Chapter 15: Coastal Hydraulic Design

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Section 1: Definitions

Below is a list of common design terminology and definitions for some of the basic coastal engineering concepts that are included in this chapter. For a more extensive list of coastal engineering terms, see the USACE Coastal Engineering Manual ( CEM) Appendix A.

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AE/VE Zone – Coastal hazard zones (flood zones) designated by the Federal Emergency Management Agency (FEMA) on Flood Insurance Rate Maps. Zone AE areas are subject to inundation by the 1-percent-annual-chance flood event (commonly referred to as the 100-year storm). Zone VE areas are areas subject to inundation by the 1-percent-annual-chance flood event with additional hazards due to storm-induced velocity wave action.

Aggradation Aggradation, also called accretion, is the increase in land elevation due to building up by natural (e.g., sediment deposition out of water onto a beach) or artificial (e.g., beach fill deposited mechanically on a beach) means.

Astronomical Tide – The astronomical tide is the rising and falling effect on sea levels caused by the gravitational effects of the Earth, sun, and moon, without accounting for atmospheric influences.

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Barrier Island – Specific to Texas, a barrier island is a landform that is separated from the mainland by a bay (typically connected to the Gulf of Mexico by one or more inlets) or lagoon. Barrier islands are usually low lying, form parallel to the mainland, and contain a beach and dune system. They are subject to morphological changes on a geologic time scale, including erosion, overwash, and migration.

Bathymetry – Bathymetry is the terrain of the seafloor including rivers, bay, estuaries, and oceans. Bathymetric elevations should be referenced to vertical and horizontal datums; for example, the North American Vertical Datum of 1988 (NAVD88) and Texas State Plane coordinates.

Breakwater – A breakwater is a coastal structure generally aligned parallel to the shoreline, usually placed within a few hundred feet of the shoreline. Breakwaters are intended to reduce the amount of wave energy reaching the shoreline by reflecting or absorbing some of the wave energy, although some of the wave energy may still pass over or through the breakwater. Breakwaters can be built using various materials, the most typical of which is rock armor stone and/or riprap.

Breaking Wave – Breaking waves occur when a wave becomes overly steep and unstable based on the wave height compared to the wavelength. This occurs when waves reach shallower waters towards coastlines in an area known as the surf zone. Wave breaking dissipates wave kinetic energy by transferring some of that energy to turbulence. Wave breaking is a nearshore process.

Bulkhead – A bulkhead is a partition placed along the shoreline as a soil retaining structure. A bulkhead is typically designed to prevent land movement only and would not be designed to protect against wave loads. Therefore, a bulkhead will typically offer less protection than a seawall.

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Coastal Processes – See ‘Nearshore Processes’

Coastal Zone – The coastal zone is the land area of the state located near the coast that may be impacted by coastal processes, extending offshore to the continental shelf and, for the purposes of this Manual, inland to the limits estimated by the FEMA-designated AE/VE zones, including a 1-mile buffer. The State of Texas (Texas General Land Office) also defines a coastal zone boundary for management purposes in the Texas Administrative Code Title 31 §503.1, which includes portions of the following Texas counties: Cameron, Willacy, Kenedy, Kleberg, Nueces, San Patricio, Aransas, Refugio, Calhoun, Victoria, Jackson, Matagorda, Brazoria, Galveston, Harris, Chambers, Jefferson, and Orange.

Continental Shelf – The continental shelf is the portion of a continent (i.e., North America) that is offshore and submerged, but significantly shallower than the open ocean. The continental shelf usually has a relatively mild slope throughout, followed by a steep drop-off to the open ocean. The limits of the outer continental shelf are approximately 200 nautical miles offshore.

Contraction Scour – Contraction scour may occur when water accelerates as it flows through a constricted area, where the downstream opening is narrower than the upstream channel or cross-sectional area. The increase in velocity can cause more sediment removal than under non-constricted conditions.

Cross-Shore – Perpendicular to the shoreline. Also called ‘shore-normal.’

Currents – Currents are the movement of water generally concentrated in a prevailing direction. Currents may be caused by the rise and fall of tides (‘periodic’ or ‘tidal’ currents), seasonal winds, or may be related to general ocean circulation.

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Datum, Vertical – A vertical datum is a permanent elevation for a location in space used as an elevation to which other elevations are referred. Tidal datums are one type of vertical datum that relate elevations to the approximate land-water intersection at a particular location (such as a tide station). Orthometric datums are another type of vertical datum that use global mean sea level to approximate a land-based plane of zero elevation from which to reference other measured elevations.

Design Life – The forecast life expectancy of infrastructure based on its design.

Diurnal Tide – A tide cycle with one high tide and one low tide each day.

Diffraction, Wave – The process of waves changing direction (bending or wrapping) as they encounter an obstacle, structure, or narrow opening. This is a nearshore process.

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Ebb Tide – The ebb is the portion of the tidal cycle when the tide is falling and water is moving out of bay systems towards (in the case of the Texas coast) the Gulf of Mexico.

Erosion – Erosion is the wearing away of land by natural forces, such as wave action or wind energy. Erosion may occur over a long period of time, such as in the case of shoreline change, or over the short-term, such as in the case of scour.

Estuary – An estuary is the transition zone between one or more rivers and the open sea. It includes the region of a river mouth in which the freshwater from the river intermixes with the salt water from the sea, creating brackish water.

Exceedance Probability – See ‘Return Period’

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Fetch – Fetch is the distance of open water that the wind acts upon to generate waves. Fetches may be a distance of a few feet to hundreds of miles. All else being equal, a longer fetch will allow larger waves to form.

Flood Tide – The flood is the portion of the tidal cycle when the tide is rising and water is moving from (in the case of the Texas coast) the Gulf of Mexico into the bay systems.

Flooding, Coastal – Coastal flooding occurs when low-lying land near the coastline is flooded by seawater.

Freeboard – Freeboard is the vertical distance between the water surface (or in some cases may be referenced from the wave crest) and the crest of the structure in consideration. For example, the freeboard of a breakwater is typically defined as the vertical distance between the structure crest and the water surface. Alternatively, the freeboard of a bridge span is the open distance between the lowest portion of the span and the water surface.

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Groin – A groin is a coastal structure aligned perpendicular to the shoreline and intended to interrupt longshore sand transport along beaches. Accretion may occur on the updrift side of a groin, accompanied by erosion on the downdrift side.

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Hindcast – A hindcast is a dataset used to forecast wave conditions based on a statistical analysis of measured wind information and past storm events. The data that is used to develop a hindcast is typically measured over a period of decades.

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Inland Water Body – In most cases, this term describes a body of water located landward of the coastal zone and/or not directly connected to a bay, gulf, or ocean. Examples include lakes or rivers, even if the river eventually extends to the coast.

Inundation – See ‘Flooding, Coastal’

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Jetty – A jetty is a structure aligned perpendicular to the shoreline intended to shelter navigation channel entrances from waves and reduce sediment deposition in the channel. Jetties are typically constructed where navigation channels pass through inlets connecting a smaller water body (such as a bay) to a larger water body (such as the Gulf of Mexico).

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LiMWA – The Limit of Moderate Wave Action (LiMWA) is a FEMA-designated line that defines the inland limit of area expected to receive 1.5-foot or greater breaking waves during the 1-percent-annual-chance flood event. Waves of 1.5 feet or higher have been shown to cause significant damage to structures. A LiMWA is shown on some FEMA Flood Insurance Rate Maps for areas along coastlines.

Littoral Drift – Littoral drift is the process by which sediment is transported along the shoreline by waves and currents. Sediment movement is typically longshore but can include cross-shore components. Littoral drift occurs within the littoral zone.

Littoral Zone – The littoral zone is the area where sediment transport either onto or away from the beach occurs within the nearshore area. This zone typically extends from the depth of closure at a beach to the high-water mark at the shoreline. The depth of closure is the approximate location at which the beach profile taken over a series of months or years converge, indicating that the net sediment transport into or out of the littoral system is approximately zero. As sediment transport is caused by the nearshore process of wave breaking, the littoral zone encompasses and extends beyond the surf (breaker) zone.

Living Shoreline – A living shoreline is a shoreline stabilization technique that is used to protect shorelines using a combination of structural (e.g., breakwater) and non-structural, nature-based (e.g., vegetation, sand) components. In many cases, the structural solution will use natural materials such as rock.

Longshore – Parallel to the shoreline.

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MHHW – Mean Higher High Water (MHHW) is a statistical tidal datum calculated using the highest water surface elevation of each day, typically over a 19-year recording period.

MHW – Mean High Water (MHW) is a statistical tidal datum calculated using the highest water surface elevation of each tidal period, typically over a 19-year recording period.

MLLW – Mean Lower Low Water (MLLW) is a statistical tidal datum calculated using the lowest water surface elevation of each day, typically over a 19-year recording period.

MLW – Mean Low Water (MLW) is a statistical tidal datum calculated using the lowest water surface elevation of each tidal period, typically over a 19-year recording period.

Morphology – The evolution of the form of a river, estuary, seafloor, or other landform over time.

MSL – Mean Sea Level (MSL) is a statistical tidal datum calculated using hourly water surface elevation values, typically over a 19-year recording period.

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NTDE – National Tidal Datum Epoch (NTDE) is a 19-year averaging period to which all tidal datums are referenced. The current NTDE spans from 1983-2001 and is actively considered for revision every 20-25 years.

Neap Tide – The neap tide is the tide when there is the smallest difference between high and low tide in a day. The neap tide typically occurs two times each month during quarter moon phases.

Nearshore Processes – Nearshore processes, or ‘coastal processes’, are physical processes that occur in the nearshore zone due to wind, waves, currents, tides, and other forces impacting the land and seabed in coastal areas. Nearshore processes include, among many others: wave breaking, refraction, diffraction, reflection and shoaling; overwashing; and sediment transport. Nearshore processes occur in the nearshore zone, although their impacts may extend beyond this area.

Nearshore Zone – The nearshore zone is located along the coastline and is typically bounded by the low-tide shoreline on the landward side and the offshore zone on the seaward side (which typically begins at water depths of about 60-65 feet). The nearshore zone is roughly equivalent to the littoral zone, although it typically extends further seaward than the littoral zone.

Numerical Modeling – Numerical modeling is the application of computer software to perform calculations. Numerical models can range in complexity from simple, one-dimensional steady state calculations to large-scale, multi-dimensional systems.

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Overtopping, Wave – Wave overtopping occurs when a wave transmits water over an existing structure such as a breakwater, seawall, or building.

Overwash – Overwash is the effect of waves overtopping a coastal structure, such as a seawall, without the water then directly flowing back to the sea or lake. Overwash often carries sediment landward, which is then lost to the beach system.

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Period, Wave – The wave period is the amount of time required for the full length of wave to pass a given point in space.

Physical Modeling – Physical modeling can be applied to better understand anticipated hydrodynamic and wave processes. An example of a physical model is a scaled re-creation of a bridge pier placed in a wave tank.

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Reflection, Wave – Reflection is the process by which wave energy is redirected seaward after impacting a structure or obstruction, such as a seawall or a steep beach face. Wave reflection can lead to complex wave interactions, increase turbulence, and generate scour.

Refraction, Wave – Refraction is the process by which waves tend to change direction and become more parallel to the shoreline as they approach the shore. This phenomenon occurs because the portion of the wave nearer to the shore that advances in shallow water moves more slowly than the portion of the wave farther from the shore in deeper water (due to bottom friction), causing the wave crests to bend.

Relative Sea Level Rise – See ‘Sea Level Rise’

Return Period – The return period is a statistical value intended to provide design context on how often a specific value or event may occur. The inverse of the return period is the statistical percentage that the value or event will occur in any given year, also known as the exceedance probability. For example, a 100-year return period wind speed has a 1% chance of occurring within any given year.

Revetment – A revetment is a structure used to protect a shoreline or embankment from wave or current impacts. It is typically installed directly on grade and is composed of concrete, stone, or other armoring.

Rip Current – A powerful, swift current flowing seaward from the shore over a narrow portion of shoreline. It is typically formed due to an onshore-directed wave breaking over submerged nearshore sandbars and then flowing back out through a narrow opening between the bars. Rip currents are commonly called ‘rip tide’ and are particularly hazardous for swimmers.

Runup, Wave – Wave runup is the vertical distance that the water surface will temporarily increase as a wave propagates up a sloped surface. The runup is caused by the wave directly interacting with a structure or the shoreline and will be affected by surface roughness, porosity, and geometry.

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Scour – Scour is the local erosion caused by swift-moving water at the interface between a structure and the sediment or substrate in which it is located. In the coastal zone, scour is of particular concern at bridge pilings or at the base of bulkheads, seawalls, and other manmade structures.

Sea Level Rise – Sea level rise is the long-term increase in mean sea level. It can be measured on a global scale, based on the global mean sea level, or at a local level. When measured locally, sea level rise is frequently computed in tandem with regional land elevation changes (such as subsidence). The long-term trend in sea level rise, accounting for regional land change, is known as relative sea level rise.

Seawall – A vertical structure built along a portion of coastline to prevent erosion and other damages due to wave impacts. A seawall typically offers more protection to an area than a bulkhead, and design will usually address storm surge, wave overtopping, toe scour, uplift caused by waves, and direct wave attack.

Sediment Budget – Sediment budget is the amount of budget added to and removed from the coastal system over an area of interest and time frame. The amount of sediment into the system less the amount of sediment out of the system yields a net sediment remaining within the system. A positive sediment budget indicates a sediment surplus and can lead to aggradation. A negative sediment budget indicates a sediment deficit and can lead to shoreline erosion.

Sediment Transport – Sediment transport is the movement of sediment within a nearshore system caused by wave breaking and the net direction of winds, waves, and currents. The sediment transport accounts for why an area may be accreting (aggrading) or eroding over time.

Semi-diurnal Tide – A semi-diurnal tide is a tide that has a cycle of approximately one-half of a tidal day (12.4 hours) and generates two high tides and two low tides each day. The predominating type of tide throughout the world is semi-diurnal.

Setup, Wave – Wave setup is the vertical increase in water surface elevation due to the presence of breaking waves. In contrast to individual wave crests which are easily observable, wave setup occurs as a relatively gradual increase in the water surface across a broad area and is typically indiscernible to the naked eye.

Setup, Wind – Wind setup is the vertical increase in water surface elevation due to the force of the wind piling up water along a shoreline. Wind setup can contribute to high tides and is a component of storm surge.

Shoaling, Wave – Shoaling refers to the phenomenon whereby wave heights increase as waves propagate towards the shoreline and enter shallower water. When waves reach a critical point where the ratio between wave height and water depth becomes too steep, the waves break.

Shoreline Change – Shoreline change is the gain or loss of land area along the coastline that classifies if a shoreline is accreting (aggrading) or eroding. Shoreline change can be measured over the long-term (decades), due to geological, morphological, or meteorological trends, or short-term, due to episodic events (e.g., tropical storms).

Spring Tide – The spring tide is the tide when there is the largest difference between high and low tide in a day. The spring tide typically occurs two times each month during new moon and full moon phases.

Stillwater Elevation – The stillwater elevation, also known as stillwater level (SWL), is the elevation of the top of water without including the crests of individual wave but including wave setup. When being used to describe storm conditions, SWL includes storm surge.

Storm Surge – Storm surge is the increase in water surface elevation due to a combination of reduced atmospheric pressure (typical before and during tropical systems) and wind and wave setup. Storm surge does not include the crests of individual waves.

Storm Tide – Storm tide is the elevation that combines the storm surge and the expected tidal elevation that would have occurred at that time without a storm.

Subsidence – Subsidence is the sinking of the ground due to many location-specific factors including sediment characteristics and reduction in amount of subsurface water.

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Tide Range – The difference between the average high tide and low tide at a particular location.

Tide Station – A tide station is a station for which tide predictions are generated for a given location. Tide stations can be harmonic, for which tide predictions are generated directly from harmonic constants and have listed tidal datums, or subordinate, for which tide predictions are generated using high/low tide predictions from a designated harmonic tidal station and tidal datums are not generally calculated.

Transect – A transect is a straight line in the field or on a map along which data are recorded, measured, observed or calculated. Within the context of coastal engineering, transects are typically aligned perpendicular to the shoreline.

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Wave Height – The wave height is the vertical distance between the wave crest and the wave trough.

Wave Height, Significant – The significant wave height is the statistical average of the largest one-third of the wave heights in a given time series, typically 17 to 20 minutes.

Wave Height, Maximum – The maximum wave height is the statistical largest wave height in a given time series.

Wavelength – The wavelength is the horizontal distance representing the length of wave. This distance is typically measured from wave crest to wave crest.

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