Lake Wanahoo - Update on Flooding
On May 20, following a significant amount of rain, some lowland flooding occurred downstream of Lake Wanahoo near Wahoo. Some fields were flooded and water ran over Highway 77/92 for a period of several hours.
This surprised some area residents and attracted a significant amount of media attention, based on the belief that downstream flooding would be completely eliminated now that the Lake Wanahoo dam is fully operational.
While any amount of flooding is unfortunate, the NRD would like to emphasize that the Wanahoo dam is working as intended. The dam did hold back a significant amount of storm water, but it was not designed to capture 100% of the runoff from a large rain event like the one that occurred in the late evening/early morning hours of May 19-20.
That rain was unusual, with more than 3 inches falling over the entire 87-square-mile Sand Creek Watershed that drains into Lake Wanahoo. Typically, rain events follow a more linear path, with higher intensity rains in a narrow band along the center and lower amounts on either side.
The heavy watershed-wide rain was compounded by the fact that soils were already saturated by rains earlier in the week, and much of the rain fell on newly-planted crops with little or no vegetation to help slow runoff.
The amount of runoff from the May 19-20 rain was equivalent to a 7- to 10-year storm (a storm that has a probability of occurring every seven to ten years). More than 2,700 cubic feet of water per second (cfs) was passing over the Wanahoo principal spillway during the peak rainfall. Prior to the construction of Lake Wanahoo, rain events as small as 500 cfs (a 1/2 year storm) were enough to cause lowland flooding similar to what occurred on May 20.
In short, the flooding would have been worse if Lake Wanahoo had not been in place. Over the long term, the frequency and intensity of lowland flooding in the area should be sharply reduced thanks to the dam.
Over the short term, with already-saturated soils and more bad weather being forecast, there is a concern that further lowland flooding could occur if another large rain event takes place. To help reduce that risk, the NRD is temporarily lowering the water level of Lake Wanahoo by two feet to provide extra storage capacity. The district and its partners will also be evaluating the lake's operational plan to see if other adjustments can be made to further minimize the chances of future flooding.
If you have questions, please contact us at 402-443-4675, toll-free 1-888-764-0543, or lpnnrd@lpnnrd.org. Get more information on Lake Wanahoo and the Sand Creek Watershed Project here.