| Abstract/Description or Keywords |
Windermere Creek is a steep mountain stream in the Rocky Mountains. Channel instabilities developed over the 2011 and 2012 freshet flows. The instabilities resulted in increased sediment generation in the reach between Windermere Loop Road and the Mine Road and deposition in downstream reaches, most notably upstream of Highway 93 crossing and at Windermere Lake. Sedimentation in the lower reach of Windermere Creek reduced channel capacity resulting in overbank flooding and the Regional District of East Kootenay (RDEK) declaring a state of emergency August 5, 2011. Reduced channel depth following the aggradation has increased the risk of local icing problems within the channel which is expected to increase the risk of flooding during seasonal freeze and thaw. Summer flow in 2012 was abnormally high across the Kootenays with numerous watersheds experiencing destabilisation and flooding; most notably the July 12 debris avalanche (in which three people died) and July 13 debris flow at Johnsons Landing, and the July 15 debris flow on nearby Fairmont Creek. High summer flow resulted in upstream degradation and downstream deposition of sediment increasing from that experienced in 2011. Sedimentation upstream of the Highway 93 culvert crossing blocked the culvert inlet in 2012. The upstream end of the culvert eventually floated and failed as water level rose above the culvert inlet. Sediment continued to deposit in the lower reach of the Windermere Creek and result in additional localised flooding. Northwest Hydraulic Consultants Ltd. (NHC) teamed with Dr. Richard McCleary to provide a hydrotechnical assessment of Windermere Creek concentrating on the reach from the Mine Road downstream to Windermere Lake. Previous studies by BC Ministry of Forests, Lands, and Natural Resource Operations (FLNRO, 2011) and R. McCleary (2012) identified a destabilising reach located roughly 1 km upstream of Windermere Loop Road. Re_assessment of this and adjacent reaches following the 2012 freshet indicated that substantially more degradation and sediment mobilization had occurred in the 2012 freshet. Following the destabilising events of 2011 and 2012 Windermere Creek has been left in a compromised state. Without mitigation work it is expected that the upper channel will continue to degrade, headcut, and widen increasing the sediment loading to downstream reaches. Increased sediment loading is expected to deposit in locations of lower shear stress, predominantly upstream of Highway 93 and at Windermere Lake. Recurring deposition in these areas will continue to lead to culvert or channel blockage and localised flooding. Mitigation measures should address both the channel instability in the upper reach and the ongoing deposition of sediment downstream. No one solution is able to address both of these issues. Therefore, it is recommended that a series of grade controls be constructed to maintain vertical grade of the channel in the most destabilised reach _ roughly 1 km upstream of Windermere Loop Road _ and a sediment trap be constructed upstream of Highway 93 to intercept and allow controlled removal of transported sediment. It is expected that the middle grade controls also be used to re_establish flow along the north side channel limiting the stress on the south channel and potentially improve habitat value. Class D cost estimates for construction of the grade controls and sediment basin are $170,000 and $315,000, respectively. Five_year operations and maintenance costs are estimated at $17,000 for grade controls and $60,000 _ $250,000 for the sediment basin. |
