Reply to thread

Message: <blockquote data-quote="Allen" data-source="post: 421582" data-attributes="member: 389">No, Garrrison Dam was not "wide open" in 2011.  It ended up needing to release around 155,000 cfs in early June.Garrison Dam was built to safely pass right around 899,000 cfs, so it was passing less than 20% of its rated discharge.Recent dam safety studies now put the maximum flood (roughly a 1,000 yr event) at right around 1.2 million cfs.  In the coming years, the dam will be modified to construct a larger capacity spillway in order to bring it back to the standard of needing to pass a 1,000 yr flood.Note, this is happening at many dams across the country.  These dams were built in the 1940-1960s with only 30-50 years worth of meteorologic and hydrologic data.  There's now 90-100+ years of data and a good share of dams no longer provide the protection originally intended.I don't know offhand to what extent (cfs) these new flood protections will buy in the way of flow in the river, but I am going to hazard a guess that there's no way anyone will pay what it would take to prevent flood damages from occurring at 1.2 million cfs.Here's the existing stage-rating curve for Bismarck:  <a href="https://waterdata.usgs.gov/nwisweb/get_ratings?site_no=06342500&file_type=exsa" target="_blank">https://waterdata.usgs.gov/nwisweb/get_ratings?site_no=06342500&file_type=exsa</a>It only goes up to 450,000 cfs at a stage of 27.0 ft, so one would have to extrapolate quite a bit to estimate how high the water would be in south Bismarck at 1.2 million cfs.The flood of 1952 which had an estimated flow of 500,000 cfs reached a stage of 27.90 ft, but that's only #4 on the <a href="https://water.noaa.gov/gauges/BIWN8" target="_blank">National Weather Service's ranking</a> of worst floods in the Bismarck area.  The 1881 flood reached 31.60 ft.  If you would like to translate that into elevation above MSL, add 1619.46 ft and then compare it to your home's base elevation.  This would be in the NAVD-88 vertical datum.  You will want to add about a 1.25 ft for every river mile you are north of Steamboat Park, and subtract the same for every river mile you are below it to account for slope of the river.Obviously, the risk of such floods in any given year are something around 0.1%, but it surely isn't 0.00000 and the consequences are quite apocalyptic.</blockquote>

Verification: What is the most common fish caught on this site?

Reply to thread

Recent Posts

Friends of NDA