,

US Army Corps of Engineers Los Angeles District

WATER CONTROL ,

iDROUGHT CONTINGENCY PLAN

HANSEN DAM TUJUNGA WASH LOS ANGELES COUNTY, CALIFORNIA

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SEPTEMBER 1992 Date of photo: March 5. 1941

DROUGHT CONTINGENCY PLAN

HANSEN DAM

LOS ANGELES RIVER BASIN, LOS ANGELES COUNTY, CALIFORNIA

SEPTEMBER 1992

Prepared

by

U.S. Army Corps of Engineers

Los Angeles District

Reservoir Regulation Section

DROUGHT CONTINGENCY PLAN FOR HANSEN DAM

TABLE OF CONTENTS

1. Authorization and Purpose . . . . . . . . . . 1-01 Statement of Authorization and Purpose 1-02 Characteristics of Drought

2. General Description of Project 2-01 Location 2-02 Purpose. . . . . . . . 2-03 Water Conservation 2-04 Regional Water Agencies

2-04a Background. . . 2-04b Existing Facilities 2-04c Future Plans

2-05 Historical Droughts

3. Water Control Plan

4. Drought Management Plan 4-01 Drought Management Plan 4-02 Environmental Considerations

5. Public Information

6. Responsibility

LIST OF TABLES

Table 1 Key Contact Agencies and Personnel for Water Control Decisions

Page

1 1 1

1 1 1 1 2 2 3 4 4

4

5 5 6

6

6

~Regarding Drought Actions . . . . . . . . . . . . . . . . . . . 7

Plate 1 Plate 2 Plate 3

LIST OF PLATES

Location Map of Hansen Dam Hansen Dam Drainage Showing LACDPW Recharge Facilities Water Control Plan for Hansen Dam and Reservoir August 1990

iii

DROUGHT CONTINGENCY PLAN HANSEN DAM

LOS ANGELES RIVER BASIN, CALIFORNIA

1. Authorization and Purpose.

1-01 Statement of Authorization and Purpose. This plan was prepared under the authority of ER 1110-2-1941, dated 15 September 1981. Its purpose is to identify how the Corps of Engineers could render assistance during a drought through flexibility of regulation and use of storage at its reservoir projects. This plan describes the project, provides a background for evaluation of drought activities and lists coordination agencies and personnel. This plan is intended only as a guide, and actual management may differ as changing conditions dictate.

1-02 Characteristics of Drought. Drought is defined in the Glossary of Meteorology as, "A period of abnormally dry weather sufficiently prolonged for the lack of water to cause a serious hydrologic imbalance (i.e., crop damage, water-supply shortage, etc.) in the affected area" (Huschke, 1959). Drought can be characterized by less than normal precipitation, lack of soil moisture, low streamflow, below normal reservoir levels, or reduced ground-water storage. The lack of water that is implicit in each of these conditions, and the effects of this shortage, must both be considered when analyzing drought. A drought characterized by less than normal precipitation does not mean there is a water supply shortage, the reservoir is low, or even streamflow is low. Many hydrologic features need to be examined and each in a site specific way. Streamflow, for example,. may originate from several sources such as natural runoff, ground water, springs, or regulated flow. Reservoir levels are subject to operating rules and the size and nature of the watershed, in addition to the amount of precipitation. And water supply shortages require consideration of the source of supply, management measures, and demand.

2. General Description of Project.

2-01 Location. Hansen Dam is located near the northern edge of the San Fernando Valley on Tujunga Wash, about one mile below the confluence of the Big Tujunga and Little Tujunga Washes, and about four miles southeast of the town of San Fernando. The boundary of the drainage area is formed by the San Gabriel Mountains on the north and west, and by the Verdugo Mountains and a secondary range of the San Gabriel Mountains on the south and east. The location of the project is shown on plate 1. The drainage area is 152 square mile.

2-02 Purpose. Hansen Dam is an essential element for flood control in the Los Angeles River drainage basin. In conjunction with Sepulveda and Lopez Dams, Hansen Dam is vital for the flood protection of lower portions of the San Fernando Valley and the City of Los Angeles. Storage regulation provided by these flood control basins permit efficient use of the Los Angeles River Channel.

2-03 Water Conservation. Hansen Dam does not contain any storage allocation for water supply, however, the Water Resources Development Act of 1986, Report 99-1013, Section 847 authorizes the Secretary of the Army to facilitate water

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conservation and groundwater recharge measures at Hansen Dam in coordination with the City of Los Angeles, California and the Los Angeles County Flood Control District (LACFCD), to the extent consistent with other project purposes. The invert of the Hansen Dam outlet works is at an elevation of 990-ft. Hansen Dam has two ungated outlets at an elevation of lOll-ft. and thus cannot conserve water above that elevation.

2-04 Regional Water Agencies.

2-04a Background.

Los Angeles County Department of Public Works (LACDPW) has maintenance responsibility for the Tujunga Channel downstream of Hansen Dam and maintains and operates a number of projects in the drainage area, including Hansen Spreading Grounds.

Hansen Dam and Reservoir was constructed by and is Federally owned and operated by the u.S. Army Corps of Engineers, Los Angeles District (LAD). A large portion of the Hansen Reservoir Federal lands, operated by the Corp of Engineers (COE) , is leased to the City of Los Angeles for recreational purposes. The Corps reserves the right to inundate this land. Also, the Department of Water and Power, City of Los Angeles (DWP) maintains and operates Tujunga Spreading Grounds.

The Los Angeles Department of Water and Power (DWP) administers water supplies for the City of Los Angeles. Under normal circumstances, 70 percent of the Los Angeles City water comes from the Eastern Sierra via their Los Angeles Aqueduct. Wells in the San Fernando Valley and other local groundwater basins supply 15 percent, and purchases from Metropolitan Water District of Southern California (MWD) consisting of State Project water and Colorado River Aqueduct Water, furnish the last 15 percent.

Conditions for LADWP water supplies have changed due to the environmental decision concerning Mono Lake water so that even in normal water years, the Los Angeles Aqueduct may be able to supply only 50-55 percent of the city's water needs. Under present drought conditions in California, LADWP has in FY 91-92, furnished 31 percent of Los Angeles's water needs with Los Angeles Aqueduct water, 15 percent with ground water and 54 percent with water purchases from MWD.

In addition to its flood control mission, LACDPW has the equally important task of conserving as much of the storm and other waste waters as practicable. The use of water conservation facilities adjacent to river channels, and in soft­bottom channels permits water to percolate into groundwater for later pumping. These water spreading facilities are located in areas where the underlying soils are composed of pervious formations.

The LACDPW operates' 2,369 acres of spreading grounds and basins and soft­bottom channel spreading areas for replenishment of local aquifers to increase water supplies. The conservation of local runoff is supplemented by spreading imported water and reclaimed water purchased by water agencies. The total gross acreage of spreading grounds in Los Angeles County is 3,294 acres.

2

The importance of this activity is apparent when it is realized about 35 to 45 percent of the water used in the Los Angeles County is pumped from groundwater supplies. The growth of the County, combined with periodic droughts, seriously depleted these supplies on numerous occasions through the years.

2-04b Existing Facilities.

1. Big Tujunga Dam. Big Tujunga Dam is located on Big Tujunga Creek, approximately 15 miles upstream of Hansen Dam (pl. 2). The concrete arch structure, completed in 1931, is operated and maintained by the LACDPW for flood control and water conservation. Big Tujunga Dam has a storage capacity at spillway crest of 6,240 ac-ft with a maximum outflow of 24,250 ft3/s. The drainage area of Big Tujunga Dam is 82.3 square miles.

2. Hansen Spreading Grounds. Hansen Spreading Grounds is owned and operated by the LACDPW (pl. 2). The facility is located approximately 1,500 ft. downstream of Hansen Dam. A radial gate across Tujunga Wash Can divert flow into the spreading grounds. Pertinent information concerning Hansen Spreading Grounds is listed below:

Maximum basin intake capacity Maximum basin outlet discharge Allowable water quality sediment limit Storage capacity Maximum percolation rate Basin gauge height limits*

400 ft3/s 150 ft3/s 400 ppm 330 ac-ft 250 ft3/s 4-5 ft

*Basin gauge height limit refers to the depth of water in the basin during spreading operations.

3. Tujunga Wash Channel. The Tujunga Wash Channel, from Hansen Dam to the Los Angeles River, was constructed in 1952 (pl. 2). The original capacity of the unimproved channel immediately downstream of the dam was 12,000 ft3/s. Following improvement of the channel in 1952, the capacity rose to 22,000 ft3/s. The channel capacity was revised in 1988 during LACDA review studies to 20,800 ft3/s to reflect current freeboard criteria. The rectangular channel is lined with reinforced concrete.

4. Tujunga Spreading Grounds. Tujunga Spreading Grounds is owned and operated by the Department of Water and Power, City of Los Angeles (DWP). It is located approximately 3 miles downstream of Hansen Dam (pl. 2). Although DWP is capable of spreading water from Tujunga Wash, it does not, and has not since 1983, due to the high sediment concentration of flood control releases. DWP spreads surplus water from DWP's reservoirs via the California Aqueduct.

5. Pacoima Diversion Channel. Pacoima Wash is an improved channel from Lopez Dam to Paxton Street where it is diverted to Tujunga Wash by the Pacoima Diversion Channel (pl. 1). This channel was constructed in 1954 as a part of the LAD Lopez Dam Project. The channel capacity is 17,000 ft3/s at the confluence of Tujunga Wash.

3

6. Branford Spreading Grounds. Branford Spreading Grounds are owned and operated by the LACDPW. The facility is located near the Pacoima Diversion Channel/Tujunga Wash confluence. Branford Spreading Grounds obtains its water from a local storm drain system. The facility discharges water into the Pacoima Diversion Channel and has no capability of diverting water from the channel. The facility's maximum inflow and outflow is 1,540 ft3/s.

7. Lopez Canyon Diversion Channel. The Lopez Canyon Diversion Channel drainage area, located between Lopez Dam and Hansen Dam, is about 2.4 square miles (pl. 1). About 80 percent of the area is on the southern slope of the San Gabriel Mountains. The gradient of the stream ranges from approximately 1,700 ft/mi (0.32) in the headwaters to 250 ft/mi (0.05) near the outlet into Hansen flood control basin.

2-04c Future Plans. All the reservoir storage space in Hansen Dam is allocated for flood control. No water is impounded behind the dam for the purpose of recreation. A l30-acre recreational lake was maintained behind the dam up until the 1970's when it succumbed to sedimentation. The Water Resources Development Act of 1986, Public Law 99-662, Section 847, authorized the Secretary of the Army to contract for the sale of dredged material at Hansen Dam for the purpose of facilitating flood control, recreation, and water conservation.

The LAD is currently in the design stages of re-establishing recreational lake facilities of Hansen Reservoir. A proposal to excavate another lake in the basin is in process at this time. The lake is to be filled with potable water supplied by the LADWP which may be reclaimed by spreading lake outflows into Hansen Spreading. Grounds immediately downstream of Hansen Dam. The water imported from this project will run through Hansen Reservoir for possible recapture by downstream spreading grounds. The project is due for completion in October 1993.

2-05 Historical Droughts. Principal drought periods in the Southwest identified because of their significant duration, magnitude, and/or areal extent are: 1897-1905, 1924-1934, 1954-1956, 1959-1960, 1976-1977, and 1987-1992.

3. Water Control Plan. The objective of the water control plan is to m~n~m~ze flood damages. Project releases are regulated to protect downstream communities. LACDPW and the Los Angeles City DWP are routinely notified of the release and of possible impacts to these agencies' spreading grounds downstream.

The project is operated according to the Reservoir Regulation Schedule in plate 3. The reservoir is allowed to build a debris pool by keeping the eight gates open at 1.0 ft. until the water surface elevation reaches 1,010.5 ft. After the water surface elevation reaches 1,010.5 ft., all gates are opened fully to 8.0 ft. until the downstream channel capacity of 20,800 ft3/s is reached at a pool elevation of 1053.0 ft. The gates are then progressively closed as the water surface elevation rises until, at elevation 1,066, the gates are fully closed. At this point, spillway flow plus ungated outflow is approximately equal to downstream channel capacity. On the falling limb of the inflow hydro graph , the same gate schedule is followed as the riSing limb down to a pool elevation of l,053.0-ft. All gates are left fully open at 8.0 ft below a water surface elevation of 1,053.0-ft until the reservoir is empty. The schedule was revised in

4

1988 to reflect the following: to conform to a revised downstream channel capacity of 20,800 ft3/s; to have no more than four gates operated at a time; and to prevent debris and sediment from building up and clogging the outlet works. Keeping the gates open at 1.0 ft. until the water surface elevation reaches l,OlO.S-ft is intended to minimize the floating debris problem, and keeping the gates fully open as the reservoir empties is intended to minimize the conduit sedimentation problem.

Hansen Reservoir is generally drained as rapidly as possible, consistent with the achievement of downstream flood control. If runoff conditions are expected to cause flow to exceed the downstream channel capacity based on downstream channel observers and anticipated side inflow, releases are reduced, so as not to contribute to the flooding. The objective is to safely empty the reservoir in preparation for the next flood and to prevent the outlet works from becoming clogged from sediment deposition.

4. Drought Management Plan.

4-01 Drought Management Plan.

Because of the shortage of water in the region, the Corps' Reservoir Regulation Section monitors water supply conditions. Monitoring of the water supply includes tracking of basin wetness, Mono Basin snowpack, Colorado River system storage, and local ground water spreading operations by the LACDPW. Support to the Chief, Reservoir Control Section is provided by the Office of Counsel, the Operations Branch, Construction-Operations Division, and the Environmental Resources Branch, Planning Division, making up the Corps Drought Management Committee (CDMC).

Hansen Dam does not contain any storage allocat,ion for water supply. however. the Water Resources Development Act of 1986, Public Law 99-662, Section 847 authorizes the Secretary of the Army to facilitate water conservation and groundwater recharge measures at Hansen Dam project in coordination with the City of Los Angeles, and the Los Angeles County DPW, to the extent consistent with other project purposes. Tujunga Wash downstream of the dam is concrete-lined but diversion possibilities exist at the Hansen and the Tujunga Spreading Grounds. When conditions are favorable, low releases from below the debris pool can be coordinated with the Los Angeles LACDPW and Los Angeles County DWP to facilitate use of their spreading grounds. Currently, reservoir storage space use for water conservation is limited until local interests formally agree to participate in the removal of sediment deposits accumulated in the reservoir that are directly attributable to water conservation operation. However, in the event of a drought, the impounding of water for water conservation would be considered further. Any such plan would be evaluated to ensure that the flood control purpose of the project would not be compromised.

The Hansen Dam is empty year around with all of its space authorized for flood control. The triggering mechanism for change of operations from flood control to drought management operations for this dam would be a request from either LACDPW or LADWP for a slower drawndown of Hansen Dam water impounded up to about lOll-ft, and an agreement to remove sediment as a result of the operation.

5

There is ~o standard evaluation criteria due to the severe debris problem at the project which has, in the past, threatened the flood control purpose of the project by plugging the gates. Thus, each water conservation operation must be evaluated on a case-by-case basis. Development of a contract would be based on the mitigation needs negotiated for that particular operation. The agencies that would be involved in a coordinated plan or deviation, thus making up the Interagency Drought Management Committee (IDMC) for this Hansen Dam are listed in table 1.

4-02 Environmental Considerations.

Depending upon the nature of any specific proposal put forward for the modification of the water control plan in response to drought, the environmental effects, and the required environmental coordination, NEPA documentation, and mitigation, could vary widely. When a proposal is made, an environmental assessment (EA) will be prepared and coordination with the appropriate resource agencies will be undertaken. The EA will briefly provide sufficient evidence and analysis for determining whether to prepare an environmental impact statement (EIS) , or a finding of no significant impact. Coordination will include the U.S. Fish and Wildlife Service and the state wildlife agency, in accordance with the Fish and Wildlife Coordination Act, and will include other resource agencies and groups depending on the nature of the proposal and the resource that might be affected. Mitigation, if any is required, will be determined through the NEPA process.

5. Public Information. General information on the Corps actions is disseminated to the public thro~gh the Chief, Hydrology and Hydraulics Branch, Los Angeles District. Furthermore, South Pacific Division representatives report Corps drought activities at the periodic meetings of the Governor's Drought Action Team and Interagency Drought Task Force, which, along with the California Department of Water Resources (DWR) State Drought Information Center, is California's designated lead agency in providing information to the public. The State Drought Center publishes a Weekly Update On Drought Conditions.

6. Responsibility. The Reservoir Regulation Section, Los Angeles District is responsible for the preparation, revision, and implementation of the subject Drought Contingency Plan. The Water Resources Division, Engineering Directorate of the South Pacific Division has the responsibility for review and approval.

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TABLE 1

Key Contact Agencies and Personnel for Water Control Decisions Regarding Drought Actions

Corps Drought Management Committee

Grigor Grigorian - Chief, Reservoir Regulation Section Joseph Evelyn - Chief, Hydrology and Hydraulics Branch Steve Temme1 - Office of Counsel Steve Dwyer - Operations Branch Ruth Villalobos - Environmental Resources Branch

Interagency Drought Management Committee

LOS ANGELES COUNTY DEPARTMENT OF PUBLIC WORKS

James Ranci1io - Flood Control Operations Ken Swanson - Flood Control Operations 24 Hour Emergency

213/894-6915 213/894-5520 213/894-5390 213/894-5620 213/894-5413

818/458-6309 818/458-6308 818/458-4357

CITY OF LOS ANGELES DEPARTMENT OF WATER AND POWER

Marty Adams - Water Control Engineer 24 Hour Emergency

U.S. FISH AND WILDLIFE SERVICE

Dick Zembe1 - Manager

SOUTH PACIFIC DIVISION

Jaime Merino - Chief, Water Resources Division

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213/481-6237 213/481-4918

619/431-9440

415/705-1521

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HANSEN DAM

TUJUNGA WASH. CALIFORNIA

LOCATION MAP

(LOS ANGELES COUNTY

DRAINAGE AREAl

U. S. ARMY CORPS OF ENGINEERS LOS ANGELES DISTRICT

PLATE 1

4" 2 S' o N

... stream Gauge Name and N~"'~er i Elevation, and Period of Re~ord

Drainage Area,

Little TUjunga Creek near San Fernando, Ca (above Hansen Dam) F19-R (11096500), 1068 ft., 21.1 sq. mi., 1928-: '74

Tujunga Creek bel~·.,f Hansen Dalll F20-~ (11097000), 94) ft., IS) sq. mi., 19)2-pr~sent

Big Tujunga Creek beloy Hil~ Creek FII1C-R, 2650 ft., 64.9 sq. ~i., 19)0-1972

Big Tujunga Creek beloy Big Tuju:"'-':Ja Dalll F168-R, 206) ft., 82.7 sq. mi., 19)1-present

Tujunga Creek near Sunland (above Gold canyon) F2';')-R (11095500), 1572 ft., 106 sq. mi., 1916-1979 (see Exhibit C)

20'

SPREADING GROUNDS

SPREADING GROUNDS

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USA

HANSEN DAM

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DRAINAGE AREA

N

MILES I 2

GINEERS

leT

PLATE 2

Step No.

\.Jhen reservoir water surface

is between elevation

Feet - NGVO

,-

No. No.2

Feet of feet of opening opening

Hansen Dam Reservoir Regulation Schedul~ (For rising and falling stages)

Gate setting for gates as indicated

No. 3

Feet of opening

No.4

feet of opening

No. 5

Feet of opening

No. 6 No. 7

Feet of feet of openi ng open; ng

No.8

feet of opening

Total C~ted

discharges

ft3/s

Downstream gauge

height** Feet

follow Step 1 during rising stages

1.. 990.0 - 1,010.5 : 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 o to 1,260 0.97 - 2.52

Follow Steps 2 to 9 during rising or fall ing stages

2 •• 1,010.5 1,053.0*: 8.0 8.0 8.0 8.0 8.0 8.0 8.0 8.0 7,920 to 20,730: 5.61 9.30 3 •• 1,053.0 1,060.0 8.0 7.0 7.0 8.0 8.0 7.0 7.0 8.0 19,370 to 20,520: 8.88 9.22

Spillway Gated &: Ungated

4 •• 1,060.0 1,061.0 8.0 6.0 6.0 8.0 8.0 6.0 6.0 8.0 19,400 to 20,430: 8.89 9.20 5 •• 1,061.0 - 1,062.0 7.0 6.0 6.0 7.0 7.0 6.0 6.0 7.0 18,960 to 20.740: 8.78 9.29 6 •• 1,062.0 1,063.0 5.0 6.0 6.0 4.0 4.0 6.0 6.0 5.0 18,160 to 20,430: 8.56 9.20 7 •• 1,063.0 1,064.0 5.0 3.0 3.0 4.0 4.0 3.0 3.0 5.0 17,580 to 20,280: 8.41 9.15 8 •• 1,064.0 1,065.0 5.0 0 3.0 4.0 0 3 0 5.0 17,590 to 20,680: 8.42 9.26 9 •• 1,065.0 1,066.0 0 0 3.0 1.0 0 3 0 0 17,300 to 20,660: 8.35 - 9.26

Spillway and ungated flow

10 •• 1,066.0 -1,067.0 0 0 0 0 0 0 0 0 18,690 to 22,420: 8.70 - 9.71 Above 1,067.0 0 0 0 0 0 0 0 0 22,420+ 9.71+

#If downstream channel flow is anticipated to exceed the channel capacity. releases from the project should be reduced to pre *During faLling stages the gates shall be left fully open to drain the reservoir completely. Then the gates shall be set at 1.0 feet.

**Source for elevations up to 8.30 feet from USGS Rating Table No.5; for elevations greater than 8.30 feet values were extrapolated from USGS data.

August 1990

DAM OPERATOR INSTRUCTI~S

1. Conmunication with the District Office is available.

a. Notify the Reservoir Operations Center when a gate change will be required according to the schedule.

b. Notify the Reservoir Operations Center if unable to set the gates as instructed.

2. COOITIUflication with the Oistrict Office is not ava; table.

a. Try to reestablish communications through the los AngeLes County Flood Control OPY (YUK470).

b. (i) Rising stages. Allow a period of one hour to pass to reestablish communications with the District Office. If after one hour communication is not reestablished follow the gate operati~ schedule.

(ii) Falling stages. Maintain current downstream gauge height until communication is reestablished~

c. If one or more of the gates cannot be operated adjust the remaining gates gradually and uniformly until the downstream gauge heightagrees with scheduled values. Keep a close check on gauge height and change the gate opening as often as required. If the downstream gaugeheight is unobtainable adjust the gates that are functioning so that the sum of the gate openings wilt equal the sum of the openings shown inthe schedule.

3. Trash Blockage.

If outlets become blocked with trash, increase gate openings to maintain scheduled downstream gauge height.

4. Notification to los Angeles County DP~.

Notify personnel at the los Angeles County O~P spreading grounds prior to making each gate change. Do not increase release until confirmation is received that their diversion gate located in the downstream channel has been adjusted.

OUTLETS

(Looking: Dow:lst t"eam)

[~I 1-<-_ Elev. 1,011.0

Elev. 990'

lJol'"ated 8' x 6'

GJGJGJGJGJGJ Each Gated Outlet 5' x 8'

PLATE 3