3.4.2.2 Potential Effects on North Platte River Downstream

3.4.2.2 Potential Effects on North Platte River Downstream

Black Canyon received comments regarding the potential for impacts on the North Platte River downstream of the Project. As discussed above, it is unlikely Project operations during pumping will resuspend significant amounts of sediment due to near-bottom velocities of close to 0 fps. Based on the results of CFD modelling, during generation a small portion (approximately 50 feet by 50 feet) of the bottom may experience near-bottom velocities up to 3 fps which has the potential to mobilize sediments. Black Canyon will use established BMPs, as outlined in the Erosion and Sediment Control Plan, to prevent and mitigate the transport of sediments and metals downstream on the North Platte River. If sediments were to be transported downstream, they are expected to settle out of the water column within Kortes Reservoir and would not be deposited within the Miracle Mile.

As discussed in Section 3.4.1.5, Black Canyon conducted temperature modeling to evaluate the potential for the Project to affect water temperatures in Seminoe Reservoir or downstream. From September to April the main body of Seminoe Reservoir is naturally well mixed (i.e., not thermally stratified) and from May to August a natural thermal stratification occurs. Under existing conditions, maximum temperature stratification in Seminoe Reservoir occurs during the summer months (June, July, August). Black Canyon’s modeling indicates that under pumped storage operations in the winter, during both pumping and generation, there will be minimal difference in the temperature stratification under existing conditions and with Project operations. In the summer, during pumping operations, there are minor differences in the temperature stratification under existing conditions and with Project operations. This indicates that the Project, when pumping, does not impact thermal stratification in Seminoe Reservoir, regardless of the natural stratification occurring throughout the year. However, in the summer during power generation, temperature becomes fully mixed (i.e., destratified) for a portion of the water column, resulting in increased absolute water temperatures under unusual low-water conditions, and a temporal shift in natural seasonal water temperatures overall.

Black Canyon’s modeling found that the greatest modelled temperature increase occurred in August of 1977, an extreme low-water year, with an average monthly temperature range of 10.7C to 11.7C under existing conditions and 12.4C to 15.1C under Project operations. However, 15.1C is substantially lower than the maximum average temperature range during August 1976 and 1978, more typical water years, under existing conditions (without Project operations). With Project operations, modeling indicates that water released by Seminoe Dam penstocks into Kortes Reservoir during summer conditions in extreme low-water years (represented by 1977) will be within the average temperature ranges during typical water years under existing conditions. As a result, Project is not expected to detrimentally affect water temperatures downstream of Seminoe Reservoir.

Depending on the hydrologic conditions of a given year, modelling results indicate that natural seasonal temperature increases are expected to occur approximately 4 weeks earlier in a typical year as a result of Project operations. Under existing conditions, peak water temperatures generally occur in September. With Project operations, similar peak

water temperatures are observed in August, with slightly reduced temperatures in September. Generally, the shift in temperature starts in late May/early June and extends through late August/early September. Modeling indicates that this period would largely occur after the spring (early April through late-May) spawning season for Rainbow Trout and prior to the initiation of fall spawning periods for Brown Trout (mid-October through December) and fall run Rainbow Trout (Scott and Crossman, 1973; Roberge et al, 2002); each of these are important species present in Seminoe Reservoir and the Miracle Mile. Based on these model results, effects on aquatic biota within Kortes Reservoir, and subsequently the downstream “Miracle Mile,” does not appear likely to be of a magnitude or duration to cause a measurable change in the aquatic system of the North Platte River downstream of Seminoe Reservoir.

Measures to Reduce Risk of Over Pumping

As noted in Section 2.1 of Exhibit A, in the event of an over-pumping scenario (i.e., if sensor equipment fails and excess water is pumped to the upper reservoir, causing the water level of the upper reservoir to rise above the maximum crest elevation), water would be released from the upper reservoir via an emergency spillway into a stilling basin, which would then overflow water into an unnamed drainage about 0.5 mile north of Dry Lake Creek that would then flow down a slope of approximately 2,700 feet into Kortes Reservoir. Over pumping and resulting spill is extremely unlikely because of the numerous design features incorporated into the Project, including pump shut-off/upper reservoir volume controls.

Discharges over the over-pumping emergency spillway will be minimized or eliminated by redundant data sensors linked to the pumping controls. A Level Control System will be used for normal plant operation and, but a completely independent Level Protection System will form be a fail-safe backup system to the Level Control System. Multiple types of instrumentation equipment will be used for both systems to avoid faults specific to one manufacturer. Redundancy, alternative cable routes and types, and battery back-up packs at the upper reservoir will also be incorporated to mitigate the consequences of equipment failure or power supply interruptions. The independent Level Protection System will consist of, at a minimum, three sets of two electrical sensing devices type switches, which will be set at least three inches higher than the normal shutdown level of the pump cycle. Each set will be connected to one of the units. If either of the pairs of sensors switch is activated, a hard-wired shutdown of the pump cycle will occur. At least two other sensors located remotely Two mechanical float type switches located away from each other will be included to back up the electrical switches and will be set at least 3 inches higher than the unit electrical sensors switches. Each of these mechanical float type switches extra sensors will trip all three pumps. Two additional electrical switches will be located within the overpumping emergency spillway – but separately from each other - to trip all pumps if any significant water volumes flows over the spillway crest. Actuation of either switch in the over-pumping emergency spillway will trip all the pump cycles and provide initiate an alarm. In summary the independent emergency Level Protection System will include at least eight discrete water level sensors physically separated from each other, plus two spillway sensors.

A literature review has been made of the reliability of water level sensors. A study by Idaho National Engineering Laboratory in 1995 indicated an average failure rate of water level sensors of between 2.2 to 6 E-7 per hour, while other selected references indicated a worst case of 2.1 E-6 per hour. The IEEE Standard of 2007 indicates a failure rate of 2.88813E7 per hour of a pressure sensor based on a rate of 0.00253 per year. From the available data and using a conservative value of failure rate of 2.0E-6, the proposed system of sensors would exhibit a combined failure rate of 1.6E-23 per hour which implies that water could be inadvertently discharged over the emergency spillway once every 7.13E+18 years.

Including the spillway sensors in the calculation (i.e., assuming that the spillway sensors fail to register the initial discharge from over pumping, and thus allow continuous over pumping), implies a failure rate of 5.606E-31 per hour (or uncontrolled release once every 1.78E+30 years).

In the event that all sensors fail to initiate a trip, the inflow to Kortes Reservoir would not exceed the Project pumping capacity, expected to be approximately 8,298 cfs. As a comparison, the capacity of the ungated Kortes spillway is 50,000 cfs. Therefore, any effect on the downstream Miracle Mile stretch of the North Platte River would be delayed if Kortes was below its top water level at the time of over pumping. If the over pumping did cause a spill from Kortes Reservoir, the flow would be lower than historical maximum flows, as described in the next section.

Irrespective, Black Canyon has analyzed the consequences of a scenario in which all three of the Project’s pumping units were operating and all sensors failed, during which a flow of up to 8,298 cfs could be discharged into Kortes Reservoir until the issue is resolved. As described below, the consequences of a release from the proposed upper reservoir to the over-pumping emergency spillway would be primarily sediment transport.

Because the slope between the over-pumping emergency spillway and Kortes Reservoir is very steep, large flows due to inadvertent over pumping would be anticipated to result in severe erosion of the unnamed drainage.

The stilling basin would discharge into a natural gulley downslope of the northwest corner of the upper reservoir site and thence into Kortes Reservoir. Riprap erosion protection for 200 feet downstream of the stilling basin would protect the upper part of the gulley and discourage erosion below the over-pumping emergency spillway.

The most direct gulley route to Kortes Reservoir from the over-pumping emergency spillway is 2,700 feet (slope distance). As noted, it is intended that significant vulnerable areas will be anchored and protected by riprap or gabions. To calculate potential debris transport, it has been assumed that such protection would be implemented over one-third of the gulley length or for 900 feet, leaving 1,800 feet unprotected. Assuming that the maximum width of the flow is 100 feet, which for over pumping of 8,298 cfs would result in a flow depth of approximately five feet, it is reasonable that an average of three feet of the bed material might be mobilized. Therefore, a total of approximately 20,000 cubic yards of

material (1,800 feet by 100 feet by 3 feet) could be displaced and possibly enter Kortes Reservoir under this extremely unlikely scenario.

Historical Data on Flows from Kortes Reservoir

In the past 40 years, there have been three naturally occurring events that resulted in daily mean flows of 13,000 to 16,000 cfs lasting for periods of 3 to 7 days (Reclamation 2019b). The following analysis compares these historic flows to the potential overflow event of the proposed upper reservoir and concludes that an emergency over-pumping event would result in a lower flow rate than these naturally occurring events.

In the unlikely event of the over-pumping scenario, an estimated 8,298 cfs would be combined with the Kortes Reservoir average monthly high daily flow of 2,187 cfs (June), totaling an anticipated flow of approximately 10,390 cfs entering the Miracle Mile. To be conservative, Black Canyon analyzed a scenario where 11,500 cfs would flow into the emergency spillway. Historically, as shown in Table 3.4-12, the Miracle Mile has experienced flows above 11,500 cfs in the years 1983, 1984 and in 2010 (Reclamation 2019b).

Table 3.4-12. Historical Kortes Reservoir Flows Recorded Above 11,500 cfs.

Kortes Reservoir – Historic High Flow Events

Date Flow in cfs

6/25/1983 6/26/1983 6/27/1983 6/28/1983 6/29/1983 6/30/1983 7/1/1983 5/26/1984 5/27/1984 5/28/1984 5/29/1984 6/7/1984 6/8/1984 6/9/1984 6/13/2010 6/14/2010 6/15/2010 6/16/2010

Source: Reclamation 2019b. 13,800 14,350 15,425 15,525 16,060 16,225 14,500 11,930 12,219 11,757 12,261 13,059 13,017 12,366 13,720 14,600 14,220 12,340