Location of Project:
Southwest corner of Jackson Park Golf Course. The golf course is located southeast of the intersection of 5th Avenue Northeast and Northeast 145th Street.
Description of Project:
Construction of three connected off-channel detention ponds on the north branch of Thornton Creek, with a total of 25 acre-feet of live storage.
Installation of aerators on the ponds, including existing Phase I pond, to prevent summer stagnation and associated odor.
Enhancement of approximately 2,300 linear feet of the north branch of Thornton Creek; creation of a low flow channel within a meandering flood plain, and installation of instream habitat elements (root wads, rocks, large woody debris)
Creation of 1.6 acres of new wetland and enhancement of 0.7 acres of existing wetland
Planting of riparian corridor and buffer area with native species
Reason for Project:
1) Reduce flooding in north branch of Thornton Creek.
2) Reduce velocities in the creek that scour and erode streambeds and banks.
3) Remove barriers within the creek that restrict fish passage.
4) Add vegetation along creek and expand riparian corridor.
Benefits of project:
1) Remove flooding to adjacent land along the creek in during various storm events up to the 25-year storm by adding detention and reducing flows downstream of golfcourse.
a) Using HSPF to model continuously the creek for the past 50 years, the results showed the Jackson Park ponds will reduce peak flows by 8 percent immediately above the confluence of the North and the South branch. Just below the golf course, peak annual flow will be decreased by 28 percent. Throughout the Jackson Park Golf Course - a section noted for flooding and erosion, the ponds will reduce the annual flood flow by 63 percent.
b) Reduced channel scouring and erosion. By reducing flows during various storm events and increasing the channel size, the velocity within the creek will be reduced, therefore, decreasing channel scouring and erosion.
2) The existing creek is channelized within golf course. The majority of the creek is a straight, narrow and shallow trapezoid channel with a portion lined with broken concrete. The existing channel will be replaced with a more natural channel with improved instream and riparian habitat. At the southern end of the project, a barrier to fish passage will be removed. Near the existing water reservoir, the creek has a 1-foot vertical drop within a short distance. By having to realign the creek for the detention ponds, this vertical drop will be removed by spreading it out over a much greater horizontal distance.
3) Currently, Jackson Park Golf Course has water rights from the state to withdraw up to 1.3 cubic feet per second from the north branch of Thornton Creek for irrigation of the golf course. Water is drawn directly from the creek during irrigation causing fluctuations in water levels in the creek. With the new, efficient irrigation system, water will be drawn from pond #3. Pond #3 will be fed by a much smaller flow diverted from the creek circulated through the ponds and stored for irrigation. This will result in higher summer flows and less fluctuation in water levels in the creek.
Eleanor A. Jackson, P.E.
Senior Civil Engineer
Drainage & Wastewater Engineering Design
Engineering Services Branch
Seattle Public Utilities