Monday, September 15, 2014

What is Atlas 14?

Is the flooding that occurred in the spring of 2014 further evidence that weather is changing, our precipitation
models are inaccurate, or a combination of both?

In 2013 the National Oceanic and Atmospheric Administration (NOAA) released Atlas 14, Volume 8, a tool that revises rainfall/precipitation frequency estimates. The estimates serve as an update to the U.S. Weather Bureau’s Technical Paper No. 40 (TP-40) published in 1961, which has served as a key document over the years for water planners and hydrologists. The Watershed District, which used TP-40 estimates in the past, is in the process of integrating Atlas 14 projections into its next management plan.

Why the change? What is the significance?

Eleven Midwestern states, including Minnesota, recognized that there was room for improvement with the 1961 TP-40 methodology and new technology to support doing so.  They pooled their funds to update this tool. TP-40 used available rainfall information from far fewer stations than exist today and included the “dust‐bowl” years of the 1930’s.  Using many more rainfall collection stations to record rainfall outside of a major drought period allowed for more accurate modeling of current climate patterns.  The maps below show how reporting locations have increased in Atlas 14.

Left: TP40 Daily Stations across MN.  Right: Atlas 14 Daily Stations.  The number of stations in Minnesota where this rainfall frequency data is collected daily. Map images courtesy of BARR Engineering. 
Left: Rainfall stations that recorded sub-daily precipitation amounts for the TP-40 model.  Right: Sub-daily stations used for the Atlas 14 model.  Green dots show stations that record at 1-hour increments, red dots record at 15-minute increments, and yellow dots record at variable increments based on rainfall volumes.

The average record length used in the tool is now over 50 years and doubles the record use in original studies. The oldest Minnesota rainfall dataset dates back to 1836 (Fort Snelling/Minneapolis St. Paul Airport)!

Rainfall frequency or recurrence intervals in Atlas 14 are provided for 1‐year, 2‐year, 5‐year, 10‐year, 25‐year, 50‐year, and 100‐year events. A new feature in Atlas 14 is an interactive web interface. Here is a link to it:

Another concern that prompted an updating of this tool was the under-projection of rainfall and depths, given the magnitude of more recent storms. Atlas 14 has revealed some significant increases in each state, particularly in the depths for 24-hour/100-year storms (a heavy rainfall that drops X-amount of rain in 24 hours that only occurs once every 100 years). For example, current estimates for 100-year storm depths in a 24 hour period for Minneapolis/St. Paul airport have risen from 6 to 7.5 inches. This represents a 25% increase. See the diagram below for a comparison of TP 40 values versus Atlas 14 estimates in 24 hour/100 year depths for Ramsey-Washington Metro Watershed District and other locations in the Twin Cities metro area.

[click to enlarge] Left: A map showing the boundary of RWMWD (pre-Grass Lake WMO merger) and the precipitation estimates for a 100-year 24-hour storm according to the old TP-40 estimates (yellow-dotted lines) and the newer Atlas 14 (blue-dotted lines).  Right: The difference in the amount of rainfall estimated in the two models varied throughout the state.  In some places, the two models estimated roughly the same, whereas in others including the Twin Cities they were upwards of 25% different.

Some surprises have also been uncovered with the application of Atlas 14. It has revealed that there are some rather large changes over short distances in terms of the difference in 24-hour/ 100-year depths (inches) across the region. Here are a couple examples: Minneapolis, MN to St. Cloud, MN: 7.9 to 6.1 (1.8” difference) and Worthington, MN to Sioux Falls, SD: 7.8 to 5.9 (1.9” difference).
The updated rainfall depths published in Atlas 14 will be incorporated into Ramsey-Washington Metro Watershed District’s hydrologic model to calculate more accurate 100-year water surface elevations on lakes and water bodies within the District. The results will be included in the District’s water management plan update and be used for planning future flood risk mitigation efforts and project implementation. Tasks included in the process include verifying and updating watershed divides, updating rainfall depth and distributions and simulating 100 –year events. Analyses of the historical frequency of heavy rainfall events will provide information for engineers and others to help us design and operate infrastructure such as culverts and stormwater runoff ponds as well as potentially assist us in potentially holding future development to higher standards.
Here are some considerations that the District may need to take into account in coming years with the use of Atlas 14:
  • The difference between 10 and 100-year events are greater: existing municipal storm sewer may not be undersized, but flood control could be undersized.
  • Freeboard requirements may need to be adjusted. This refers to the elevation of a building compared to the 100-year flood level of a waterbody. District standards are currently 2ft above the base flood level.  While the standard may remain, the elevation at which a building is considered "high enough" may have to change.
  • Some other possibilities may need to be considered including mitigating impacts due to larger storms and flooding, providing safe overflow routes, larger easements and changes to ordinances, permitting, policies and standards.

Thanks to this effort put together by Midwestern states to update our rainfall modeling, we look forward to using Atlas 14 to improve our methods and practices to do all we can to keep our citizens, their property, and our ecological systems safe.

Credits: Information and graphics for this article provided by Brandon Barnes at BARR Engineering.

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