Coastal Water Levels
Total Page:16
File Type:pdf, Size:1020Kb
Guidance for Flood Risk Analysis and Mapping Coastal Water Levels May 2016 Requirements for the Federal Emergency Management Agency (FEMA) Risk Mapping, Assessment, and Planning (Risk MAP) Program are specified separately by statute, regulation, or FEMA policy (primarily the Standards for Flood Risk Analysis and Mapping). This document provides guidance to support the requirements and recommends approaches for effective and efficient implementation. Alternate approaches that comply with all requirements are acceptable. For more information, please visit the FEMA Guidelines and Standards for Flood Risk Analysis and Mapping webpage (www.fema.gov/guidelines-and-standards-flood-risk-analysis-and- mapping). Copies of the Standards for Flood Risk Analysis and Mapping policy, related guidance, technical references, and other information about the guidelines and standards development process are all available here. You can also search directly by document title at www.fema.gov/library. Water Levels May 2016 Guidance Document 67 Page i Document History Affected Section or Date Description Subsection Initial version of new transformed guidance. The content was derived from the Guidelines and Specifications for Flood Hazard Mapping Partners, Procedure Memoranda, First Publication May 2016 and/or Operating Guidance documents. It has been reorganized and is being published separately from the standards. Water Levels May 2016 Guidance Document 67 Page ii Table of Contents 1.0 Topic Overview .................................................................................................................. 1 2.0 Astronomic Tide ................................................................................................................. 1 2.1 Tides and Tidal Datums .................................................................................................. 1 3.0 Storm Surge Modeling ....................................................................................................... 4 3.1 General Considerations .................................................................................................. 5 3.2 Mesh Considerations ...................................................................................................... 6 3.3 Boundary Forcing ........................................................................................................... 7 3.4 Astronomical Tidal Effects in Storm Surge Analysis ....................................................... 8 3.5 Wind Drag and Bottom Drag ........................................................................................ 13 3.6 Land Cover Data .......................................................................................................... 14 3.7 Wave Setup .................................................................................................................. 14 3.8 Storm Climatology ........................................................................................................ 14 3.9 Measured Water Level Data ......................................................................................... 14 3.10 Ice Cover ...................................................................................................................... 16 3.11 Model Validation ........................................................................................................... 17 4.0 Water Levels in Sheltered Waters .................................................................................... 20 4.1 Variability of Tide and Storm Surge in Sheltered Waters ............................................. 20 4.2 Seiche ........................................................................................................................... 22 4.3 Estimating Sheltered Water Levels Using Existing Flood Insurance Study Data ......... 22 5.0 Nonstationary Processes ................................................................................................. 24 5.1 Relative Sea Level – Sea Level Rise ........................................................................... 25 5.2 Relative Sea Level – Land Subsidence and Rebound ................................................. 25 5.3 Astronomic Tide Variation ............................................................................................ 25 6.0 References ....................................................................................................................... 26 List of Figures Figure 1. Tidal datum information for Los Angeles, CA. ............................................................... 2 Figure 2. Predicted, Observed, and Residual Tides at Panama City, Florida ............................. 15 Water Levels May 2016 Guidance Document 67 Page iii 1.0 Topic Overview This guidance document supports the standards related to the determination of coastal stillwater levels (SWLs). The SWL is a coastal water surface resulting from astronomical tides, storm surge, and, depending on the location, other effects such as El Niño effects or seiching. The SWL does not include wave heights and runup. The stillwater elevation (SWEL) is the statistical elevation of the SWL relative to a specified datum. The statistical methods needed to determine the 1%-annual-chance SWEL are discussed in FEMA’s Coastal Flood Frequency Analysis Guidance document. The mean water level (MWL) includes all components contributing to the SWL plus static wave setup. The dynamic water level (DWL) is the combination of SWL plus static and dynamic wave setup; this generally applies only to studies on the Pacific coast. Coastal Base Flood Elevations (BFEs) are a combination of waves or runup on top of the SWELs. Depending on the coastal location, the SWL historically included wave components. On the Atlantic and Gulf coasts, the SWL included astronomic tides as well as storm surge and wave setup that are developed during large tropical and extra-tropic storm events. On the Great Lakes, the tidal component is so minimal to be excluded from the SWL and only storm surge and wave setup were included in the calculations. On the Pacific coast, the surge components driven by barometric pressure changes and El Niño effects can be captured in tide station records. In the Pacific studies dynamic wave setup was added. Inclusion of wave setup would include the word ‘Total’ in the definition of SWL or SWEL. This guidance document discusses astronomic tides and the process of extracting SWL data from those records in Section 2. For areas where storm surge processes dominate, guidance on how to determine the SWL from storm surge effects is provided in the Section 3. Section 3 includes a discussion on the inclusion of tides for surge-dominated coastal areas. Sheltered waters and non-stationary effects are discussed in Sections 4 and 5, respectively. 2.0 Astronomic Tide The astronomic tide is the regular rise and fall of the ocean surface in response to the gravitational influence of the moon, the sun, and the Earth. Because the astronomic processes are entirely regular, the tides, too, behave in an entirely regular, though complex, manner. The statistical analysis of tide gage data is discussed in the Coastal Flood Frequency Analysis Guidance document. 2.1 Tides and Tidal Datums The tides along the Atlantic are semi-daily or semidiurnal, meaning that there are two highs and two lows each day, while in the Gulf of Mexico the tides are mix of diurnal, meaning that there is only one high and low each day, and semidiurnal. The tides along the Pacific Coast are mixed and semi-diurnal; conventionally, mixed tides are semi-diurnal tides for which the magnitudes of successive highs or successive lows have large variation. The average of all the highs is denoted as mean high water (MHW) while the average of all the lows is mean low water (MLW). Averages are taken over the entire tidal datum epoch, which is a particular 19-year period explicitly specified for the definition of the datums; a full astronomic tidal cycle covers a period of 18.6 years. The average of all hourly tides over the epoch is the mean sea level (MSL). Water Levels May 2016 Guidance Document 67 Page 1 The daily highs are generally unequal, as are the lows, and are identified as Higher High, Lower High, and so forth. At a given coastal location, each of these has a mean value identified as mean higher high water (MHHW), mean lower high water (MLHW), mean higher low water (MHLW), and mean lower low water (MLLW). In addition to these, there is the mean tide level (MTL) which is the average of MHW and MLW, and which is also called the half-tide level. These several levels are important because they constitute the datums to which tide data have traditionally been referred. Local charts and recorded tide gage data are generally referenced to local MLLW or MLW. This introduces some ambiguity because MLLW and MLW vary from place to place and from epoch to epoch. For use in most FEMA Flood Insurance Studies, these tidal datums are transformed to a standard vertical datum such as the North American Vertical Datum of 1988 (NAVD88) or the National Geodetic Vertical Datum of 1929 (NGVD29). For example, the National Oceanic and Atmospheric Administration (NOAA) maintains tidal benchmarks for many stations that are now tied to a standard vertical datum. Benchmark sheets for active and historic stations are available at the NOAA Website, http://tidesandcurrents.noaa.gov/. NOAA’s Vertical Datum Transformation Program (VDATUM) provides transformation information at areas