D.4.6 Coastal Erosion
This section provides methods for Mapping Partners to define the shape and location of eroded
beach profiles, upon which the 1% flood conditions (waves and water levels) will act and from
which flood hazard zones and Base Flood Elevations (BFEs) will be mapped.
D.4.6.1 Overview
Erosion processes and consequences of erosion can either be “episodic” or “chronic.” These two
descriptors assign a very important temporal component to erosion processes and their results.
Episodic erosion is the shore and backshore adjustment that results from short duration, high
intensity meteorologic and oceanic storm events. This type of event response results in shore
adjustment and occurs during a single storm or during a series of closely spaced storm events
within a storm season. Shore and backshore profile changes during intense storms and hurricanes
can result in dramatic beach and dune erosion, retreat, breaching, or removal of backshore dunes;
cause retreat and collapse of bluff and cliff formations; and culminate in greater landward
encroachment of waves and flooding from the ocean. Chronic erosion is associated with slow,
long-term processes such as gradual shoreline adjustment associated with: (1) sea level rise, (2)
land subsidence, (3) changes in sediment supply due to watershed modifications or dam building,
and (4) decadal adjustments in rainfall, runoff, and wave climate associated with global
warming.
Current Federal Emergency Management Agency (FEMA) regulations are limited to risks and
losses occurring as the direct result of a storm event. The National Flood Insurance Program
(NFIP) does not address long-term gradual chronic erosion but focuses on flood-related erosion,
episodic erosion, due to storm events . FEMA does not currently map long-term erosion hazard
areas as some local or state agencies do. FEMA Flood Insurance Rate Maps (FIRMs) do not
inform property owners of erosion risks. FIRMs only indicate risks from flooding hazards in the
form of BFEs and flood hazard zones. Therefore, flood assessment guidelines in this section only
include methods for estimating eroded shore and backshore profiles during single large storm
events, so runup and overtopping computations can be made to determine flood risks associated
with those events. Section D.4.9 discusses how results from event-based erosion assessments are
to be used by Mapping Partners to determine flood risks and delineate hazard zones.
D.4.6.2 Pacific Coast Characteristics Related to Storm-induced Erosion
Pacific storms track from the Pacific Ocean toward the coasts of California, Oregon, and
Washington. Unlike hurricanes that occur along the Atlantic and Gulf coasts, Pacific Coast
storms are frontal storms. These storms have smaller peak force wind speeds and surges than
hurricanes, but have much longer durations (often on the order of several days). Intense Pacific
storms are capable of generating 40- to 60-foot waves, and Pacific storms often “line up” in a
series of back-to-back events that track thousands of miles across the Pacific Ocean to attack the
West Coast of the United States for weeks with elevated tides and high surf. A series of storms
with only short periods of time between their peaks are capable of causing significant beach
profile recession. One, two, or more storms may occur in a winter season before a severe storm
event occurs. The periodic occurrence of El Niño oceanic conditions significantly amplifies the
effects of Pacific storms with increased sea level and wave heights. This change in oceanic
temperature, weather, and wave climate during El Niño periods is unique to the Pacific and
usually represents meteorological and oceanic conditions when wind, waves, total water levels,
and coastal erosion are the greatest.
Pacific Coast beaches undergo typical seasonal changes in profile and location from summer to
winter conditions. During winter months, increased total water levels along with high-energy,
steep waves tend to move sand offshore, adjusting the beach profile and its cross shore location.
By the end of the summer or early fall after months of calm seas, the beach has recovered and the
berms and back beach dunes may be well developed again. Figure D.4.6-1 provides a sketch of
generalized seasonal profile changes that occur on sand beaches of the Pacific Coast.
Figure D.4.6-1. Typical Pacific Coast Summer and Winter Beach Profiles
(after Bascom, 1964)
D.4.6.3 Background and Definitions
By their nature, coastlines are extremely complex and dynamic environments. The type and
magnitude of coastal erosion are closely related to general coastal exposure and beach setting.
D.4.6.3.1 Coastal Exposure
Coastal exposure refers to: (1) whether the coastline and beach are situated on the open coast,
e.g., exposed to the undiminished waves, water levels, tides, winds, and currents associated with
the open coast, or (2) whether the coastline is located within a sheltered area that is fully or
partially protected from the direct action of ocean waves, winds, tides, water levels, and currents.
The latter condition is referred to as a sheltered water area. Beach erosion processes resulting
from changes in total water level and wave action are similar along the open coast and within
sheltered water areas; however, the magnitude, rate, and ultimate beach response may be quite
different for sheltered water areas due to dramatic differences in total water-level changes and
wave energy during large storms. Sheltered water areas typically have reduced wave energy and
smaller runup. Some sheltered water areas found in confined embayments or estuaries may,
however, experience higher still water elevations resulting from the combined effects of
astronomical tides and fresh water runoff from streams and rivers and modified tidal and surge
conditions.
The primary differences in estimating coastal erosion for these two types of beach exposures
relate to how waves and water levels are determined for the 1% response storm condition. Refer
to Section D.4.2 for guidance on how the 1% annual chance storm is determined and to Sections
D.4.4 and D.4.5 for guidance on how waves and water levels are estimated for these two coastal
exposures.
D.4.6.3.2 Beach Setting
Beach setting refers to localized geomorphic characteristics of the shore and backshore zone
related to site-specific geology, profile shape, material composition, and material erodibility;
proximity to other dominant features such as coastal inlets, storm outfalls, streams, and creeks;
harbors and coastal structures; littoral sediment supply; and pocket beaches; and seasonal
changes in beach width due to changes in wave direction. Six common beach settings
representative of those along the California, Oregon, and Washington coastlines are addressed in
these guidelines:
1. Sandy beach backed by a low sand berm or high sand dune formation
2. Sandy beach backed by shore protection structures
3. Cobble, gravel, shingle, or mixed grain sized beach and berms
4. Erodible coastal bluffs
5. Non-erodible coastal bluffs or cliffs
6. Tidal flats and wetlands
Figures D.4.6-2 through D.4.6-7 provide sketches and define terms for these six common beach
settings found along the Pacific Coast. Table D.4.6-1 describes these settings, and lists
recommended methods and data necessary for estimating beach profiles for use during runup
computations. For the most part, these six settings are found in both open coast and sheltered
water areas. However, the magnitude and net effects of tides, waves, currents, and erosion often
differ between open coasts and sheltered areas. Policy and criteria for evaluating the stability and
performance of costal beach nourishment projects are not yet developed, and only basic guidance
is provided in Section D.4.1, Pacific Coast Guidelines Overview.
Beach settings (1) and (2) are likely to be the most important coastal settings from a hazards
mapping perspective. These two settings tend to experience the most erosion and flooding during
large storm events. The following sections describe procedures for estimating storm-induced
erosion for all six Pacific Coast beach settings listed in Table D.4.6-1. Two different
Figure D.4.6-2a. Sand Beach Backed by High Sand Dune
(Beach Setting No. 1) (after Griggs, 1985)
Figure D.4.6-2b. Sand Beach Backed by Low Sand Berm
(Beach Setting No. 1) (after Bascom, 1964)
Figure D.4.6-3. Sand Beach Backed by Shore Protection Structures
(Beach Setting No. 2)
Figure D.4.6-4. Cobble, Gravel, Shingle, or Mixed Grain Sized Beach and Berms
(Beach Setting No. 3)
Figure D.4.6-5. Erodible Coastal Bluffs
(Beach Setting No. 4) (after Griggs, 1985)
Figure D.4.6-6. Non-Erodible Coastal Bluffs and Cliffs
(Beach Setting No. 5)
Figure D.4.6-7. Tidal Flats and Wetlands
(Beach Setting No. 6)
Table D.4.6-1. Common Beach Settings Found Along the California,
Oregon, and Washington Coastlines
Beach
Setting
Reference to
Sketch or
Photo of
Beach
Setting
Materials
Recommended
MLWP Methods
Recommended
Eroded Profile
Methods
Data
Requirements
1. Sandy beach
backed by low
sand berm or high
sand dune
formation
Figures D.4.6-1,
D.4.6-2a, D.4.6-2b
D.4.6-8, D.4.6-9,
D.4.6-10, D.4.6-11,
D.4.6-12, D.4.6-13,
D.4.6-14, D.4.6-15,
D.4.6-16, D.4.6-17,
D.4.6-18, D.4.6-19
Fine to coarse
beach and dune
sands
See Subsection D.4.6.4
1. Begin with (surveyed)
existing beach profiles.
2. Develop MLWP and overlay
on existing beach profile:
2.a. If have good post-large
storm eroded beach profile
data: Use those data as initial
winter beach profile, e.g., the
MLWP.
2.b. If not: Use MLWP
methods listed in Subsections
D.4.6.5 and D.4.6.5.2 if using
MK&A geometric erosion
model, or use methods listed in
Subsections D.4.6.5 and
D.4.6.5.5 if using the K&D
geometric erosion model.
See Subsection D.4.6.5
1. Begin with the
MLWP.
2. Select the MK&A or
K&D method for
computing beach
erosion/recession that
occurs to the MLWP
during the 1% storm.
3. Apply the (a) MK&A
model (Subsection
D.4.6.5) primarily for
open coast OR and WA
beaches backed by high
dunes, and (b) the K&D
model (Subsection
D.4.6.5) for CA Open
Coasts or Sheltered
Waters.
- Local wave and water-
level information
- Local geology and
beach and dune material
characteristics
- Historical beach profile
data.
- Recent data for project
study area
- LIDAR or surveyed
profile data
2. Sandy beach
backed by shore
protection
structures
Figures D.4.6-3,
D.4.6-20, D.4.6-21
-Fine to coarse
beach sands
-Need
characteristics
of other fill or
revetment
materials
See Subsections D.4.6.4 and
D.4.6.6
1. Estimate beach slope m from
beach profile measurements
immediately following winter
storms, or
2. Estimate m from median
grain size and beach exposure
relationships (see Fig. D.4.6-9).
See Subsections
D.4.6.6 and D.4.7
1. Use post large-storm
beach profile data.
2. Compute local scour
depths at toe in front of
structures, (see CEM).
3. Determine whether
structure fails or is
overtopped (D.4.7).
4. Adjust profile from
step 1 to account for
effects computed in
steps 2 and 3.
- Local wave and water-
level information
- Local geology and
beach and dune material
characteristics
- Historical beach profile
data
- Recent data for project
study area
- LIDAR or surveyed
profile data
- Characteristics and
dimensions of
foundations and
structures
Table D.4.6-1. Common Beach Settings Found Along the California,
Oregon, and Washington Coastlines
(cont.)
Beach
Setting
Reference to
Sketch or
Photo of
Beach
Setting
Materials
Recommended
MLWP Methods
Recommended
Eroded Profile
Methods
Data
Requirements
3. Cobble, gravel,
shingle or mixed
grain size beach
and berms
Figures D.4.6-4,
D.4.6-22, D.4.6-23,
D.4.6-24, D.4.6-25,
D.4.6-26
Medium gravel
to large cobble
and small
boulders
See Subsection D.4.6.7
Assume cobble beach and
berm profile is stable;
however, fronting beach sands
may erode during winter
storms (see Figs. D.4.6-15 and
16)
Therefore, assume MLWP is
same as eroded winter storm
profile.
See Subsection D.4.6.7
Determine eroded winter
storm profile from
measured historical
beach profiles,
supplemented by probing
or trenching to cobble
bottom
-Local wave and water-
level information
-Local geology and beach
and cobble berm material
characteristics
-Historical beach profile
data
-Recent data for project
study area
- LIDAR or surveyed
profile data
4. Erodible
coastal bluff
fronted by narrow
sandy beach
Figures D.4.6-5,
D.4.6-27, D.4.6-28,
D.4.6-29, D.4.6-30,
D.4.6-31
Bluff materials:
Loosely
cemented sands
and gravels
Beach
materials:
Fine to coarse
sands, some
small gravels,
cobbles
See Subsection D.4.6.8
-Develop MLWP from
measured historical beach
profiles following large storm
events; seek historical profiles
during El Niño years if
possible.
-Supplement profile data with
probing or trenching to
confirm seasonal depth of
scour.
- If available, use historic
eroded winter profile
measured right after large
storm.
See Subsection D.4.6.8
-Develop eroded beach
profile from measured
historical beach profiles
following large storm
events during El Niño
years if possible.
-Supplement profile data
with probing or
trenching to confirm
seasonal depth of scour.
-If bluff is erodible,
apply Noble Engineers
Bluff Erosion Method
for bluffs (USACE,
2003) using estimate
beach slope and
elevations from eroded
beach profile, above.
-Local wave and water-
level information
-Beach and bluff material
characteristics and
erodibility information
-Perform field inspections
and sampling to
determine geotechnical
bluff erosion parameters
-Historical beach profile
and bluff retreat data
-Recent data for project
study area
5. Non-erodible
coastal bluffs or
cliffs
(Note: This
setting is often
fronted by a rocky
beach or rock
platform capped
with a thin layer
of sand)
Figures D.4.6-6,
D.4.6-32, D.4.6-33
Bluff materials:
Erosion-
resistant rock or
cemented sands
and gravels
Beach
materials:
Thin layer of
fine to coarse
sands with
some small
gravels, over
rocky bottom or
rock platform
See Subsection D.4.6.9
Assume the winter beach
profile is stable and is
estimated from measured
historical beach profiles,
supplemented by probing to
rocky bottom
See Subsection D.4.6.9
-Verify that bluff or cliff
setting is non-erodible.
-Determine winter beach
profile from measured
profile information and
probing in March-April.
-Local wave and water-
level information
-Geologic information for
study area
-Historical beach, bluff
and cliff profile data
-Recent data for project
study area
6. Tidal flats and
wetlands
Figures D.4.6-7,
D.4.6-34, D.4.6-35
Tidal Flats:
Cohesive
sediment and
organic
materials;
cohesive clays
and silts;
Wetlands:
cohesive clays,
silts and
organic
materials often
capped with
marsh
vegetation
See Subsection D.4.6.10
-Assume tidal flats and
wetland profiles are stable
during single storm events.
-Examine historical site
information to determine
whether profiles are stable,
receding or accreting.
-Determine winter profiles
from LIDAR and/or other
measured historical profiles.
See Subsection D.4.6.10
Tidal flats and wetland
surfaces typically do not
erode during storm
events; check local
history of site; use
LIDAR surveys or
measured profiles to
develop final winter
profiles.
-Local wave and water-
level information
-Sediment & geologic
information for study area
-Historical profile data
-Recent data for project
study area
-RU, OT and wave
propagation computations
will require estimates of
vegetation density and
roughness
methods are proposed for Beach Setting No. 1, depending on whether the backshore is a berm or
dune and if there is overtopping during the 1% storm event.
D.4.6.3.3 Data Sources
Estimation of coastal erosion during storm events typically requires the following types of site-
specific beach information and data:
1. Summaries and photos of historical coastal erosion
2. Aerial photos of study area
3. Local geology and shore and backshore material characteristics
4. Previous Flood Insurance Study (FIS) mapping and reporting
5. Historic and recent beach survey data: Light Detection and Ranging (LIDAR) topography
and profile data
The following list provides references and websites where pertinent data may be obtained for use
in event-based erosion analyses:
* Allan, J. C. and P. D. Komar. August 2004. Morphologies of Beaches and Dunes on the
Oregon Coast, with Tests of the Geometric Dune-Erosion Model. Technical Memo.
August 2004, 43 pp.
* Barton, C. C. 2004. U.S. National Coastal Assessment, USGS, Geologic Division, St.
Petersburg, FL, website: .
* Beach Morphology Monitoring Program Beach Profiles, 2004:
.
* California Coastal Zone Conservation Commission. 1975. California Coastal Plan.
Prepared by the California Coastal Zone Conservation Commission, State of California,
San Francisco, CA. 443 pp.
* Carr, E. E. 2002. Database of Federal Inlets and Entrances. U.S. Army Corps of
Engineers, Coastal Inlets Research Program. . June 19.
* Coastal Natural Hazards Policy Working Group. 1994. Improving Natural Hazards
Management on the Oregon Coast, Oregon Sea Grant, Oregon State University,
Publication No. ORESU-T-94-002, 128 pp.
* Department of Navigation and Oceanic Development. 1977. Assessment and Atlas of
Shoreline Erosion Along the California Coast, State of California Resources Agency,
Department of Navigation and Ocean Development, Sacramento, CA, 305 pp.
* Elliott, D. L., C. G. Holladay, W. R. Barchet, H. P. Foote, and W. F. Sandusky. 1986.
Wind Energy Resource Atlas of the United States, DOE/CH 10093-4, DE86004442.
Prepared by Pacific Northwest Laboratory, Richland, WA. Prepared for the U.S.
Department of Energy, Assistant Secretary, Conservation and Renewable Energy, Office
of Solar Electric Technologies, Wind/Ocean Technologies Division. Published by the
Solar Technical Information Program, Solar Energy Research Institute [now the National
Renewable Energy Laboratory], Golden, CO. October. .
* Flick, R. E. (ed.). 1994. Shoreline Erosion Assessment and Atlas of the San Diego
Region, Volume II. California Department of Boating and Waterways, Sacramento, CA.
* Gerstel, W. J., M. J. Brunengo, W. S. Lingley, Jr., R. L. Logan, H. Shipman, and T. L.
Walsh. 1997. Puget Sound Bluffs: The Where, Why, and When of Landslides Following
the Holiday 1996/97 Storms. Washington Geology, vol. 25, no. 1. March.
.
* Good, J. W. (ed.). 1992. Coastal Natural Hazards, Science, Engineering and Public
Policy. Oregon Sea Grant, Oregon State University, Publication No. ORESU-B-92-001,
162 pages.
* Gornitz, V., Beaty, T., and R. Daniels. 1997. A Coastal Hazards Data Base for the U.S.
West Coast. ORNL/CDIAC-81, NDP-043C, Oak Ridge National Laboratory, Oak Ridge,
Tennessee. .
* Komar, P.D. 1997. The Pacific Northwest Coast: Living with the Shores of Oregon and
Washington. Duke University Press.
* Nichols, M. D., 2003. Draft Review of California Coastal Erosion Planning and
Response: A Strategy for Action. California Resources Agency, Ocean Resources
Management Program, Sacramento, CA.
* NOAA. 2000a. Tidal Datums and Their Applications. NOAA Special Publication NOS
CO-OPS 1, Silver Spring, MD. June. .
* NOAA. 2000b. Nautical Chart Symbols, Abbreviations and Terms, Chart No. 1, Eleventh
Edition. Lighthouse Press, Annapolis, MD. 99 pp. NOAA Nautical Chart Users Manual.
.
* NOAA. 2003. Computational Techniques for Tidal Datums Handbook, NOAA Special
Publication NOS CO-OPS 2, Silver Spring, MD. September. .
* Peterson, C. D, M. E. Darienzo, D. Hamilton, D. J. Pettit, R. K. Yeager, P. L. Jackson, C.
L. Rosenfeld, and T. A. Terich. 1994. Cascadia Beach-Shoreline Database, Pacific
Northwest Region, USA. State of Oregon, Department of Geology and Mineral Industries,
Open File Report 0-94-2, Portland, OR.
* Southwest Washington Coastal Erosion Study, 1996: .
* U.S. Army Corps of Engineers (USACE-LAD). 2003. Encinitas and Solana Beach
Shoreline Feasibility Study, San Diego County, California - Coastal Engineering
Appendix Without Project Conditions, Los Angeles District Corps of Engineers. August
2003.
* ———. 1998. Coastal Erosion Along the U.S. West Coast During the 1997-98 El Nino:
Expectations and Observations. USGS, Center of Coastal Geology. .
* Important Links to Other Information Sites Regarding California Coastal Zone
Management Topics: .
* : Coastal Erosion (NOAA).
* : Remote Sensing (NOAA).
* : Assessment and Atlas of
Shoreline Erosion along the California Coast (Calif. Dept. of Boating & Waterways).
* : Listing of GIS Data Bases for Various Types of Data.
* : California Coastal Records Project-Aerial
Photographic Survey of the California Coastline.
* : Shoreline Data (NOAA).
* : Topographic Data (NOAA).
D.4.6.4 Estimating Eroded Beach Profiles
D.4.6.4.1 The Concept of the Most Likely Winter Profile
To estimate beach erosion and profile changes for a specific coastal setting that occurs during a
particular winter storm event, it is important to first estimate the initial beach profile conditions
that exist just before the occurrence of the storm (see Figure D.4.6-8). This initial beach profile
represents the likely winter profile conditions for a particular coastal setting, defined as the Most
Likely Winter Profile (MLWP). These initial conditions must be estimated before determining
beach profile changes for a particular storm event. Once determined by the Mapping Partner, the
MLWP is then modified according to the amount of erosion that occurs during a specified storm
event as a result of increased water levels and wave action. Figure D.4.6-9 provides a generalized
definition sketch of the MLWP for a typical sand beach backed by high sand dunes.
D.4.6.4.2 General Approach for Estimating Eroded Beach Profiles from Single
Storms
The first step is to locate the site on a large-scale map. Next, determine the coastal exposure, open
coast, or sheltered water area. Next, obtain and review mapping and published information
regarding the site and its geologic, morphologic, seasonal water levels and wave climate, coastal
processes, and erosional characteristics (e.g., refer to Subsection D.4.6.3.3 for references to sources
for these types of information). Next, review historic summer and winter beach profile data if
available, then determine the type of coastal setting(s) and the seasonal erosion characteristics that
best represent the study area. Select from the six common beach settings applicable to coastlines of
California, Oregon, and Washington that are described in Subsection D.4.6.3 and listed in Table
D.4.6-1. Several different setting types may exist within the same study area depending on the
aerial size and complexity of the study area. Therefore, a large project area with more than one
type of beach setting, a variety of coastal exposures, and spatially varying material composition
may require different data and the application of different procedures to estimate the MLWP or
eroded profile for each representative setting. Mapping Partners should always establish
subreaches within the larger study area that typify representative shore and backshore conditions
within a particular subreach. If a series of cross-shore profiles is used to represent the shore and
backshore for the study area, profiles must be carefully located to best capture the morphologic and
potential erosional, runup, and overtopping aspects of each subreach.
After the study area is divided into representative subreaches, Mapping Partners must estimate the
initial pre-storm event beach profile (MLWP) for each cross-shore profile in the subreaches.
Methods for establishing the MLWP for each of the six primary settings defined above are
presented in Subsections D.4.6.5 through D.4.6.10. Once determined by the Mapping Partner, the
MLWP is then modified according to the amount of erosion that may occur for a particular setting
and profile location during a specified storm event. Beach Setting No. 1 will always
Figure D.4.6-8. Evolution of the Initial Beach Profile Before
Occurrence of Large Storm Event (after SPM, 1984)
Figure D.4.6-9. Definition Sketch of MLWP for
Sand Beach Backed by Sand Dunes (Beach Setting No. 1) (after Bascom, 1964)
require development of the MLWP, followed by computation of the amount of additional profile
adjustment (erosion) that may occur to the MLWP during any specified storm event. Depending
on beach material properties and width of the beach, Beach Setting No. 2 may require a similar
two-step process to determine the eroded beach profile in front of shore protection structures.
Beach Setting Nos. 3 and 4 should be checked for erosion potential, but they are less likely to
experience significant erosion beyond what one would estimate as the MLWP for those settings.
Beach Setting Nos. 5 and 6 are typically stable and erosion-resistant, so once the Mapping
Partner establishes the winter profiles, analyses of further erosion is not required. The amount of
erosion and profile adjustment that occurs for Beach Setting Nos. 1 and 2 depends on the
magnitude and duration of the event and is related to the total water level and wave
characteristics. Methods and procedures for estimating beach profile changes for each of the six
primary settings are presented next.
D.4.6.5 Estimating Profile Changes for Sand Beaches Backed by Low Sand
Berms or High Dunes (Beach Setting No. 1)
The main erosion-related factors affecting beach profiles during storms are: (1) antecedent
conditions of the beach and back beach (profiles and beach-dune juncture elevation) before the
occurrence of the specified storm event (this issue of initial beach conditions is addressed by the
MLWP); (2) forcing processes that include the duration and time histories of the wave
characteristics, water levels, and runup; and (3) response elements that include the beach setting
and the dune/bluff characteristics, including material erodibility. Mapping Partners need methods
that account for the general effects of these processes for estimating the change in profile that the
beach and back beach dunes will experience during the event in order to compute runup and
overtopping and set BFEs, and establish depth and velocity hazard zones.
For Beach Setting No.1, a sandy beach backed by a low sand berm provides some buffer against
storm wave attack. These beaches typically exhibit a very significant change in beach profile due
to seasonal changes. The range of the seasonal change in berm width can vary from 50 to 250
feet as an extreme. Figures D.4.6-10 and D.4.6-11 show broad sandy beaches backed by low
sand berms. Figure D.4.6-2b provides a sketch of a typical beach profile for broad sandy beaches
backed by low sand berms (Beach Setting No. 1).
Figure D.4.6-10. Sandy Beach Backed by Low Berm, Newport Beach, CA
(Source: Noble Consultants, Inc.)
Figure D.4.6-11. Sandy Beach Backed by Low Berm, Huntington Beach, CA
(Source: Noble Consultants, Inc.)
Several event-based erosion assessment models are available, including simplified geometric
models, simple process-based numerical simulation models, and more complex process-based
simulation models. At the present time, process-based models have not been refined or calibrated
for general application to Pacific Coast conditions. Therefore, use of process-based models is not
recommended unless Mapping Partners have site-specific model calibration and validation data.
Otherwise, Mapping Partners shall use the simplified geometric models discussed below.
The long-standing 540 ft3/ft criterion previously adopted by FEMA for estimating Atlantic and
Gulf coast dune erosion should not be used on the Pacific Coast. The Technical Working Group
(2004) determined that general application of the 540 criterion is not applicable for the Pacific
Coast and found that there are simple geometric models that are more reliable and applicable for
assessing dune and berm erosion on the Pacific Coast. Two geometric models are recommended:
the K&D model developed by Kriebel and Dean (1993) and the MK&A model that was
developed by Komar et al. (2002) and further modified by McDougal and MacArthur (2004).
The MK&A model was developed and tested for the Oregon and Washington coast, so it is most
applicable to Type 1 beach settings found in Oregon and Washington. The K&D model is more
generalized and has been regularly applied to Type 1 beach settings in California, with
successful test applications in Oregon and Washington.
D.4.6.5.1 General K&D and MK&A Model Characteristics and Applicability
Regardless of the simplicity of these geometric models, both the K&D and the MK&A models
produce reasonable estimations of sand beach and dune recession during single storm events.
Both models were tested using measured beach profile and wave data in southern California and
in Oregon. Model results agreed well with observed conditions. However, the determination of
reliable input parameters is crucial to the accuracy of model results.
The erosion potential in the MK&A model is determined entirely by the change in the total water
level and the beach slope, and is very sensitive to the slope. For the K&D model, the wave setup,
event duration, D50 of the beach material, and profile characteristics (beach face slope and surf
zone profile) determine the maximum beach erosion potential. The K&D model considers the
conservation of sand volume between the erosion from the upper portion of the beach and its
deposition offshore. For both models, the storm duration directly affects the maximum beach
erosion and must be determined carefully.
The K&D model (Kriebel and Dean, 1993) was developed for four different beach profiles: (1) a
square berm, (2) a sloping backshore, (3) a sand beach backed by high dunes (15 to 50 feet high),
and (4) a sand beach backed by a low berm with a wide backshore. Therefore, the K&D model is
applicable to a wider variety of beach conditions and settings. For the purposes of these
guidelines, we only consider one of the typical equilibrium beach conditions (Beach Setting No.
1) available with the K&D model.
Southern California has many broad sand beaches backed by relatively low sand berms and
broad back beach terraces. Beaches backed by high sand dunes are more common in northern
California, Oregon, and Washington. The crest elevations of the low sand berms generally range
between +5 to +15 feet high. If wave runup is included in the total water level, as is done in the
MK&A model, the water level can easily exceed the low berm crest during moderate to large
storms. The MK&A geometric model was developed primarily for applications along the Oregon
and Washington coasts and is most appropriate for application to high dunes that are unlikely to
be overtopped during a storm event. The K&D model is recommended for either case, where low
berms can be easily overtopped during the storm event, or for high dunes that are unlikely to be
overtopped. Figure D.4.6-12 shows a typical section of open Oregon coastline fronted by a long
broad sand beach backed by high dunes where the MK&A model was successfully applied. All
model results should be checked against observed post-storm data for reasonableness.
Figure D.4.6-12. Photograph of Netarts Bay, Oregon
(Photo Provided by Jonathan Allan)
D.4.6.5.2 MK&A and Its Application to Beach Setting No. 1
A geometric model for foredune erosion has been employed by Komar et al., (2002) on the
Oregon and Washington coast to establish coastal setback distances for sandy beaches backed by
dunes. This model was modified by McDougal and MacArthur (2004) to provide estimates of
beach profile recession due to large storm events. The model is based on the underlying
assumptions of an MLWP and the characteristic shape of shoreline recession that will result
during a large wave and water-level event. The shoreline recession profile is characterized by the
beach face slope, m, the beach-dune juncture elevation, Ej, and cross-shore location of the beach-
dune juncture, yj. These are shown in Figure D.4.6-13A. The juncture elevation is taken to occur
at the maximum extent of the total runup plus the measured tide. The measured tide includes all
processes that influence the water surface elevation such as surge and El Niño. The total runup is
defined to include static and dynamic wave setup. The sum of the total runup plus the tide
(including surge and El Niño) is referred to as the total water level (TWL). The sum of the
astronomical tide, El Niño, and surge is the still water level (SWL) and is typically obtained from
measurements. The setup and runup are calculated using methods described in Section D.4.5.
D.4.6.5.2.1 MK&A Methods for Estimating the MLWP
The first step for determining eroded beach profiles is to estimate the MLWP for each cross-
shore profile. When using the MK&A method, the upper profile is specified by the beach face
slope in the swash zone, m and the beach-dune juncture elevation and cross-shore location, Ej
MLWP and yj MLWP as shown in Figure D.4.6-13A. Because both the elevation and location of the
juncture may be associated with different magnitudes of TWL events, the notation ()MLWP is used
to denote the MLWP case. The juncture elevation in the MK&A model is taken to occur at the
maximum extent of the still water plus the total runup. The measured tide includes all processes
that influence the water surface elevation such as the astronomical tide, surge, and El Niño. The
runup is defined to include wave setup. The beach face slope is determined in the swash zone at
high water levels. For the MLWP, m and Ej MLWP are determined from beach profile
measurements following a significant storm or at the end of the winter season, or they may be
determined from typical winter wave and water-level conditions (as explained below).
The MLWP should be determined from profile data immediately following a significant storm or
series of winter storms. The greater the time between the end of the storm conditions and the
measurement of the profile, the less reliable the estimates of the MLWP. During this time,
aeolian transport, sloughing of the dune face scarp, and re-construction of the upper profile all
tend to mask the MLWP beach face (swash zone) slope and in particular, the beach-dune
juncture. As these processes proceed, the elevation of the beach-dune juncture actually increases.
Profiles taken during the summer and fall should not be used. Profiles measured later in the
winter season are preferred as they should represent the maximum beach response due to the
seasonal cycle.
D.4.6.5.2.2 Estimating Ej MLWP and m from Profile Data
Komar and Allan (2004) suggest that Ej MLWP can be determined from LIDAR data and field
verification. Unfortunately, this requires data collected immediately following significant storm
events in order to capture the most likely eroded profile before other processes occur that may
mask Ej MLWP or adjust its location. This procedure is also best supported by detailed site
inspections immediately following storm events in order to survey and photo document beach
profile conditions. McDougal and MacArthur (2004) discuss sensitivities and difficulties
estimating the MLWP based solely on survey data.
D.4.6.5.2.3 Estimating m from Median Diameter of Beach Sands
Bascom (1964), Wiegel (1964), and others have shown that there are strong correlations between
the beach face slope and the median diameter of the beach sands as shown in Figure D.4.6-14.
These types of relationships can be used to estimate the beach face slope. The user must select
the curve that best matches the coastal exposure, beach material characteristics, and settings
represented by the curves prepared by the original authors. Open coasts along Oregon and
Washington experience beach slopes approximately two times as steep as one would estimate
using Wiegel’s regional relationship as shown in Figure D.4.6-14, or approximately 1:25-30
(v:h). Therefore, Mapping Partners shall check estimated slope values from Figure D.4.6-14 with
observed data. It is recommend that regional relationships similar to these be developed and
tested for the different coastal exposures and settings found in California, Oregon, and
Washington for estimating winter beach face slope.
Figure D.4.6-13. Definition Sketches for Terms and Dimensions
Required by the Modified Komar & Allan Geometric Model
(after Komar et al., 2002, and McDougal and MacArthur, 2004)
Figure D.4.6-14. Relationships Between Beach Slope and Median
Diameter of Beach Sands (from Wiegel, 1964)
D.4.6.5.2.4 Ej MLWP from Wave and Water Levels
Given the difficulty in identifying a single value to select for Ej MLWP based on beach profile data
alone, it may be possible to supplement the estimate with information about the waves and water
levels that are typically responsible for producing the dune-beach juncture elevation, Ej . The
juncture elevation can be estimated for the typical winter wave conditions as:
Ej = (R + ET)winter storm average (D.4.6-1)
where the runup includes the setup and the tide includes surge and El Niño (see Figure D.4.6-
13A). In Equation D.4.6-1, Ej represents the average of the sum of R and ET from 10 to 20 largest
storms per year, averaged over the storm duration for the entire wave data record.
The following subsection provides detailed procedures for estimating beach profile changes for
sandy beaches backed by berms and sand dunes (Beach Setting No. 1, Table 4.6-1) using the
MK&A model.
D.4.6.5.2.5 MK&A Model for Estimating Beach Profile Changes
The key assumption in the MK&A model is that when a large storm event occurs, the upper
beach face slope remains constant but the beach-dune juncture adjusts in response to the higher
level of waves and tides. This is shown in Figure D.4.6-13B. A typical winter pre-storm
(MLWP) condition is shown as the solid line. This is taken as the initial condition (the MLWP)
to establish m and EjMLWP. If a storm event elevates the TWL, then the shoreline erodes and
retreats. The beach-dune juncture location associated with this retreat (point B in Figure D.4.6-
13B) is estimated as the projection along the beach face slope onshore to the elevation of the
storm’s TWL. Above this elevation, the sand is assumed to remain at the angle of repose
(approximately 30o) up to the surface elevation of the dune. The recession in the MK&A model
due to Ej Storm is calculated as the recession in excess of the MLWP. The maximum potential
recession is given by:
(D.4.6-2)
where EjStorm and EjMLWP correspond to Equation D.4.6-1 beach-dune juncture elevations
evaluated at the storm conditions and for the MLWP. The MK&A method gives the maximum
potential equilibrium cross-shore change in shoreline position landward from the MLWP
resulting from a storm event. However, the actual amount of beach erosion and dune recession
depends on wave conditions, TWL, and storm duration. Therefore, the amount of beach erosion
and dune recession for a particular storm event is less than the maximum potential cross-shore
change represented by R?Storm by a factor referred to as the storm duration recession reduction
factor, ?, as discussed in Subsection D.4.6.5.3. Figure D.4.6-13D shows a sketch of this where
the profile ACBD represents the maximum potential equilibrium cross-shore change in shoreline
position, R? Storm and ACE is the actual eroded profile for the storm related to its duration. This is
discussed further in Subsection D.4.6.5.3.
The cross-shore location of the juncture point, yj is the initial location for the MLWP and may
change with time (Figure D.4.6-13A). This can be in response to chronic erosion, sea-level
changes, or other long-term effects. It may be necessary to adjust yj for the MLWP if the time
between the MLWP determination and the analysis of the recession is significant or if chronic
shoreline position changes are significant.
If a beach consists of a thin layer of sand capping a wave-cut terrace or other erosion-resistant
materials, then the MLWP occurs at the location and profile of the erosion-resistant layer.
Figure D.4.6-13C shows a second beach-dune juncture at point D. This additional recession is
associated with other processes such as chronic erosion or local hot spots. Hot spots may develop
when the profile is located in a rip current embayment or in the lee of a littoral barrier. Following
Komar et al. (2002), where an adjustment was allowed for hot spots, the recession may be
written as:
(D.4.6-3)
where EHotSpot is the localized lowering of the profile due to shoreline recession during a
significant storm event due to local hot spots. Effects of site-specific hot spots and the amount of
local beach lowering at that location is estimated from seasonal monitoring data from past large
storm events.
D.4.6.5.2.6 Dune Overtopping with the MK&A Model
If Ej Storm exceeds the dune crest elevation, then the dune will be overtopped. This occurs
independently of the storm duration or profile recession. The overtopping volumes may be
estimated using the excess runup, which is the height that the predicated runup exceeds the dune
elevation. If Ej Storm is less than the dune crest elevation, the dune crest may still be removed as
shown by profile ACD in Figure D.4.6-15.
Note that the MK&A method is best applied to sand beaches with high dunes where overtopping
is not expected to occur. The K&D method (discussed in Subsection D.4.6.5.5) will
accommodate some overtopping, and therefore is more suitable for sand beaches with dunes and
lower berms. However, neither model addresses the changes in the berm nor dune shape when
overtopping occurs. Breaching typically results in a significant lowering of the dune profile and
development of an overwash fan. The present methodologies do not provide a direct mechanism
to address this breaching process. When overtopping occurs, the dune profile is adjusted by
extending the MLWP slope m to the backside of the dune. Figure D.4.6-15B shows this scenario
as profile ACE continuing seaward from the heel of the dune. Relationships like those shown in
Figure D.4.6-14 by Wiegel (1964) can be used to estimate the ultimate beach face slope
following significant dune breaching. If this approach is used, Mapping Partners should check
the reliability of their results with observed information and data.
The following subsection describes how the effects of storm duration and seasonal responses are
considered by the two recommended geometric models (MK&A and K&D).
Figure D.4.6-15. Schematics of Dune Overtopping with the MK&A Model
D.4.6.5.3 Time Dependency of Profile Response (Within the MK&A and K&D Models)
The geometric models (MK&A and K&D) provide an estimate of the maximum potential cross-
shore displacement of the profile. The wave and water levels must persist long enough to achieve
this maximum. This is often not the case because a single storm event may have a shorter
duration than is required to achieve the maximum potential cross-shore recession. Kriebel and
Dean (1993) proposed a method to include the duration effects of a storm with respect to the
response time scale of a beach profile.
The time scale for the beach profile was estimated from numerical model results to be:
(D.4.6-4)
in which Ts is the time scale, C1 is an empirical constant (=320), Hb is the breaker height, hb is
the breaker depth, g is the acceleration due to gravity, B is the berm elevation, m is the beach
face slope, Wb is the surf zone width, and A is the beach profile parameter that defines an
equilibrium profile according to Equation D.4.6-5.
h = A y2/3 (D.4.6-5)
The beach profile parameter, A, depends primarily upon sediment grain size, D50. Table D.4.6-2
summarizes A over a range of sediment sizes. The values in Table D.4.6-2 are well approximated
by the equations:
(D.4.6-6)
in which D50 is the sand diameter in mm and A is in m1/3 or ft1/3. Table D.4.6-3 gives estimates of
the time scale for several representative conditions. It is seen that typical times are on the order
of 10 to 100 hours. As the surf zone width increases, the response time also increases. Properties
that increase the surf zone width include larger wave height, smaller sand size, and a milder
slope. The response time also increases as the berm height increases. The longer profile response
time associated with larger wave heights has the interesting result that the largest wave height
may not yield the largest recession because it takes longer for the larger waves to achieve the
maximum potential recession. Consider the first two waves in Table D.4.6-3, which only differ
in wave height and the associated breaker depths. Assuming the period in both cases is 13 s and
the storm duration is 24 hours and employing methods discussed below, the 10-foot wave height
has a recession of 70 feet and the 20-foot wave has a recession of 55 feet.
Table D.4.6-2. Equilibrium Beach Profile Coefficients
(Dean and Dalrymple, 2002)
D50 (mm)
A (m1/3)
A (ft1/3)
0.1
0.063
0.0936
0.2
0.100
0.1486
0.3
0.125
0.1857
0.4
0.145
0.2155
0.5
0.161
0.2392
0.6
0.173
0.2571
0.7
0.185
0.2749
0.8
0.194
0.2883
0.9
0.202
0.2987
1.0
0.210
0.3120
Table D.4.6-3. Estimates of the Beach Profile Time Response
Hb (ft)
hb (ft)
D50 (mm)
A (ft1/3)
m
B (ft)
Wb (ft)
Ts (hrs)
10
13
0.2
0.1486
0.05
10
801
28
20
25
0.2
0.1486
0.05
10
2267
53
30
38
0.2
0.1486
0.05
10
4164
77
20
25
0.2
0.1486
0.05
1
2267
14
20
25
0.2
0.1486
0.05
10
2267
53
20
25
0.2
0.1486
0.05
20
2267
64
20
25
0.2
0.1486
0.01
10
2267
96
20
25
0.2
0.1486
0.02
10
2267
80
20
25
0.2
0.1486
0.10
10
2267
34
20
25
0.1
0.0936
0.05
10
4533
138
20
25
0.2
0.1486
0.05
10
2267
53
20
25
0.5
0.2392
0.05
10
1110
18
The beach profile response is determined by a convolution integral. It is assumed that the time
dependency of the storm hydrograph may be approximated as:
(D.4.6-7)
where t is time from the start of the storm and TD is the storm duration. The convolution integral
is:
(D.4.6-8)
which integrates to:
(D.4.6-9)
where and is the maximum potential recession that would occur if the storm
duration was infinite as yielded by Equations D.4.6-2 and D.4.6-3 (Figure D.4.6-13D) for the
MK&A method. If the storm duration is long with respect to the profile time scale, then a
significant portion of the maximum potential shoreline response will occur. As the ratio of
TS / TD decreases, less of the maximum shoreline change will be realized. The time of the
maximum recession is determined by setting the derivative of Equation D.4.6-9 equal to zero and
solving for the time. This yields:
(D.4.6-10)
in which tm is the time that the maximum occurs with respect to the start of the storm.
Unfortunately, this is a transcendental equation and must be solved by approximation or
numerical methods. The maximum recession that occurs as the result of a single storm or
duration limited response is:
(D.4.6-11)
where ? is the storm duration recession reduction factor, Rm is the maximum recession that
occurs for the given storm duration that occurs at time tm. Figure D.4.6-16 gives the solution to
Equation D.4.6-11 in graphical form. Therefore, duration limited recession is:
Rm = ? R? (D.4.6-12)
Multiple Storm Responses
The maximum recession observed on the Pacific Coast often results from the occurrence of
several storms in succession. Unless there is site-specific information or guidance for using
multiple storms, it is recommended that a single storm analysis be used. If multiple storms are to
be considered, then the cumulative recession may be estimated by summing the contribution of
each storm to the recession beyond the previous profile. McDougal and MacArthur (2004b)
discuss methods for conducting cumulative recession analyses in their report entitled EBE
MLWP Discussion. Before initiating a seasonal response investigation, Mapping Partners should
check with the FEMA study representative to confirm that this level of analysis is necessary and
that there are sufficient historical data to confirm the results.
Figure D.4.6-16. Storm Duration Recession Reduction Factor
D.4.6.5.4 Summary of the MK&A Geometric Modeling Approach for Sand Beaches
Backed by Sand Berms and Dunes (Beach Setting No. 1)
Mapping Partners evaluating sand beaches backed by sand berms and dunes (Beach Setting
No. 1) using the MK&A model approach shall complete the following steps to estimate the
beach erosion and recession associated with storm events. Figure D.4.6-17 shows the sequence
of key activities and computational considerations required to determine storm-induced beach
profile changes for Beach Setting No. 1.
Figure D.4.6-17. Key Activities to Determine Beach Profile Changes for Beach
Setting No. 1
Develop Data:
1. Obtain wave and water-level data necessary to define the waves and water levels for the
10-20 largest storms each year.
2. Determine existing shoreline location and conditions.
3. Define reaches alongshore in which wave, beach, and backshore conditions are nearly
uniform. Data and calculations must be conducted for at least each subreach.
4. Obtain beach profile data required to establish the MLWP or the annual winter wave and
water-level conditions to develop an MLWP for each subreach.
5. Determine median sand diameter, D50, on the beach face for each subreach.
6. Obtain historical beach profile data required to estimate the magnitude of local hot spot
erosion and site-specific beach lowering with each subreach being evaluated within the
study area.
7. Seek historical data for use in validating results from the application of the simple
geometric models.
Determine MLWP:
1. Estimate beach slope m from measured post-storm winter profile data as discussed in
Subsection D.4.6.5.2 or use a relationship such as Wiegel’s (1964) (Figure D.4.6-14) to
relate median beach sand diameter to beach slope, m.
2. Estimate Ej MLWP based on measured winter profile data following the occurrence of large
storms or make estimates using winter wave and water-level conditions as outlined in
Subsection D.4.6.5.2.
3. Estimate the cross-shore location for the MLWP, yj, related to existing beach profile
conditions.
Determine Beach Recession for Each Storm Event:
1. Determine static setup and/or TWL as required for the geometric recession model to
calculate the potential recession for the storm, R? storm.
2. Determine storm duration recession reduction factor for the storm, ? (Figure D.4.6-16).
3. Determine duration limited recession for storm, Rstorm, and if the berm/dune is breached,
modify beach and berm/dune profile to account for breaching or local hot spot erosion if
necessary according to Subsection D.4.6.5.3.
4. If runup is different on the modified profile, re-compute runup.
5. If runup results in overtopping, then compute overtopping. Save the maximim
overtopping value. Also compute the overtopping volume as V = integral Q dt over
duration of storm.
6. For each year, save conditions corresponding to the largest annual TWL storm event:
TWL, Q, V, ?, H, T, D, ?, Rstorm , etc.
Use Measured Profiles to Validate Results:
Mapping Partners shall always locate the best and most reliable measured data for their project
site. They should also use measured beach profile data wherever possible: (1) to aid in estimating
the MLWP, and (2) to determine, calibrate, and validate the eroded beach profile for a specified
storm event. The eroded beach profile estimated for a particular storm event is the profile
required for computing runup and overtopping associated for that event.
Determine the 1% Storm Event:
Mapping Partners shall follow procedures outlined in Section D.4.2 for determining the 1%
percent storm conditions for use in determining flood hazards.
D.4.6.5.5 K&D Geometric Modeling Approach for Sand Beaches Backed by Sand
Berms and Dunes (Beach Setting No. 1)
Kriebel and Dean developed an analytical solution to approximate the temporal beach-profile
response to a single storm (Kriebel and Dean, 1993). The maximum potential recession of a sand
beach profile, R?, was established based on the equilibrium principle proposed by Bruun (1962)
for erosion due to long-term, sea-level rise. Kriebel and Dean assumed an equilibrium beach
profile with respect to the prevailing water level and wave climate. Typically, the prevailing
water level is referred to the mean sea level (MSL). The eroded profile is then shifted upwards
by an elevation equal to the water-level rise caused by storm surge and wave setup, and landward
by an amount of beach recession potential (R?) until a volume balance is achieved between sand
eroded from the upper portion of the beach and sand deposited offshore. Based on this
conservation of sand volume, the maximum erosion potential (R?) can be defined as a function
of the water-level rise (S) during a storm, breaking wave depth (hb), surf zone width (Wb), berm
or dune height (B or D), and the slope (m) of the upper foreshore beach face. Along the Pacific
Coast, the water-level rise during a storm event is mostly influence by wave setup, as the storm-
induced surge tends to have a minor effect.
Kriebel and Dean presented analytical solutions to estimate the maximum erosion potential R?
for four different beach settings. The solution for one of the four settings that is typically
observed on the Pacific Coast, as shown in Figures D.4.6-18 and D.4.6-19, is presented as
follows:
? Maximum erosion potential for a beach backed by a low sand berm:
(D.4.6-13)
Figure D.4.6-18. Definition Sketch for K&D Geometric Model
Figure D.4.6-19. Sketch for K&D Geometric Model for Case Where Historical Beach
Profile Data Are Available to Prepare the MLWP
? Maximum erosion potential for a beach backed by a high sand dune:
(D.4.6-14)
where S is the water-level rise representing the sum of the peak storm surge (wind effects and
barometric pressure effects) and the wave setup, hb is the breaking water depth, Wb is the surf
zone width, m is the slope of the foreshore fronting face, and B and D are the berm and dune
heights above the prevailing water level, respectively. Equations D.4.6-13 and D.4.6-14 estimate
the maximum recession potential, assuming that the storm event lasts indefinitely. The actual
storm-induced recession (Rm), which depends strongly on the duration of each storm event, must
be multiplied by a storm duration recession reduction factor, ?, as stated in Subsection D.4.6.5.3.
For backshore profiles that are not well approximated by the analytical solutions given in Kriebel
and Dean (1993), a conservation of sand volume equation (i.e., a simple balance of cuts and fills)
may be solved numerically. Further discussion of this computational procedure is provided in the
following guidelines.
Because the elevated storm water level is based on the magnitude of wave setup and storm surge
only to a small extent on the Pacific Coast, it is likely that the storm water level is below the
berm or dune height. In the event that the elevated storm water level is higher than the crest of
the berm or dune, the K&D model may no longer be applied and the profile must be adjusted for
overtopping. When the K&D model is applied to estimate the storm-induced erosion, various
model input parameters are required. The calculation of storm-induced erosion, using the K&D
convolution method, is delineated as follows:
Acquire Wave and Water-level Data:
1. Obtain hindcasted wave data (e.g., Global Reanalysis of Ocean Waves [GROW] data)
and measured historical water levels necessary to define the oceanographic conditions
including waves and water levels for 10-20 largest storm events for every hindcasted
year.
2. Acquire historical beach profiles to establish the MLWP (i.e., pre-storm beach profile
conditions).
3. Seek historical pre- and post-storm profiles to validate the application of the simple K&D
geometric models.
Determine MLWP for K&D Method:
First, determine existing shoreline location and conditions. Then, establish representative reaches
within the shoreline area being analyzed that are similar in coastal morphology (average
offshore/nearshore bathymetry, wave exposure, onshore beach slope, beach materials, etc.). This
may consist of only one typical reach or several different typical reaches for the shoreline area
being analyzed. Following are procedures for establishing the MLWP for a sandy beach backed
by either a low sand berm or a high sand dune for application with the K&D geometric model.
1. Procedure for a Study Site Without Previously Surveyed Historical Profiles
a. Determine existing shoreline location and conditions.
b. Always use measured historical post-storm winter beach profile data when available
to establish the MLWP. However, if there are no historical post-storm winter beach
profile data, conduct a basic wading survey from the crest of the berm or dune to the
approximate mean low low water (MLLW) line (see National Oceanic and
Atmospheric Administration [NOAA] tidal datum) following a series of winter
storms in March or April to prepare a surveyed beach profile from the berm crest to
approximately MLLW.
c. Collect sediment samples, preferably in late March or early April to determine the
median sand diameter (D50) for use in Equations D.4.6-5 and D.4.6-6.
d. Determine the MSL from NOAA’s tidal datum, and identify the MSL location across
the beach profile. This location divides the beach profile into an upper foreshore
berm/dune section and the surf zone section.
e. Plot the measured upper foreshore profile section above the MSL line based on the
basic wading survey, site photographs, and available historical information (see
Figure D.4.6-18).
f. Determine the berm or dune height (B or D) above the MSL line and foreshore slope
(m) from the estimated upper foreshore section (see Figure D.4.6-18).
g. Approximate the surf zone section of the MLWP from Kriebel and Dean’s
equilibrium beach profile, based on the measured D50 and the application of Table
D.4.6-2 and Eqs. D.4.6-5 and D.4.6-6.
h. Assemble the entire MLWP based on the surveyed upper foreshore and surf zone
sections linked at the MSL, as illustrated in Figure D.4.6-18.
i. Document data sources, assumptions, and conversions.
2. Procedure for a Study Site With Previously Surveyed Historical Profiles:
a. Determine existing shoreline location and conditions.
b. Select a representative surveyed winter profile (see Figure D.4.6-19) from historical
post-storm beach profile data, which was surveyed during the end of the winter
season (March-April) and represents the typical winter beach profile conditions.
c. Determine the MSL from NOAA’s tidal datum, and identify the MSL location across
the beach profile. This location divides the beach profile into an upper foreshore
berm/dune section and the surf zone section.
d. Determine the berm or dune height (B or D) above the MSL line and compute the
foreshore slope (m) through linear curve fitting to the upper foreshore section.
e. Determine the surf zone section of the MLWP by curve-fitting Kriebel and Dean’s
equilibrium profile from Eqs. D.4.6-5 and D.4.6-6 to the surf zone section of the
surveyed beach profile below the MSL line (see Figure D.4.6-19).
f. Assemble the entire MLWP based on the approximated upper foreshore and surf zone
sections linked at the MSL, as illustrated in Figure D.4.6-19.
g. After the MLWP is defined, determine the K&D model input parameters including
the berm or dune height (B or D) and the foreshore slope (m) (see Figures D.4.6-18
and D.4.6-19).
Quantify Peak Storm Conditions of a Selected Storm Event:
The peak storm conditions for a selected storm event shall be used to determine the storm surge
including the wave setup, wave breaking, and storm duration. Storm surge resulting from the
fluctuations in the wind speed and atmospheric pressure is usually small in the Pacific Coast, and
thus the wave setup is the primary parameter to determine the water-level rise. The increase in
water level induced by El Niño events should also be included, if applicable (see Section D.4.4,
Waves and Water Levels). The peak storm conditions include wave height, period, and incoming
direction at the peak of the storm, as well as the storm duration that is characterized in Section
D.4.4. These wave conditions are used to determine the wave setup (S), water depth of breaking
wave (hb), and the surf zone width (Wb) needed by the K&D geometric model. The MSL water
depth can be used as a representative water depth to calculate wave transformation. The
procedures are listed as follows:
1. Determine the breaking water depth (hb) and the surf zone width (Wb), based on the
MLWP and the selected wave event.
2. Estimate the wave setup using the formula presented in Subsection D.4.5.1.
3. Based on the procedures described in Section D.4.4, calculate the storm surge induced by
wind effects and barometric pressure effects, if applicable.
4. Estimate the increase in water level induced by the El Niño Southern Oscillation (ENSO)
events in accordance with the procedures stated in Section D.4.4, if applicable.
5. Determine the total increase in water level (S) induced by the storm (see Figures D.4.6-18
and D.4.6-19).
Calculate Storm-induced Beach Erosion:
1. Calculate the maximum beach erosion potential R? using Equation D.4.6-13, if the
subject beach is backed by sand berms with height B.
2. Calculate the maximum beach erosion potential R? using Equation D.4.6-14, if the
subject beach is backed by sand dunes with height D.
3. Calculate the time scale (TS) from Equation D.4.6-4.
4. Determine the storm duration (TD) from Subsection D.4.4.1, and compute the storm
duration recession reduction factor, ?, from Figure D.4.6-16 for the given value of TD/TS.
5. Multiply the maximum recession potential (R? ) by the storm duration recession reduction
factor to estimate the storm-induced beach erosion and recession distance (Rm).
Prepare Eroded Post-Storm Beach Profile:
1. Set back the upper foreshore profile above the elevated storm wave level (see Figures
D.4.6-18 and D.4.6-19) landward by the calculated berm or dune recession distance Rm
with the same fronting-face slope (m).
2. Place the new link point between the upper foreshore section and the surf zone section at
the elevated storm water level (see Figures D.4.6-18 and D.4.6-19).
3. Shift the surf zone section of the MLWP below the MSL landwards and upwards to the
link point (see dashed curve below the MSL line in Figures D.4.6-18 and D.4.6-19).
4. The adjusted profile from Steps 1 to 3 produces the “eroded storm profile” for a specified
location and beach profile. Mapping Partners shall perform these steps for all beach
profiles needed to describe the spatial adjustments to the beach and dune system being
evaluated.
5. Document results and assumptions.
D.4.6.5.6 Potential Future Use of Process-based Models
Process-based models are typically more complex and have greater data input requirements than
simplified geometric models, but are formulated to include the effects of more of the physical
processes affecting beach erosion and coastal sediment transport. While conserving sand
volumes, as done by the K&D geometric model, process-based models also compute the cross-
shore transport of beach sand induced by nearshore storm waves, and determine the change in
beach profile based on material grain size, wave energy, and the variation in sand transport rate.
Two simple process-based models, SBEACH (Larson and Kraus, 1989, 1998; Larson et al.,
1990) and EBEACH (Kriebel, 1984a, 1984b; Kriebel and Dean, 1985), have been widely used in
the eastern United States to calculate storm-induced beach erosion. Other, more complex
process-based models, such as the COSMOS model (Southgate and Nairn, 1993) developed in
England, have also been used to calculate storm-induced beach erosion. More complex models
can be data-intensive, time-consuming, and costly to use. However, for certain settings,
application of simple or complex process-based models presents a significant opportunity to
improve how beach profile changes are depicted over simplified geometric-based models.
Both SBEACH and EBEACH have been calibrated to the large-scale laboratory wave-tank
experiments and field data on the Atlantic and Gulf coasts. They have been applied to numerous
field case studies on the Atlantic and Gulf coasts, and to a lesser degree in the Great Lakes,
environments that more closely fit the conditions for which they were developed and calibrated.
However, several less-successful experiences using SBEACH, EBEACH, and COSMOS have
occurred on the coasts of California (Noble Consultants, 1994) and Oregon (Komar et al., 1999;
Komar, 2004). Documentation of these attempted applications indicates that these process-based
models may underpredict the erosion during storms on the Pacific Coast, where the beach
morphology and storm characteristics differ from the beach settings that were used in developing
these models.
In August 2004, the U.S. Army Corps of Engineers (USACE) officially recognized the limitation
of SBEACH to predict erosion on Pacific Coast beaches and funded a research program to
explore the causes of the model’s underprediction of erosion on the Pacific Coast so as to
improve its applicability for the Pacific Coast region. The Coastal and Hydraulics Laboratory
(CHL) is currently modifying the SBEACH model, so it can be applied to the specific site
characteristics and wave climate of the Pacific Coast region. Noble Consultants has provided the
identical database to CHL for the four southern California shoreline locations that were used for
field verification of the K&D (Kriebel and Dean, 1993) and MK&A (McDougal and MacArthur,
2004) simplified geometric models discussed above.
Eventually, simple and complex process-based models will become more reliable and will
ultimately provide additional means for estimating event-based erosion along the Pacific Coast.
However, further reformulation and validation are required before they can be used in FEMA
coastal flood hazard assessments. At this time, without accurate calibration to local conditions,
process-based models are not recommended for general use.
D.4.6.6 Estimating Profile Changes for Sandy Beaches Backed by Protective
Structures (Beach Setting No. 2)
Figure D.4.6-20 shows a photograph of a typical sand beach backed by coastal development and
protective structures in Southern California. This setting is often subject to dramatic beach
erosion and profile adjustment during single events. Figure D.4.6-3 shows a typical beach profile
for this setting, while Figure D.4.6-21 shows the sequence of key activities and computational
considerations required to determine storm-induced beach profile changes for Beach Setting
No. 2.
The Mapping Partner shall perform the following steps to develop the MLWP for this type of
setting:
1. Review the references listed in the support documents and literature on assessing
performance and erosion at coastal structures; review Section D.4.7.
2. Determine existing shoreline location and condition of structures.
3. Examine photos and historical pre- and post-storm event LIDAR and beach profile data
for the study area and develop a MLWP for the beach by examining the envelope of
seasonal post-storm event beach profile data.
Figure D.4.6-20. Sand Beach Backed by Shore Protection Structures,
Crystal Cove, California (Source: Noble Consultants)
Figure D.4.6-21. Key Activities to Determine Beach Profile Changes for Beach
Setting No. 2
4. The toe of a structure often becomes buried beneath sand deposits during calm sea
conditions. Try to determine whether the MLWP profile is formed by the structure at its
toe by probing through the sand along the toe of the structure to measure the depth of
sand at the toe. Inspection trenches can also be dug and profiled by an experienced
coastal geologist (here profiling means identifying and mapping the vertical location and
thickness of distinct sediment lenses along the cut face of an inspection trench). Results
from this activity should provide information on the historical depth of scour that has
occurred in front of the structure.
5. If a relatively broad sandy beach is located in front of the coastal structure, the MLWP
for the sand beach portion must be estimated from historical winter beach profile data,
supplemented with probing or inspection trench data down to an elevation of
approximately MLLW.
6. Survey the elevation of the top of structures and back-beach profile. Depending on
available data, some types of LIDAR data may be suitable for this purpose.
7. Next, splice the structural profile, average winter sand beach profile, and back beach
profiles together to create a continuous beach profile that represents the complete MLWP
for the beach, structure profile, and back beach areas.
8. Next, determine if the MLWP can experience additional erosion during a selected storm
event.
9. If further erosion is possible during a large storm, determine likely depth of local scour in
front of structures and compare to the scour depths determined from probing or trenching
and smooth MLWP to reflect this local change; refer to Section D.4.7 and to the CEM
(USACE, 2003).
10. Use this continuous beach profile for subsequent runup computations.
11. Determine whether the structures are overtopped, damaged, or removed during the storm
event being evaluated according to methods prescribed in Section D.4.7.
12. Try to validate assumptions and results using observed data from previous large storm
events.
13. Document assumptions, data that were used, and results.
D.4.6.7 Estimating Profile Changes for Gravel and Cobble Beaches (Beach
Setting No. 3)
Explicit procedures for determining beach and back beach profile changes on gravel and cobble
beaches are not as well developed or documented as for sand beaches. Figures D.4.6-22 and
D.4.6-23 show typical Pacific Coast cobble berm-backed beaches. Figure D.4.6-4 provides a
sketch of a typical cobble beach profile (Beach Setting No. 3) and shows the different material
layering and composition present for these settings. Cobble beaches and berms are often quite
stable features as indicated by the measured cross-beach profiles shown in Figures D.4.6-24 and
D.4.6-25. The Mapping Partner shall assume that the cobble beach and cobble berms are stable
features during storms and act like natural shoreline protection features, but that the toe and
apron become partially covered with sands during summer months and mild winter seasons.
Figure D.4.6-22. Sand Beach Backed by Cobble Berm and Bluffs, South Carlsbad,
California (Photo by Noble Consultants)
Figure D.4.6-23. Sand Beach Backed by Cobble and Shingle Berm and Sandy Terrace,
Batiquitos Lagoon, California (Photo by Noble Consultants)
Figure D.4.6-24. Measured Beach Profile Change Due to January 1988 Storm,
Batiquitos Lagoon, California
Figure D.4.6-25. Measured Beach Profile Change Before and After January 1988 Storm,
Batiquitos Lagoon, California
Perform local probing and inspection trenching from the berm face out onto the beach to validate
this assumption and to determine the typical eroded winter profile. If it is determined from other
measured data from past storm events that the eroded winter profile underestimates the amount
of cobble beach retreat during large storm events, then use the observed data for the eroded
beach profile for runup and overtopping. Otherwise, use the eroded winter profile for subsequent
runup and overtopping computations.
Figure D.4.6-26 shows the sequence of key activities and computational considerations required
to determine storm-induced beach profile changes for Beach Setting No. 3.
Figure D.4.6-26. Key Activities to Determine Beach Profile Changes for Beach
Setting No. 3
Mapping Partners shall use the following procedure to establish the typical eroded winter profile
for Beach Setting No. 3, Cobble, gravel or shingle beaches and berms:
1. Review the references listed in the support documents and literature on the design of and
construction of dynamic revetments and cobble berms.
2. Examine photos and historical pre- and post-storm event LIDAR and beach profile data
for the study area and develop a typical winter profile from observed data (refer to Figure
D.4.6-25 for an example of historical profiles).
3. Verify this profile by probing or trenching through the sand along the toe of a cobble
berm to measure the profile of the cobble berm left by the previous period of high wave
conditions. (See Figures D.4.6-4 and D.4.6-25 that show how the toe of a cobble berm
often becomes buried beneath sand deposits during calm sea conditions.) The eroded
profile (circles) was surveyed only a few days following a major storm event in January
1988.
4. If a relatively broad sandy beach is located in front of the cobble berm, determine
whether there is a history of significant erosion of the sand beach portion and include that
information in the beach profile data.
5. Survey the top of berm and back-beach profile.
6. Splice the cobble berm profile, winter sand beach profile, and top-of-berm and back
beach profiles together to create a continuous beach profile that represents the complete
profile for the beach, cobble berm, and back beach areas.
7. Use this eroded beach profile for subsequent runup computations during the selected
storm event, unless other information indicates the profile may need further adjustment
during large storm events.
8. Check results and try to validate them with observed information.
9. Document assumptions and results.
D.4.6.8 Estimating Profiles for Beaches Backed by Erodible Bluffs or Cliffs
(Beach Setting No. 4)
Significant portions of the California coast have narrow to nonexistent beaches backed by high,
steep, erodible coastal bluffs and cliffs, as shown in Figures D.4.6-27, D.4.6-28, and D.4.6-29
and illustrated in Figure D.4.6-5. The evolution of this bluff-type shoreline is significantly
different from that of the sandy beaches backed by either dunes or low-lying berms. A thin sand
lens often overlays a rocky beach or bedrock platform fronting the bluff. These thin deposits of
sand are removed each winter storm season. If storm water levels reach sufficient elevations to
intersect the toe of the bluffs, storm waves can directly impinge upon the bluff face causing bluff
toe erosion. If enough material is eroded from the toe during a storm, the upper portion of the
bluff can fail, resulting in bluff retreat (Figures D.4.6-28 and D.4.6-29). This type of bluff failure
and retreat is common along the Pacific Coast, and is particularly important in the highly
developed coastal communities of central and southern California. It should be noted that
significant bluff failure may not occur during all storm events. However, if the bluff materials
are erodible, toe erosion and bluff failure is possible during single storm events. The rate and
extent of bluff erosion and failure depends on the site-specific bluff profile conditions at the time
of the event (toe elevation and setback distance from the surf zone) and on the erodibility of the
bluff materials. In some locations, it may take several storms to cause sufficient toe erosion to
lead to bluff failure, or only one significant event with sufficient TWL, duration, and wave
orientation to result in significant storm erosion.
Previous estimates for coastal bluff retreat have typically resorted to temporally averaged rates
over a long period. Though the average annual rate of coastal cliff erosion is a reasonable
indicator of the gradual retreat of the bluff top, it does not adequately predict the episodic nature
of bluff failure that can result in 3 to 50 feet of retreat during a single storm event. The average
annual retreat rate provides a misleading indication of the hazards of coastal bluff or cliff erosion
because the occurrence of storms of sufficient magnitude and duration to cause significant buff
retreat are episodic. At some locations, coastal bluffs have fairly low elevations and may be
overtopped by large wave events. Therefore, assessment of coastal flood and erosion hazards in
coastal settings (Beach Setting No. 4) with erodible bluffs requires special methods and data.
Figure D.4.6-27. Wave Cut Coastal Bluff, Encinitas, California
(Source: USACE, Los Angeles District)
Figure D.4.6-28. Bluff Failure and Retreat During 1998 El Niño Storms, Pacifica, California
(Photo by Kevin Coulton)
Figure D.4.6-29. Bluff Failure and Retreat, Encinitas, California
(Photo by Noble Consultants, Inc.)
During the reconnaissance phase of a coastal flood assessment, Mapping Partners shall
determine whether the study area includes erodible coastal bluffs and cliffs (Beach Setting No. 4)
and whether the bluff elevations are low enough for overtopping.
D.4.6.8.1 General Approach for Beach Setting No. 4
Figure D.4.6-30 shows the sequence of key activities and computational considerations required
to determine storm-induced beach profile changes for Beach Setting No. 4.
Determine the coastal setting and history of episodic and chronic bluff erosion for the study area;
then:
1. Obtain reliable beach and bluff profile data (surveyed cross-shore profiles or LIDAR) for
existing conditions. Try to obtain these data near the end of the winter season in March or
April.
2. Determine whether bluff erosion and failure monitoring data are available for the study
area. Obtain and examine that information to determine the magnitude of episodic toe
erosion and bluff retreat.
3. Estimate top-of-bluff elevations and compare to potential significant storm TWL and
whether the bluff is subject to overtopping or frequent wave attach or toe erosion.
Figure D.4.6-30. Key Activities to Determine Beach Profile Changes for Beach
Setting No. 4
4. Perform a site inspection to confirm general historical information related to episodic
erosion or overtopping hazards associated with the site. Determine relative erodibility of
the bluff materials using standard geologic/geotechnical field procedures (Sunamura
1983; USACE-LAD, 2003; and Williams et al., 2004).
5. If potential damage to structures or public safety are determined not to be significant, the
Mapping Partner shall document those results and whether further analyses are
recommended.
6. If further analysis of bluff erosion or overtopping is not recommended, or the site is
determined to be non-eroding, assume the bluff or cliff is classified as Beach Setting No.
5 (and document why) and that the bluff or cliff is non-eroding during large events. Then
apply methods listed in Subsection D.4.6.9 for analyses of non-erodible bluffs.
7. Perform all further runup and overtopping analyses on the surveyed existing winter
conditions beach and bluff profiles for the site.
8. Document results, and summarize the data, methods used, and assumptions associated
with the analyses.
If it is determined that the study site experiences significant erosion and retreat during large
storm events, then the Mapping Partner shall document those findings and discuss with the
FEMA study representative whether there are sufficient data, time, and budget to perform a more
detailed bluff erosion analysis. Depending on the site-specific characteristics of the setting and
bluff materials, a detailed bluff erosion analysis is likely to require detailed geologic sampling,
bluff erosion monitoring data, and bluff erosion simulation analyses. Data requirements and
procedures for conducting detailed bluff erosion analyses are presented in the next subsection.
D.4.6.8.2 Detailed Bluff Erosion Analyses
As part of the effort for the USACE feasibility study along the Encinitas/Solana Beach shoreline,
Noble Consultants, Inc., developed a statistical modeling procedure to characterize the bluff
failure induced by storm wave attacks (USACE-LAD, 2003). The approach and statistical model
have been certified by the USACE, Coastal Engineering Research Center as the preferred
method to statistically quantify the random bluff failure for shoreline studies that include a bluff
failure component. Information on this approach is available from USACE Los Angeles District
and has been published in the Journal of the American Shore & Beach Preservation Association
(Williams, Lu, and Qin, 2004).
Given wave and TWL characteristics and the erodibility of buff materials, the statistical bluff
failure model estimates bluff toe erosion induced by impinging waves and predicts random
episodic bluff failures for varying storm conditions. A semi-empirical formulation developed by
Sunamura (1982, 1983) is used to quantify the short-term bluff toe erosion rate as a function of
the intensity of impinging waves and the site-specific erosion resistance of bluff materials:
(D.4.6-15)
where X is the accumulated bluff toe erosion depth from N waves acting on the bluff toe, Xi is the
individual erosion by the i-th wave with height of Hi and duration of ?ti, Sc is the compressive
strength of the bluff material, ? is the density of water, g is the gravitational acceleration, C is a
non-dimensional constant, k is a constant with dimensions of length over time [L/T], and
subscript j is the group number of the critical wave height Hj to initiate the toe erosion, which is
given by .
This procedure requires regional and site-specific data. A statistical Monte Carlo simulation
approach is used to characterize the correlation between bluff toe erosion and bluff failure for
temporally varying wave conditions. If the cumulative depth of the bluff toe notch induced by
storm waves exceeds a locally determined threshold value for triggering a bluff failure, the
individual upper bluff retreat is determined by a randomly selected retreat value from an historic
database for the site. The threshold value is empirically determined from historical bluff failure
events. It may vary from one coastal bluff region to another. In the Encinitas/Solana Beach
region, the threshold value of the cumulative toe erosion at which a bluff failure is likely to occur
is approximately 8 feet.
The methodology may be applied in any situation where undermining of the bluff toe triggers
upper bluff block failure; however, substantial field data are required to determine several of the
required parameters and for proper calibration of the bluff failure model. Therefore, if the
Mapping Partner determines that a detailed bluff erosion study is necessary, he/she must provide
the following field data, at a minimum:
1. The type of rock formation and/or bluff soil materials from which stability and the
erosion-resistant force of the bluff material can be quantified.
2. Field measurements of bluff toe erosion in response to cumulative wave energy
associated with past storm events for determining and calibrating empirical coefficients
required by the Sunamura formula used by the model.
3. Historical data of upper bluff failures, indicating approximate horizontal length and
transverse width of bluff top land loss during past storm events for formulating the
probability distribution of the severity of bluff failure.
Two sets of field data are required to establish and calibrate the wave-induced toe erosion and to
establish the statistical representation of bluff failure events. To calibrate the toe erosion
produced by the statistical model, the depth of the toe erosion shall be measured before and after
significant storm events and correlated to the cumulative wave energy during those events at the
bluff toe. At least two full years of data are required to capture seasonal variability of toe
erosion, and up to five years of data may be needed to calibrate the correlation between the
impinging waves and the resultant toe erosion. Longer monitoring periods are desirable and will
include more storm events and more cumulative wave energy statistics, and thus result in higher
accuracy in model calibration.
To assemble the representative statistics of random occurrences, adequate observations of upper
bluff failures are required. At least two to five years of monitoring data are required to provide a
reasonable representation of the size distribution of the failures. The larger the database, the less
uncertainty there is in the predicting toe failure. There are no known analytical methods for
forecasting bluff toe erosion and failure; therefore, a statistical approach is the only means of
forecasting bluff failure and retreat due to the random temporal wave action during large storms.
To capture any seasonal variability, at least 5 years of data are required, and to ensure a
statistically valid database, up to 10 years of data may be needed if failures are uncommon. This
is likely to limit the applicability of this approach for traditional FEMA coastal flood studies,
unless these data are readily available at the beginning of the project. If it is determined that data
are available and that the application of the statistical bluff failure model is necessary, use the
following procedures:
Characterize Fronting Beach Conditions
The Mapping Partner shall perform Steps 1 through 6 under general approach, above. Assess
whether the potential damage to the bluff-top developments resulting from bluff failure is highly
probable. If a subject bluff is fronted by either a sand berm or dune with a sufficient width to
separate the bluff from direct wave impingement during the winter months, the storm-induced
erosion to the berm and dune, as stated in Subsection D.4.6.4, should be applied. If the protective
sand berm or dune is removed during the winter months, use this eroded condition as the winter
beach profile and apply the bluff failure model for that beach condition. If eroded winter profile
data are not available, then the profile should be developed by probing down to erosion-resistant
materials along beach transects to establish the bedrock profile. A typical eroded winter profile
should be developed from surveyed and probing data of the underlying bedrock layer for each
typical reach of shoreline area being analyzed. A sketch of a typical erodible bluff fronted by a
rock platform capped with a thin sand layer is shown in Figure D.4.6-31. If the fronting beach is
a broad sandy beach with berms or dunes, apply beach erosion methods prescribed in Subsection
D.4.6.5.
Figure D.4.6-31. Typical Erodible Bluff Profile Fronted by Narrow Sand-capped Beach
Apply Bluff Failure Model
Following are procedures for applying the statistical bluff failure model:
1. Collect field data for each setting and subreach along the study area.
a. Measure bluff toe erosion during at least two separate periods.
b. Conduct field probing to determine the bedrock layer across the beach area.
c. Determine the elevation of the bluff toe and bedrock layer intercept and cross-shore
slope.
2. Assemble historical bluff failure events.
a. Determine bluff failure characteristics in terms of the length along the bluff crest-line
and the transverse recession dimensions.
b. Formulate probability distribution of the magnitude of various bluff failure events.
c. Determine the threshold value of toe notch depth when the upper bluff failure occurs
(see USACE, 2003; Williams et al., 2004).
3. Calibrate Sunamura’s empirical equation.
a. Determine the wave histogram including storm events within the two separate
historical wave and erosion periods.
b. Estimate the temporal histogram of wave heights at the bluff base for each of the
selected periods with synchronized tide levels.
c. Determine the bluff resistance force for the type of bluff material at the site.
d. Calibrate Sunamura’s empirical equation (i.e., Equation D.4.6-15) from the
cumulative toe erosion measured in these two separate periods and quantify the total
impinging wave energy during each period from hindcast data.
4. Calibrate bluff retreat model by simulating past bluff failure events.
a. Assemble historical wave characteristics at the bluff base and synchronize with
measured tide levels.
b. Determine the probability distribution of wave characteristics at the bluff base.
c. Apply the Monte Carlo sampling technique to randomly select the histogram of wave
characteristics at the bluff base.
d. Estimate the cumulative notch depth at the bluff toe using the calibrated toe erosion
equation (Equation D.4.6-15).
e. Apply the same Monte Carlo sampling technique to randomly select a bluff failure
event if the accumulative notch depth is deeper than the prescribed threshold value
deduced from Step 2. Assemble historical bluff failure events.
f. Perform multiple simulations for a required long-term duration (e.g., 10 years) until a
statistical representation regarding the occurrence of bluff failure is achieved.
g. Derive the statistical mean and other pertinent properties, such as the exceeding
probability of a cumulative bluff retreat distance at the end the modeled duration.
h. Compare results with observed data from the site and adjust coefficients as necessary.
5. Apply calibrated model for 1% annual storm event.
a. Determine winter profiles for fronting beach conditions and elevation of bluff-beach
intercept.
b. Apply calibrated model for entire 1% annual storm.
c. Determine amount of toe erosion and bluff crest line recession for the 1% storm.
d. Use this adjusted profile for all further runup and overtopping analyses associated
with the 1% annual storm.
6. Document results, data, and assumptions.
D.4.6.9 Estimating Beach Profiles for Beaches Backed by Erosion-Resistant
Bluffs or Cliffs (Beach Setting No. 5)
Erosion-resistant bluffs and cliffs are often fronted by rock terraces, rocky beaches, or narrow
rock platforms capped with thin layers of sand or gravel. Once the thin sand cap is eroded from
the rocky beach, this beach setting is stable; see Figure D.4.6-6 and Figure D.4.6-32. Therefore,
Mapping Partners shall assume the sand cap is removed from the beach profile before a
significant storm event and use the adjusted rocky beach profile along with measured profiles for
the non-erodible bluffs or cliffs for all subsequent runup and overtopping computations.
Document assumptions, methods, data resources, and results. Figure D.4.6-33 shows the
sequence of key activities and computational considerations required to determine storm-induced
beach profile changes for Beach Setting No. 5.
D.4.6.10 Profiles in Tidal Flats and Wetlands (Beach Setting No. 6)
Tidal flats and wetlands are low-gradient coastal features, usually found in sheltered water areas
and comprised of fine cohesive silts and clays; see Figures D.4.6-7 and D.4.6-34. Figure D.4.6-
35 shows the sequence of key activities and computational considerations required to determine
storm-induced beach profile changes for Beach Setting No. 6.
Figure D.4.6-32. Photo of Erosion-resistant Cliff
Figure D.4.6-33. Key Activities to Determine Beach Profile Changes for Beach
Setting No. 5
Sedimentation processes in this beach setting are typically depositional. Over time, these coastal
landforms may become capped with wetland vegetation and detrital deposits and debris from
overland wave propagation during storm events. Therefore, Mapping Partners may assume that
tidal mudflats and wetland profiles do not erode over the time-scale of single storm event.
Mapping Partners should compare existing tidal flat and wetland profiles with recent post-storm
profiles to verify this assumption. If it is determined that no measurable erosion occurs during
single storm events, then the Mapping Partner shall use the existing profile information to
determine runup, overtopping, and overland propagation. However, if it is found that measurable
changes can occur during a single storm, the Mapping Partner should document the observed
changes experienced at the site and propose to the FEMA study representative a procedure for
using that information to adjust the existing profiles before determining runup, overtopping, and
overland propagation. The Mapping Partner shall document assumptions, data used, and methods
implemented to prepare the final profiles, and summarize the results.
Figure D.4.6-34. Photograph of Tidal Flats and Wetlands Complex
(Photo by Northwest Hydraulic Consultants, Inc.)
Figure D.4.6-35. Key Activities to Determine Beach Profile Changes for
Beach Setting No. 6
Discussions of long-term erosion and the potential consequences of chronic erosion are found in materials listed in
the reference section of this document and in many of the support documents referenced herein.
Guidelines and Specifications for Flood Hazard Mapping Partners [November 2004]
D.4.6-1 Section D.4.6