so steep coastal floodprotection structures usually face attenuated storm
waves and/or have rough surfaces. The basic effects of those differences can be outlined for use
in simplified overtopping assessments.
For sloping structures sited within the surf zone (dt < 2Hs), Design of Seawalls Allowing for
Wave Overtopping indicates that basic overtopping guidance in Figure D.2.813 can be used with
attenuated rather than incoming wave height (Owen, 1980). A simple estimate basically
consistent with other analyses of the base flood is that significant wave height is limited to
H '
s = dt/2at the structure toe. The value of 2F/dt describes the effectively increased freeboard in
entering Figure D.2.813, and the indicated Q* value is then converted to
Q
using
H's
. The presumed wave attenuation ignores any wave setup as a small effect with the partial
barrier, and dt should always correspond to the scour condition expected in wave action
accompanying the base flood.
Figure D.2.813 might also be made applicable to rough slopes, using a roughness coefficient (r)
from Table D.2.81 to describe the effectively increased freeboard with greater wave dissipation
on the structure. Design of Seawalls Allowing for Wave Overtopping proposed formulating effect
of structure roughness as F/r, and Beach and Dune Erosion during Storm Surges confirmed a
similar dependence of overtopping on roughness in measured results for irregular waves (Owen,
1980; Vellinga, 1986). The overtopping relation reported as reliable in Wave Runup and
Overtopping on Coastal Structures is
Q* = 8•105 exp[3.1(rR* F / Hs)] (D.2.818)
where R* = [1.5 m/(Hs/Lop)0.5], up to a maximum value of 3.0, is an estimated extreme runup
normalized by Hs, for a barrier slope given as the tangent m (de Waal and van der Meer, 1992).
Equation 3 is meant to pertain to very wide ranges of test situations with moderate overtopping,
but it appears very approximate in comparison with specific results for r = 1, shown in
Figure D.2.813. It may be advisable to evaluate Equation D.2.8.118 for both smooth and
rough barriers, then to use the ratio to adapt a value from Figure D19 for the case with
roughness. Design of Seawalls Allowing for Wave Overtopping (Owen, 1980) and Wave Runup
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
D.2.827 Section D.2.8
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
and Overtopping on Coastal Structures (de Waal and van der Meer, 1992) provide further
overtopping guidance on the effects of composite profiles, oblique waves, and shallow water
with sloping structures.
Figure D.2.813. Schematic Summary of StormWave Overtopping at Structures of
Various Slopes and Freeboards,
D.2.828 Section D.2.8
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
For overtopped vertical walls, the effects of wave attenuation appear relatively complex, but
Random Seas and Design of Maritime Structures (Goda, 1985) provides extensive empirical
guidance on various structure situations with incident waves specified for deepwater. Figure
D.2.814 converts basic design diagrams for wave overtopping rate at a vertical wall, to display
wall freeboard required for rates of 1 cfs/ft and 0.01 cfs/ft with various incident wave heights.
Goda (1985) also provides a convenient summary on the effect of appreciable fronting roughness
in storm waves: the required freeboard of a smooth vertical wall for a given overtopping rate is
approximately 1.5 times that needed when a sizable mound having concrete block armor is
installed against the wall. With this information, a specific vertical wall can be categorized as
having only modest overtopping ( Q < 0.01 cfs/ft), intermediate overtopping, or severe
overtopping ( Q > 1 cfs/ft) expected for the base flood. Likely runoff or ponding behind the wall
must then be identified; severe overtopping requires a delineation of the landward area
susceptible to wave action and velocity hazard.
Considering Figure D.2.814 with respect to common wall and wave heights, wave overtopping
that endangers structural stability appears usual during the base flood.
An assessment of failure during the base flood for typical walls would be fully consistent with
one recommendation of Criteria for Evaluating Coastal FloodProtection Structures, which
states that “FEMA not consider anchored bulkheads for floodprotection credit because of
extensive failures” (Walton et al., 1989).
D.2.8.2.3 Overtopping Rate Considerations for Establishing Flood insurance risk
zones
An interpretation of the estimated overtopping rate in terms of flood hazards is complicated by
the projected duration of wave effects, the increased discharge possible under storm winds, the
varying inland extent of water effects, and the specific topography and drainage landward of the
barrier. However, Table D.2.86 provides guidance that is potentially applicable to typical
coastal situations.
Table D.2.86. Suggestions for Interpretation of Mean Wave Overtopping Rates
Flood insurance risk zone Behind Barrier
<0.0001 cfs/ft Zone X
0.00010.01 cfs/ft Zone AO (1 ft depth)
0.010.1 cfs/ft Zone AO (2 ft depth)
0.11.0 cfs/ft Zone AO (3 ft depth)
>1.0 cfs/ft*
30ft width+ of Zone VE
(elevation 3 ft above barrier crest),
landward Zone AO (3 ft depth)
*With estimated Q much greater than 1 cfs/ft, removal of barrier from transect representation may be appropriate.
+Appropriate inland extent of velocity hazards should take into account barrier characteristics, incident wave
conditions, overtopping flow depth and velocity, and other factors.
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
D.2.829 Section D.2.8
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
Figure D.2.814. Required Freeboard of Vertical Wall to Limit Mean Overtopping Rate
to Certain Values, Based on Design Curves of Random Seas and Design of Maritime
D.2.830 Section D.2.8
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
Guidelines and Specifications for Flood Hazard Mapping Partners [February 2007]
D.2.8.2.4 Ponding Considerations
Once the mean overtopping rate has been estimated for the base flood, determining the resultant
flooding landward of the barrier will require the Mapping Partner to evaluate several parameters,
includ ng the duration of overtopping, topography, and drainage landward of the overti opped
barrier. By integrating the volume of overtopping (mean rate times duration) and comp aring this
to the available storage landward of the barrier, an estimated ponding elevation can be
determined. This elevation should be adjusted by the Mapping Partner depending upon rainfall
rates associated with the overtopping event, drainage features and systems landward of the
barrier, and crest elevations of any features that may allow ponded water to escape. Ponding
assump tions and calculations should be reviewed carefully to ensure that overtopping and other
pot i
ent al sources of water trapped behind the barrier are accounted for appropriately.
The duration of overtopping can vary widely, depending on the coastal flood cause, from a
fastmoving hurricane to a nearly stationary extratr opical storm. The final guidance is offered: a
minimu m assumption for the duration of floodpeak overtopping would generally be 2 hour s.
Durations of 10 hours or more could be appropriate for the cumulative effects of an extratrop ical
storm c ausing flooding over multiple high tides.
D.2 2 Overtopping Depth and Velocity Considerations
.8. .5
In cases where the potential runup exceeds a barrier crest by 3.0 feet or more, the MappingPartner will map a VE splash zone landward of the crest (s ee Subsection D.2.8.1.7). The
Mapping Partner may consider the overtopping depth and velocity as one factor to determine th e
landa .
w rd limit of the VE splash zone.10
10 This new mapping procedure was introduced in the Pacific Mapping Guidelines. More details are provided in
Sections D.4.9.2.1 and D.4.5.2.5.
All policy and standards in this document have been superseded by the FEMA Policy for Flood Risk Analysis and Mapping.
D.2.831 Section D.2.8