Hydraulic Structures. Notes and Handouts

Transcription

1 Hydraulic Structures Notes and Handouts

2 Introduction Free surface flows in particular often have hydraulic structures to control flow Dams Spillways Stilling Basins Weirs Gates Can have hydraulic/structural/geotechnical aspects

3 Purposes of Structures 1. Storage Structures (Reservoir) 2. Conveyance (Culvert) 3. Waterway and Navigation (Navigational Canal) 4. Coastal (Breakwater, Jetties) 5. Measurement and Control (Sharp-edged weir) 6. Energy-Conversion (Hydroelectric Dam and Turbines) 7. Sediment & Fish Control Structures (Fish Weir) 8. Energy-Dissipation (Stilling Basin) 9. Collection (Surface Drainage Inlet)

4 Dams More than 79,000 significant dams in US (over 25ft high, 50 acre-feet or dangerous) Indiana 1047 many are old (>100 years) Flood control Impoundment (drinking water, electricity, navigation) Diversion (irrigation)

5 Types of Dams in US Earthfill (72,300+ of 79,000) Rockfill (2366) Concrete (2392) Masonry (761) Stone (480) Timber Crib (257) Other (1044) Oroville Dam (California) 754 ft high

6 Earthfill Dams

7 Earthfill Dams By far most common type of dam - Trapezoidal profile Levee is also type of earthfill dam Homogeneous or with more impermeable core Careful about seepage Oldest construction type - Use local materials Avoid Sudden Drawdown Should never be overtopped Separate spillways

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10 Grand Teton Dam Failure

11 Concrete Gravity Dams Remains stable through gravitational forces Usually in narrow cross-sections Careful of foundation for high head Common for hydroelectric plants

12 Gravity Dam Stability Need to make sure that concrete gravity dams stay in place Resistance to sliding FS Resistance to overturning about toe FS 2 All soil pressures on dam must be positive Negative pressures lead to high pressure intrusions instability Earthquakes can provide additional loading in seismically active areas

13 F Fr F R Hydrostatic Force, F HS Weight, W Fluid Uplift on Dam Base, F u Sedimentation, F s Earthquake on Dam, F EQ (any direction) Earthquake on Water, F EW (horizontal) Friction at Base, F Fr Soil Vertical Force, F R Example

14 Dam Soil Pressures After other weights and moments are summed, the magnitude, direction, and location of resultant soil force are just what is left over to sum to zero First approximation: trapezoidal force along base To avoid negative soil pressures, need to make sure resultant is in the middle 1/3 of base

15 Arch Dams Volume of gravity, earthfill dams increases strongly with height In narrow valley with strong side walls, build narrow arch dams Walls take arch thrust and resist overturning More economical use of materials Need good foundation

16 Arch Dam Stresses Arch dams have normal stresses against a curved surface Arch stresses usually larger than vertical compressive stresses Hoop stress depends on dam radius Optimization for total amount of concrete used in arch dam Minimum amount for arc to span gap

17 Flow Under Dams Dams combine large changes in head with short distances for groundwater to travel Potential for high gradients of porewater pressure Piping leads to catastrophic failure Extremely careful geotechnical investigation needed for dams even for rock Improvements grout to reduce permeability plus drainage curtain The narrower the dam, the greater the potential for failure arch dams must have good geotechnical aspects

18 Use of Grout Curtains Images from Grouters.org

19 Dam Construction Stages 1. Divert water from construction site Tunnel, Channel 2. Cofferdam (upstream and downstream) to seal off work area 3. Foundation work and build dam 4. Close diversion, remove cofferdam Details vary greatly depending on circumstances

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21 Diversion tunnel construction Cofferdam construction Construction in blocks Concrete lined tunnel

22 Dam Benefits/Adverse Effects Benefits Flood control Water supply Power Navigation Recreation Adverse Effects Resettlement Fish habitat/spawning Sediment impoundment Downstream erosion Temperature changes Loss of flooding cycles Maintenance/End of life issues

23 Dam Effects on Long-Term River Valley Landforms How do civil engineering activities affect long term processes? Widespread stream damming for power in pre-steam engine days In Pennsylvania, approx 0.15 dams/km 2 What effect does this have, even when old dams are gone?

24 Fig. 2. Historic 19th-century milldams (triangles) on Piedmont streams in York, Lancaster, and Chester counties, southeastern Pennsylvania, located from >100 large-scale township maps dating to 1876 (York), 1875 (Lancaster), and 1847 (Chester). R C Walter, D J Merritts Science 2008;319: Published by AAAS

25 Effect of Damming a Valley Upstream of dam, water in millpond/reservoir becomes much slower Sediment load carried by stream/river tends to be deposited in slower water Over time (decades to centuries), sediment builds up in reservoir, increasing the bed elevation Smaller dams <10 acre/ft, typical ~3% per year loss of storage Stream bed becomes near-level with time upstream of dam

26 Fig. 3. Streams throughout the mid-atlantic region R C Walter, D J Merritts Science 2008;319: Published by AAAS

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28 Sequence of Fill Original stream valley Dammed valley delta of coarse-grained material at beginning of pond, finer grained material throughout pond Gradual siltation to marsh Final conversion to valley meadows, occasionally flooded Often quickened by other human activities: agriculture, deforestation leading to larger sediment loads

29 Fig. 4. Methods for locating dams and assessing their impact on reservoir sedimentation, subsequent channel incision, and bank erosion with high-resolution LIDAR. Ground Elevation Existing Dam Stream Elevation Removed Dam Original Stream R C Walter, D J Merritts Science 2008;319: Published by AAAS

30 Beaver Dams Perform Similarly

31 Gradually Filling in the Valley

32 Leading to Beaver Meadows

33 Dam Removal Remove a dam, instantly change water levels Stream/river valley erodes downward through millpond sediments Steep banks, often near-vertical Gradual stream channel widening as pond sediments erode How long will it take to return to original state? Centuries, or never? Should it return to the original state? What was the original state?

34 Decisions for Dam Removal Net Benefit: Benefits minus costs (financial and operational) Costs Loss of hydropower/navigation Loss of flood control/impoundment/recreation Disruption to existing roads/houses/infrastructure Increased downstream sediment load Large financial outlay: Who Pays can be many millions of dollars!

35 Benefits for Dam Removal Increased downstream sediment loads More pronounced high/low flow cycles Removes chance of sudden failure for poorly maintained dams Water temperature changes Generally shows ecological improvements Fish passage and spawning Usually increased biodiversity

36 Implications 1. Civil and Environmental Engineering activities have effects that will last for centuries 2. What appears to be a natural state may actually have been created by humans, and may not be sustainable, or in equilibrium 3. This process also occurs in larger dams and reservoirs, with expensive consequences

37 Dam Failures Why do dams fail? Overtopping ~34% Foundation Defects ~30% Piping and Seepage ~20% Conduits and Valves ~10% Flow path beside conduit Other ~6% Includes human error What are the consequences? What can be done to prevent this? Examine case studies of dam failure

38 IVEX Dam Failure, Ohio Dam first constructed 1842 for hydropower Partially or completely failed five times 7.4m high Catastrophic Failure August 13, year rainfall event Reservoir emptied in 2-3 minutes Large flood wave downstream Peak discharge ~500m 3 /s Not likely for such a small stream in the absence of dam failure

39 Site Characteristics Chagrin River Ohio Mean flow downstream of dam ~400cfs (~11 m 3 /s) Overall slope 3m/km (0.003 slope) Sand and gravel banks and bed, fine material transported through system

40 Dam History Dam First Constructed in 1842, unanchored Failed immediately as toppling failure Rebuilt as earth dam with masonry spillway Failed completely in 1877 Rebuilt 1890 Partial failures in 1913, 1985 Seepage piping at spillway-dam contact Now used for water supply Spillway could not pass the Probable Maximum Flood of 1640m 3 /s

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42 Dam Failure August 13, 1994, 13.5cm (5.3 inches) rainfall in 24 hours 70 year event Flows 1.9m above base of spillway Active gullying on downstream toe of dam from seepage piping Led to catastrophic failure widening gap between masonry spillway and dam May have been worse because of trees on dam

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45 Failure Characteristics Estimated 466m 3 /s (16,500ft 3 /s) flow through dam 7.5m high breach, 15m wide Wide debris fan 1km downstream 1.5m over bankfull depth Strong erosional damage Road inundated 24,000-31,000m 3 of fine sediment swept downstream Downstream millpond filled in over years

46 Reservoir Sedimentation Initial storage capacity 274,000m 3 Reduced by 86%, 0.62%/year More than 2.6m sedimentation in some areas of reservoir

47 Lessons Design not adequate Spillway capacity, spillway-earth dam junction Built a century ago without engineering design Poor maintenance Trees growing on dam Introduces path along roots for water to travel Partial previous failures patched but not rebuilt No one seemed to pay attention to the dam

48 Testalinden Dam Failure Built in 1930s British Columbia for water storage and irrigation Privately owned Failed June 13, 2010 Large debris and mud torrent impacting homes and agricultural areas

49 History Built 1937 to store snowpack water in drier months Outlet culvert, spillway Seepage at outlet culvert noted in : road built across dam, spillway filled in with culvert under road 1961: spillway culvert not adequate for design flood 1977: Engineer s report dam condition deplorable, should be removed or rebuilt 1978 Dam is a hazard to life and property Continued problems 1980, 1985, 1988, 1992 report minimal freeboard, poor condition, spillway culvert was covered by rock slab

50 Failure June 11 (2 days before failure) Hiker noticed lake full and water flowing across the road. 12 inch channel across roadway Reported to Local Tourist Information Tourist information called police Technician only considered possible road washout, not dam failure not important road so nothing done

51 Failure June 13, 2010 Locals noticed dam was failing Everyone evacuated Large debris torrent flooded houses and farms, destroyed property No one injured

52 Before After After After

53 Why did this happen? Little to no engineering design of dam Poor capacity to convey large quantities of water Poor maintenance (covering culvert with rock slab) No remediation of known problems No clear path of accountability for dam safety hiker s warning not fully acted upon