![Frequently-Asked Questions (FAQs)](https://webarchive.library.unt.edu/web/20161103075948im_/http://cirp.usace.army.mil/_images/FAQ-title.jpg)
This webpage provides answers to Frequently-Asked Questions (FAQs) about
coastal inlets and the Coastal Inlets Research Program (CIRP). The purpose of
this FAQ compendium is to assemble concise and informative responses to common
questions. However, the responses are necessarily short; and in many cases
significant additional explanation may be warranted. Selected references have been given for additional
information. Suggestions for additional FAQs are welcomed (see this FAQ for details).
Most of the FAQs listed on this web page were originally included in the
technical note (CHETN) "Frequently-asked questions (FAQs) about coastal
inlets and
U.S.
Army Corps of Engineers' Coastal Inlets Research Program (CIRP)" by
Steven A. Hughes and Nicholas C. Kraus. [Click here to
download the original CHETN in PDF format.]
POINTS OF CONTACT: Questions about the FAQs listed on this web page
can be addressed to Mitchell Brown (Voice: 601-634-4036, e-mail: Mitchell.E.Brown@usace.army.mil),
or to the CIRP program manager, Dr. Julie D. Rosati (Email: Julie.D.Rosati@usace.army.mil.
GENERAL QUESTIONS ABOUT COASTAL INLETS
- Where Can I Look Up
Terminology About Coastal Inlets?
- What Is a Coastal
Inlet?
- What Is the Interest
of the U.S. Army Corps of Engineers in Coastal Inlets?
- How Many Federally
Maintained Inlets Are There in the United States?
- How Much Does the
Corps Spend Annually Maintaining Coastal Inlets?
- What Are the Largest
and Smallest Coastal Inlets Maintained by the Corps?
- What Are Deep-Draft
And Shallow-Draft Navigation Channels?
- Who Do I Contact to
Obtain Advice About My Inlet?
COASTAL
INLET HYDRODYNAMICS
- What Is the Tidal
Prism of an Inlet?
- Why Is Tidal Prism
Significant?
- What Can Change Tidal
Prism?
- How Is Tidal Prism
Related to Inlet Channel Cross-Sectional Area?
- What Are Ebb-Dominant
And Flood-Dominant Inlets?
- What Are Ebb-Biased
And Flood-Biased Inlets?
COASTAL
INLET MORPHOLOGY
- What Causes Ebb And
Flood Shoals to Form?
- What Is a Stable
Inlet?
- What Makes an Inlet
Unstable?
- Is There a Method for
Determining Whether an Inlet Will Be Stable or Unstable?
- What Is the Natural
Bypassing of an Inlet?
- What Is the Mechanical
Bypassing of an Inlet?
- How Do Stabilized
Inlets Interact with Adjacent Shorelines?
COASTAL
INLET CHANNELS
- What Are the
Mechanisms for Channel Infilling?
- Will Deepening a
Navigation Channel Reduce Dredging Rate?
- What Are Ways to Reduce
Dredging Frequency of a Navigation Channel?
- What Are the
Differences Between Natural Inlet Channels and Those That Are Maintained
Through Dredging?
COASTAL
INLET STRUCTURES
- Why Are Jetties Built?
- Why Do Some Inlets
Have Only One Jetty, Whereas Others Have Two?
- What Factors Determine
Distance Between Parallel Jetties?
- What Determines
Seaward Length of Jetties?
- How Are Rubble-Mound
Jetties Designed?
COASTAL
INLETS RESEARCH PROGRAM (CIRP)
- What Is the Objective
of CIRP?
- What Are the Products
of CIRP?
- How Is CIRP
Structured?
- How Is Research
Planned in CIRP?
- Who Determines the
Research Direction for CIRP?
- Who Can I Contact to
Suggest Tools That Are Needed and Problems or Inlets With Problems That
Might Be Addressed by CIRP?
- Does CIRP Partner with
Corps Districts to Solve Specific Inlet Problems?
- Can CIRP Support
Non-Federal Inlet Projects?
- Does CIRP Sponsor
External Research?
- What Mechanisms Are
Used for Submitting a Research Proposal to CIRP?
- What Are the Criteria
for Successful Proposals?
AVAILABLE
TOOLS FOR SOLVING COASTAL INLET PROBLEMS
- What Types of Tools
Are Available for Addressing Coastal Inlet Processes and Problems?
- What Problems Are Best
Addressed by Physical Models?
- Who Conducts Physical
Model Studies of Inlets?
- What Facilities Are
Available at CHL for Coastal Inlet Physical Model Studies?
- Where Can I Find
Information about Available PC-Based Computer Programs?
- What Is the Cost for
these PC-Based Computer Programs?
- Do I Need any
Additional Special Software to Run PC-Based Programs?
- What Is the Coastal Modeling System (CMS)?
- What Sophisticated
Numerical Models Are Available in CMS?
- Can Anyone Obtain
these Models?
- How Does a Corps
Employee Obtain the SMS or a Particular Numerical Model?
- How Does Everyone
Else Obtain a Particular Numerical Model?
- Are There Manuals and
Training Available for Applying Numerical Model?
- Who Do I Contact if I
Am Having Problems Running a Particular Numerical Model?
- What Tools Are
Available for Estimating Scour at Jetties?
- Is There a Method for
Estimating Equilibrium Scour Depth?
- Is There a Method for
Estimating Navigation Channel Infilling Rates?
- How Do I Construct a Sediment
Budget for an Inlet?
- How Can I Obtain CIRP
Publications?
- Where Can I Find
Information About a Specific Inlet?
- Are Data Available
from CIRP?
MISCELLANEOUS
INLET TOPICS
- My Question Is Not
Listed Above, What Can I Do?
- Where Can I Send My
Suggestion for a FAQ?
- What if I Do Not Agree
With the Answer to a FAQ, or I Have Additional Comments?
ANSWERS
- GENERAL QUESTIONS ABOUT COASTAL INLETS
- Where Can I Look Up Terminology About
Coastal Inlets?
Glossaries of coastal terminology can be found in the Corps' Coastal
Engineering Manual at http://chl.erdc.usace
.army.mil/CHL.aspx?p=s&a=ARTICLES;104 and in CIRP's Web-based
general information system called "Inlets Online," http://www.oceanscience.net/inletsonline/ that also houses aerial photographs of many Federal coastal inlets.
Return to FAQ Index
- What Is a Coastal Inlet?
A coastal inlet connects an ocean, sea, or lake through a typically narrow
landmass to the water body behind it, such as a bay, estuary, lagoon, or
river. Hydrodynamic forcing contributing to the water exchange that
maintains the coastal inlet may be the tide, river flow, wind, or
seiching. Bruun and Gerritsen (1960) gave
this definition for a tidal inlet, which is the most common type of
coastal inlet: "A tidal inlet is the waterway connection between the
sea and a bay, a lagoon, or a river entrance through which tidal and other
currents flow." They distinguish three inlet classifications based on
origin: geological origin such as the
Golden Gate
;
hydrological origin where a river enters the sea; and littoral origin such
as openings through barrier islands. A tidal inlet is distinguished from a
bay entrance, for example, such as New York Harbor in that the tidal flow
is responsible for maintaining the channel of the inlet; without tidal
flow, an inlet would close.
Return to FAQ Index
- What Is the Interest of the
U.S.
Army
Corps of Engineers in Coastal Inlets?
The Corps is the Federal agency responsible for maintaining navigable
inlets that have been authorized by the
U.S.
Congress, as well as for
missions in federally authorized shore-protection and environmental
sustainability projects, which often involve coastal inlets as a
component. The Corps has, therefore, several responsibilities concerning
Federal inlets. Inlets provide the entry point to coastal harbors and
inland waterway systems, through which passes more than 95 percent of
international freight and 2.3 billion tons of domestic and foreign
commerce (http://www.iwr.usace.army.mil/ndc/wcsc/wcsc.htm).
Inlets also serve as entry points to harbors of refuge during storms,
provide access to the ocean for military vessels for national defense,
serve fishing and recreational vessels, and promote water exchange and
water quality in estuaries and bays.
Return to FAQ Index
- How Many Federally Maintained Inlets Are
There in the
United
States
?
According to the CIRP-developed Federal Inlets Database,
154 inlets are maintained at some level by the Corps. This number does not
include more than 200 primarily shallow-draft recreational inlets, many of
which are found in the
Great Lakes
. There
are numerous other inlets along the coasts of the United States that are
not maintained by the Federal government. Some of the non-Federal inlets
are maintained by state or local governments, whereas others are not
maintained for navigation and are left to evolve naturally.
Return to FAQ Index
- How Much Does the Corps Spend Annually
Maintaining Coastal Inlets?
Annual spending for maintenance dredging navigation channels through
coastal inlets and maintenance of coastal inlet structures is in excess of
$1 billion. These activities maintain the authorized depth and location of
the navigation channel, promoting reliable navigation.
Return to FAQ Index
- What Are the Largest and Smallest Coastal
Inlets Maintained by the Corps?
"Large" is defined here by reference to the tidal prism, which
is the amount of water entering or exiting an (tidal) inlet on flood or
ebb tide, respectively, excluding river flow. The largest inlet maintained
by the Corps (through occasional dredging of the channel through the ebb
shoal) is the entrance to
San
Francisco
Bay
.
This inlet has a tidal prism of about 5.1 x (10)10 cu ft and a
minimum cross-sectional area of about 9.3 x (10)5 sq ft. It is
difficult to determine the smallest federally maintained tidal inlet
because of limited data on tidal prism and cross-sectional area for many
of the smaller inlets. As an example on the smaller side, Mattituck Inlet
is located on the north
shore
of
Long Island
, NY.
It is a federally maintained navigation project authorized in 1896 and has
a tidal prism of 4.32 x (10)7 cu ft (Morgan et al. 2005), three orders of
magnitude less than that of San Francisco Bay. Jarrett (1976) lists tidal prisms for 106
inlets.
Return to FAQ Index
- What Are Deep-Draft And Shallow-Draft
Navigation Channels?
Deep-draft navigation channels are those with depths greater than 15 ft (Headquarters, U.S. Army Corps of
Engineers 2004). Depths for navigation channels are typically
referenced to mean lower low water (mllw), which is a tidal datum
expressing the average of the lowest periodic tidal elevation occurring in
a day. Typically, the tide is semidiurnal, meaning that there are two
highs and two lows in a tidal day (24 hr, 50 min). The mllw datum is the
average of the lowest of the lows in a day. On rivers and in the
Great Lakes
, other navigation depth datums are
defined. The majority of deep-draft channels pertain to oceangoing ships
with drafts of 30 ft or more. The most common deep-draft channel depths
are between 36 and 45 ft, with an increasing number of inlets being
maintained to 50-ft depths to accommodate the largest class of oceangoing
vessels termed "Post-Panamax." Shallow-draft channels have
maximum depth of 14 ft, but many small coastal inlets have shallow-draft
channel depths averaging between 6 and 9 ft.
Return to FAQ Index
- Who Do I Contact to Obtain Advice About My
Inlet Problem?
Corps employees and those working for other Federal agencies should
contact the CIRP program manager, Dr. Nicholas Kraus (contact information
given in the "Points of Contact" paragraph at the top of this
web page). Questions about a Federal inlet from the private sector should
be directed to the Public Affairs Office of the Corps District in which
the subject inlet is located.
Return to FAQ Index
ANSWERS
- COASTAL INLET HYDRODYNAMICS
- What Is the Tidal Prism of an Inlet?
The tidal prism P of an inlet is the volume of water that enters
through the inlet channel during the flood tide, or exits during the ebb
tide, excluding river flow and other nontidal flow sources. Tidal prism is
sometimes estimated as the planform area of the bay times the magnitude of
the water level increase in the bay during flood tide, P = 2 ab Ab, where ab is the amplitude of the tide in the
bay, and Ab is the bay area. In modern times, the tidal prism
is measured with acoustic-Doppler velocity current meters that record the
current through the water column. By making boat transects across an inlet
entrance, the discharge (water velocity through the channel cross section,
giving a volume of water per second) can be measured and related to the tidal
prism.
Return to FAQ Index
- Why Is Tidal Prism Significant?
The tidal prism promotes circulation in the bay and flushing of water from
the bay through the inlet to the sea. As water moves in and out of the bay
through the inlet channel, it creates a current that removes sediment
deposited in the channel by waves and currents. This self-scouring reduces
blockage of the inlet by sand moving along the coast, thus keeping the
inlet open or maintaining stability of the inlet channel cross section.
Numerous geomorphic properties of tidal inlets, such as the volume of ebb
and flood shoals, have been related to the tidal prism.
Return to FAQ Index
- What Can Change Tidal Prism?
The tidal prism can be reduced by either reducing the bay surface area
(e.g., through land reclamation or by building causeways across a bay (Davis and Zarillo 2003) or by stabilizing
the inlet channel with jetties in such a way that the bay tide amplitude
is reduced due to flow resistance in the entrance channel. Tidal prism can
be increased by excavating to increase the bay area or by modifying
(typically, deepening) the inlet channel to reduce flow resistance and
increase the bay tide amplitude.
Return to FAQ Index
- How Is Tidal Prism Related to Inlet Channel
Cross-Sectional Area?
Because the tidal prism water volume must enter and exit through the
relatively constricted inlet, flow increases and sediment is scoured until
the inlet erodes to a stable channel cross-sectional area. Measurements of
stable inlet minimum cross-sectional area below mean sea level show good
correlation to the magnitude of the tidal prism. This relationship was first
formalized into an equation by M. P. O'Brien (O'Brien
1931) and more thoroughly investigated for the Atlantic Ocean, Gulf of
Mexico, and
Pacific Ocean
coasts by Jarrett (1976). A relation between tidal
prism and inlet channel cross-sectional area was first quantitatively
inferred by LeConte (1905).
Return to FAQ Index
- What Are Ebb-Dominant And Flood-Dominant
Inlets?
Consider a tidal inlet system that has no other source of water inflow or
outflow except through the inlet channel. If the duration of the falling
tide (ebb tide) exceeds that of the rising tide (flood tide), leading to a
larger peak flood current (greater cross-sectional averaged peak flood
velocity), the system is referred to as flood dominant or flood
asymmetric. Conservation of mass requires that the same amount of water
must move through the inlet during flood tide as during ebb tide. Because
the flood tide occurs over less time, the flow rate must be greater.
Similarly, if the duration of the falling tide is shorter than that of the
rising tide, leading to stronger peak ebb current, the system is referred
to as ebb dominant or ebb asymmetric. More information can be found in Walton (2002). See this
FAQ for discussion of ebb bias and flood bias.
Return to FAQ Index
- What Are Ebb-Biased And Flood-Biased Inlets?
For a single inlet serving an otherwise closed bay system, if there are no
river or other sources or sinks of water, the amount of water entering the
inlet on flood and exiting on ebb must be the same. If, however, in a
single-inlet system, water enters the bay through river discharge,
freshwater aquifers in the bay, runoff from the land, or another source,
there can be a net ebb current over a tidal cycle. Such an inlet is said
to have an ebb bias. For bays served by two or more inlets, it is possible
for one or more inlets to tend to be ebb biased, and the other or others
to be flood biased. Such a bias can be caused by tidal phasing in the
ocean, different depths in the inlet channels, or wind blowing over the bay
in a predominant direction. See this FAQ for
discussion of ebb dominance and flood dominance.
Return to FAQ Index
ANSWERS
- COASTAL INLET MORPHOLOGY
- What Causes Ebb And Flood Shoals to Form
Water from the bay or ocean accelerates as it enters the inlet channel due
to the Venturi effect. If the flow velocity in the entrance channel is
greater than that of the adjacent water when exiting the entrance, a
turbulent jet is formed. The jet spreads laterally into the adjacent
regions, and jet velocity decreases as the jet widens. The stronger flow
velocity in the jet mobilizes and transports sediment until eventually the
jet velocity decreases to the point that sediment is deposited. This jetting
mechanism is responsible for both ebb and flood shoals. (River deltas are
created in the same way.) If sand is available for transport by the tidal
jet as through wave action and the longshore current, the ebb and flood
shoals can store a large volume of sediment. Waves also influence the
morphology of the ebb shoal by opposing the current, causing the sand to
deposit in a more restricted area than the flood shoal. Ebb shoals serve
as a pathway for alongshore-moving sand to bypass the tidal inlet and
continue moving downdrift.
Return to FAQ Index
- What Is a Stable Inlet?
Inlet stability can refer to either its location or to its channel cross
section. Inlets without jetties, whether dredged or not, tend to migrate
along the shore. Jetties stabilize the inlet location and navigation
channel. Concerning channel cross-sectional stability, a stable inlet is
one where the tidal flow passing through the inlet is sufficiently strong
to maintain a minimum equilibrium cross section by flushing littoral
sediment that enters the channel by currents and wave action. Observation
shows that for inlets on sand coasts, the long-term mean-maximum current
(the mean of maximum tidal current that typically occurs at spring tide)
must exceed about 1 m/sec or 3.3 ft/sec to maintain channel
cross-sectional stability (Bruun 1968).
For stable inlets, a decrease in cross-sectional area causes increased
flow velocity to maintain the same tidal prism exchange, and the increased
water velocity erodes the sediment deposited in the channel.
Return to FAQ Index
- What Makes an Inlet Unstable?
Inlets become unstable if the tidal flow cannot adequately clear the
channel free of littoral sediment, and the inlet cross-section decreases.
Littoral sediment is sediment moving along the coast, typically
transported by wave-induced currents. For unstable inlets, reduced
cross-sectional area results in weaker flow velocity and a decrease in the
tidal prism that further decreases the flow velocity until the entire
inlet is choked with sand, and the inlet closes. One example of where this
might happen is on a coast with a predominant littoral drift in one
direction. Alongshore-moving sand builds a barrier that eventually extends
past the original inlet and diverts the inlet channel in alongshore. This
lengthened channel has greater flow resistance, decreasing the flow
velocity and tidal prism. Geomorphic responses of inlets and inlet stability
are discussed by FitzGerald et al.
(2000) and by Davis and Zarillo (2003).
Return to FAQ Index
- Is There a Method for Determining Whether an
Inlet Will Be Stable or Unstable?
Yes, Escoffier (1940, 1977) proposed an analytical technique
based on the intersection of the tidal prism - inlet area relationship
with a curve representing the maximum inlet velocity variation with
minimum cross-sectional channel area. The maximum velocity curve increases
from zero to a maximum value as the cross-sectional area increases, then
it decreases with further increase in channel cross-sectional area. If the
velocity curve and the tidal prism - inlet area curves intersect, there
will typically be two intersection points, one representing a stable
condition and one representing an inlet that will close. See Seabergh and Kraus (1997) for additional
information.
Return to FAQ Index
- What Is the Natural Bypassing of an Inlet?
Natural inlet bypassing, the process by which sediment moves from the
updrift to the downdrift side of an inlet, can be attributed to waves and
tidal currents driving the longshore transport around the peripheral edge
of the ebb-tidal shoal, thus promoting stability of downdrift shorelines (Dean 1988). This persistent transport of
sediment along the outer edge of the ebb-tidal shoal by waves and tidal
currents is referred to as a continuous bypassing mechanism. Discontinuous
bypassing mechanisms involve the downdrift and onshore migration of bar
complexes (FitzGerald 1988; Gaudiano and Kana 2000). Two of these
mechanisms, stable inlet processes and ebb-tidal shoal breaching, are
based upon the migration of large bar complexes formed on the downdrift
side of the ebb-tidal shoal. A third discontinuous bypassing method is
inlet migration and spit breaching. All discontinuous bypassing mechanisms
result in the bypassing of large discrete packets of sediment. The
mechanism by which inlets bypass sediments (continuous or discontinuous)
determines erosion and accretion patterns on downdrift beaches. Refer to FitzGerald et al. (2000) for further
discussion of natural mechanisms of sediment bypassing at tidal inlets.
Return to FAQ Index
- What Is the Mechanical Bypassing of an
Inlet?
Mechanical inlet bypassing is the movement of sediment from the updrift to
the downdrift side of an inlet by artificial means. Material may be taken
from the updrift fillet, a deposition basin, the dredged channel, the ebb
shoal or flood shoal, or other nearshore shoals. The mechanism for
bypassing the material can be a hopper dredge, split hull dredge, pipeline
dredge, or truck.
Return to FAQ Index
- How Do Stabilized Inlets Interact with
Adjacent Shorelines?
The influence of a stabilized inlet on adjacent shorelines is determined by
the magnitude and direction of net longshore transport. In situations
where the inlet is sited at a nodal, or near-zero point in the net
longshore transport rate, the adjacent shorelines will equilibrate in a
relatively short time after the construction of a stabilized inlet with
minimal changes to natural processes (Komar
et al. 1976). In situations where an inlet is located where net
longshore transport is strongly biased in one direction, sediment is
impounded by the updrift jetty, advancing the updrift shoreline and
creating a triangular-shaped deposit referred to as an accretion fillet.
Over the long term, this creates a deficit in the sediment budget
downdrift of the inlet and results in recession of the downdrift
shoreline, which can be reduced by sediment bypassing. The near field
shoreline, located between the downdrift jetty and bypassing bar
attachment point, experiences high rates of shoreline recession (Bruun 1995). Beneficial placement of
material dredged from the navigation channel can mitigate near field
shoreline recession. The far field shoreline, located downdrift of the
bypassing bar attachment point, typically experiences recession rates
greater than the long-term regional trend. The extent and magnitude of
downdrift shoreline recession are dependent on the percentage of longshore
transport bypassing the inlet.
Return to FAQ Index
ANSWERS
- COASTAL INLET CHANNELS
- What Are the Mechanisms for Channel
Infilling?
Inlet entrance and approach channels have five characteristic signatures
of shoaling (Pope 2000).
- Bank Encroachment. Near the ends of jetties
and along the sides of permeable jetties, channels can infill due to
longshore transport of sediment entering the channel from the adjacent
beaches. A shoal forms on the updrift side of the channel and constricts
it. The classical jetty tip shoal is an example. In areas experiencing persistent
strong wind, such as the Texas coast, wind blown transport of sediment
can form similar shoals where the dry beach meets the inlet or jetty.
Sediment contributing to bank encroachment by longshore transport
typically represents the coarser material available at the site (e.g.,
sand, gravel, shell fragments). Wind-blown sediment usually consists of
finer sands and silts.
- Shoaling. In regions with weaker
currents, such as offshore and back-bay portions of the channel, shoaling
may occur through deposition of, primarily, suspended sediment in the
bottom of the channel. Sediment around coastal inlets typically consists
of fine to medium sand, silt, and mud. Some inlets may also contain
gravel, such as on glacial coasts and in the Great Lakes. Uniform infilling
may also occur due to a loss of hydraulic gradient, such as locations
where the channel widens.
- Migration. Migrating channels may
maintain depth and width, but move away from the original dredged location.
In some cases, such as after a severe storm, a channel may be entirely
abandoned and switch to a new location.
- Channel slope failure. Slope failure of the
channel sidewalls can contribute to decreasing navigable depth of the
channel. Slumping of the sidewalls can occur after deepening of a
channel, if the side slopes exceed the stabile angle of repose of the
sediment, or if loadings increase due to storm waves or earthquakes.
- Bed forms. Although not necessarily
related to shoaling in the channel, large bed forms caused by reworking
of channel sediment can decrease navigable depth. Pope (2000) gives an example estimating that
for a channel with mean grain size 0.3 mm, the channel would have only
ripples with a flow speed of 0.4 m/sec, increase to sand waves (which
could impede navigation) at 0.7 m/sec, and become planar at 1.5 m/sec.
Return to FAQ Index
- Will Deepening a Navigation Channel Reduce
Dredging Rate?
There have been reports (formal and informal) concluding that deepening of
a channel will improve hydraulic efficiency through it and, therefore,
decrease or at least not change past dredging maintenance volumes. Such a
conclusion should be viewed with caution and skepticism. Although a deeper
channel may increase the current velocity in it because of reduced
friction, a deeper channel decreases the potential for sediment
resuspension and transport out of the channel. In addition, even if a
channel is not widened after it is deepened, it will tend to widen due to
channel side slope failure, again making sediment bypassing across it more
difficult. A deepened channel may increase in effective length, thereby
increasing the length to be dredged. Any increase in current velocity,
which is typically minor, gained through channel deepening will not likely
offset the improved sediment trapping capacity of a deeper and possibly
wider channel. For design, combined modeling of waves, current, sediment
transport, and channel deposition must be conducted to assess relative
maintenance requirements. For certain situations, desk evaluation methods
are available in support of reconnaissance estimates.
Return to FAQ Index
- What Are Ways to Reduce Dredging Frequency
of a Navigation Channel?
The method used to reduce dredging frequency largely depends on the
mechanisms causing the channel infilling (see this
FAQ).
- Bank encroachment. For sediment moving over
and through permeable jetties, raising and sand-tightening the structures
will reduce channel infilling. If wind-blown transport of sediment into
the channel is significant, vegetation and sand fences can reduce
infilling.
- Infilling and migration. The dredging frequency
for channels that are infilling and/or migrating may be reduced by
deepening the channel. Note that the terms "advance
maintenance" and "over-depth dredging" are procedures that
increase channel depth. Advance maintenance is an increased amount of dredging
greater than the authorized depth to increase the time between required
dredging, thereby reducing channel maintenance cost, because mobilization
of equipment is an appreciable cost. Over-depth dredging is a small
amount of allowed dredging beyond the authorized or specified depth in
recognition of limitations in the measurement and dredging process in
varying waves and water level. A deeper channel may decrease the dredging
frequency; however, field experience indicates that the dredging quantity
will increase.
- Channel slope failure. Failure of side slopes
due to channel designs exceeding the sediment stability angle can be
alleviated through modifying the design, or anticipated as a mechanism
for initial channel adjustment.
- Bed forms. Channel bed forms that
locally limit navigable depth are most practically dealt with by dredging
to remove the material. Sometimes it is possible to create deposition
basins or channel wideners to capture migrating bed forms (e.g., Johnston et al. 2002).
Return to FAQ Index
- What Are the Differences Between Natural
Inlet Channels and Those That Are Maintained Through Dredging?
Table 1 lists selected differences between natural
and maintained channels, and Figure 1 is a conceptualized drawing of both
types. Many of the differences are related to the relative instability of
natural channels, which can change course over time scales of days, weeks, and
years. As the natural channel meanders and migrates, shoals in the vicinity of
the channel also change, creating hazardous conditions for navigation. Waves
may break on the shoals, and natural inlets can close for periods of time. The
advantage of natural inlet channels is that they bypass sediment to the
adjacent beaches. This bypassing may be regular, via the ebb-tidal shoal or
inlet channel, or on irregular time intervals (taking years to decades) through
welding of shoals to the beach.
Maintained channels are much more reliable and
stable than natural inlet channels, and they provide safe navigation. However,
they represent more of a sediment sink to the regional sediment budget;
bypassing of sediments may take decades to evolve as the stabilized inlet
reaches a new equilibrium, and may not occur at all. The maintained channel
seeks to return to its original condition through shoaling. Both of these
problems can be alleviated through dredging of the maintained channel and
mechanical placement of the dredged material on adjacent beaches.
![FAQ #2](FAQ-Figure-1.gif)
Figure 1. Conceptualized drawing of natural and maintained inlet channels
Return to FAQ Index
ANSWERS
- COASTAL INLET STRUCTURES
- Why Are Jetties Built?
Jetties are constructed primarily to stabilize the location of the inlet
and its channel, preventing inlet migration and protecting the channel
against nearshore waves and currents. The jetties direct tidal flow in the
navigation channel in a predictable manner, and inlet flow velocity is
usually increased because of confinement of the tidal current, thus making
conditions less favorable for sand deposition in the channel. The scouring
action of the tidal current reduces required maintenance dredging. Jetties
also provide wave protection for navigation channels for wave directions
other than straight into the channel. This protection is essential in
high-energy wave climates because it provides safe passage for vessels
through the energetic surf zone and across the dangerous offshore bar with
shallow water and breaking waves.
Return to FAQ Index
- Why Do Some Inlets Have Only One Jetty,
Whereas Others Have Two?
Inlets located where longshore sediment transport is primarily in one
direction may be adequately stabilized with a single updrift jetty. The
channel through inlets stabilized by a single jetty tend to migrate toward
the structure, which can result in a channel maintenance and navigation
concern (Kieslich 1981). Some
single-jetty inlets were originally envisioned as dual-jetty inlets, but
after construction of the first (updrift) jetty either the second
(downdrift) jetty was not needed, or funding was not available to complete
the original plan. Inlets located where longshore transport has similar
magnitude from both directions will not be protected by a single jetty.
Return to FAQ Index
- What Factors Determine Distance Between
Parallel Jetties?
The leading factor determining minimum width between jetties for a
dual-jetty (parallel) system is assuring adequate clearance for the
intended navigation usage, involving consideration of channel width and
adequate distance to preserve a buffer of bottom material near the
jetties. Once this criterion has been met, a second width criterion
relates to inlet cross-sectional area versus tidal prism (see this FAQ. Limiting the width between jetties can
result in greater tidal flow velocity that scours the channel and reduces channel
maintenance dredging. The goal is to select a jetty separation that
promotes increased channel scouring without creating an excessively strong
inlet current that could degrade navigation reliability. Pragmatically, if
an inlet existed prior to construction of the jetties, these structures
are typically placed on the interior shallow margins of the natural inlet
to reduce construction costs by minimizing structure length in deeper
water (requiring greater stone volume).
Return to FAQ Index
- What Determines Seaward Length of Jetties?
As a first estimate, jetties extend to the offshore contour corresponding
to the depth of the dredged navigation channel. The appropriate seaward
extent of jetties is a compromise between navigation benefits gained by
longer jetties, and reduced natural sand bypassing to the downdrift
beaches offered by shorter jetties. Longer jetties protect navigation
until vessels reach deeper water outside the surf zone under typical or
moderately stormy wave conditions. They also prevent longshore-moving sand
from shoaling the navigation channel, thus extending time between dredging
cycles. However, longer jetties block more of the longshore sediment
transport, increasing potential for the downdrift beaches to erode more
than they would with shorter jetties that allow more sediment to be
bypassed via the ebb shoal bar. This potential erosion must be remedied
through mechanical bypassing of sediment trapped at the updrift jetty and
dredged from the channel. In addition, construction costs increase with
jetty length, not only because of additional material, but also because
stable armor stone sizes increase as jetties are built into deeper water.
Jetty construction cost is proportional to the volume of placed stone.
Return to FAQ Index
- How Are Rubble-Mound Jetties Designed?
There are two components to rubble-mound jetty design: functional design
and structural design. Functional design corresponds to the planform
layout of the jetty system needed to achieve the required functionality
being sought. Functional design includes locations and orientations of the
jetties; distance between the jetties; seaward extent of the jetties;
landward jetty terminus necessary to prevent flanking; and assessment of
impacts to the tidal prism, navigation channel, and adjacent shorelines.
Also included in functional design might be criteria related to wave
reduction in the navigation channel by minimizing wave overtopping. The second
component, structure design, consists of determining the jetty
cross-section dimensions necessary to meet the functionality criteria
while remaining structurally intact during the design storm event.
Structure design includes setting crest elevation to prevent overtopping,
determining type of armor unit (stone or concrete units), calculating the
stable armor unit weight, designing underlayers and core, designing the
seaward round-head, and providing necessary scour protection. Geotechnical
engineers assess the foundation material and design a mattress to
distribute the structure weight. As a jetty progresses into deeper water,
armor weight and crest elevation are increased, as necessary.
Return to FAQ Index
ANSWERS
- COASTAL INLETS RESEARCH PROGRAM (CIRP)
- What Is the Objective of CIRP?
The primary objective of CIRP is to conduct research and develop tools
that reduce costs associated with operation and maintenance (O&M) of the
nation's Federal navigable coastal inlets and channels. A central aspect
of this objective is transfer of knowledge and tools developed by CIRP to
Corps District and Division engineers, cost-sharing partners with the
Corps, and to the private sector that supports the Corps' O&M mission.
Return to FAQ Index
- What Are the Products of CIRP?
CIRP produces a variety of products that support Corps coastal inlet
operation and maintenance activities directly and indirectly. These
include predictive numerical models of waves, currents, sediment
transport, morphology change, and water-structure interaction; physical
modeling technology; computer programs implementing new and existing
analytical techniques; periodic technology-transfer workshops, and
training with CIRP technology; and technical publications describing both
applied and basic research advances. In addition, CIRP staff members
advise and consult with District engineers on specific projects. CIRP
products are listed at http://cirp.usace.army.mil/cirp/cirp.html.
Return to FAQ Index
- How Is CIRP Structured?
CIRP consists of six technical work units, each guided by a principal
investigator and one administrative work unit. Overall technical direction
within the CIRP is provided by a program manager, with oversight from the
Navigation Program technical directors in the
U.S.
Army Engineer Research and
Development
Center
and from
Corps of Engineers Headquarters.
Return to FAQ Index
- How Is Research Planned in CIRP?
Research focus varies as new technology developments mature and are
transitioned to Corps field offices (Districts). Constant interaction with
engineers throughout the Corps in review of CIRP and in studies performed
for Corps Districts identifies new needs for tools and technology,
stimulating research paths and applied engineering development to fulfill
those needs. CIRP is continuing to improve numerical modeling technology
by addition of sediment transport capability and morphology change across
several length and time scales, allowing prediction of navigation channel
infilling, sand bypassing at inlets, and coastal morphology change in the
vicinity of inlets. Capabilities are also being added to the newest
Boussinesq wave model, and laboratory tests are planned to develop clear
guidance for selecting Boussinesq model coefficients appropriate for
representing a wide range of coastal structures. New emphasis is being
directed at problems related to maintenance and repair of inlet jetties,
and CIRP is developing PC-based tools for predicting such things as
channel infilling rates and shoal development. CIRP strives to develop
practical and usable technology for Districts to solve problems of concern
to the Corps' Federal navigation mission.
Return to FAQ Index
- Who Determines the Research Direction for
CIRP?
Research direction of CIRP is a collaborative effort involving the
principal investigators; program manager; technical directors at the U.S.
Army Engineer Research and Development Center's Coastal and Hydraulics
Laboratory (CHL); Corps Headquarters; and Corps Division and District
engineers. Corps Division and District engineers (and their consultants)
define problems and express needs. Headquarters has the national view of
which needs are critical and of national priority. The principal
investigators keep abreast of new developments, and they propose viable
research and development that will result in practical solutions. The CIRP
program manager serves as the catalyst that monitors the quality of the
research and products, and assures everyone is making progress toward the
solutions. The program manager also allocates appropriate resources to
each project. The CHL technical directors assure linkages are made among
Corps research programs and those of other Federal agencies to leverage
expertise to the best advantage across all programs. CIRP welcomes suggestions
for research needs, as well as bringing attention to particular problems
at coastal inlets.
Return to FAQ Index
- Who Can I Contact to Suggest Tools That Are
Needed and Problems or Inlets With Problems That Might Be Addressed by
CIRP?
Contact the CIRP Program Manager, Dr. Nicholas Kraus, at the phone number
or e-mail address listed in the "Points of Contact" paragraph at
the top of this web page. Alternately, contact any of the CIRP staff
listed on the CIRP Web page, or bring your suggestion to the attention of
any CHL employee you meet at Corps meetings, technical conferences, or
while discussing other projects or studies.
Return to FAQ Index
- Does CIRP Partner with Corps Districts to
Solve Specific Inlet Problems?
There are many examples of District inlet problems being studied by a
joint team of District and CIRP engineers and scientists. Most of these
studies are conducted as reimbursable work involving one or more
experienced CIRP engineers or scientists. Usually the CIRP staff is not
supported by CIRP while participating in the studies. However, there have
been situations when CIRP recognized a benefit to be gained by
contributing funds or staff time to a reimbursable project to acquire
additional data or test predictive modeling technology that would advance
ongoing research, particularly if applicable on a national level.
Short-term consulting support by a CIRP staff member, typically limited to
less than 2 weeks, can be attained through a request to the Dredging
Operations Technical Support (DOTS) Program (http://el.erdc.usace.army.mil/dots).
Return to FAQ Index
- Can CIRP Support Non-Federal Inlet Projects?
Participation is typically limited to projects for which a Federal
interest has been formally recognized. However, there are two exceptions.
CIRP can partner with the private sector to compete for work outside the
United States. The second exception is if CIRP identifies a non-Federal
inlet to study that offers a particular set of problems or situation that
would be of national interest to existing Federal inlet navigation
projects. An example is study of the evolution of a newly opened inlet.
Return to FAQ Index
- Does CIRP Sponsor External Research?
CIRP has a long history of supporting research conducted by academic
institutions and private sector firms. Some of the research and
development efforts successfully completed under contract include major
numerical model development, development and enhancement of the
Surfacewater Modeling System (SMS), geomorphologic analysis and toolkit
development, data collection covering a wide range of parameters, and Web-based
tools. CIRP prides itself on its support of leading researchers in the
arena of inlet processes.
Return to FAQ Index
- What Mechanisms Are Used for Submitting a
Research Proposal to CIRP?
Unsolicited research proposals on any topic related to coastal inlets can
be submitted to CIRP via the Broad Agency Announcement (BAA) process. The
BAA is similar to a request for proposals, so any proposal received under
the BAA is considered to be a competitive proposal that can be funded if
the proposal is found to be technically sound, relevant to CIRP
objectives, and if funds are available. Guidelines and forms for
submitting a BAA proposal can be found at http://www.mvk.usace.army.mil/contract/docs/2002BAA.pdf.
Return to FAQ Index
- What Are the Criteria for Successful
Proposals?
The most important criterion for a successful proposal is that the
research produces one or more end products that have practical application
for solving the real-world inlet and navigation problems faced by the
Corps. Whereas CIRP recognizes the value of pure research and advancing
understanding of inlet processes, our mission is to support the Corps
O&M activities at federally maintained inlets and entrances. Other
rating criteria include originality of the proposal, records of the
proposers, probability of success within the proposed time and budget, and
funding requested relative to available funds. BAA proposals are reviewed
internally by at least three senior technical staff members.
Return to FAQ Index
ANSWERS
- AVAILABLE TOOLS FOR SOLVING COASTAL INLET PROBLEMS
- What Types of Tools Are Available for Addressing
Coastal Inlet Processes and Problems?
Depending on the particular inlet problem, available tools include
analytical techniques, simple PC-based computer programs, sophisticated
numerical models solved over spatial grids, and small-scale physical
models. Inlets are complex hydrodynamic and geomorphologic systems, and in
many studies several types of tools may be required to answer specific
problems. For example, analytical techniques and simple PC-based programs
might be applied in the early stage of an inlet study to develop
preliminary estimates of key inlet hydrodynamic parameters. GIS analysis
of aerial photographs, bathymetry surveys, and shoreline position provides
information of morphologic response to inlet modifications. More precise
detail can be determined by applying sophisticated numerical models or
physical models. Project cost and potential benefits contribute to any
decision on what tools are brought to bear on any specific study. Kraus (2006) gives an overview of
morphologic response to creation and modification of coastal inlets,
including the navigation functioning of inlets.
Return to FAQ Index
- What Problems Are Best Addressed by Physical
Models?
Physical models reproduce a portion of the inlet at small scale.
Generally, the modeled area is no larger than about 1 or possibility 2
square miles. Physical models can represent complex hydrodynamics
phenomena such as wave interaction with jetties, wave overtopping of
jetties, wave diffraction and reflection inside an entrance, complex
three-dimensional (3-D) flow separation at structures, and wave breaking
over complex bathymetry. Physical models are commonly employed for
verifying rubble-mound jetty stability (2-D and 3-D), determining
overtopping and wave transmission, assessing scour potential, and
investigating navigability issues. Physical models include all nonlinear
physics without simplification, but care must be taken to minimize scaling
distortion and laboratory effects that can render results questionable.
Practical operational constraints typically limit the range of water level
and wave conditions that can be represented in a physical model. Further
information about physical model advantages and disadvantages can be found
in Hughes (1993).
Return to FAQ Index
- Who Conducts Physical Model Studies of
Inlets?
Physical models of inlet processes can be divided into two categories,
research studies and site-specific studies. Researchers work with physical
models to understand physical processes and, in some cases, develop
relationships that support numerical model simulations. Physical models
are also established to verify numerical models and assess the strengths
and weaknesses of numerical approximations. Site-specific physical model
studies are sponsored by Corps Districts or other agencies with the goal
of solving particular problems at a project site or modifying the existing
projects to gain additional functionality. Design and operation of an
inlet physical model must be conducted by engineers and technicians
experienced with the difficulties in achieving realistic simulations and
valid results. These engineers work in conjunction with sponsors to
develop an optimal testing plan to address the problems and issues. Wave
flumes are limited to examining breakwater and jetty stability and wave
processes at structures. Planform inlet models require substantial wave
basins equipped with both wave and tidal current generation capability.
Few large-scale facilities exist in the
United
States
, and the largest, by far, is located in
Vicksburg
,
MS
,
at the Coastal and Hydraulics Laboratory.
Return to FAQ Index
- What Facilities Are Available at CHL for
Coastal Inlet Physical Model Studies?
The principal basin facility is the CIRP-operated generic inlet physical
model (http://cirp.usace.army.mil/cirp/cetns/cetniv-19.pdf).
This basin was designed to represent characteristics common to many
inlets, but in an idealized manner. The basic model bathymetry was
constructed in concrete (fixed-bed) with an offshore slope molded in the
shape of an equilibrium beach profile. A navigable inlet with rubble-mound
jetties cuts through the barrier beach to a back-bay area. The idealized
model can simulate constant or varying ebb or flood tide, and waves can be
added using an irregular wave generator. Typical studies include examining
the benefits of jetty spurs to reduce movement of sediment into the inlet,
understanding wave/current interaction, simulating formation of
crenulate-shaped erosion at the landward jetty terminus, and developing
equilibrium inlet cross sections. Other CHL facilities include wave basins
for constructing and testing site-specific inlets and entrances, wave
flumes for testing of jetty and breakwater cross-section design and armor
stability, and a precision flow table ( http://cirp.usace.army.mil/cirp/cetns/chetn-iv55.pdf)
for rapid preliminary assessment of hydrodynamic changes brought about by
inlet structure modifications.
Return to FAQ Index
- Where Can I Find Information about Available
PC-Based Computer Programs?
CIRP and other research programs at CHL produce PC-based computer programs
that automate calculation procedures for solving specific problems. These
programs generally are simple to operate, and the computerized versions
have undergone testing and feature user-friendly interfaces. The PC-based
programs differ from the more sophisticated numerical models that solve
governing equations over a spatial grid. When a program becomes available
as a stand-alone code, it is described in a Coastal and Hydraulics
Engineering Technical Note (CHETN) together with information about whom to
contact to receive a copy of the code. The formal public release of a
completed PC-based program is through the CEDAS suite of programs ( http://chl.erdc.usace.army.mil/CHL.aspx?p=s&a=Software;11)
being developed through a cooperative agreement between CHL and a private
company. The CEDAS package replaces the former ACES computing software,
but the ACES components are included in CEDAS. The private company assists
in developing interfaces, help information, and other tasks that allow
CIRP to conserve research funding.
Return to FAQ Index
- What Is the Cost for these PC-Based Computer
Programs?
PC programs developed in CIRP and announced initially through a CHETN are
typically free of charge and available to anyone who asks. Operation of
these codes and evaluation of accuracy of results are the responsibility
of the user. Because of the large and varied use of CIRP technology, our
staff cannot typically answer questions about application, model input
requirements, how to operate the code, or how to interpret results. If you
find an error in a particular PC-based program, we would appreciate
receiving a description of the problem. Codes that are included in the
CEDAS software suite are more thoroughly tested within the interface.
CEDAS is available free-of-charge to Corps staff. Additional information,
software downloads, documentation, and registration information can be
found at ( http://chl.erdc.usace.army.mil/CHL.aspx?p=s&a=Software;11).
The registration process involves generation of a software key specific to
the computer on which CEDAS will be used. For those needing to purchase a
license, contact Veri-Tech, Inc. (http://www.veritechinc.net/).
Return to FAQ Index
- Do I Need any Additional Special Software to
Run PC-Based Programs?
Generally, PC-based programs are developed to run stand-alone on computers
under some version of the Windows operating system, and no additional
software need be installed or licensed to operate the codes. The exception
is any GIS-based analysis program that is built to be operated in
ARCVIEW(r). Documentation about the program will give specific
requirements.
Return to FAQ Index
- What Is the Coastal Modeling System (CMS)?
The CMS is an integrated numerical modeling system
for simulating waves, current, water level, sediment transport, and
morphology change at coastal inlets and entrances. Emphasis of the CMS is
on navigation channel performance and sediment exchange between the inlet
and adjacent beaches. A key objective of this work is to develop, test,
and transfer the CMS to Corps Districts and industry for use on specific
engineering studies.
Return to FAQ Index
- What Sophisticated Numerical Models Are
Available in CMS?
Sophisticated numerical models in the CMS include codes that solve the
governing differential equations over a gridded spatial domain using
either a finite-difference or a finite-element solution scheme. Models
being actively developed by CIRP or developed at CHL and applied to
address coastal inlet problems include: (a) CMS-Flow is a 2D hydrodynamic and morphology change model that is computationally efficient, easy to set up, and has features required for many coastal engineering applications at coastal inets and at the bays and beaches adjoining inlets; (b) CMS-Wave is
an easy-to-apply, flexible, robust, model for nearshore wind-wave growth
and propagation. Other models are in research
development and lack complete full interfaces and documentation. All the above-listed models can be run on a modern PC-based workstation.
Return to FAQ Index
- Can Anyone Obtain these Models?
The models contained within the Coastal Modeling System are included in the
Surfacewater Modeling System (SMS). The SMS provides a uniform interface
for various gridded models, and the system features easy grid development,
preprocessing of input files, model operation, data and output
visualization, and toolboxes to support modeling efforts. Specific models
can be set up and run within the SMS. The SMS package is commercially
available, but because CIRP and CHL provide funding for its support, SMS
is provided at no cost for Corps staff. Others are required to purchase an
SMS license, thereby sharing in SMS development cost. Models not yet
released in the SMS are typically not available. However, specific studies
requiring the capabilities of models still in development may be able to
access a model under arrangement with the CIRP investigator.
Return to FAQ Index
- How Does a Corps Employee Obtain the SMS or
a Particular Numerical Model?
Corps users can obtain the SMS package by going to the CHL Web site ( http://chl.erdc.usace.army.mil/CHL.aspx?p=s&a=Software;4),
then selecting SMS Documentation and Registration. Once the software is
downloaded and installed, you will need to obtain a password to enable the
SMS components. Double-click the SMS icon and select Enable; then you can
either e-mail the security string to SMS@usace.army.mil or select the On-line registration button. This procedure generates a
password that binds the SMS software to the computer on which you have
installed the package. Your password will be sent in a separate e-mail.
Use this password to unlock all the features of the SMS package, including
the models in the Inlet Modeling System.
Return to FAQ Index
- How Does Everyone Else Obtain a Particular
Numerical Model?
Non-Corps users can obtain the SMS package and password commercially from
Environmental Modeling Systems, Inc. (http://www.ems-i.com/).
After receipt of the licensing fee, a password will be generated to unlock
all the package features. The password ties the software to the single
machine on which the password request was generated.
Return to FAQ Index
- Are There Manuals and Training Available for
Applying Numerical Model?
CIRP and other research programs at CHL offer periodic hands-on training
sessions on the use and application of the CMS and other numerical models
developed at CHL. The focus of the workshops varies from introductory to
advanced, and often workshops are presented when major new features or
capability become available. CIRP-sponsored workshops are announced in
both the "News & Events" (http://cirp.usace.army.mil/cirp/cirpnews.html)
section of the CIRP Web site, and in the quarterly CIRP internal Corps
newsletters (http://cirp.usace.army.mil/cirp/newsletters.html).
Workshops have been held in Vicksburg at CHL, at Corps District offices,
and at some technical conferences. Training in SMS model use is also
offered commercially by Environmental Modeling Systems, Inc. (http://www.ems-i.com/). Model
documentation is usually a combination of published technical reports,
technical notes, training tutorials, and Web-based documentation. The SMS does
provide on-line help.
Return to FAQ Index
- Who Do I Contact if I Am Having Problems
Running a Particular Numerical Model?
CIRP staff attempt to answer questions from Corps staff applying models to
District projects. In some cases, the questions can be answered
immediately or with little effort. Examples include range of input
variables and how to display results. Questions requiring a significant
expenditure of staff time are answered as feasible, and it may be
necessary to provide funding in some instances. Often, such support is
covered through District contributions to a numerical model maintenance
program. The CIRP staff expects the person asking the question to be
knowledgeable about the basic operation and applicability of the model of
interest.
Return to FAQ Index
- What Tools Are Available for Estimating
Scour at Jetties?
Scour at jetties can be caused by several mechanisms involving the
combination of waves and tidal currents. Research in CIRP examined scour
caused by jetting of tidal flows such as deflected ebb jets, jets formed
by jetty orientation, and flow constriction jets. For specific jetty
orientations, inviscid flow theory is used to create flow maps that show
streamlines and lines of constant discharge per unit width. These flow
maps indicate where the flow is strongest. The flow maps can be combined
with a technique for estimating equilibrium scour depth. Two on-line flow
net calculators are provided on the CIRP Web site under "Products
& Tools" (http://cirp.usace.army.mil/cirp/products.html).
Example applications are also given.
Return to FAQ Index
- Is There a Method for Estimating Equilibrium
Scour Depth?
A simple technique for estimating the maximum equilibrium scour depth for
flow through an inlet was described by Hughes
(1999) in a CHETN titled "Equilibrium scour depth at tidal
inlets" (http://cirp.usace.army.mil/cirp/cetns/cetniv-18.pdf).
Equilibrium scour depth is estimated as a function of maximum discharge
per unit width, sediment diameter, and sediment specific density. The
methodology is intended only for sand-sized noncohesive sediment. An
on-line calculator is provided on the CIRP Web site under "Products
& Tools" (http://cirp.usace.army.mil/cirp/products.html).
Return to FAQ Index
- Is There a Method for Estimating Navigation
Channel Infilling Rates?
There are several levels of technology available. (a) For reconnaissance
studies or small projects with limited budgets, a morphology-based
numerical model has been developed for estimating infilling of a dredged
navigation channel by cross-channel sediment transport. The model and
methodology are applicable to channels on the open coast and in estuaries,
bays, and lakes. A central principle of the model is that an equilibrium
or natural channel depth can be identified, and the rate of channel
infilling is proportional to the deviation between the dredged depth and
the natural depth. The model requires information readily obtained or
estimated in engineering analysis as the channel depth in its natural
condition prior to dredging and the rate of sediment transport approaching
normal to the channel. A paper describing the model will be available
soon. (b) At more advanced and computation-intensive level, the Inlet
Modeling System (CMS) M2/3D model (Militello
et al. 2004) is available to estimate channel infilling in either two-dimensional
mode, which is suited for long-term simulations, and three-dimensional
mode, which is more suited for detailed studies or complex areas.
Return to FAQ Index
- How Do I Construct a Sediment Budget for an
Inlet?
CIRP has described the process for creating a sediment budget (Rosati and Kraus 1999) and developed and
refined a computer program to assist creation of sediment budgets for
inlets and adjacent shorelines. The PC-based program called the Sediment
Budget Analysis System (SBAS) can be downloaded from the CIRP Web site
under "Products & Tools" (http://cirp.usace.army.mil/cirp/products.html).
The program has a user-friendly interface that enables rapid development
of a sediment budget directly on top of aerial imagery of the area of
interest. The SBAS eases construction of a sediment budget. The validity
of the resulting budget depends on the quality and accuracy of the values
entered. Kraus and Rosati (1998) discuss
how to estimate uncertainty in sediment budgets.
Return to FAQ Index
- How Can I Obtain CIRP Publications?
A complete listing of technical reports, journal articles, and conference
papers published by CIRP staff is included on the CIRP Web site under the
"Publications" link (http://cirp.usace.army.mil/cirp/cirppubs.html).
Electronic versions of the more recent reports and articles are available
for free downloading on the Web site. Full versions of some of the older
publications are not available, but all publication entries include the
citation and abstract. Reprints of some journal articles and conference
papers might be obtained directly from the authors upon request. Sweeping
requests for hard copies of all publications (e.g., "Please send me a
copy of every CIRP report.") cannot be fulfilled. Electronic versions
of all Coastal and Hydraulics Engineering Technical Notes (CHETNs) related
to inlets and CIRP can be downloaded from the CIRP Web site under the
"Technical Notes" link (http://cirp.usace.army.mil/cirp/cirpcetns.html).
Return to FAQ Index
- Where Can I Find Information About a
Specific Inlet?
CIRP maintains two databases related to tidal inlets and coastal
structures that are accessible from the CIRP Web site. The Database of
Navigation Projects and Structures ( http://cirp.usace.army.mil/cirp/structdb/structdbinfo.html)
contains information on over 1,200 coastal structures along with a few
parameters related to associated tidal inlets. Many of the inlet records
include aerial photographs. The second database provides additional
information on 154 federally maintained tidal inlets ( http://cirp.usace.army.mil/cirp/inletsdb/inletsdbinfo.html).
Where both databases have an entry for the same inlet, the records are
cross-linked. Some inlet record fields are sparsely populated. Work is
underway to consolidate both databases and expand the amount of
information contained in the records. Another on-line source for specific
inlet photographs is "Inlets Online" (http://www.oceanscience.net/inletsonline/).
Many inlets have been studied extensively by CIRP, by universities, and by
consulting engineers. CIRP studies are typically published and available
in electronic form from the CIRP Web site. Availability of studies
conducted by others varies.
Return to FAQ Index
- Are Data Available from CIRP?
Data sets obtained by CIRP as part of the research program are usually
described in the resulting technical publications. Data sets may be
included as an appendix or they may be summarized. Data collected as part
of a supported project study of an inlet may be similarly included in
publicly available reports and technical articles. Requests for specific
data should be sent to the person who authored the technical document.
Return to FAQ Index
ANSWERS
- MISCELLANEOUS INLET TOPICS
- My Question Is Not Listed Above, What Can I
Do?
Tidal inlets and entrances encompass a wide range of topics, knowledge,
problems, solutions, and analysis techniques. It is not possible to list all
the questions that might arise. If your questions are not included in this
technical note, the first place to look is the CIRP Web site (http://cirp.usace.army.mil/cirp/cirp.html).
The Web site contains a cornucopia of inlet information including
publications, technical notes, photographs, presentations from past
workshops, and more. The FAQs included in this CHETN are duplicated on the
CIRP Web site, and new FAQs are added as they arise. Technical papers
describing new research findings about tidal inlets can be found in a
variety of mainstream journals and in the proceedings of technical
conferences. Several books contain authoritative chapters about tidal
inlets, inlet hydrodynamics, and inlet sedimentation.
Return to FAQ Index
- Where Can I Send My Suggestion for a FAQ?
If you have a question or a suggestion for a FAQ, please contact Dr.
Steven Hughes at the phone number or e-mail address given in the
"Points of Contact" paragraph at the top of this web page. We
will do our best to provide an answer and include your FAQ on the CIRP Web
site.
Return to FAQ Index
- What if I Do Not Agree With the Answer to a
FAQ, or I Have Additional Comments?
We are interested in hearing from you. The CIRP team has attempted to
provide short and accurate responses to all the FAQs listed in this
technical note. However, we appreciate having mistakes identified or
differences of opinion expressed; and we are open to discussing these.
Whereas changing the answer to a FAQ in this technical note is difficult,
the corresponding version on the CIRP Web site can be updated to reflect
corrections. Such updates will be clearly labeled to avoid confusion when
compared to the original technical note version. Contact Dr. Steven Hughes
at the phone number or e-mail address given in the "Points of
Contact" paragraph at the top of this web page, and he will refer you
to the CIRP team member most knowledgeable on the subject matter of the
FAQ in question.
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REFERENCES
Bruun, P. (1968). "Tidal inlets and
littoral drift." University Book Company,
Oslo
,
Norway
.
[Return to FAQ Answer]
Bruun, P. (1995). "The development of
downdrift erosion," Journal of Coastal Research, 11(4),
1,242-1,257.
[Return to FAQ Answer]
Bruun, P., and Gerritsen, F. (1960).
"Stability of coastal inlets." North-Holland Publishing Company,
Amsterdam
, 140 pp.
[Return to FAQ Answer]
Davis, R. A., and Zarillo, G. A. (2003).
"Human-induced changes in back-barrier environment as factors in tidal
inlet instability with emphasis on
Florida
," Coastal and Hydraulics Laboratory
Engineering Technical Note ERDC/CHL CHETN-IV-57,
U.S.
Army Engineer Research and
Development
Center
,
Vicksburg
,
MS
.
[Return to Hydrodynamics FAQ Answer]
[Return to Morphology FAQ Answer]
Dean, R. G. (1988). "Sediment
interaction at modified coastal inlets: Processes and policies," in Hydrodynamics
and Sediment Dynamics of Tidal Inlets, D. G. Aubrey and L. Weishar, ed.,
Springer Verlag, NY, 412-439.
[Return to FAQ Answer]
Escoffier, F. F. (1940). "The
stability of tidal inlets," Shore and Beach, 8(4), 114-l 15.
[Return to FAQ Answer]
Escoffier, F. F. (1977).
"Hydraulics and stability of tidal inlets," GITI Report 13,
U.S.
Army Engineer Waterways Experiment
Station,
Vicksburg
,
MS
.
[Return to FAQ Answer]
FitzGerald, D. M. (1988).
"Shoreline erosional-depositional processes associated with tidal
inlets," in Hydrodynamics and Sediment Dynamics of Tidal Inlets, D.
G. Aubrey and L. Weishar, ed., Springer Verlag, NY, 186-225.
[Return to FAQ Answer]
FitzGerald, D. M., Kraus, N. C.,
and Hands, E. B. (2000). "Natural mechanisms of sediment bypassing at
tidal inlets," Coastal and Hydraulics Laboratory Engineering Technical
Note ERDC/CHL CHETN-IV-30,
U.S.
Army Engineer Research and
Development
Center
,
Vicksburg
,
MS
.
[Return to Third Morphology FAQ Answer]
[Return to Fifth Morphology FAQ Answer]
Gaudiano, D. J., and Kana, T. W.
(2000). "Shoal bypassing in
South Carolina
tidal inlets: Geomorphic variables and
nine empirical predictions for nine mesoscale inlets," Journal of Coastal
Research, 17(2), 280-291.
[Return to FAQ Answer]
Headquarters, U.S. Army
Corps of Engineers. (2004). "Hydraulic design of deep-draft navigation
projects," EM 1110-2-1613,
Washington
,
DC
.
[Return to FAQ Answer]
Hughes, S. A. (1993). "Physical
models and laboratory techniques in coastal engineering." World Scientific
Publishing Co., Pty, Ltd.,
Singapore
.
[Return to FAQ Answer]
Hughes, S. A. (1999). "Equilibrium
scour depth at tidal inlets," Coastal and Hydraulics Laboratory
Engineering Technical Note ERDC/CHL CHETN-IV-18,
U.S.
Army Engineer Research and
Development
Center
,
Vicksburg
,
MS
.
[Return to FAQ Answer]
Jarrett, T. (1976). "Tidal prism
- inlet area relationships," GITI Report 3,
U.S.
Army Engineer Waterways Experiment
Station,
Vicksburg
,
MS
.
[Return to General FAQ Answer]
[Return to Hydrodynamics FAQ Answer]
Johnston, S., Kraus, N. C., Brown,
M. E., and Grosskopf, W. G. (2002). "DMS: Diagnostic modeling system:
Report 4: Shoaling analysis of St. Marys Entrance,
Florida
," Coastal and Hydraulics Laboratory
Engineering Technical Report ERDC/CHL TR-99-19,
U.S.
Army Engineer Research and
Development
Center
,
Vicksburg
,
MS
.
[Return to FAQ Answer]
Kieslich, J. M. (1981). "Tidal
inlet response to jetty construction," GITI Report 19, General
Investigation of
Tidal Inlets
,
U.S.
Army Corps of Engineers, 63 pp.
[Return to FAQ Answer]
Komar, P. D., Lizarraga-Arciniega, J. R.,
and Terich, T. A. (1976). "Oregon coast shoreline changes due to
jetties," Journal of Waterways, Harbors and Coastal Engineering Division,
102(WW1), ASCE, 13-30.
[Return to FAQ Answer]
Kraus, N. C. (2006). "Coastal inlet
functional design: Anticipating morphologic response," Proceedings
Coastal Dynamics '05, ASCE.
[Return to FAQ Answer]
Kraus, N. C., and Rosati, J. D. (1998).
"Estimation of uncertainty in coastal sediment budgets at inlets,"
Coastal and Hydraulics Laboratory Engineering Technical Note ERDC/CHL
CETN-IV-16, U.S. Army Engineer Research and Development Center, Vicksburg, MS,
(http://cirp.usace.army.mil/cirp/cetns/cetniv-16.pdf).
[Return to FAQ Answer]
LeConte, L. J. (1905).
"Discussion on river and harbor outlets, 'Notes on the improvement of
river and harbor outlets in the
United States
,'" Paper No. 1009, by D. A. Watts, ASCE
Transactions, 55, 306-308.
[Return to FAQ Answer]
Militello, A., Reed, C. W., Zundel,
A. K., and Kraus, N. C. (2004). "Two-dimensional depth-averaged
circulation model M2D: Version 2.0, Report 1, technical documentation and
user's guide," ERDC/CHL TR-04-2,
U.S.
Army Engineer Research and
Development
Center
,
Coastal and Hydraulics Laboratory,
Vicksburg
,
MS
.
[Return to FAQ Answer]
Morgan, M. J., Kraus, N. C., and
McDonald, J. (2005). "Geomorphic analysis of Mattituck Inlet and Goldsmith
Inlet, Long Island, New York," Coastal and Hydraulics Laboratory Technical
Report ERDC/CHL TR-05-2, U.S. Army Research and Development Center, Vicksburg,
MS, 332 pp.
[Return to FAQ Answer]
O'Brien, M. P. (1931). "Equilibrium
flow areas of inlets on sandy coasts," Journal of the Waterways and
Harbors Division, 95(WW1), ASCE, 43-52.
[Return to FAQ Answer]
Pope, J. (2000). "Where and why
channels shoal: A conceptual geomorphic framework," Coastal and Hydraulics
Laboratory Engineering Technical Note ERDC/CHL CHETN-IV-12,
U.S.
Army Engineer Research and
Development
Center
,
Vicksburg
,
MS
.
[Return to FAQ Answer]
Rosati, J. D., and Kraus, N. C. (1999).
"Formulation of sediment budgets at inlets," Coastal and Hydraulics
Laboratory Engineering Technical Note ERDC/CHL CETN-IV-15 (Revised September
1999),
U.S.
Army Engineer Research and
Development
Center
,
Vicksburg
, MS.
(http://cirp.usace.army.mil/cirp/cetns/cetniv-15.pdf).
[Return to FAQ Answer]
Seabergh, W. C., and Kraus, N. C.
(1997). "PC program for coastal inlet stability analysis using Escoffier
method," Coastal Engineering Technical Note CETN IV-11, U.S. Army Engineer
Research and Development Center, Vicksburg, MS, 7 pp, (http://cirp.usace.army.mil/cirp/cetns/cetniv-11.pdf).
[Return to FAQ Answer]
Walton, T. L., Jr. (2002). "Tidal velocity
asymmetry at inlets," Coastal and Hydraulics Laboratory Engineering
Technical Note ERDC/CHL CHETN IV-47, U.S. Army Engineer Research and
Development Center, Vicksburg, MS, ( http://cirp.usace.army.mil/cirp/cetns/chetn-iv47.pdf).
[Return to FAQ Answer]
Last Updated March 12, 2008