05/04/2018
POSSIBLE FLOODING, OKANOGAN RIVER
3 May 2018
The Okanogan River is now expected to reach flood stage around noon on Friday, 4 May 2018. It is forecast to reach “moderate flood” stage sometime late in the evening on Monday, 7 May, and reach “major flood” stage late in the evening on Thursday, 10 May. THIS MAY CHANGE if the weather warms up or cools down more than expected, if the rain moving into Canada is heavier or lighter than expected, and so on.
At “major flood” stage, the river is expected to reach its highest level in the past forty-four years. What does this mean for those living along the river on the reservation?
The last significant floods on the Okanogan River were in 1972 and 1974. The 1972 flood set a new record, and1974 was the third highest Okanogan River year on record. Since those floods, though, a levee was built in 1979 to protect Omak. Flood levels are measured at the river gage in Tonasket, but that number may not translate well to locations downstream. Actual water depth changes with the width of the river at any particular point prior to flooding.
1. Flood Stage, 15 foot level:
There are many years where the Okanogan River reaches the basic flood stage but does not cause damage, only inconvenience. As the river rises, so does the ground water on low-lying lands near the river. On many, possibly even most years, the rising ground water floods low-lying fields near the river. This affects primarily those who keep livestock pastured or corralled in those fields, and they may need to move their stock if the river levels rise. If you have a basement and are close to the river, you may get seepage into your basement.
At 16 feet, records indicate that low-lying lands between Rodeo Track Road and Benton Street, south of Omak, will begin to flood.
2. Moderate Flood Stage, 17 foot level:
At this level there will be significant flooding of low-lying fields along the river, partly due to rising ground water levels, from Oroville to Okanogan. Water will rise to the shoulder of Omak Eastside Road near Wanacut Creek. Records indicate that seepage will cause flooding of basements near the river in Okanogan. At 17.5 feet, seepage through the levee will affect low-lying lands along the off-reservation (west) bank of the river south of Okanogan.
3. Major Flood Stage, 18 foot and above:
National Weather Service information is that at and above the 18 foot level, “Major flooding of surrounding croplands is likely. Some homes and buildings adjacent to the river will experience some basement and first floor flooding.”
It has been more than forty years since the river has been this high. The levee in Omak is built to withstand this level of flood. However, isolated homes and buildings along the river may be vulnerable. We don’t have any firm information as to which buildings may flood, or which may simply be isolated as access roads across fields flood from rising water. If you live in an isolated home near the river, you may need to look at maps showing your elevation compared to the expected river levels.
The FEMA flood maps are available on line at https://msc.fema.gov/portal They are not easy to interpret, and they show “hundred year flood” zones. This flood is expected to be less than the hundred-year level.
4. What can you do/What should you do?
If you live behind the levee in Omak, just be prepared for inconvenience as low-lying fields and nearby roads flood. The Omak levee was inspected in 2016 (west side) and in 2017 (east side) and was found to be in adequate condition to protect against a flood. If you have a basement, it may get wet. You may wish to move anything valuable out of the basement if you live near the river.
If you own livestock pastured or corralled in low-lying fields near the river, you should make plans now to move them if there is no higher portion of the field they can move to. Field flooding will likely be higher than anyone has seen in the past forty-four years. If your livestock are normally fine during the yearly field flooding, they may not be this year.
If you live in or own a structure outside of a levee-protected community, near to the river or in a low-lying area that has flooded in the past during high water, it is time to think about how to protect your home and property. That’s a decision you have to make for yourself. It may be best to think of this like you would if a fire is approaching: if you have to go, what should you take when you leave?
If it is a two-story building and the first floor may flood, what can you move to the second floor to keep it out of flood waters? If you think your home is safe but that all of the access roads may flood, do you want to stay there (possibly without power or running water) until the water goes down, or will you be better off evacuating? If you live in a one-story home that you expect to flood, can you move your valuables to someplace higher before the waters rise? Friends and relatives may be able to store some of your property for you.
5. Sandbags
If you live outside of a levee-protected community, and you want to sandbag to protect your home, sandbags will be available to members and residents at the Omak Community Center beginning Friday morning, 4 May. They come in bundles of 100, and we ask you to take no more than one bundle each to start.
Here are some general rules of thumb for sandbagging:
• Sand is best, but you can fill sandbags with available dirt.
• Sandbags should be filled no more than 2/3 full, and should be dropped into position. That allows the sand or dirt to shift so that the bag can seal against adjacent sandbags. If the bag is too full, it doesn’t work well.
• One layer of sandbags protects to a height of no more than four to six inches, so a wall of sandbags two high that is properly laid will protect no more than 8 inches to one foot in height. If you plan for four inches of height per layer, that would be best.
• A single sandbag, filled 2/3 full, will be approximately one foot from end to end.
To protect an area 25 feet long to a depth of one foot, you would need at a minimum approximately 75 sandbags.
Sandbagging does require some skill. To learn more, you can look at a pamphlet found here:
http://www.nws.usace.army.mil/Portals/27/docs/emergency/NWD_Sandbag_Pamphlet.pdf
Updates on potential flooding will be posted here as information becomes available. Replies to this site will not be answered, but those messages will be monitored for useful response information.
