High tide flooding

Miami Beach, Florida, Joe Raedle / Getty Images

"Sunny day flooding"

High Tide Flooding

  • Tides: For most cities on the ocean, sea level goes up and down about twice a day in a regular pattern. These Tides are due to the Moon's gravity acting on Earth's oceans as Earth rotates (once a day) and the moon rotates around Earth (about once each 27 days)
  • Sunny day flooding / High tide flooding: It's not unusual that there is some variation in the height of "high tide": The moon's gravitational pull is the most important factor for high tides. But the sun also pulls on us with gravity.

    For example: When the sun and the moon are both on the same side of Earth, their pulls add together, and you get a "spring tide", which is higher than the average high tide.

    Also... "king tides" when Sun and Earth and Moon and Earth are particularly close and aligned...

    When the tide is unusually high the ocean is more likely to flood low-lying coastal areas, even without any recent rainfall or storms or climate change.

  • Global average sea levels, 1880-2015


    epa.gov

  • Concern about sunny day flooding: In recent years people have been noticing more and more flooding... "Business Insider" photo feature.
  • NOAA's 2018 State of High Tide Flooding and 2019 Outlook website tries to quantify what high-tide-flooding means, and how to measure it.
  • NOAA uses 3 national flooding thresholds to classify high tide flooding events. In order to understand what this means, you'll need to understand a bit about the terms "MHHW" and "MLLW" and how "normal" tides are measured.
  • Charleston: Flooding in Charleston Harbor, and Charleston gets ready for Dorian (NPR): They get plenty of practice!
  • Reasons for sea level rise (Smithsonian Institute)

Explore and report

Using the links above (or others you find), write up answers to the following questions:

  1. What does NOAA stand for? What kind of organization is it (in a few sentences)?

    "National Oceanic and Atmospheric Administration" - a U.S. government agency. NOAA also runs the National Weather Service, which ran afoul of Donald Trump in "Sharpiegate".

  2. The report uses the terms "MHHW" and "MLLW". So, a little bit about those: The "M" stands for "mean" which is another word for "average".

    MHHW - 'Mean highest high water' - is the Mean=average highest tide level. Typically, there are *two* high tides (and two low tides) each day. So, the idea is to measure the sea level at the highest high tide from each day, and then average that highest sea level during over a period of time.

    MLLW - 'Mean lowest low water'

    Now, read that article (above) about the definitions of MHHW and MLLW.

    In your own words, how would describe what MHHW and what MLLW mean? Include in your descriptions something about the time period that was used for whatever averages are important.

    In this case, over the 19 years from 1983-2001. MLLW - 'Mean lowest low water'.

    Question: If sea levels are rising, would you expect that high tide sea levels in 2019 would on average be:

    1. below MHHW
    2. about the same as MHHW
    3. above MHHW
    4. can't say

    Question: If sea levels are rising, would you expect that low tide sea levels in 2019 would on average be:

    1. below MHHW
    2. about the same as MHHW
    3. above MHHW
    4. can't say
    MHW - Mean high water: take the average of the two high tides, and then average that over the same 19 year period.

    Question Would you expect that MHW is:

    1. lower than MHHW
    2. about the same as MHHW
    3. higher than MHHW
    4. can't say
    See the NOAA datum page.
  3. Reading from those links above...

    What are the 3 national flooding thresholds that NOAA uses to describe high tide flooding? Give them in both meters and feet. Put in your own words both the meaning of "moderate flooding" and how you'd measure it.

    The report finds that, on average, U.S. coastal infrastructure is vulnerable to minor, moderate, and major flooding at heights of about 0.5, 0.8, and 1.2 meters [1.6 feet, 2.6 feet, and 3.9 feet] above the average daily highest tide (Mean Higher High Water).

    A few of you used a table....

    When a high tide is this much above MHHW
    Mild floodingModerate floodingMajor flooding
    0.5 to 0.8 m0.8 to 1.2 m more than 1.2 m
    1.6 to 2.6 ft2.6 to 3.9 ft more than 3.9 ft
  4. Find out how many days had "moderate or greater" high-tide flooding at one location, during one month:

    On the "State of high Tide Flooding..." site, read the first part of the report. When you get to the interactive diagram with a map of the US, pick one monitoring station on the East or Gulf coast to look at in more detail. Note the name.

    Go to the Tides and Currents "Products" page. Then look for the Water Levels link, and find your monitoring station.

    In one 24 hour period, how many high tides do you see? How many low tides?

    Most of you saw 2 high tides per day, e.g. Trident Pier, Cape Canaveral, FL. Though Galveston was a bit weird.

    Adjust the dates to run from Sept 1 2019 to Sept 30 2019. (The site will only plot up to 30 days of data). To check for high-tide flooding, would it be easier to plot levels relative to the "MLLW" or "MHHW" datum? Choose the one that's easier!

    Are there any tides which would exceed the NOAA damage threshholds in this month? If so, count how many days would meet the moderate threshhold or higher. Write down the number, the month, and the station. If there weren't any, switch to 30 days in a different month.

    It's not enough to just go *over* MHHW. For moderate flooding you need to exceed 2.6 feet (0.8 m) above MHHW. In this graph, of Trident Point (Cape Canaveral, FL, March, 2018):
    The highest tide was 1.1 feet above MHHW. This does not reach the "minor flooding" threshold.

    On the other hand, Boston, March of 2018 shows

    • 8 days of minor flooding
    • 2 days of moderate flooding
    • 1 day of major flooding
  5. Return to the "2019 report ...and outlook" and read further. What is the concern in this data that NOAA is trying to raise?
  6. When you get to the page on "Trends in Annual High Tide Flood Frequencies": Which part of the USA seems to be suffering the most from the changes in flood frequencies?

    It appears the Gulf and East coasts are seeing greater increases in high tide flooding than the West coast.

  7. Continue reading the report and outlook. Other questions or comments you have about this?
  8. What are some of the reasons that sea levels are rising? Which are related to climate change? Does this explain all the trends you saw?
    M. C. Morton both offers some answers and shows how to refer to someone else's work (while avoiding plagiarism):
    Over the last century, sea level in Boston Harbor rose by about 28 centimeters, due to both thermal expansion of seawater as the oceans warm (SN Online: 9/28/18) and the melting of distant ice sheets. Conservative projections for Boston place sea level about 15 centimeters higher by 2030, 33 centimeters higher by 2050 and 149 centimeters higher by 2100. In a worst-case scenario, if greenhouse gas emissions continue at the current pace, sea level could rise by as much as three meters by 2100.

    New England and the eastern shore of Canada have a unique combination of geographic factors that push water farther inland in response to high tides: The region’s shallow seafloor topography tends to funnel water higher inland, and its proximity to the Gulf Stream — a major ocean current that runs from the Gulf of Mexico up along the East Coast — also helps magnify tides. Due to rising ocean temperatures, the Gulf Stream is slowing down, causing even more water to pile up along the East Coast and boosting high tides, physical oceanographer Tal Ezer of Old Dominion University in Norfolk, Va., reported in June in Earth’s Future.

    -Mary Caperton Morton, Science News "With nowhere to hide from rising seas, Boston prepares for a wetter future"

    Better than enclosing large amounts of text from an article is summarizing key pieces in your own words. One of you wrote about this same article:

    Another great adaptation being made is more “greenspaces”. This refers to things like parks and wetlands being added to coastlines to absorb floods. Not only will this help control water, but it works as a native habitat restoration, and is great for tourism and recreation.
  9. What is one city trying to do to adapt to high-tide flooding? Find an article (online is fine) about some Atlantic Coast (East Coast) US city and and summarize what the city is trying to do to adapt to rising sea levels and high-tide flooding. (Include the title and URL of the article).