DAMS AND RESERVOIRS
CHAPTER 6 -- ROBERSON ET AL,. WITH ADDITIONS
DAMS AND RESERVOIRS
- PLANNING CONSIDERATIONS
- DETERMINE FLOOD
AND DROUGHT FLOWS TO DETERMINE
THE REQUIRED CAPACITY AND OPERATING
PROCEDURE FOR THE RESERVOIR.
- ALSO, THE
REQUIRED SPILLWAY CAPACITY.
Definition sketch of reservoir storage volumes.
Emergency spillway, El Capitan Dam and reservoir,
San Diego County, California.
Emergency spillway, Morena Dam and reservoir,
San Diego County, California.
Emergency spillway, Turner Dam and reservoir,
San Diego County, California, spilling on February 24, 2005.
- ON-SITE INSPECTION, GEOLOGIC MAPPING, DRILLING OF EXPLORATORY
HOLES, COLLECTION OF CORE-SAMPLE DATA
BY QUALIFIED ENGINEERING GEOLOGISTS.
THESE DATA REVEAL STRUCTURAL ABILITY OF
THE FOUNDATION MATERIAL TO WITHSTAND
- LEAKAGE OR EROSION PROBLEMS MAY BE ENCOUNTERED.
- SOILS HIGH IN ESP (EXCHANGEABLE SODIUM PERCENTAGE) SHOULD BE AVOIDED.
Failure of Teton Dam, Snake River, Idaho, June 5, 1976.
- A COMPLETE ASSESSMENT
OF THE AREA TO BE INUNDATED.
- TOPO MAPS, LAND OWNERSHIP, LAND CLASSIFICATION, LOCATION OF ROADS AND PUBLIC UTILITIES, FOR RELOCATION AND ACQUISITION OF
- AVAILABILITY OF MATERIALS FOR DAM BUILDING, MANPOWER, ENVIRONMENTAL ASPECTS,
SEDIMENTATION, DAM SAFETY.
Turner reservoir, San Diego County, California.
TYPES OF DAMS:
- ACCORDING TO THE MATERIAL (EARTH, ROCK,
CONCRETE, WOOD), AND
- ACCORDING TO THE WAY IN WHICH THEY RESIST THE FORCES IMPOSED ON THEM (GRAVITY, ARCH, BUTTRESS).
- GRAVITY DAM IS ONE IN WHICH THE GRAVITATIONAL FORCES (WEIGHT) ARE GREAT ENOUGH
TO RESIST THE OVERTURNING MOMENT AND
SLIDING FORCE OF THE HYDROSTATIC FORCES
IMPOSED ON IT.
San Vicente gravity dam and emergency spillway,
San Diego County, California.
- BUTTRESS DAM IS ONE IN WHICH REINFORCED
CONCRETE SLABS CONSTITUTE THE DAM FACE,
AND ARE SUPPORTED BY VERTICAL BUTTRESSES AT INTERVALS OF 50 TO 100 FT.
Hodges multiple-arch/butress dam,
San Diego County, California.
- ARCH DAM IS DESIGNED TO TRANSFER THE IMPOSED LOADS TO ADJACENT ROCK WALLS ON
EITHER SIDE OF CANYON.
Monticello Dam, Sacramento, California.
- EARTH AND ROCKFILL DAMS ARE SPECIAL
TYPES OF GRAVITY DAMS BUILT OUT OF EARTH
WORK AND ROCKWORK.
- EARTH DAM IS USUALLY FAVORED OVER THE CONCRETE DAM IF SUITABLE EARTH MATERIALS ARE
AVAILABLE NEAR THE SITE, AND IF A SUITABLE
SPILLWAY CAN BE PROVIDED.
Typical design of an earth dam.
San Luis dam, California 
Tinajones dam, Peru .
Leakage downstream of Tinajones dam, Peru .
- IT IS NOT SAFE TO SPILL WATER OVER TOP OF AN
Fuse spillway, La Leche river at Huaca de la Cruz intake, Peru .
- IN EARTH DAMS, SPECIAL ATTENTION MUST BE
GIVEN TO THE DESIGN, CONTRUCTION, AND
MAINTENANCE TO RESIST INTERNAL EROSION.
CHEMISTRY OF SOILS IS VERY IMPORTANT.
Sheep Creek Barrier Dam, Utah.
- HYDRAULIC FILL DAMS ARE OLD.
- THEY WERE PLACED
BY PUMPING A SLURRY AND PLACING IT WET.
- ROLLED FILL EARTH DAMS ARE NOW THE RULE.
- ROLLER COMPACTED CONCRETE (RCC) DAMS ARE
Olivenhain Dam (RCC), San Diego County.
- THE BUILT-UP SECTION OF THE EARTH OR
ROCKFILL DAM IS THE EMBANKMENT.
- THE EMBANKMENT MUST RESIST THE HYDROSTATIC PRESSURE IN THE RESERVOIR AND
MUST CONTAIN AN IMPERVIOUS SECTION TO DISCOURAGE SEEPAGE.
- HOMOGENEOUS EMBANKMENT IS CONSTRUCTED IF THE SOIL MATERIAL IS RELATIVELY FINE AND ABUNDANT.
- WITH A VARIETY OF MATERIALS, BOTH COARSE
AND FINE, THE USUAL PRACTICE IS TO DESIGN A ZONED EMBANKMENT.
- THE FINER MATERIAL IS COMPACTED TO PRODUCE A RELATIVELY IMPERVIOUS CORE AT THE
- THE COARSE MATERIAL, PLACED ON THE U/S AND D/S, PROVIDES STABILITY.
- THE INTERFACE ZONE BETWEEN FINE AND
COARSE MATERIAL MUST BE CAREFULLY DESIGNED: FILTERS.
- FILTERS ALLOW PASSAGE OF SEEPAGE WATER
BUT PREVENTS DISLODGEMENT OF FINE PARTICLES IN THE CORE.
- OTHER FACTORS:
-- ADEQUACY OF FOUNDATION
-- EMBANKMENT STABILITY
-- SLOPE PROTECTION
ADEQUACY OF THE FOUNDATION
STEPS MUST BE TAKEN IF THE FOUNDATION IS
-- THE SLOPES OF THE FOUNDATION MAY BE
FLATTENED TO DISTRIBUTE THE LOAD OVER A
-- IF THE WEAK SOILS IN THE FOUNDATION ARE
NOT TOO THICK, THEY MAY BE EXCAVATED.
-- THE EMBANKMENT MAY BE CONSTRUCTED AT
A SLOWER PACE THAN NORMAL SO THAT THE
WEAK SOILS WILL HAVE TIME TO CONSOLIDATE WITHOUT EXCESSIVE DIFFERENTIAL
SETTLEMENT TO THE EMBANKMENT.
HELP SPEED UP SOIL CONSOLIDATION.
- SOMETIMES A CUTOFF TRENCH IS EXCAVATED
IN THE FOUNDATION SO THAT CONTACT IS MADE
BETWEEN CORE MATERIAL AND THE BEDROCK.
- CONSTRUCTING A CUTOFF ENSURES THAT ANY
OLD DRAINS, PERVIOUS ZONES, OR ABANDONED
PIPES ARE FOUND AND REMOVED.
- GROUTING THE FOUNDATION MAY BE NECESSARY.
- GROUTING PRESSURE TO BE DETERMINED CAREFULLY.
-- THE DESIGN OF THE EMBANKMENT SHOULD
BE BASED ON THE AVAILABLE SOILS, THEIR
WATER CONTENT, AND THE NEED FOR DRYING
AND WETTING OF THE SOILS TO ACHIEVE OPTIMUM CONDITIONS FOR COMPACTION.
-- ANY LARGE BOULDERS IN THE SOIL SHOULD
BE REMOVED BECAUSE THEY WILL MAKE
ROLLING OF THE SOIL DIFFICULT.
-- IT IS IMPOSSIBLE TO PREVENT ALL SEEPAGE
THROUGH THE DAM.
-- PERMEABILITY OF THE MATERIAL WILL DETERMINE THE RATE AT WHICH WATER SEEPS
THROUGH THE EMBANKMENT.
-- A RELATIVELY IMPERVIOUS CORE IS USED TO
-- THE WIDTH OF THE CORE
DEPENDS ON THE AMOUNT OF IMPERVIOUS
-- CURRENT PRACTICE: 1/4 OF
-- TOP WIDTH SHOULD NOT BE LESS
THAN 10 FT.
-- IF THE FUNDATION IS PERVIOUS, A CUTOFF
TRENCH MAY BE USED TO REDUCE SEEPAGE.
-- UPSTREAM BLANKETS MAY ALSO BE USED.
-- DOWNSTREAM CONTROL OF SEEPAGE WATER
IS HANDLED BY MEANS OF IMPERVIOUS DRAINAGE BLANKETS, OR BY CONSTRUCTING MOST
OF THE EMBANKMENTS OF IMPERVIOUS MATERIAL.
- THE DOWNSTREAM SLOPE REQUIRES LESS
PROTECTION THAN THE UPSTREAM SLOPE.
- 1-FT TO 2-FT LAYER OF ROCK OR COBBLES.
- GRASS TURF IS ALSO USED FOR DOWNSTREAM
- A ROCKFILL DAM USES ROCK TO PROVIDE STABILITY AND A THIN UPSTREAM-FACE MEMBRANE
OR A COMPACTED EARTH CORE IN THE EMBANKMENT.
- ROCKFILL DAMS: THE MAIN EMBANKMENT CONSISTS OF ROCK.
- THE FOUNDATION OF A ROCK FILL DAM:
ALMOST ALL ROCKFILL DAMS ARE BUILT ON
FAIRLY SOLID ROCK FOUNDATIONS.
- THE UPSTREAM AND DOWNSTREAM SLOPES OF
THE EMBANKMENT ARE CONSTRUCTED TO HAVE
THE ANGLE OF REPOSE OF THE ROCK, I.E., 1.3
TO 1.4 H TO 1 V.
- THE ROCK IS LAID DOWN IN LAYERS AND EACH
LAYER IS COMPACTED.
- THE UPSTREAM FACE OF THE EMBANKMENT
REQUIRES SPECIAL TREATMENT TO PROVIDE A
SUITABLE SURFACE ON WHICH THE IMPERVIOUS
CONCRETE FACE WILL REST.
- LAYER OF GRAVEL AND SAND ABOUT 12 FT WIDE
(HORIZONTAL), FOR DAMS LESS THAN 300 FT.
- FOR HIGHER DAMS, THE WIDTH IS INCREASED
TOWARD THE BASE.
- THE MATERIAL IN THE FACE-SUPPORTING ZONE
IS PLACED IN LAYERS ABOUT A FOOT-THICK AND
COMPACTED WITH SEVERAL PASSES OF A VIBRATORY ROLLER.
- UPSTREAM FACE: AN ANCHOR IS REQUIRED FOR
THE IMPERVIOUS FACE.
- COMMON PRACTICE IS TO EXCAVATE A PORTION
OF THE FOUNDATION ROCK AND POUR A CONCRETE WALL (A FOOTWALL OR A TOE SLAB).
- THE FACING OF MODERN CONCRETE FACE DAMS
CONSISTS OF RENFORCED SLABS WITH VERTICAL JOINTS BUT NOT HORIZONTAL JOINTS.
- THE FACE OF A 300-FT DAM MIGHT CONSIST OF SLABS 50 FT WIDE AND 500 FT LONG.
- THE SLAB IS ABOUT 1-FT THICK, AND INCREASES TOWARD THE BOTTOM AS: 1 + 0.003H.
- THE AMOUNT OF STEEL REINFORCEMENT USED
IN THE FACE SLAB IS USUALLY ABOUT 0.4% OF VOLUME.
- THE REINFORCING RUNS BOTH HORIZONTALLY AND VERTICALLY.
- THE IMPERVIOUS FACE MUST BE ABLE TO CONFORM TO THE CHANGES IN THE UPSTREAM SURFACE OF THE DAM.
- THIS IS ACCOMPLISHED BY THE VERTICAL CONTRACTION JOINTS WITH WATER STOPS OF RUBBER, PLASTIC, OR EXPANDABLE MATERIAL.
The Journal of San Diego History, Winter 2002, Vol. 48, No. 1.
At the turn of the twentieth century, a population of about 18,000 people were being supplied with water from a system that captured a major part of the runoff from rain in the area.
However, San Diego was still dependent on local rain to supply the reservoirs with water, and drought conditions prevailed in the first 15 years of the century. City fathers offered
a local rainmaker named Charles Hatfield $10,000 if he could cause enough rain to fill Morena Reservoir. The first day of January 1916, the dry weather continued. Hatfield had built
towers near Morena Reservoir and was releasing strange and secret vapors into the atmosphere. The need for water was becoming more desperate each day.
On January 14, 1916, a massive
storm rolled in from the Pacific, and for the next six days, most of Southern California was deluged. Old Town received more than four inches of rain, and Cuyamaca received 18 inches,
amounts that equaled nearly one half of their usual annual rainfall. Every river and creek was choked with floodwater, and Lake Morena was full. No doubt Hatfield was ecstatic,
probably celebrating his success that had saved San Diego.
The rain ceased on January 19th, and the area began to dry out. Then, just a few days later, the skies clouded again,
the wind blew from the southeast, and a second deluge began, which pounded down sheets of rain, day after day. The storm added another three inches of rain at Old Town and more
than 14 inches at Cuyamaca. The already saturated watershed retained little of the rain, and raging torrents of water raced down the rivers and creeks, destroying nearly every
object in its path. One thing that remained structurally sound was Derby's dike. It prevented the onrushing river from devastating Old Town and filling San Diego Bay with tons
Morena Reservoir has a large watershed, and the dam survived the large flood runoff without major damage. At Sweetwater Dam, the spillway was not large enough, and
the floodwater overtopped the dam and then washed out a large section of the south abutment. The dam had been constructed with enough structural stability, so that it did
not wash out. When the dam was later repaired, a second spillway was incorporated in the south abutment.
Otay Dam, however, was a total loss. The spillway did not have sufficient capacity for the flood flows and the dam was overtopped. Water filled the observation shafts on
the downstream side of the steel core, and the pressure blew out the rock that provided the structural stability. The steel core swung out like a gate, releasing the full
depth of water, which created a flood wave in the canyon of gigantic proportions. The dam was about 130 feet high, and the depth of the wave in the canyon a short distance
below the dam site was about 100 feet high. The force of the water was so great that it stripped every bit of vegetation from the canyon walls, leaving a clear trace of the
wave crest. The lower canyon is much wider, and the wave height decreased to approximately 20 feet, which was still devastating to the people living in the valley below.
Every structure was destroyed and many people lost their lives. At the end of the flood, San Diegans found that every bridge over every river or creek was impassible, and the only way to travel to Los Angeles was by boat.
After the flood, Hatfield was nowhere to be found. Some time later, he appeared at the City Council to claim his $10,000. However, the city attorney determined that either
Hatfield was responsible for the rain, or he was not. If he collected the $10,000 for filling Morena Reservoir, he was also responsible for the flood damage. Hatfield did
not collect the money, and he disappeared from the area.
- BECAUSE THE SITES FOR FUTURE DAMS ARE
USUALLY LESS SUITABLE, POTENTIAL FOR
PROBLEMS IS GREATER.
- EXPERIENCE COMES FROM STUDYING DAMS
THAT HAVE FAILED.
- SEVERAL PROBLEMS CAN LEAD TO FAILURE OF
- IF FACTORS OF SAFETY ARE FOLLOWED, DAMS OUGHT TO BE SAFE.
- EARTH DAMS PROPERLY COMPACTED SHOULD
- MOST FAILURES HAVE TO DO WITH FOUNDATIONS OR SEEPAGE PROBLEMS.
- ASCE HURRICANE KATRINA REPORT.
Soil shear strength under 17-Street canal failure, New Orleans, Louisiana.
- ALSO, PROBLEMS WITH INADEQUATE SPILLWAY
CAPACITY, EROSION OF EMBANKMENT, AND
MASSIVE SLIDES UPSTREAM INTO THE RESERVOIR.
- PIPING: ALL EARTH DAMS WITH A CENTRAL
CORE HAVE SOME SEEPAGE.
- IF SEEPAGE OCCURS WITH MOVEMENT OF SOIL
MATERIAL, PARTICLES ARE WASHED AWAY AND
EVENTUALLY LEADS TO FAILURE.
- PIPING CAN DEVELOP IN EARTH DAMS WHEN
THE FILTER BETWEEN FINE SOIL AND COARSE
MATERIAL IS NOT PROPERLY DESIGNED.
- CAREFUL DESIGN AND CONSTRUCTION IS REQUIRED.
- TYPE OF SOIL MAY ALSO BE IMPORTANT: SOIL
SHOULD HAVE LOW SODIUM (EXCHANGEABLE SODIUM PERCENTAGE OR ESP).
- SOILS IN ARID REGIONS HAVE HIGH ESP
- SOILS IN HUMID REGIONS HAVE LOW ESP.
- YET IT IS IN ARID REGIONS THAT WE NEED TO
BUILD THE DAMS.
- SOLUTION: BRING THE SOIL FROM HUMID REGIONS INTO ARID REGIONS: EXPENSIVE.
- PIPING CAN ALSO OCCUR WHERE THE FILL MATERIAL JOINS THE ABUTMENT MATERIAL, OR
WHERE IT JOINS A SOLID STRUCTURE SUCH AS
AN OUTLET CONDUIT.
- COLLARS, AS IN FIG. 6-24 WILL REDUCE POSSIBILITY FOR PIPING.
- PROPER COMPACTION IS DIFFICULT AROUND COLLARS, AND SOME AUTHORITIES DISCOUNT THEIR EFFECTIVENESS.
- IN SMALL DAMS, BURROWING ANIMALS CAN BE
A SOURCE OF PIPING PROBLEMS.
- PIPING CAN BE DETECTED BY OBSERVING
- IF LEAKAGE IS MUDDY, DAM IS IN DANGER OF
- ONCE PIPING OCCURS, RAPID REMEDIAL ACTION
IS CALLED FOR.
- FIRST, RESERVOIR POOL SHOULD BE DRAWN DOWN AS FAST AS POSSIBLE.
- OUTLET WORKS SHOULD BE FAST ENOUGH SO
THAT THE RESERVOIR CAN BE DRAWN DOWN QUICKLY.
- DAM MAY DEVELOP PROBLEMS RELATED TO
DIFFERENTIAL SETLEMENT, SLIDING, AND LEAKAGE.
- THE BEST WAY TO AVOID FOUNDATION PROBLEMS IS TO CONDUCT A THOROUGH GEOLOGICAL SURVEY OF THE SITE.
- INFORMATION SHOULD BE STUDIED BY COMPETENT GEOLOGISTS AND ENGINEERS.
The Vaiont reservoir landslide
Vaiont Dam, 262 m high, is on the Vaiont River, a tributary of the Piave River, in
northeast Italy, in the south-eastern part of the Dolomite Region of the Italian Alps, near Belluno,
about 100 km north of Venice. The dam, one of the highest in the world, was completed in 1961
and is used to generate hydroelectric power for the northern cities of Milan, Turin and Modena.
When the level in the reservoir was raised above a threshold level,
a landslide of 270 million cubic meters developed and fell into the Vaiont reservoir, causing 50 million cubic meters of stored
water to spill over the dam, sweeping away the village of Longarone and flooding nearby hamlets.
Lessons from the Vaiont disaster
The Vaiont reservoir disaster is a classic example of the failure of engineers and geologists
to understand the nature of reservoir design. During the filling of the
reservoir, a block of approximately 270 million cubic meters detached from one side and slid into the lake at
velocities of up to 30 m/s (110 km/h).
The resulting wave overtopped the dam by 250 m
and swept onto the valley downstream, with the loss of about 2500 lives. Remarkably, the dam remained unbroken.
- GEOLOGIC INVESTIGATIONS SHOULD BE MADE
OF THE RESERVOIR AREA TO LOCATE POTENTIAL SLIDE HAZARDS.
- ALTERNATIVE INCLUDES CHANGING DAM SITE.
- DAMS CAN BE OVERTOPPED IF THE SPILLWAY
CAPACITY IS NOT ENOUGH TO CARRY THE
- EARTH DAMS ARE SUSCEPTIBLE TO EROSION
Failure of Barragem Norte cofferdam, Brazil, 1982.
Erosion of stilling basin near ski-jump spillway, Poechos Dam, Peru.
Protection of the chute spillway, Tarbela Dam, Pakistan.
Protection of the chute spillway, Oros Dam, Brazil.
Emergency spillway, Gallito Ciego Dam, Peru.
Baffle structure in spillway, upstream of stilling basin,
Gallito Ciego Dam, Peru.
Stilling basin, Gallito Ciego Dam, Peru.
- RIPRAP OF ADEQUATE SIZE AND GRADATION
MUST BE USED IN THE STILLING BASIN ITSELF
TO PREVENT EROSION OF THE BASE MATERIAL.
- CHUTE SPILLWAYS HAVE ALSO FAILED DUE TO
MISALIGNMENT OF JOINTS OF THE SPILLWAY
FLOOR, WHICH CAN LEAD TO CAVITATION AND
EVENTUAL DESTRUCTION OF THE SPILLWAY.
- EXAMPLE: TARBELA DAM FAILURE (1974).
- A RESERVOIR IS A MANMADE LAKE OR STRUCTURE USED TO STORE WATER.
- PEOPLE HAVE MAJOR CONTROL OVER THE USE OF THE WATER.
- FOR WATER TANKS, INFLOW IS CONTROLLED, BUT OUTFLOW IS DICTATED BY CONSUMER DESIRES.
- RIVER RESERVOIRS HAVE UNCONTROLLED INFLOW BUT CONTROLLED OUTFLOW.
- QUESTIONS IN DESIGN OF RESERVOIRS:
-- WHAT HEIGHT OF DAM?
Oroville Dam, Feather River, California, 770 ft (235 m),
the highest earth dam embankment in the U.S.
Failure of Teton Dam, Snake River, Idaho, on June 5, 1976.
-- WILL EVAPORATION BE A PROBLEM?
Calculate the annual loss to evaporation (in percentage of runoff) for the Sobradinho reservoir,
in Northeastern Brazil (Lat. 9o 30' S), with reservoir area 4,225 km2, mean annual discharge
1,060 m3/s, and the following monthly series
of potential evaporation (oC)
[January to December]: 23.1 27.3 26.6 26.5 25.8 24.7 24.4 25.0 26.9 28.0 28.2 27.6
Use temperature data to calculate annual potential evapotranspiration by Thornthwaite method.
Annual PET = 158 cm = 1.58 m.
The annual runoff volume is: Q = 1,060 m3/s × 86,400 s/d × 365 d/y = 3.343 × 1010 m3.
The annual evaporated volume is: E = 4,225 km2 × 10002 m2/km2 × 1.58 m = 0.668 × 1010 m3.
The percentage of evaporation/runoff is: (0.668/3.343) × 100 = 20.0%
This is one of the highest reservoir evaporation volumes in the world, attributed to the shallow (mean depth = 8.6 m)
and large reservoir (4,225 km2)
and semiarid climate, with relatively high potential evapotranspiration (1.58 m).
-- WILL EARTH STABILITY IN RESERVOIR BE A PROBLEM?
See Vaiont Dam above.
-- WILL INCOMING SEDIMENT BE A PROBLEM?
Filling of Tarbela Dam with Sediment
The Tarbela dam construction was completed in 1974 and while test-filling the site,
sink holes in the alluvium bed were noticed. This led to costly remedial measures and
delay in filling up by two years. Since then, every year, 200 million tons of silt
has been deposited in the reservoir. Predictably, a silt delta formed and crept towards
the dam. It was envisaged that the silt delta would be 48 km from the dam by 1983, but it
actually loomed to within 19 km. By 1991, the delta crest was just 14 km from the dam and
creeping towards the dam at the rate of one kilometer per year.
In 1997, Tippetts-Abbett-McCarthy-Stratton International Corporation recommended,
that if remedial measures were not taken for the management of sediments, the delta would cross the danger limit line by as early as 2006.
Decaying vegetation in Balbina Dam, Amazon rainforest, Brazil.
-- RELOCATION OF UTILITIES BE A PROBLEM?
-- PURCHASE OF LAND TO BE INUNDATED?
-- DOWNSTREAM POLLUTION?
-- RECREATION SITES?
-- IMPACT OF NATURAL RESOURCES AND
-- ANTHROPOGENIC FAILURES: BALDWIN HILLS DAM
Failure of Baldwin Hills Dam
In December of 1963 the newly constructed Baldwin Hills Reservoir, in Los Angeles, failed with five deaths and a great deal of property damage.
The cause of failure was graben subsidence, accelerated by oil extraction, supplemented by reinjection of waste brine into the
ground, which caused fault movement beneath the reservoir, as seen in the animation. Injection pressures
exceeded hydrofracture pressures; the recorded timing of the fault offset indicate injection as being the decisive factor.
- VOLUME OF WATER THAT CAN BE STORED IN
THE PARTICULAR RESERVOIR.
- NORMAL MAXIMUM POOL: MAXIMUM POSSIBLE
LEVEL WHEN THE SPILLWAY GATES ARE
- ASSUMED TO BE AT THE SPILLWAY CREST.
- MINIMUM LEVEL: LOWEST LEVEL AT WHICH
FLOW CAN BE RELEASED.
- USEFUL STORAGE: STORAGE BETWEEN MAXIMUM
AND MINIMUM LEVELS.
- DEAD STORAGE: NOT ACCESSIBLE.
- SURCHARGE STORAGE: DURING FLOODS,
RETAINED ABOVE SPILLWAY CREST.
Definition sketch of reservoir storage volumes.
- HOW TO DETERMINE WHAT STORAGE CAPACITY
- FLOW-MASS CURVE OR RIPPL CURVE.
- FIGURE 6-28 IS THE MASS CURVE FOR THE SALMON RIVER DURING A 2-YR DROUGHT (1976-78).
- WHAT RESERVOIR STORAGE VOLUME IS NEEDED TO GUARANTEE A MINIMUM FLOW OF 6,000 CFS?
- THE SLOPES OF THE MASS CURVE ARE DISCHARGES (CFS).
- TO DETERMINE THE AMOUNT OF STORAGE TO GUARANTEE 6,000 CFS, DRAW A LINE TANGENT TO THE MASS CURVE AT POINT a.
- THE STORAGE NEEDED IS THE MAXIMUM SPREAD: 10,000 SEC-FT-MO (CFS ⋅ MONTH)= 595,000 AC-FT.
- ANOTHER PROBLEM: IF Y STORAGE IS AVAILABLE, WHAT FLOW CAN BE GUARANTEED?
- IF 2,000,000 AC-FT = 33,600 SEC-FT-MO WERE AVAILABLE, RUN LINE cd UNTIL MAXIMUM DIFFERENCE IS Y = 33,600.
- COMPARED THE SLOPE OF LINE cd WITH DISCHARGES FOR VARIOUS SLOPES.
- A DISCHARGE OF 7,500 CFS CAN BE GUARANTEED FOR 2,000,000 AC-FT OF STORAGE, BASED ON 1977-78 DROUGHT DATA.
- CONSIDER THAT WITHDRAWALS MAY NOT BE A CONSTANT.
- CONSIDER EVAPORATION AND PERCOLATION (LEAKAGE).
- U.S. ARMY CORPS OF ENGINEERS HEC-5 MODEL.
- STORAGE USUALLY ALLOCATED TO MANY USES.
- WATER SUPPLY, POWER GENERATION, FLOOD
CONTROL, FISH AND WILDLIFE, RECREATION.
- STORAGE TYPES:
-- INSTREAM FLOW
-- FLOOD CONTROL
-- POWER PRODUCTION
- STORAGE REQUIREMENTS OFTEN CONFLICT
WITH ONE ANOTHER
- FOR MAXIMUM POWER, RESERVOIR SHOULD BE
KEPT AS FULL AS POSSIBLE.
- FOR FLOOD CONTROL, RESERVOIR SHOULD BE
KEPT AS LOW AS POSSIBLE.
- IN MANY STREAMS, HYDROPOWER OPERATION
REQUIRES THAT FLOW VARY GREATLY
THROUGH THE DAY.
- ANNOYANCE TO USERS OF RIVERS DOWNSTREAM.
- INSTREAM FLOW NEEDS AND IRRIGATION REQUIREMENTS.
- USBR WILL ASSIGN SPECIFIC VOLUMES IN THE
RESERVOIR FOR FLOOD CONTROL, JOINT USE,
- THE TOP PART IS RESERVED FOR FLOOD CONTROL.
- THE NEXT LEVEL IS RESERVED FOR JOINT USE,
ASSIGNED FOR FLOOD CONTROL DURING WET
SEASON; CONSERVATION DURING DRY SEASON.
- CONSERVATION INCLUDES IRRIGATION, POWER,
MUNICIPAL AND INDUSTRIAL WATER SUPPLY,
FISH AND WILDLIFE, RECREATION, NAVIGATION,
AND WATER QUALITY.
- RESERVOIR OPERATION PROCEDURES MUST BE
- MUCH CARE AND ANALYSIS IS NEEDED TO DEVELOP OPERATING PLANS FOR A MULTIPURPOSE PROJECT.
WIND-GENERATED WAVES, SETUP AND FREE
- WAVES DEVELOP IN OPEN STRETCH OF WATER SUCH AS IN RESERVOIR.
- CREST OF THE DAM MUST BE MADE HIGHER
THAN THE MAXIMUM POOL LEVEL TO PREVENT
OVERTOPPING WITH WAVES.
- ADDITIONAL HEIGHT IS CALLED FREEBOARD.
- SETUP CALCULATION (WIND TIDE):
F = fetch (in the direction of wind movement)
U = wind velocity
D = reservoir depth
S = setup
L = dimension normal to page
γw = unit weight of water
ρa = density of air
HEIGHT OF WIND WAVES
- ARMY CORPS OF ENGINEERS STUDIED WAVES HEIGHTS THAT MIGHT BE
EXPECTED WITH A GIVEN FETCH AND WIND
- GIVEN FETCH AND WIND VELOCITY,
DETERMINE WAVE HEIGHT (FIG. 6-31).
- ALSO, FIG. 6-31 GIVES THE MINIMUM TIME THE
WIND MUST BLOW TO DEVELOP THAT PARTICULAR WAVE HEIGHT.
- HEIGHTS INDICATED ARE JUST THE LARGER WAVES.
- THEY ARE CALLED SIGNIFICANT WAVES:
AVERAGE HEIGHT OF THE HIGHEST ONE-THIRD OF THE WAVES.
- A WAVE 1.67 TIMES THE SIGNIFICANT WAVE
HEIGHT HAS ONLY 0.4% CHANCE OF OCCURRING.
- SOME SPLASHING MAY BE ALLOWED.
- AMOUNT OF FREEBOARD DEPENDS ON THE
AMOUNT OF WAVE RUNUP ON THE FACE OF THE DAM.
- RUNUP: DIFFERENCE BETWEEN MAXIMUM ELEVATION OF WAVE RUNUP AND STATIC ELEVATION.
- THE AMOUNT OF RUNUP FOR A GIVEN WAVE IS A
FUNCTION OF THE EMBANKMENT SLOPE, ROUGHNESS, AND RELATIVE STEEPNESS OF THE WAVE Ho/Lo.
- WAVE LENGTH IS:
- Lo = 0.159 g T2
- WAVE PERIOD T IS A FUNCTION OF WIND SPEED AND
- T = 0.429 U 0.44 F 0.28 /g0.72
- THIS FORMULA FUNCTIONS WITH THE UNITS OF g.
- THESE FORMULAS ARE FOR DEEP-WATER
WAVES, THAT IS, FOR DEPTHS GREATER THAN
ONE-HALF OF THE WAVELENGTH.
- RESERVOIR DEPTH HAS TO BE GREATER THAN
0.50 (0.159 g T2)
- IF SHOAL WATER EXISTS NEAR THE FACE,
THE SHALLOW WAVE CASE EXISTS, AND OTHER FORMULAS SHOULD BE USED.
- FREEBOARD IS CALCULATED BY SUMMING THE
AMOUNT OF SETUP AND THE RUNUP THAT MIGHT BE
EXPECTED, AND ADDING ANOTHER INCREMENT
FOR INTANGIBLE EFFECTS.
- INTANGIBLES MAY INCLUDE POSSIBLE SETTLEMENT OF THE EMBANKMENT.
CONSIDER A RESERVOIR WITH FETCH F = 10 MILES,
AND WIND VELOCITY U = 80 MPH.
THE EMBANKMENT HAS U/S SLOPE
OF 1 V TO 3 H.
THE UPSTREAM EMBANKMENT IS FACED WITH WELL-DESIGNED RIPRAP.
THE FREEBOARD IS TO BE BASED ON THE WAVE HEIGHT THAT WILL BE EXCEEDED
BY ONLY 2% OF THE WAVES.
RESERVOIR DEPTH IS 100 FT, AND THE DAM IS 200 FT HIGH.
CALCULATE THE FREEBOARD.
- THE FREEBOARD WILL BE EQUAL TO SETUP PLUS RUNUP PLUS ALLOWANCES FOR
SETTLEMENT PLUS AN AMOUNT FOR CONTINGENCIES.
- TO DETERMINE RUNUP:
- THE SIGNIFICANT WAVE HEIGHT IS OBTAINED
FROM FIG. 6-31:
- FOR F = 10 MILES AND U = 80 MPH, THE SIGNIFICANT WAVE HEIGHT IS: Hs = 10.6 FT.
- THE WIND MUST BLOW FOR 75 MINUTES FOR
THIS WAVE TO DEVELOP.
- FROM TABLE 6-4, FOR 2% EXCEEDANCE,
THE RATIO OF SPECIFIC WAVE HEIGHT TO SIGNIFICANT WAVE HEIGHT IS 1.4.
- THE SPECIFIC WAVE HEIGHT IS:
H = Ho = 1.4 × 10.6 = 14.8 FT.
- DETERMINE WAVE PERIOD:
- T = 0.429 U 0.44 F 0.28 /g0.72
- U = 80 mph = 117.3 ft/s
- F = 10 miles = 52,800 ft.
- T = 0.429 (117.3) 0.44 (52800) 0.28 /(32.2)0.72
- T = 6.02 seconds.
- THEN: Lo = 0.159 g T2
- Lo = 0.159 (32.2) (6.02)2
- Lo = 186 FT.
- Ho/Lo = 14.8/186 = 0.08
- FROM FIG 6-33, FOR Ho/Lo = 0.08 AND A SLOPE OF
0.33, IT IS ESTIMATED THAT R/Ho = 0.5, BY EXTRAPOLATION, FOR RELATIVELY PERMEABLE RUBBLE MOUNDS (RIPRAP)
- RUNUP: R = 0.5 × 14.8 = 7.4 FT.
- CALCULATION OF SETUP
- S = K F [U2/(gD)]
- and K = 0.000002025 (dimensionless)
- S = 0.000002025 × 52800 × [(117.3)2/(32.2 X 100)] =
- ASSUME 1% OF DEPTH FOR EMBANKMENT SETTLEMENT.
- SETTLEMENT FREEBOARD WILL BE 0.01 Ddam = 0.01 × 200 = 2 FT.
- FOR CONTINGENCIES, ALLOW: 1.5 FT.
- TOTAL FREEBOARD = RUNUP + SETUP + SETTLEMENT + CONTINGENCIES
- TOTAL FREEBOARD: 7.40 + 0.46 + 2.0 + 1.5 = 11.36 FT.
- SEDIMENTS ARE EITHER CARRIED IN SUSPENSION (SUSPENDED LOAD) OR BY ROLLING AND SLIDING ALONG THE BED (BED LOAD).
- PROBLEMS RELATING TO SEDIMENTATION ARE:
- ESTIMATING THE RATE OF ACCUMULATION OF SEDIMENT, SO THAT ENOUGH VOLUME CAN BE RESERVED FOR SEDIMENT ACCUMULATION.
- INCLUDING THE ANTICIPATED SEDIMENT ACCUMULATION IN THE OPERATING PLAN.
- PLANNING FOR THE LOCATION OF RECREATIONAL SITES (AVOID DELTAS).
- SEDIMENT YIELD IS THE TOTAL SEDIMENT OUTFLOW IN A SPECIFIED PERIOD (YEAR).
- ENGINEER MUST ESTIMATE SEDIMENT YIELD.
- OBTAIN RECORDS OF MEASURED SEDIMENT DISCHARGE. PREFERABLY 10 YEARS OR MORE.
- OBTAIN RECORDS OF SEDIMENT DISCHARGE IN BASINS OF SIMILAR HYDROLOGIC CHARACTERISTICS.
- CORRELATE LONG-TERM RECORDS IN NEIGHBORING BASINS WITH SHORT-TERM RECORDS IN BASIN OF INTEREST.
- STUDY RECORDS OF SEDIMENTATION OF EXISITING RESERVOIRS THROUGHOUT THE REGION.
- USE ESTABLISHED FORMULAS, SUCH AS DENDY AND BOLTON.
- THE TRAP EFFICIENCY IS A MEASURE OF THE EFFECTIVENESS OF THE RESERVOIR IN TRAPPING AND HOLDING INCOMING SEDIMENT.
- THE TRAP EFFICIENCY DEPENDS ON THE CAPACITY/INFLOW (C/I) RATIO AND THE TYPE OF SEDIMENT (COARSE, MEDIUM, FINE).
- RATIO C/I IS DIMENSIONLESS.
- C IS THE CAPACITY OF THE RESERVOIR IN L3 UNITS.
- I IS THE TOTAL ANNUAL INFLOW IN L3 UNITS.
- BRUNE CURVES DETERMINE TRAP EFFICIENCY AS A FUNCTION OF C/I RATIO AND TYPE OF SEDIMENT (FIG. 6-35).
SEDIMENTATION IN RESERVOIRS: EXAMPLE
- A PLANNED RESERVOIR HAS A TOTAL CAPACITY OF 10 MILLION M3.
- THE CONTRIBUTING CATCHMENT AREA IS 250 KM2
- THE MEAN ANNUAL RUNOFF AT THE SITE IS 400 MM.
- THE ANNUAL SEDIMENT YIELD IS 1000 M.TONS/KM2.
- THE SPECIFIC WEIGHT OF SEDIMENT DEPOSITS IS 12000 N/M3.
- THE SEDIMENTS ARE FINE-GRAINED.
- HOW LONG WILL IT TAKE THE RESERVOIR TO FILL
UP WITH SEDIMENTS?
- USE 5 INCREMENTS OF RESERVOIR CAPACITY (DISCRETE INTERVALS).
- HAND CALCULATION OF RESERVOIR DESIGN LIFE.
- ONLINE CALCULATION OF RESERVOIR DESIGN LIFE.