A Tale of Two Dams and a dry river. November 1, 2011 Planning, Resources and Technology Committee Mojave Water Agency

Transcription

1 A Tale of Two Dams and a dry river November 1, 2011 Planning, Resources and Technology Committee Mojave Water Agency

2 The Mojave River Headwaters San Bernardino Mountains Terminus (mouth) Soda Lake (Baker, CA) Starting Elevation ~ 3,000 ft Ending Elevation ~ 900 ft Length ~ 110 miles Rainfall at the Headwaters ~41 ½ inches per year (Lake Arrowhead)

3

4 15,780 cfs ~12,000 cfs Jan 11, :43 AM Dec. 21, 2010 ~ 1:00 AM Note: gauge stopped functioning ~ noon 12/22. Actual peak occurred 12/23 Historic Floods Mojave River Dam designed maximum flow 23,500 cfs Forks Dam Outflow

5 THE DAMS ALONG THE MOJAVE Mojave River Dam (Forks Dam) Purpose Flood control Cedar Springs Dam Purpose Reservoir for State Water Project

6 Mojave River Dam and Cedar Springs Dam Flood Control vs. Reservoir Planned and built by different entities Completed at nearly the same time Approximately the same size and construction Located in close proximity to each other Purposes and operational practices are commonly confused

7 Mojave River Dam Cedar Springs Dam Attenuates stormwater pulse events Does not have control gates All stream flow below the attenuating threshold passes through immediately into the Mojave River Excess stream flow impounded and released as soon as possible Maintains a constant reservoir level for State Water Project purposes Has control gates All watershed runoff entering Silverwood Lake is released to the Mojave River Dam as soon as possible typically within 24 hours

8 Cedar Springs Dam

9 Mojave River Dam

10

11

12

13 Mojave River Dam Earliest investigation of damming of the Mojave River 1875 Dam project endorsed by the State of California in 1958 West Fork Flood Control and Reservoir Project authorized in 1960 Army Corps of Engineers and Bureau of Reclamation planned 1960 to 1965

14 Mojave River Dam continued General Design Memorandum No. 2, General Design for the Mojave River Forks Reservoir signed October 1966 First construction contract awarded June 1967 Dam completed May 1971 Permanent operating equipment completed November 1974

15 Mojave River Dam Specifications Type: Rolled earthfill Height above original streambed: 200 feet Crest Length: 2,223 feet Top Width: 20 feet Peak Outflow: 23,500 cfs Reservoir Design Flood: 154,000 acre-feet over 3 days

16 Cedar Springs Dam A planned part of the California Department of Water Resources State Water Project Completed 1971 Edmonston Pumping Plant activated Jan 21, 1972 Pumping water over the Tehachapi s toward Sliverwood Lake First water behind dam Jan 22, 1972 Serves as a regulating reservoir for the SWP Several State Water Contractors take SWP deliveries from Silverwood: Mojave Water Agency Metropolitan Water Dist. Crestline-Lake Arrowhead Water Agency San Bernardino Valley Municipal San Gorgonio Pass Water Agency

17 Cedar Springs Dam continued Water is siphoned from the CA Aqueduct into Silverwood through the Mojave Siphon The aqueduct at the siphon is higher in elevation than the lake Power is generated at a hydro plant along the Mojave Siphon. Mojave Water Agency has taken delivery of 107,000 acre-feet of SWP water from Cedar Springs Dam since it was constructed

18 Cedar Springs Dam Specifications Type: Zoned earth and rock filled Height: above original streambed 213 feet Crest Length: 2,250 feet Top Width: 42 feet Storage Capacity at Top of Dam: 78,000 acrefeet Lake Surface Area: 995 acres

19 How Does Cedar Springs Dam Operate? Flow from the watershed into the reservoir is released as soon as possible.

20

21

22

23

24

25 How Does Mojave River Dam Operate? Peak storm flow is attenuated.

26

27

28

29

30

31

32

33

34

35 Interesting relationship between Fork s Dam outflows and Lower Narrows flow Dec , 2010

36

37

38 Local Rainfall

39 Some observations The Dec 21 st peak at the Lower Narrows Gauge was 30,000 cfs while the corresponding outflow from the Mojave River Dam on Dec 21 st was about 12,000 cfs. This implies over half of the peak flow at the Narrows was due to local runoff occurring downstream of the Mojave River Dam and upstream of the Narrows.

40 General Facts and Conclusions On average, about 1/3 of the flows through Mojave River Dam come from West fork drainage Of that about 2/3 comes from Silverwood Lake Silverwood (Cedar Springs Dam) is not designed as a flood control facility and is not operated as such Mojave River Dam is designed to attenuate flows by temporarily storing water behind the dam Mojave River Dam outlet is un-gated, no one can control outflows up or down Inflows/Outflows from Mojave River Dam are the most critical data for flood response Over time, contributing flows downstream of Mojave River Dam may be increasing from urbanization and should be monitored