Section 3: Stillwater Levels
Anchor: #i1025277Consideration of Water Levels in Coastal Roadway Design
Stillwater level represent the water surface absent wave height and wave runup. Fluctuations in stillwater levels along the Texas coast are influenced by astronomical tides, storm surges, and long-term sea level changes. Tides and storm surge reflect dynamic processes that move water in a fairly short time period, but their effects are still incorporated into the overall term “Stillwater level.” The stillwater level should be considered for all roadway projects located in coastal areas to determine the landward extent and elevation of inundation and wave forces.
Identification of a stillwater level is a critical element in the design of a roadway infrastructure located in coastal areas.
This section seeks to:
- Anchor: #FNGTRYNL
- Synthesize – at a high level – the various factors that contribute to water level fluctuations along the Texas coastline; and Anchor: #OFJUSWKK
- Provide guidance on the estimation of appropriate stillwater levels for design of coastal transportation infrastructure.
Astronomical Tides
Water levels fluctuate throughout the day, primarily due to astronomical tides caused by the gravitational pull of the moon and sun. In Texas, long-term water level stations are available online through the National Oceanic and Atmospheric Administration (NOAA) Tides and Currents: Center for Operational Oceanographic Projects and Services and the Texas Coastal Ocean Observation Network (Figure 15 7). The Texas Gulf Coast generally experiences a diurnal pattern of one high and one low tide each day; however, there are times each month where semi-diurnal tides occur (two high and two low tides each day). The average daily tide range (difference between the average high tide and low tide) along the Texas coast averages less than 2 feet. Table 15-2 shows the tide ranges at representative locations along the coastline.
Figure 15-7. NOAA tide stations along the Texas Coast (USACE Galveston District Southwestern Division, 2018).
|
NOAA Station Name |
Station Number |
Tidal Range (feet) |
|---|---|---|
|
Rainbow Bridge, Port Arthur-Orange |
8770520 |
1.1 |
|
Texas Point, Sabine Pass |
8770822 |
2.0 |
|
Eagle Point, Galveston Bay |
8771013 |
1.1 |
|
Galveston Pleasure Pier |
8771510 |
2.0 |
|
Galveston Pier 21 |
8771450 |
1.4 |
|
Matagorda Bay Entrance Channel |
8773767 |
1.2 |
|
Aransas Pass, San Patricio County |
8775241 |
1.4 |
|
USS Lexington, Corpus Christi |
8775296 |
0.6 |
|
South Padre Island, Brazos Santiago |
8779749 |
1.4 |
|
NOTE: Tidal range reflects the great diurnal tidal range and is defined as the difference between Mean Higher High Water (MHHW) and Mean Lower Low Water (MLLW). |
||
Large differences in the tide range can occur at the same location throughout the month. During full and new moons, spring tides occur because the sun and moon are aligned with respect to the earth. Their combined gravity causes a larger than average tidal range: high tides are higher and low tides are lower. During quarter moons, neap tides occur when the gravity of the sun and moon are opposed, creating a smaller than average tidal range. More information on tides can be found the NOAA reference included in this section.
Tidal elevations also vary spatially and are influenced by local bathymetry, shoreline orientation, and wind/current patterns. Due to the complexities of the Texas shoreline, observed tide range varies locally, with some areas experiencing tidal amplification (e.g., Galveston Pleasure Pier), while others experience a dampening of the tidal signal (e.g., USS Lexington).
Annual High Tides
Annual high tides, also known as king tides, occur several days each year and produce ocean levels over a foot higher than average high tides along the Texas coastline.
Annual high tides often result in temporary flooding of low-lying coastlines, particularly if they coincide with a storm event or onshore wind that elevates tides above predicted levels. Because these tides can often cause nuisance flooding and reduce stormwater drainage efficiency, considering typical annual high tide elevations is especially important when designing low-lying roads and roadway drainage systems. In addition to temporary road closures, frequent flooding caused by these tides could lead to recurring damages to the roadway and increase the rate of scheduled maintenance.
Although most roadway infrastructure being constructed will be based on minimum design elevations that exceed the annual high tide, there may be cases where roadway assets remain vulnerable to flooding from annual high tide exposure (e.g., road repair or roadway culvert designs in rural areas). Therefore, it is recommended that the designer evaluates the design of the roadway project for potential exposure to annual high tide water level elevations and, where possible, construct roadway and drainage features at a higher relative elevation or design for the road base and support to account for frequent soil saturation. Care should also be taken to consider the hydraulic connectivity and ensure the proposed roadway does not obstruct the flow during or after high tide flooding events. The designer may consider incorporating some drainage features that will allow water to flow on either side of the asset.
Evaluating water levels collected at local tide stations during known annual high tide events provides a general range of expected water level events. Several approaches can be used to determine water elevations associated with an annual high tide event. Annual high tide events can be extracted using extreme value analysis of historical tide data observed at a local tide station.
Tidal Data Sources
- Anchor: #HMFXIISX
- NOAA Center for Operational Oceanographic Products and Services (CO-OPS) — NOAA Tides and Currents is a collection of wind, water level, and current data from NOAA recording stations along U.S. coastlines. Water level observations and tidal datums are available at some of these recording stations. Anchor: #UWQORJCC
- Texas Coastal Ocean Observation Network (TCOON) — TCOON is a network of wind and water level observation stations along the Texas coast, with data collected by a combination of federal, state, local, and academic institutions. Water level observations and tidal datums are available at most TCOON recording stations.
Storm Tides
Tropical and extratropical cyclones can be hazardous for projects located in coastal areas both along the shoreline and several miles inland. Storm tides, which represent the stillwater elevation inclusive of storm surge, can bring high water, strong currents, and large waves to locations that are not normally exposed to coastal forces. Storm tides are difficult to predict, as it is highly sensitive to small changes in storm track, size, approach speed and angle, and atmospheric pressure. Characteristics of the coastline, including the width and slope of the offshore continental shelf and the presence of inlets, such as bays and estuaries, also affect the elevation of a storm tide.
To account for the water elevations associated with storms during the infrastructure planning and design phase, storm surge is added to the astronomical tide (typically represented by the normal high tide) to derive a storm tide elevation (Figure 15-8). Storm tides are stillwater elevations, meaning they do not include wave effects, such as wave amplitude and wave setup. Storm tides are often expressed in terms of an exceedance probability, or the likelihood that the water level will surpass a given elevation based on a statistical analysis of historical observations or model simulations.
For example, a 100 year flood event identifies a stillwater elevation that has a 1% annual exceedance probability (AEP). It is commonly used as an indicator to inform assessments of flood risk or design criteria (FHWA, 2005). Refer to Chapter 4 (Hydrology), Section 2 (Probability of Exceedance) for commonly used terminology regarding exceedance probabilities. In this section, it is also referred to as a return period or return interval. For planning and design phases, care should be taken in understanding how the storm tide elevation was computed, based on the type of approach, modeling methodology, and models adopted, as it could include or not include wave setup and/or tides.
Figure 15-8. Storm Surge vs. Storm Tide (modified from NOAA Storm Surge Overview, https://www.nhc.noaa.gov/surge/)
Storm tides are an important consideration for transportation infrastructure located in the coastal environment because these events allow waves to reach farther inland, thus exposing areas at higher elevations to the risk of flooding and wave action. It can also influence the hydrodynamics along tidal inlets, increasing risk of scour around bridges and coastal structures.
Storm Tide Sources
The following list describes readily-available storm tide information that may be applicable for project design. The approach to derive storm tides varies in terms of the numerical equations used, assumed boundary conditions, quality of topographic and bathymetric data, and selection of statistical models used to estimate return intervals. Therefore, it is no surprise that the results between studies are likely to differ from each other, in some cases, significantly.
The storm tide sources described below, while useful for understanding relative extreme water elevations during storm events, will require review by a TxDOT Precertified Coastal Engineer as they may not be adequate for design of site-specific projects. For a Level 1 or 2 analysis, the designer may be able to obtain appropriate storm tide elevations from the sources listed below. These storm tide sources may also be applicable for a Level 3 analysis for initial project evaluation. Otherwise, numerical modeling (refer to the Numerical Modeling of Water Elevations subsection) may be required to calculate local storm tide conditions. FEMA flood insurance study data can also be useful for storm tide data, but the base flood elevation data they provide includes additional coastal factors such as wave and run-up data. Additional interpretation of this data would be necessary to determine stillwater levels.
- Anchor: #IWIUYRWI
- U.S. Army Corps of Engineers (USACE) — The USACE Galveston District is responsible for public navigation and engineering projects along the entire Texas coast. Prior USACE projects may have developed stillwater levels that could be used for transportation infrastructure design. For transportation projects planned near areas where USACE structures are located, it is recommended that the USACE District be contacted to inquire about any previous water level analyses that may have been performed for the area. Results will likely have been developed based on specific past project needs and should be carefully evaluated prior to use, but may be applicable for project design, particularly for a Level 1 analysis. https://www.swg.usace.army.mil/ Anchor: #WOSCNSTM
- NOAA Extreme Water Levels — NOAA publishes storm tide estimates based on statistical analysis of historical tide data. Published values are often consistent with the stillwater level component of the FEMA base flood elevation. For NOAA-operated tide stations along the Texas coast, annual exceedance probability levels have been calculated for the 1-, 10-, 50, and 99-percent annual chance storm tides. https://tidesandcurrents.noaa.gov/est/
Extreme Storm History
In addition to obtaining numerical water level data, understanding the history of extreme storms and associated impacts at a project location is important for design. Although it is a qualitative assessment, reviewing prior storm history can provide valuable insight for project design based on observations of local impacts posed by past significant events.
For all analysis Levels, it is recommended that the local storm history be reviewed to consider local storm impacts. For Level 1, it may be reasonable to simply reference and compare high water marks or other recorded data to published flood frequency elevations. In particular, the designer should consider large storms that occurred after the FEMA maps were published. For example, Hurricane Harvey occurred in 2017. If the FEMA maps for the project area are dated 2010, then clearly the effects of Harvey potentially influencing the 100-year flood level estimates were not included. While a complete re-analysis may not be feasible for a given transportation project, multiple high stormwater events or even one very large storm after the FEMA elevations were published could indicate that the FEMA levels are underrepresenting the risk. Further consideration may be warranted and may elevate what was planned to be a Level 1 analysis into Level 2 or Level 2 to Level 3 analysis. For Levels 2 and 3, a more in-depth investigation of historical storm effects should be performed.
- Anchor: #AKSRKDHV
- NOAA Historical Hurricane Tracks — NOAA documents historical tropical system tracks based on user input. Options for input include location and radius of interest, timeframe, ocean basin, category, among others. The database includes hurricanes, tropical storms, tropical depressions, and extra-tropical storms. https://coast.noaa.gov/hurricanes Anchor: #UCVAQUBI
- NOAA Storm Events Database — NOAA maintains a database of recorded tropical and extra-tropical systems that can be searched by location, storm strength, and other factors. https://www.ncdc.noaa.gov/stormevents/ Anchor: #FDOHGEPC
- Hurricane Reports — NOAA, USACE, the University of Texas–Bureau of Economic Geology, the Texas A&M University–Corpus Christi Harte Research Institute, and other public research institutes have published reports documenting the strength and impacts of many of the historical hurricanes that have impacted the Texas Gulf Coast. These reports can generally be obtained online and/or from library archives. Anchor: #NRORBCVP
- Sea, Lake, and Overland Surges from Hurricanes (SLOSH) Model — NOAA’s database compiles the results of the SLOSH numerical model developed by the National Weather Service. The SLOSH model provides estimates of storm surge heights for historical, hypothetical, and predicted hurricanes by modeling different storm tracks, approach angles and speeds, hurricane categories, and tide levels. Risk-based information (e.g., exceedance probability) and specific flood elevations are not provided, and location-specific storm surge elevations are provided as a function of storm intensity (e.g., Category 1, Category 2, etc.). The database can be viewed using the SLOSH Display Program (SDP). http://www.nhc.noaa.gov/surge/slosh.php.
The SDP is intended for use by emergency managers to understand anticipated storm surge vulnerability. The results presented in the SDP can be used to understand worst possible surge levels. Although SLOSH model results are not likely to be used in roadway project design, they can be useful in the planning phase when assessing if the project site is vulnerable to hurricane storm surge exposure. SLOSH model results should be evaluated at each level of analysis for projects located in coastal areas to ascertain the risk from storm surge based on project location. If SLOSH results indicate that the site is exposed, it is recommended that additional freeboard or armoring be incorporated into the project design, particularly if it is a Level 2 or Level 3 analysis.
Anchor: #i1025516Numerical Modeling of Water Elevations
Where adequate long-term tide data are not available or when it is necessary to capture the dynamics of a complex shoreline or large-scale project (Level 3), numerical simulation of water levels can provide higher confidence in the selection of a design stillwater level. Numerical models account for site specific details and processes that give rise to complex interactions between water and the surrounding natural and built environments. The use of numerical models reduces the uncertainty associated with the representation of relevant coastal processes needed to design roads, bridges, and any other facilities TxDOT may develop. In general, a numeric modeling effort will need to be applied to appropriately capture the complexity of the study area. Table 15-3 describes the applicability of commonly used numerical models for roadway design.
|
|
Level 1 Analysis |
Level 2 Analysis |
Level 3 Analysis |
|---|---|---|---|
|
Design or Modeling Inputs |
FEMA flood map elevations, NOAA tide station data, USACE sea level maps, CHAMP outputs |
FEMA flood map elevations, NOAA tide station data, USACE sea level maps, CHAMP outputs |
2D and 3D hydrodynamic models including coupled wave, storm surge, and morphologic inputs (ADCIRC, Delft3D, MIKE21) |
Additional information regarding model specifics (e.g., inputs, outputs) for each example can be found in the FHWA’s A Primer on Modeling in the Coastal Environment or through consultation with a TxDOT Precertified Coastal Engineer.
Anchor: #i1025532Vertical Datums
Tide elevations are measured relative to a vertical datum, a reference system that allows one to locate a point on the Earth’s surface. Without a common datum from which measurements are referenced, surveyors would calculate different elevation values for the same location. There are two main types of vertical datums for coastal applications: orthometric and tidal, defined below. Many cities also establish a city-specific vertical datum relative to a local point of reference (for example, high water line or mean sea level). Most coastal water level data is collected in reference to a local tidal datum, while most transportation projects are designed based on an orthometric datum. As a result, it is common to require conversion between datums to successfully evaluate coastal conditions.
Tidal Datums
Tidal datums are based on tidally-derived surfaces of high or low water elevations defined by phases of the tide ( NOAA Vertical Datum Transformation). They are used to describe the average location where the water and land intersect for each major tidal phase. The hydrodynamics of tidal fluctuations are controlled by local processes; thus, it is important to remember that tidal datums provide a local tide characterization and will vary along the coastline.
|
Commonly Used Tidal Datums |
|---|
In areas with mixed semi-diurnal tides, such as the Northern Texas coast, two additional datums are defined:
|
All tidal datums are referenced to a 19-year averaging period known as the National Tidal Datum Epoch (NTDE). This 19-year period is significant, as it encompasses the length of time necessary for variations in lunar cycles (which influence tide levels) to occur. The current NTDE spans from 1983-2001 and is actively considered for revision every 20-25 years. Use of NTDE allows tidal datums throughout the U.S. to have a common reference. Tidal datums are commonly reported relative to the Mean Lower Low Water (MLLW), which is the lowest reported tidal datum. For example, when a report states the Mean High Water (MHW) at the Galveston Pier 21 station (NOAA #8771450) is 1.32ft, it means that, on average, over the 19-year period of 1983-2001, the average high tide was about 1.32ft above MLLW ( NOAA Tides and Currents, 2019). However, tidal elevations can also be easily expressed relative to other datums reported at the same tide station. Refer to the Relationships Among Datums subsection for more information and common conversions.
Orthometric Datums
Orthometric datums use the Earth’s gravity field to reference heights. The North American Vertical Datum of 1988 (NAVD88) is the current national standard vertical datum. It replaced the National Geodetic Vertical Datum of 1929 (NGVD29), which was the previous national standard for 60 years. Refer to NOAA for the most current orthometric datum information.
Relationships Among Datums
The relationship among datums often varies from one area of the shoreline to another. To make a comparison with land surveyed data, all data must be converted to a standard reference, such as NAVD88. As an example, Table 15-4 presents the relationship between tide heights referenced to NAVD88 and MLLW datums at the Galveston Pier 21 tide station.
To evaluate water surface elevations relative to the various datums reported for a tide station, visit the NOAA or TCOON tidal datum section of the tide station’s website. A list of tidal datums, similar to those listed in Table 15-4 provided as an example for Galveston Pier 21 gauge, will be provided. Conversions between the datums can be calculated by making the datums relative to each other through subtraction. For example, to convert MHHW referenced to MLLW to be relative to NAVD88, subtract 0.14 feet from 1.41 feet in the right-hand column to obtain 1.27 feet in the left-hand column. This comparison indicates that MHHW is 1.27 feet above NAVD88 and 1.41 feet above the MLLW.
|
|
Relative To |
|
|---|---|---|
|
Datum |
NAVD88 (feet) |
MLLW (feet) |
|
MHHW |
1.27 |
1.41 |
|
MHW |
1.18 |
1.32 |
|
MSL |
0.69 |
0.83 |
|
MLW |
0.16 |
0.30 |
|
MLLW |
-0.14 |
0.00 |
|
NAVD88 |
0.00 |
0.14 |
The relationship between NAVD88 and tidal datums has been calculated by NOAA for many of the tide gages along the Texas coastline. The conversion is different for every tide station. Therefore, investigating the relationship between tidal datums, orthometric datums, and site-specific upland surveys used for project design is very important. If a conversion for NAVD88 has not been calculated for a tide station of interest, refer to a professional surveyor or the USACE District office to obtain the necessary offset.
Tidal Datum Sources
- Anchor: #QILTSPGC
- NOAA CO-OPS — Tidal datums are available at many of NOAA’s tidal recording stations. https://tidesandcurrents.noaa.gov/ Anchor: #ICYITNCI
- TCOON — Tidal datums are available at many TCOON recording stations. http://cbi.tamucc.edu/TCOON/
Relative Sea Level Rise
Many of the roadway infrastructure networks located along the Texas shoreline have been in place since the early- to mid-1900s and were constructed with the underlying assumption that coastal water levels are stationary through time. Contemporary data trends in sea level rise call this assumption into question, resulting in uncertainty about the future performance of existing roadway infrastructure. Future sea level rise will increase the existing baseline elevations upon which daily tidal variations are measured and are therefore important to consider in project design.
Relative sea level rise reflects the combination of large-scale changes in global ocean levels (global mean sea level) and local changes in land elevations (for example, due to subsidence, or sinking, of land). Rates of relative sea level rise along the Texas coast are some of the highest in the nation: 4.0 mm/year (0.16 inches/year) at Port Isabel (1944-2018), 5.62 mm/year (0.22 inches/year) at Rockport (1948-2018), and 6.51 mm/year (0.24 inches/year) at Galveston Pleasure Pier (1908-2018) as shown in Figure 15-9 ( NOAA Tides and Currents, 2019). These high rates of relative sea level rise are partially linked to subsidence exacerbated by the historical extraction of subsurface groundwater, oil, and gas.
Figure 15-9. Historical Relative Sea Level Rise Rates at Galveston Pier 21 ( NOAA Tides and Currents, 2019).
Figure 15-10 illustrates the projected sea level rise for points along the Texas coast in comparison to other national rates. Increases in relative sea level rise may have the following impacts on Texas transportation assets located in coastal areas:
- Anchor: #YXXKDSBB
- Roadway flooding. As coastal water levels increase, low-lying transportation routes may become exposed to more frequent and more intense flood events, interrupting roadway access and increasing maintenance costs. Anchor: #SKIJNIMI
- Efficiency of stormwater drainage. As low-lying stormwater outfalls become partially or completely inundated by rising coastal water levels, stormwater drainage may be impeded, resulting in roadway flooding. In addition, increases in coastal groundwater levels due to sea level rise may reduce the efficiency of existing stormwater drainage systems. Anchor: #XPERGMIE
- Erosion damage. Elevated coastal water levels will allow wave action to reach higher elevations, which may cause erosion or scour near transportation features, such as unarmored embankments, causeways, and bridge pilings.
Figure 15-10. U.S. RSLR Trends
Due to the nature of transportation infrastructure (fixed alignment and elevation, linear features, long lifespan, etc.), assets are often limited in their ability to adapt to future sea level conditions. It is therefore important to incorporate future changes into project design whenever possible to cope with potential impacts of changing coastal conditions and minimize future cost and risk to these assets. Table 15-5 presents additional screening criteria that should be considered.
|
|
Factor to Consider |
More Critical to Incorporate Sea Level Rise |
Less Critical to Incorporate Sea Level Rise |
|---|---|---|---|
|
1 |
Project Design Life |
Long (20+ years) |
Short (less than 20 years) |
|
2 |
Redundancy/ |
No redundant/alternative route |
Redundant/alternative route exists |
|
3 |
Alternative Routes |
Substantial delays |
Minor or no delay |
|
4 |
Anticipated Travel Delays Due to Sea Level Rise |
Critical route for commercial goods movement |
Non-critical route for commercial goods movement |
|
5 |
Goods Movement |
Vital for emergency evacuations |
Minor or no delay in event of emergency |
|
6 |
Evacuation/Emergency |
Non-safety project |
Safety project and delay would be substantial |
|
7 |
Traveler Safety |
Large investment |
Small investment |
|
8 |
(Delaying the Project to Incorporate Sea Level Rise would Lead to On-going or New Safety Concerns) |
Minor or no effect – adjacent street and roads would not need to be modified |
Substantial interconnectivity issues |
|
9 |
Expenditure of Public Funds |
Minor or no increase in project footprint in environmentally sensitive area |
Substantial increase in project footprint in environmentally sensitive area |
|
10 |
Interconnectivity Issues with Local Streets and Roads |
Incorporating sea level rise may extend design into adjacent water bodies and/or properties or affect drainage of adjacent properties |
Incorporating sea level rise will not have an impact on adjacent water bodies and/or properties |
Anchor: #i1025656
Selecting a Sea Level Rise Value for Design
|
Relative sea level rise should be incorporated in project design if project is characterized as having:
|
When incorporating relative sea level rise into project design, it is important to select a future sea level rise projection that considers the project design life and risk tolerance to flooding. Climate change modeling and sea level rise projections have continued to evolve with significant advances in the understanding of global and regional factors that contribute to relative sea level rise. In 2019, the Texas General Land Office (GLO) published an updated Texas Coastal Resiliency Master Plan (GLO Plan), which includes detailed relative sea level rise scenarios for the state. The GLO Plan relies on the global sea level rise projections released in the 2017 NOAA report Global and Regional Sea Level Rise Scenarios for the United States, which reflects the latest published and peer-reviewed sea level science. The 2017 NOAA report includes six global sea level rise scenarios (low, intermediate low, intermediate, intermediate high, high, and extreme) to examine the full range of potential future water levels.
The 2017 NOAA projections have the added advantage of providing risk-based (probabilistic) planning capabilities, which were not previously available. The range of probabilities for each planning timeframe is dependent on modeled future climate conditions (referred to as Representative Concentration Pathways – or RCP), as described in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5).
To address the impacts of relative sea level rise during the next 50 to 100 years, the GLO Plan adjusted NOAA’s intermediate scenario of a 3.3-foot global mean sea level increase by 2100 to account for local effects (e.g., vertical land movement, tectonics, and sediment compaction) through statistical analysis of local tide stations along the Texas coast. This intermediate scenario has a 2 to 17 percent chance of being exceeded by future global sea levels by the year 2100. These local effects are what differentiate sea level rise from relative sea level rise and account for the relative portion of the rates.
Because of the diversity and expanse of the Texas coastline, the GLO Plan divides the coastal area into four regions to provide a more focused assessment within each region (Figure 15-11). The regions contain unique environmental characteristics, land use patterns, and vertical land movement that affect local water levels. Table 15-6 provides average relative sea level rise projections for each region. When considering future conditions for project design, a regional projection representative of the project location should be selected.
Figure 15-11. The GLO Texas Coastal Resiliency Master Plan’s Four Coastal Regions (Texas General Land Office, 2019).
|
Planning Time Horizon |
Relative Sea Level Rise (feet) |
|||
|---|---|---|---|---|
|
(Year) |
Region 1 |
Region 2 |
Region 3 |
Region 4 |
|
2020 |
0.8 |
0.8 |
0.7 |
0.6 |
|
2030 |
1.3 |
1.2 |
1.1 |
1.0 |
|
2040 |
1.7 |
1.6 |
1.5 |
1.3 |
|
2050 |
2.2 |
2.1 |
1.9 |
1.7 |
|
2060 |
2.8 |
2.6 |
2.4 |
2.2 |
|
2070 |
3.4 |
3.2 |
3.0 |
2.8 |
|
2080 |
4.1 |
3.9 |
3.6 |
3.3 |
|
2090 |
4.8 |
4.6 |
4.3 |
4.0 |
|
2100 |
5.5 |
5.2 |
5.0 |
4.6 |
|
Notes:
|
||||
The design life of transportation infrastructure describes the time period the structure is expected to sustain usability under normal loads and conditions and varies depending on the particular asset type. TxDOT refers to the American Association of State Highway and Transportation Officials (AASHTO) Bridge Design Specifications for bridge design life, and bridges are typically designed considering a 75-year period. Roadways are typically designed considering a 50-year period. In both instances, the relative sea level rise over the lifespan of the asset may be significant and should be considered in the design. Quantifying relative sea level rise into the project design will be dependent on the design life.
For example, if a project takes place in Region 1 and entails the installation of flexible pavement with a design life of 20 years starting in the year 2020, 0.9 feet of relative sea level rise is recommended in the design. This is calculated by subtracting 0.8 feet of relative sea level rise (year 2020) from 1.7 feet of relative sea level rise (year 2040) to account for the relative sea level rise that is expected to occur since the start of the project.
To consider a more complex example, consider a project in Region 2 starting in 2025 that will install concrete pavement roadway, which has a 30-year design life, connecting to a bridge, which has a 75-year design life. The relative sea level rise projection used for the road would be 1.4 feet (2.4 feet [year 2055] minus 1.0 feet [year 2025]). The relative sea level rise projection for the bridge would be 4.2 feet (5.2 feet [year 2100] minus 1.0 feet [year 2025]). The designer will need to consider whether elevating the roadway to the elevation of the bridge is feasible and appropriate for the project to conserve the life of the bridge asset.
To allow incremental adjustments to manage the impacts of relative sea level rise, the design of some transportation assets (e.g., causeway heights, pavement surfaces, facility protection design, and roadside vegetation) could also target a shorter intended design life. For example, a causeway could consider a 30-year design life rather than a 75-year design life so that future climate conditions are more moderate and achievable based on project cost restrictions and efficiencies. The Consideration of Sea Level Rise in Project Design section below describes additional criteria to consider when incorporating relative sea level rise into transportation projects.
In Summer 2019, FHWA is expected to release an update to their Hydraulic Engineering Circular 25 (HEC-25) Highways in the Coastal Environment: Assessing Extreme Events report. The updated edition will include recommendations for incorporating sea level rise into project design and should be reviewed prior to development of stillwater levels.
Considering Relative Sea Level Rise in Project Design
Once projections for relative sea level rise have been identified for the project, the following procedures should be followed:
- Anchor: #BFUXLNJH
- Obtain elevation data for project site features (e.g., roadway, culvert, bridge). Project elevations can be obtained from as-built drawings for maintenance projects or from land surveys completed for planned projects. Anchor: #JWQUVOFN
- Select a relative sea level rise projection from Table 15-6 based on the appropriate region to assess potential impacts. Anchor: #HQOCENLM
- Using the project survey elevation data collected in step 1 and relative sea level projections from step 2, assess the relative sea level rise over the project lifespan. Anchor: #XXIETTHQ
- If applicable, identify the possible negative impacts of relative sea level rise on the project related to asset function or operation. Possible impacts include scour and/or erosion due to tidal action, reduced efficiency of drainage culverts due to higher tailwater conditions, and exposure to saltwater. Anchor: #OJKKHBKV
- For identified impacts, assess if adaptive measures will be necessary. In many cases, the project footprint may be impacted, but no adaptive measures may be required. Impacts may also be temporary (e.g., wave splash during high tide events or during storms). Not all adaptive measures require physical alteration to the roadway design. Temporary impacts may be addressed through operational modifications, such as short-term road closures. Anchor: #VGXXWIFC
- Identify the cost of potential relative sea level rise adaptive measures. Due to cost limitations, not all relative sea level rise adaptive measures may be included in the project design. For example, raising a roadway could cause a larger fill slope to encroach into an environmentally sensitive area. Assessments of potential adaptation measures and any limitations should be documented to indicate what can be achieved through evaluating the cost of adaptation vs. the cost of inaction. Costs should be considered in terms of economic, environmental, and social/human impacts. Anchor: #YJDXOWSA
- Where feasible, adaptive measures for relative sea level rise (e.g., roads elevated on berms, bridge height and on ramp adjusted for future sea levels, long-term planned retreat, enhanced erosion protection along roadway) should be incorporated into project design, particularly where future impacts are anticipated. Anchor: #HIXNUONX
- Consider unintended hydraulic impacts when designing relative sea level rise adaptive measures. For example, elevating roadways may impede floodwater drainage or affect flooding of adjacent properties. To offset these impacts, incorporating additional drainage mechanisms into project design can alleviate flooding of low-lying areas located near the modified roadway.
Sea Level Rise Projection Sources
- Anchor: #ICVBTXAI
- Texas General Land Office — 2019 Texas Coastal Resiliency Master Plan, published February 28, 2019. http://coastalstudy.texas.gov/resources/files/2019-coastal-master-plan.pdf Anchor: #YUXEUKAE
- FHWA — Highways in the Coastal Environment: Volume 3, anticipated publish date in Summer 2019 Anchor: #HHHWFMQY
- NOAA – Global and Regional Sea Level Rise Scenarios for the United States, published January 2017. e 2017 NOAA report Global and Regional Sea Level Rise Scenarios for the United States
Selecting Stillwater Levels for Project Design
Selecting appropriate stillwater levels for each project is critical for transportation infrastructure located in coastal areas. Stillwater levels are used as input for the design wave height (Section 3), which when combined, determine the design elevation. This cumulative elevation is discussed further in Section 4 of this chapter. The stillwater level at a project site is composed of a combination of the appropriate astronomical tide and storm surge, as previously discussed. If sea level rise is to be accounted for, it will be cumulatively combined with the tidal information.
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Stillwater level is a combination of:
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While the 1% AEP storm tide is the primary flood zone mapped by FEMA as either AE or VE zones, coastal transportation projects (e.g., local roadway) frequently justify a lower AEP (e.g., 50% to 10% AEP) and others (e.g., freeways or critical evacuation routes) that may justify a higher exceedance probability (e.g., 2% or 1% AEP). Knowing when, where, and how to appropriately select and apply a stillwater level comes from experience and sound judgement. Refer to Chapter 4 (Hydrology), Section 6 (Design Flood and Check Flood Standards) as a starting point for consideration by roadway classification and structure type. As demonstrated in this section, this decision is dependent on numerous factors and can be a subjective process unique to the coastal zone. Proficiencies in this topic typically reside with the Precertified Coastal Engineer; however, it is important that the designer demonstrate knowledge and competence assessing the project risk tolerance, budget restraints, and social and environmental impacts to select the most appropriate stillwater level for a safe, yet cost-effective project design.
For a project relying on a Level 1 or 2 analysis (e.g., local roads/street or other non-critical assets), it may be acceptable to develop a stillwater elevation by adding an appropriate sea level rise projection, as determined by the project’s design life and risk tolerance, to a FEMA return period storm tide collected from the latest FIS for the project area (after excluding wave heights).
More complex projects that require a Level 3 analysis are likely to require additional effort. To obtain a site-specific water elevation needed for design, the designer may need to work with a Precertified Coastal Engineer to perform risk-based modeling of water level conditions that capture the local variability experienced at the project site. These models will simulate a number of possible independent storm scenarios that include a variety of storm parameters, including wind speed, pressure, and landfall angle, among others, combined with statistical analysis to determine the probability of the storm tide occurring at the project site. These models will also be capable of incorporating changes in relative sea level to evaluate how the return period design events chosen may evolve over time. Some descriptions of how these models are developed and utilized are described in subsequent sections.