Part iii-d – Dam Failure



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Prado Dam was completed in April 1941. It is located at the upper end of the Lower Santa Ana River Canyon, which is a natural constriction controlling 2,255 square miles (5,840 square kilometers) of the 2,450 square mile (6,345 square kilometer) Santa Ana River watershed. Authorization for the project is contained in the Flood Control Act of June 22, 1936 (PL 74-738). Modifications to the dam affect the basin below 566 feet elevation. The basin comprises more than 11,500 acres, 4,100 acres of which are riparian habitat (mostly willow woodland), a 4,823-acre recreation area (1,041 developed, 3,782 undeveloped) and 2,400 acres owned by the Orange County Water District.prdo_5


Prado Dam provides flood control and water conservation storage for Orange County. It is the downstream element of the Santa Ana River flood control system. The purpose of the project is to collect runoff from the uncontrolled drainage areas upstream along with releases from other storage facilities. Generally, when the water surface elevation in the reservoir pool is below the top of the buffer pool elevation (494.0 feet NGVD during the flood season, 505.0 feet NGVD during the non-flood season), water conservation releases are made. These releases are coordinated with the Orange County Water District and are based upon the capacity of their groundwater recharge facilities and agreements with other agencies. If the water surface in the reservoir exceeds the top of the buffer pool, flood control releases commence. The objective of the flood control operation is to drain the reservoir back to the top of the buffer pool as quickly as possible without exceeding the capacity of the channel downstream. In current practice, when the water surface in the reservoir exceeds the top of the buffer pool, releases are increased to match inflow up to 5,000 cfs (142 cms). When inflows exceed 5,000 cfs, the excess water is stored in the reservoir.


When the water surface elevation in the reservoir reaches 543.0 feet NGVD, uncontrolled releases from the spillway will commence. The 5000 cfs limit on controlled releases from Prado Dam is based upon the old non-damaging capacity of the downstream channel. In the event of Prado Dam failure, floodwaters would flow through the Santa Ana Canyon on its way to the Pacific Ocean. The flood would range from about 3,000 feet wide in the canyon to over 15 miles wide downstream at the Santa Ana Freeway (Interstate 5). The flooding would impact over one million people and 110,000 acres. Within 8-10 hours, the intersection of Beach Boulevard and Edinger Avenue in Huntington Beach would experience severe flooding. The peak elevation would be 32 feet with 7 feet average over the river’s bank depth. The greatest flooding would occur in the area between the Bolsa Chica Mesa and the Newport Beach Mesa where flood depths can vary from one to nine feet.




Once the downstream channel improvements that were part of the Corps of Engineers’ Santa Ana River project were completed, the downstream channel capacity was increased dramatically to over 30,000 cfs (850 cms). The Santa Ana River project also increased the capacity of the reservoir behind Prado Dam. The modifications to the dam, which took place in three phases consisted of:


Raising the height of the dam 30 feet; building a new intake tower; and, constructing improvements to the dam’s outlet works (March 2003 – October 2006)

Constructing dikes in the basin to protect property (September 2004 – September 2007)

Raising the height of the adjacent spillway 20 feet (July 2006 – January 2008)

The modifications provided an additional 140,000 acre-feet to the reservoir. (One acre-foot is the volume of water that would cover one acre with one foot of water.) These changes will increase Prado Dams’ current 70-year level of protection to 190-year protection. These new improvements would prevent $15 billion in damages.




The total cost of the improvements to Prado Dam was approximately $430 million ($221 federal, $209 non-federal). The non-federal sponsor for this project is Orange County Flood Control District.




The project was authorized under the 1986 Water Resources Development Act (WRDA), the 1988 Energy and Water Appropriation Act (San Timoteo), and Section 309 of WRDA 1996. In 1997 federal officials, following the appropriate laws, concluded that Prado Dam was distinct from the Santa Ana River project. (See “Santa Ana Mainstem Project” on page 24 & 25.) These improvements enabled the dam to take full advantage of the improved channel capacity downstream and greatly increased the level of flood protection to the Orange County communities in the Santa Ana River floodplain. This includes most of the CCCD facilities and service area.

Figure - Prado Dam Downstream Inundation Map


http://www.spl.usace.army.mil/resreg/images/prdofi1.gif

The CCCD facilities are located in Plates 5 and 7 which is better depicted in Figure 1, page 22. (Note Plate maps are extremely difficult to read.)
Table - Prado Dam Physical Data

Embankment

Type

Earth Fill

Crest Elevation

566 feet NGVD

172.5 meters NGVD

Maximum height above streambed

106 feet

32.3 meters

Crest Length

2,280 feet

695 meters

Freeboard during PMF

(-4.3) feet

(-1.3) meters

Spillway

Type

Concrete Ogee Crest with Converging Chute

Spillway Crest

543.0 feet NGVD

165.5 meters NGVD

Crest Length

1000 feet

305 meters

Outlet Works

Number of Passages

6

Gate Type

Vertical Lift

Height x Width (each)

12 x 7 feet

3.7 x 2.1 meters

Entrance Invert Elevation

460.0 feet NGVD

140.2 meters NGVD

Maximum Capacity

17,000 cfs

481 cms

Reservoir

Debris Pool

Elevation of top of Pool

490.0 feet NGVD

149.4 meters NGVD

Area at top of Pool

768 Acres

311 ha

Gross Storage at top of Pool

4,689 Acre-feet

5.8 MCM

Flood Season Buffer Pool

Elevation of top of Pool

494.0 feet NGVD

150.6 meters NGVD

Area at top of Pool

1,081 Acres

438 ha

Gross Storage at top of Pool

8,437 Acre-feet

10.4 MCM

Non-Flood Season Buffer Pool

Elevation of top of Pool

505.0 feet NGVD

153.9 meters NGVD

Area at top of Pool

2,123 Acres

859 ha

Gross Storage at top of Pool

25,760 Acre-feet

31.8 MCM

Top of Dam

Elevation of top of Pool

566.0 feet NGVD

172.5 meters NGVD

Area at top of Pool

11,030 Acres

4,468 ha

Gross Storage at top of Pool

383,500 Acre-feet

473.0 MCM

Historic Maximum Water Surface

Date

22 February 1980

Maximum Elevation

528.0 feet NGVD

160.9 meters NGVD

Historic Maximum Release

Date

22 February 1980

Maximum Release

5,992 cfs

169.7 cms

Figure 1 – Orange County Prado Dam Inundation Map


Santa Ana River

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