MANAGEMENT of IMPOUNDED RIVERS (Part 4)

MANAGEMENT OF IMPOUNDED RIVERS (Part 4)

Zhao-Yin Wang and Baosheng Wu Lessons from the Sanmenxia Reservoir Sanmenxia reservoir is the first large reservoir on the Yellow river. Following completion of the dam, the reservoir began to function as a storage basin from September 1960 to March 1962, when the maximum pool level reached 332.6 m. Severe sedimentation, especially in the lower Weihe River, became evident immediately after impoundment. During the first 18 months, 93 percent of the incoming sediment was trapped in the reservoir, representing a release efficiency of only 7 percent. The reservoir lost 17% of its capacity due to sedimentation. Sanmenxia Dam

Yellow River Basin

• Drainage area above dam: 690,000 km2 • Annual mean discharge: 1,400 m3/s • Height of dam: 106 m • Normal pool level: 335 m • Storage capacity: 9.6 billion m3 Original Face of the Damsite Overview of Sanmenxia Reservoir Overview of Sanmenxia Reservoir Sedimentation Problems in the Reservoir • Annual sediment load: 1.6 billion tons • Average sediment concentration: 35 kg/m3 • Max sediment concentration: 911 kg/m3

Loess Plateau Sanmenxia Dam Sedimentation Problems in the Reservoir

Dam Wei River

Tongguan

• The Loss river of 20% is ofconstricted the effective from storage a width capacity of within more one than and 10 half km years, to less60% than in 6 years.1 km at Tongguan, forming a naturally constricted •river Rapid reach. upstream extension of sediment deposition in the backwater zone: • The– bed 1960 elevationto 1962, the channelat Tongguan bed at Tongguan servers station as a was hydraulic raised 4.5 control m. for both the Yellow and Wei Rivers upstream. – Backwater sediment deposition extended over about 74 km in the lower Wei River, upstream of Tongguan. Aggradation in a Tributary of Wei River

1974

The bridge piers have been increased two 1969 times, adding total height of 6.4m. Abandoned Drainage Outlet to Wei River

3m 2003 Flood in the lower Wei River

• A 2-year flood on Sept.1, 2003 – peak discharge of 3,570 m3/s – the highest stage record of 342.76 m

• Five levee breaches occurred around Huaxian city; • More than 20,000 people had to be evacuated; • Loss of 280 million US dollars. A 20 m wide of dike breach occurred on Sept. 1, 2003, at the right bank of Luowen River, a tributary of the Wei River

A 30 m wide of dike breach occurred on Sept. 1, 2003, at the right bank of Fangshan River Inundated floodplains

A part of the inundated Weinan city, Sept. 2, 2003 Inundated Huaxian hydrologic station, Sept. 1, 2003

Tunnel 1-2 N

Bridge

Yellow River

Deep hole & bottom sluice

Scoured pit Separation wall

Flushing pipe

God island

Zhongliu island Unit 1-7

Railway

Ghost island . Dam Reconstruction

• First stage reconstruction from 1965 to 1968 • Second stage reconstruction from 1970 to 1973 • Supplementary works from 1984 to 2000 First stage of reconstruction from 1965 to 1968 • Two tunnels at elevation 290 m were added, and four penstocks remodeled into outlets for sluicing sediment. • The discharge capacity had been increased from 3,080 m3/s to 6,100 m3/s at a water level of 315 m. • However, the sills of the outlet structures were too high and the capability of the reservoir to release floodwater was inadequate. • The ratio of outflow-inflow sediment reached 80%, the amount of backwater deposition increased accordingly and the bed elevation at Tongguan continued to rise. Second stage of reconstruction from 1970 to 1973 • 8 bottom outlets at elevation 280 m previously used for diversions were reopened for sluicing sediment; the intake elevation of the penstocks No. 1-5 were lowered from 300 m to 287 m. • The releasing capacity of all the outlets increased from 6,100 m3/s to about 9,100 m3/s at an elevation of 315 m. – No significant backwater could accumulate immediately behind the dam in medium or minor flood conditions, and the outflow and inflow ratio of sediment had reached 105%. – A part of the reservoir capacity was restored, and the bed elevation at Tongguan dropped by 2 m. Supplementary works from 1984 to 2000 • Due to surface abrasion and cavitation, bottom sluices no. 1 to 8 underwent repairs from 1984 - 1988. – the total discharge capacity of 8 bottom sluices was reduced by about 471 m3/s due to compression. • Two more bottom sluices, no. 9 and 10, were opened in 1990 to compensate for the reduction resulting from bottom sluice repairs. • Penstocks no. 6 and 7 were converted back to power generation in 1994 and 1997, respectively. • The last two bottom sluices, no. 11 and 12, were opened in 1999 and 2000, respectively. Front View of the Outlet

Structures Before

300

12 deep holes 8 penstocks

After

12 deep holes 1 flushing pipe 2 penstocks

300 300 290 280 2 tunnels 5 penstocks 12 bottom sluices 27 Outlets at Sanmenxia Dam

Outlet Dimensions Total Serial Invert Q at 335m number number elevation3 (m) (m /s) Deep holes w×d=3×8 m 12 1-12 300 503/each Bottom sluices w×d=3×8 m 12 1-3 280 497/each Tunnels D = 11 m 2 1-2 290 1,410 Flushing pipe D = 7.5 m 1 8 300 290 Penstocks 2 6-7 300 230/each 5 1-5 287 210/each

27 16,620 Reservoir Outlet Capacity

340 1960 1968 1973 330

320 315m 310

300 Elevation (m)

290

280 0 2000 4000 6000 8000 10000 12000 14000 16000 Outlet Discharge (m^3/s)

30806100 9101 Two Tunnel Outlets Four Remodeled Penstocks Eight Bottom Outlets at 280 m Releasing Sediment Through the Bottom Sluices

No significant backwater would occur immediately behind the dam in case of medium and minor floods Different Modes of Operation

1. Storage ( Sep. 1960 to Mar. 1962 ) Initial period of reservoir impoundment, when the reservoir was operated at high storage level throughout the whole year. 2. Flood detention ( Mar. 1962 to Oct. 1973 ) Period of flood detention and sediment sluicing, water being released without any restrictions. The reservoir was operated at low storage level throughout the year. Different Modes of Operation 3. Controlled release (Nov. 1974 to present) Period of impounding relatively clear water in non-flood seasons (Nov.-June) and discharging the turbid water in flood seasons (July-Oct.). The reservoir has been operated at high water level in non- flood seasons, and at low storage level during flood seasons, and all outlets were to be opened in time of flood peaks to sluice the sediment as much as possible. Typical Operation Schemes

340 Non-flood season Flood season 335

330 1960-1961 325 320 1974-2001 315

Elevation (m) Elevation 310 1963-1973 305 300 11 12 1 2 3 4 5 6 7 8 9 10 295 0 50 100 150 200 250 300 350 Time (day) Variation of the Pool Levels

I II III 335

330

325

320 Non-flood season 315

310 Water year 305 Pool level / m

300 Flood season

295 Storage 290 Flood detention Controlled release 285 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 Time / year Accumulated Deposition in the Reservoir I II III 4.0 ) 3 3.5 m

9 Tongguan to dam 3.0

2.5 Longmen to Tongguan 2.0

1.5

1.0 Lower Wei River 0.5 Accumulated deposition (10 0.0 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 Time (year) Variation of Reservoir Capacity 7 I II III ) 3

m 6 9

5 4 Pool level < 330 m 3 2 Pool level < 323 m 1 Reservoir storage capacity storage (10 Reservoir 0 1960 1965 1970 1975 1980 1985 1990 1995 Time (year) Cross-Sectional Profiles

Cross Section No. 22 Cross Section No. 31

345 345 April, 1960 April, 1960 340 Oct., 1961 340 Oct., 1961 335 Oct., 1964 Oct., 1964 335 330 Sept., 1973 Sept., 1973 ）） Oct., 1995 m 330 Oct., 1995 m 325 （（ 320 325

315 320 Elevation Elevation 310 315 305 310 300 305 295 500 1000 1500 2000 2500 3000 3500 4000 1700 1900 2100 2300 2500 2700 2900 Distance（m） Distance (m) Longitudinal Profiles

340 )

330 CS 48 320 CS 41 310 CS 37 April, 1960 CS 31 300 Oct., 1961 Oct., 1964 CS 22 290 Sept., 1973 Oct., 1995 Average channelAverage bottom elevation (m CS 12 280 150 125 100 75 50 25 0 Distance from dam (km)

Tongguan’s elevation and sedimentation in the Weihe River The Weihe River is the largest tributary of the Yellow River, which empties itself into the Yellow River at Tong-guan. The lower Weihe River is in the backwater region of Sanmenxia Reservoir and the lower Weihe River basin is a densely populated area. The so called Tong-guan elevation is defined as the stage of flood discharge 1000 m3/s at Tong-guan station, which is in fact the datum of the bed profile of the Weihe River. Tong-guan elevation has been increasing since operation of the Sanmenxia Reservoir, which resulted in the fast sedimentation in the Weihe River and flooding risk to Xian, (ancient capital city of China).

Volume distribution of sedimentation per length in the lower Weihe River

草滩堤防-临背差连泊湾土地盐碱化—地下水位长期抬高 7

Flood stage rise (m) rise stage Flood 250m3/s 1 3000-5000m3/s 0 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 year

Flood stage rise (present flood stage minus the stage at the same discharge before the reservoir) due to sedimentation in the lower weihe river • Sanmenxia Reservoir demonstrates the perils of giving inadequate consideration to sediment management in the planning and design of a reservoir. • Because the problems of siltation and induced flooding risk to the lower Weihe river has not been solved, decommission of Sanmenxia dam is under discussion as an alternative strategy to eventually solve the problem. • Assume the Tongguan’s elevation will be reduced by 2 m, can the sedimentation and flooding in the lower weihe river be solved and mitigated? Table1 Computed bank-full discharge after reduction in Tongguan’s elevation

Bank full discharge(m3/s) Section No. Present 5 years 10 years 15 years

Huaxian 1800 2872 3085 3239

Weinan 1650 2104 2309 2555

Lintong 3244 3772 3824 3915 347 10 Yr 15Yr 345 5Yr 5500 7050 8650 10800 343 Present 341 3500 Present 2500 5 Year 339 10 Year

Water stage(m) 1000 337 (a) WY10 15 Year Bankfull discharge 335 250 0 2000 4000 6000 8000 10000 12000 Discharge (m 3/s) 362 10Yr 15Yr 360 5Yr 10800 5500 7050 8650 358 Present

Present 356 3500 5 Year 2500 10 Year

354 Water stage (m) (c) WY26 1000 15 Year 352 250 Bankfull discharge 0 2000 4000 6000 8000 10000 1200

0 Discharge (m3/s) Fluvial processes in the Weihe River and lower Yellow River 3.0 'I' 2.5 1960-1969

2.0

1.5 /(km.a) 3 1.0 m 6

0.5 Vs/10 0.0

-0 .5 Huaxian Lintong Xianyang -1 .0 0 5 10 15 20 25 30 35 40 N o. of cross section (W Y N o.)

1.5 1969-1973 1.0 'I '

0.5 /(km.a) 3 m 8 0.0 Vs/10 -0 .5 '1 '

Huaxian Lintong Xianyang -1 .0 0 5 10 15 20 25 30 35 40 N o. of cross section (W Y N o.)

0.6 'II' (c ) 1976-1980 0.4 /(km.a) 3 0.2 'I ' m 6

10 0.0 s/ V -0 .2 '1 ' Huaxian L intong Xianyang -0 .4 0 5 10 15 20 25 30 35 Sedimentation and erosion per length in 1960-1969, 1969-1975 and 1975-1980 Sedimentation in the Weihe river due to rising Tongguan’s elevation Equilibrium Sedimentation Model

Assume there is an equilibrium sedimentation volume, Ve,, for a given increment of Tongguan’s Elevation. The rate of sedimentation is proportional to the difference between the equilibrium and real sedimentation volume: dV (6.3) = K(V −V ) dt e V=+ e−Kt[] KV e Kt dt const ∫ e (6.4)

The equilibrium sedimentation volume Ve is proportional to the enhancement of Tongguan’s ElevationΔZt,, which is given by ΔZt=Zt -323.5, in which Zt is Tongguan’s Elevation at time t and 323.5 m is the Tongguan’s Elevation before the dam. Simply the equilibrium sedimentation volume can be imagined to have a shape like a cone, then it may be assumed

Ve= AΔZt/2 (6.5) In which A is a representative area of riverbed and floodplain on which sedimentation occurs. Substituting equation (6.5) into (6.4) yields, (6.6) t V = 1 AKe −Kt [ ΔZ e Kt dt − ΔZ ] 2 ∫ t t 0 1.5 ) 3

1.2

0.9

0.6 Measurement Calculation with ΔZ=5m t Sedimentation volume (bi m 0.3 Calculation with Varying ΔZ t

0.0 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 Year Fig. 6.49 Calculated cumulative sedimentation volume with Eq. (4) (solid curve) in comparison with the real sedimentation volume (pyramids). The dashed curve in the figure is the calculation result with the value ofΔZt remaining unchanged at 5 m Fluvial processes in the lower Yellow River N % Tiexie Peiyu

(a) Date: 1960 8.27~29

Tiexie

Peiyu

(b) Date: 1963 7.24~26

Tiexie Peiyu

0 24km (c) Date: 1964 11.17~22

Fig. 6.51 Channel morphology of the Tiexie-Peiyu reach (157-189 km from Sanmenxia) pre- and post-Sanmenxia Dam 9 6 1970s Number of 3 meanders 60 3—6 6—9 9—12 12—15 15—18 18—21 21—24 24—271980s 27—30 3 河弯跨度(km) 0 3—615 6—9 9—12 12—15 15—18 18—21(c) 90-02 21—24年河弯跨度统计图 24—271990s 27—30 12 河弯跨度(km) 9 6 3 0 3—6 6—9 9—12 12—15Wavelength 15—18 of meanders 18—21 (km) 21—24 24—27 27—30 河弯跨度(km)

Fig. 6.52 Numbers of meanders with different wavelengths in a 400 km long reach downstream of Sanmenxia Reservoir in the 1970s, 1980s, and 1990s