2016-10-10-minutes-public-works-study-session

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90 -degree bend (identified by the `600' of damage 1996" on 1996" on Figure 1). At a minimum. these river conditions have damaged the levee and necessitated repair at least three times, including this year, since the Corps first inspection of the levee in 1991. Our past experience with the Corps in Yakima County is that they are willing to consider a levee setback if the non -Federal owner prefers it, however they will still only pay 80 percent of the cost of the Corps' identified repair. Based on our interest in this levee and the adjacent property, Reclamation might be able to assist Kittitas County with the repair, particularly if the setback results in increased cost -share to the County. For your consideration and discussion, we have formulated the following recommendations that we feel will benefit the County and Reclamation in the long term: The levee alignment should follow a path similar to what is shown in Figure 3 (red line). This setback is approximately 80 feet at the greatest setback distance, whereas the Corps is recommending 20 feet. This alignment will alleviate the constriction at this location in the river, Figure 3 shows this quite well. Our recommended alignment will create a river top width similar to what exists at the downstream of the levee (also shown in Figure 3) and will act to reduce the erosive forces against the levee itself and on the river bed (which presumably scoured and caused failure of the riprap toe). 2. We recommend a flatter side slope than what the Corps has proposed (21-1:1 V). Decreasing the side slope to 3H:1 V or to 411:1 V will reduce the erosive forces on the levee and the river bed. 'These gentler slopes are also consistent with Corps guidance for riprapped slopes in rivers with heavy wood/debris loads. as the Yakima River has. Also, as owner of the protected property, Reclamation acknowledges and accepts that some loss of real estate would occur as a result of the lower sideslopes and revised alignment. 3. Based on past failures of the levee. we recommend a thicker blanket of riprap than what is being proposed by the Corps. This would increase the resilience of the levee and is consistent with Corps guidance to increase the thickness of riprap blankets where debris impacts are expected-, it is also consistent with Corps guidance to increase the thickness of riprap blankets when riprap is placed underwater. 4. It appears that the Corps is planning to use the same size riprap as has been used in the past. -]'his may be acceptable if the failure mechanism of this winter's damage was related to undermining of the toe. However, conditions at this location may warrant an increased size of riprap to resist the high-energy of the river at this locations. We have offered hydraulic modeling results to the Corps to assist in the sizing of riprap. At a minimum. we suggest that the riprap blanket is extended to the current riverbed and the launchable toe extended from that point, whereas in Figure 2 it appears the Corps proposes to restore the riverbed to a pre -flood condition. The scour that occurred at this location this past winter is a natural process and we have no reason to expect that it would not occur again. Otherwise. the proposed repair may function no better than the existing slope did this past winter.