DAMS AND RESERVOIRS CHAPTER 6 -- ROBERSON ET AL,. WITH ADDITIONS DAMS AND RESERVOIRS PLANNING CONSIDERATIONS HYDROLOGICAL DATA: 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. GEOLOGIC DATA: 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 THE LOADS. 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. RESERVOIR DATA: 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 LAND. AVAILABILITY OF MATERIALS FOR DAM BUILDING, MANPOWER, ENVIRONMENTAL ASPECTS, SEDIMENTATION, DAM SAFETY. Turner reservoir, San Diego County, California. TYPES OF DAMS: CLASSIFIED 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 [090920] Tinajones dam, Peru [080216]. Leakage downstream of Tinajones dam, Peru [080216]. IT IS NOT SAFE TO SPILL WATER OVER TOP OF AN EARTH DAM. Fuse spillway, La Leche river at Huaca de la Cruz intake, Peru [080216]. 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 BECOMING POPULAR. 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 CENTER. 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 SPECIAL STEPS MUST BE TAKEN IF THE FOUNDATION IS WEAK. -- THE SLOPES OF THE FOUNDATION MAY BE FLATTENED TO DISTRIBUTE THE LOAD OVER A GREATER AREA. -- 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. -- DRAINS 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. EMBANKMENT STABILITY -- 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 REDUCE SEEPAGE. -- THE WIDTH OF THE CORE DEPENDS ON THE AMOUNT OF IMPERVIOUS SOILS AVAILABLE. -- CURRENT PRACTICE: 1/4 OF NET HEAD. -- 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. -- FILTERS MUST BE CAREFULLY DESIGNED. THE STRENGTH OF RELATIVELY IMPERVIOUS SOILS DEPENDS ON THEIR COMPACTED DENSITY, WHICH DEPENDS ON THE WATER CONTENT AND KIND AND USE OF COMPACTION EQUIPMENT. THE USUAL PRACTICE FOR SLOPES IS TO CHOOSE THOSE THAT HAVE BEEN FOUND TO BE STABLE. SIDE SLOPES ARE USUALLY 2 H : 1 V TO 3 H : 1 V. THESE SLOPES ARE CHECKED FOR STABILITY USING THE SWEDISH CIRCLE METHOD. SOIL PROTECTION ERODIBILITY OF EARTH DAMS DICTATE THAT SLOPE PROTECTION BE DONE. UPSTREAM SLOPE MUST BE PROTECTED AGAINST RAIN AND WAVE EROSION. THE MOST COMMON TYPE OF PROTECTION IS DUMPED RIPRAP. THE ROCK FOR THE RIPRAP SHOULD BE HARD, DENSE, AND DURABLE TO RESIST WAVE ACTION AND NORMAL WEATHERING OVER A LONG PERIOD. TABLE 6-2 SHOWS RIPRAP THICKNESS AND GRADATION. 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 PROTECTION. ROCKFILL DAMS 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. HEIGHT. 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. SHERARD DAMS 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 of sediment. 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. DAM SAFETY 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 DAMS. IF FACTORS OF SAFETY ARE FOLLOWED, DAMS OUGHT TO BE SAFE. EARTH DAMS PROPERLY COMPACTED SHOULD BE SAFE. 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 LEAKAGE. IF LEAKAGE IS MUDDY, DAM IS IN DANGER OF IMMEDIATE FAILURE. 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. SLIDES ON RESERVOIR SLOPES WHEN A RESERVOIR IS FILLED OR DRAWN DOWN, PHYSICAL CHANGES ON THE EARTH IN THE RESERVOIR CAN LEAD TO SLIDES VAIONT DAM: 315 MILLION CU YD AND CAUSED A 300-FT WAVE OF WATER TO WASH OVER THE CREST OF THE 869-FT HIGH DAM. ABOUT 2600 PEOPLE DIED. THE CONCRETE STRUCTURE WAS NOT DAMAGED. 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. SPILLWAY PROBLEMS DAMS CAN BE OVERTOPPED IF THE SPILLWAY CAPACITY IS NOT ENOUGH TO CARRY THE FLOOD. EARTH DAMS ARE SUSCEPTIBLE TO EROSION DURING OVERTOPPING. Failure of Barragem Norte cofferdam, Brazil, 1982. UNDERESTIMATION OF THE PEAK FLOW RATE OR VOLUME OF THE DESIGN FLOOD. ONLY COMPETENT, EXPERIENCED HYDROLOGISTS SHOULD BE GIVEN THE RESPONSIBILITY OF DETERMINING THE DESIGN FLOOD. DESIGN FLOOD SHOULD BE CHOSEN CAREFULLY. RETURN PERIOD FOR DAM DESIGN ONE COMMON CAUSE OF FAILURE HAS BEEN THE INADEQUATE DESIGN OF THE STILLING BASIN. EROSION AT THE END OF CHUTE SPILLWAYS HAS ALSO CAUSED FAILURES. STILLING BASIN MUST BE LARGE ENOUGH TO DISSIPATE THE ENERGY OF THE HIGH-VELOCITY WATER FROM THE CHUTE. ADEQUATE WING WALLS MUST BE BUILT AT THE END OF THE CHUTE TO ELIMINATE POSSIBLE UNDERMINING. 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). RESERVOIRS 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. -- WILL LEAKAGE BE A PROBLEM? Failure of Teton Dam, Snake River, Idaho, on June 5, 1976. -- WILL EVAPORATION BE A PROBLEM? 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 Solution.- 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. Source: J. Simpson, Columbia University -- WILL EXISTING VEGETATION IN THE AREA TO BE INUNDATED BE A PROBLEM? Decaying vegetation in Balbina Dam, Amazon rainforest, Brazil. OTHERS: -- RELOCATION OF UTILITIES BE A PROBLEM? -- PURCHASE OF LAND TO BE INUNDATED? -- DOWNSTREAM POLLUTION? -- RECREATION SITES? -- IMPACT OF NATURAL RESOURCES AND MITIGATION MEASURES. -- 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. Baldwin Hills Dam     RESERVOIR CAPACITY VOLUME OF WATER THAT CAN BE STORED IN THE PARTICULAR RESERVOIR. NORMAL MAXIMUM POOL: MAXIMUM POSSIBLE LEVEL WHEN THE SPILLWAY GATES ARE CLOSED. 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 IS NEEDED? 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 ALLOCATIONS STORAGE USUALLY ALLOCATED TO MANY USES. WATER SUPPLY, POWER GENERATION, FLOOD CONTROL, FISH AND WILDLIFE, RECREATION. STORAGE TYPES: -- INSTREAM FLOW -- FLOOD CONTROL -- IRRIGATION -- POWER PRODUCTION -- NAVIGATION 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, AND CONSERVATION. 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 DEVELOPED. MUCH CARE AND ANALYSIS IS NEEDED TO DEVELOP OPERATING PLANS FOR A MULTIPURPOSE PROJECT. WIND-GENERATED WAVES, SETUP AND FREE BOARD 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 BALANCE OF STATIC FORCES LEADS TO:   ∑Fx = 0 (γw/2) (D - S)2 L - (γw/2) (D + S)2 L + τo F L = 0 SHEAR STRESS ON THE SURFACE CAN BE EXPRESSED AS τo = Cf ρair U2 (γw/2) (D - S)2 L - (γw/2) (D + S)2 L + Cf ρa U2 F L = 0 WHICH LEADS TO: S = (γa/γw) (Cf/2) [U2/(gD)] F S = K [U2/(gD)] F and K = 2.025 × 10-6   (dimensionless) S AND F ARE IN LENGTH UNITS (M OR FT). HEIGHT OF WIND WAVES ARMY CORPS OF ENGINEERS STUDIED WAVES HEIGHTS THAT MIGHT BE EXPECTED WITH A GIVEN FETCH AND WIND SPEED. 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 FETCH: 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 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. EXAMPLE 6-4 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. THE AVERAGE RESERVOIR DEPTH IS 100 FT, AND THE DAM IS 200 FT HIGH. CALCULATE THE FREEBOARD. SOLUTION 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)] = 0.46 ft. 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. SEDIMENTATION IN RESERVOIRS ALL STREAMS CARRY SEDIMENT THAT ORIGINATE FROM EROSION PROCESSES IN THE BASINS THAT FEED THE STREAMS. RESERVOIRS TRAP SEDIMENTS BECAUSE BACKWATER AND REDUCED VELOCITIES. RESERVOIRS SHOULD BE DESIGNED WITH A USEFUL LIFE, SAY 100 YEARS. SEDIMENT ACCUMULATION SHOULD NOT IMPAIR THE OPERATION OF THE RESERVOIR THROUGH ITS DESIGN LIFE. 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 SEDIMENT YIELD IS THE TOTAL SEDIMENT OUTFLOW IN A SPECIFIED PERIOD (YEAR). ENGINEER MUST ESTIMATE SEDIMENT YIELD. PROCEDURES: 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. TRAP EFFICIENCY 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. 091108