**4. Flood control in Dong-Ting Lake, China (too much)**

Different from the Yellow River basin where droughts (too little) and high turbid water (too turbid) are two key challenges for water resources managers, the Yangtze river basin where the Dong-Ting lake is located often has the threat of flood (too much). Yangtze River is the third longest river in the world, and its catchment area is about 1.91×106km2. The average annual water discharge is 918km3 and average annual load of suspended sediment is 500×106 t (1956-2003, Zhang 1995), respectively the fifth and fourth in the world (Milliman and Meade, 1983; Milliman and Syvitski, 1992). Its middle reach is known as the country of a thousand lakes that serve as an efficient flood regulator, among which Dongting is the largest lake. Due to sedimentation and reclamation over the past 50 years only few lakes have been left in this region, and the total capacity has been significantly shrunk. This region plays an important role in China's economic development, and this can be inferred from the historical proverb "If southern and northern regions of Dongting Lake harvest, the whole China's prosperity is ensured"; but if this region suffers from flooding, the whole China's economical development will be badly affected.

Dongting Lake, used to be the largest freshwater lake in China before 1950. Now it has been shrunk to be the secondary largest lake in China due to high sedimentation rate. The lake valley is surrounded by mountains on the east, south, west and Yangtze River on the north. The population in this basin in 1997 was 15 million. Many rivers drain into the lake from mountainous south and west, besides three channels connect the lake to the Yangtze River, which dumps a major proportion of the sediment suspended from Yangtze River to the lake when the lake accommodates floodwater. The lake has only one outlet on the east side.

The engineers and decision makers realized that there was no hope to solve the sedimentation problem without separation, then they divided the dam's operation into two

1. in flood seasons (July–October), turbid water is discharged to the downstream by lowing the water level at 305m, thus the flow velocity is high enough to carry the highly suspended load. In this period, if the flood discharge is greater than 2500 m3/s, all the

2. in non-flood seasons (November–June), the relatively clear water is stored and the reservoir is operated at a high operational level, with the maximum level not exceeding a value between 322 m and 326 m, to store water for irrigation and hydropower

After the separation, the new design was found effective, till now, the dam is still providing the basin with flood control, irrigation, and hydropower generation, even though some benefits are lower than the original design. The engineering experiences and management practices of Sanmenxia Dam are valuable assets to the sustainable management of reservoirs/lakes on sediment-laden or nutrient-rich rivers. It can be seen that the above operation strategy is similar to the SPP introduced in this chapter as the unwanted water (high turbid floodwater) has a short residence time in the reservoir, and the clean water in non-flood season (wanted water) has a relative long residence time for the eco-system. This project not only demonstrated the perils of existing water management, but also demonstrated that a sediment balance may be achieved across an impounded reach even in

Different from the Yellow River basin where droughts (too little) and high turbid water (too turbid) are two key challenges for water resources managers, the Yangtze river basin where the Dong-Ting lake is located often has the threat of flood (too much). Yangtze River is the third longest river in the world, and its catchment area is about 1.91×106km2. The average annual water discharge is 918km3 and average annual load of suspended sediment is 500×106 t (1956-2003, Zhang 1995), respectively the fifth and fourth in the world (Milliman and Meade, 1983; Milliman and Syvitski, 1992). Its middle reach is known as the country of a thousand lakes that serve as an efficient flood regulator, among which Dongting is the largest lake. Due to sedimentation and reclamation over the past 50 years only few lakes have been left in this region, and the total capacity has been significantly shrunk. This region plays an important role in China's economic development, and this can be inferred from the historical proverb "If southern and northern regions of Dongting Lake harvest, the whole China's prosperity is ensured"; but if this region suffers from flooding, the whole China's

Dongting Lake, used to be the largest freshwater lake in China before 1950. Now it has been shrunk to be the secondary largest lake in China due to high sedimentation rate. The lake valley is surrounded by mountains on the east, south, west and Yangtze River on the north. The population in this basin in 1997 was 15 million. Many rivers drain into the lake from mountainous south and west, besides three channels connect the lake to the Yangtze River, which dumps a major proportion of the sediment suspended from Yangtze River to the lake when the lake accommodates floodwater. The lake has only one outlet on the east side.

outlets of the dam will be opened to flush the sediment as much as possible.

a large reservoir built on a river with an extremely high sediment load.

**4. Flood control in Dong-Ting Lake, China (too much)** 

economical development will be badly affected.

time periods or two different modes of operation:

generation in non-flood seasons;

This lake annually receives 173 million tons of sediment, 83% of which are carried into the lake from the Yangtze River, and 17% from other inflow rivers. Over the past 150 years, the lake area has reduced from about 6000km2 to about 2600km2 due to both natural siltation and human activity, consequently flood modulation of the lake capacity has been reduced significantly from about 30km3 in 1949 to 17km3 in 1995, about 50% of the lake's capacity has been silted. It is certain that the lake will continue to shrink, and in the near future it may serve only as a river channel during the non-flood reasons (Du et al. 2001). In another 50-100 years, this lake may totally disappear if there is no human's interference.

Similar to other plain lakes, Dongting Lake has suffered many problems, if not solved properly, these problems may cause serious consequences to environment and bring about huge economic and life loss. These problems can be summarized as flood disasters in wet seasons (too much), water shortage for the ecosystem in dry seasons (too little), lake's shrinkage (too turbidity) and water pollution (too dirty).

Apart from too much and too turbid problems, the lake water was polluted by the incoming wastewater due to rapid industrial development and population growth and agriculture fertilization. In this fertile "land of fish and rice", farmers every year use 18k tons of pesticide and 1700k tons excessive fertilizer, which flows into the lake via the rivers. After 1990s eutrophication tendency has been accelerated, according to the monitoring data, 1996- 2005 main pollutants are total phosphorus and total nitrogen (Guo et al., 2007).

Li et al (2009) summarized the problems in the Donting Lake area as a chain:

Disaster in wet seasons : sedimentation marshland expansion water space shrinkage flood level raised and flood disaster aggravated. Disaster in dry seasons : sedimentation marshland expansion water space → → → → → shrinkage water pollution and fish resource depletion wetland and biodiversity reduction. → →

The average reoccurrence interval (ARI) of flood disaster was 83 years from 276 B.C.-1525, and ARI = 20 years from 1525-1853; ARI = 5 years from 1853-1949 and ARI = 4 year from 1949-2000, recently this area suffers the flood disaster almost every year. Obviously this is caused by the sedimentation that keeps reducing the lake's capability for flood retention (Yang, 2004).

All problems that the lake is facing can be well solved if the proposed SPP strategy is applied as shown in Fig. 6, in which the length of inner levee is about 240km. This is feasible because:

*1) Sedimentation*: in the current condition, almost all incoming sediment particles are deposited in the lake, i.e., 1730 million tons of sediment, and the lake functions like a sedimentation tank where the clean water run out of the tank. But with the inner dike, the incoming flow can be separated into wanted/unwanted waters, i.e., high turbid but safe floodwater will by-pass the lake and drain to the downstream. Only a small part of excessive floodwater is allowed to enter the lake, which can mitigate the flood disaster, and the majority of the water stored in the lake comes from the falling limb in the hydrograph shown in Fig. 2, thus clean and low turbid water is stored. Currently the lake's storage capacity is only 1/10 of the annual incoming water volume, if SPP is used and then the

Novel SPP Water Management Strategy and Its Applications 251

region was struck by severe drought, affecting residents and agriculture, and more than a million people did not have adequate supplies of water. About 709,000 hectares of farmland in the basin had been hit and no water for irrigation. The central region of the lake had become a vast grassland. On May, 2010, the water surface area was only 780km2, about 30%

But 10 months ago, in July, 2010, this lake suffered the flood disaster with the highest water level reading 32.9m above the sea level, and it had to rely on the upstream reservoir's regulation to mitigate the flood disaster. Thus, it is obvious that, without control, the lake cannot play a dominant role for droughts as the water has lost quickly after the previous flood. 2011 droughts can be avoided if the SPP strategy was applied and the floodwater in

Taihu Lake, the third largest freshwater lake in China, is located in the highly developed and densely populated Yangtze River Delta. The lake is shallow with an average depth of 1.89 m, but a large surface area of 2,238 km2, and a volume of 4.66×109 m3. The annual water input averages 7.66×109 m3 and the residence time of its waters is about 300 days. There are over 219 inflow rivers or tributaries but only three main outflow rivers. The average of rainy days is 132 days/year and the average annual rainfall is about 1,145 mm. Rainfall varies seasonally with wet seasons between June and September, i.e., during the typhoon season or flood period, and the dry seasons from October to next May and constitutes the long dry

Taihu Lake plays multifunctional roles including floodwater storage, irrigation and navigation. It also serves as a major water resource for drinking, aquaculture and industrial needs, as well as being a source of entertainment and tourist interest. Its drainage basin extends over 37,000 km2 and is bounded by the Yangtze River to the north, the East China Sea to the east and mountainous areas to the west. While the basin accounts for 0.4% of the total area of China and 2.9% of the nation's population, it provides more than 14% of China's Gross Domestic Production (GDP). The GDP per capita is 3.5 times as much as the contry's average and its urbanization level ranks the first in China. This basin is vital for eastern China, where the lake water supports more than 60 million people (about 600–900 person/km2 on average), including the water supply to cities such as Wuxi, Suzhou, and

With the tremendous economic growth and increased population in its basin, Taihu Lake has begun to suffer from various environmental stresses, including deterioration of its water quality with increasing nutrient and other chemical inputs. The lake is becoming increasingly eutrophicated and has experienced annual lake-wide cyanobacterial blooms in recent decades; this has affected the drinking water supply of surrounding cities. Taihu Lake now receives annually approximately 30,635,000 kg total nitrogen (TN) and 1,751,000 kg total phosphorus (TP) from a combination of municipal and industrial wastewaters and agricultural soil runoff; chemical oxygen demand on chromium (CODCr) is 131,223,000 kg

Consequently, algae blooms have appeared and continued. The lake is often covered by algae blooms in summer, autumn and even spring. In 2007, a severe algae bloom caused a

2010 was impounded by the inner levees and gates shown in Fig. 6.

**5. Algal blooms control in Tai-Hu Lake, China (too dirty)** 

Shanghai, one of the largest cities in the world.

of normal water surface area.

period.

(Qin et al, 2007).

sediment carried by this part of water will be deposited in the lake, approximately only 1/10 of the total incoming sediment, the remaining sediment will by-pass the lake and go to the downstream. Hence, if the SPP is applied, every year only 173 million tons of sediment will deposit in the lake, therefore the reduction of sedimentation rate will be

$$\frac{173 - 17.3}{173} = 90\%$$

Thus the SPP strategy can reduce significantly the sedimentation rate, and the life span of the lake can be extended.

Fig. 6. Simplified river-lake system for the Dong-Ting Lake

*2) Flood disaster*: it has been mentioned that the cause of flood disaster is that the lake's storage is occupied by the runoff before the arrival of peak discharge, hence the excessive flood water incurs disasters. With the strategy of SPP, the floodwater can be separated as safe water and excessive floodwater. The former refers to that flow in BPC is safe, the incoming flow rate is less than BPC's design capacity and water level is below the BPC's design highest water level. The excessive floodwater is water that overflows the rivers and incurs the flood disaster if no retention area is provided for its temporally storage. All sluice gates are closed before the arrival of the excessive floodwater, thus in the rising limb in Fig. 2, all river flows together with sediment and non-point pollutants will by-pass the lake. Once the excessive peak flow that needs a retention area appears, the sluice gates are opened, at this time the lake is still empty, thus the flood disaster can be mitigated. After that, the water level in BPC will be lowered to the safe level, then all gates can be closed again to wait for the next flood wave, or if the weather prediction shows no more heavy rains in the catchment, the gates can be opened again to store as more clean water as possible for the next drought. Thus, at the end of wet season, the lake is full of good quality water.

*3) Water shortage*. Every year, after the flood season, this basin has a long dry period, and the basin often suffers the water shortage problem in this period. For example, in 2011, this

sediment carried by this part of water will be deposited in the lake, approximately only 1/10 of the total incoming sediment, the remaining sediment will by-pass the lake and go to the downstream. Hence, if the SPP is applied, every year only 173 million tons of sediment will

> 173 17.3 90% 173 <sup>−</sup> <sup>=</sup>

Thus the SPP strategy can reduce significantly the sedimentation rate, and the life span of

Inner dike

By-pass canal

Sluice gate

(BPC)

outlet

*2) Flood disaster*: it has been mentioned that the cause of flood disaster is that the lake's storage is occupied by the runoff before the arrival of peak discharge, hence the excessive flood water incurs disasters. With the strategy of SPP, the floodwater can be separated as safe water and excessive floodwater. The former refers to that flow in BPC is safe, the incoming flow rate is less than BPC's design capacity and water level is below the BPC's design highest water level. The excessive floodwater is water that overflows the rivers and incurs the flood disaster if no retention area is provided for its temporally storage. All sluice gates are closed before the arrival of the excessive floodwater, thus in the rising limb in Fig. 2, all river flows together with sediment and non-point pollutants will by-pass the lake. Once the excessive peak flow that needs a retention area appears, the sluice gates are opened, at this time the lake is still empty, thus the flood disaster can be mitigated. After that, the water level in BPC will be lowered to the safe level, then all gates can be closed again to wait for the next flood wave, or if the weather prediction shows no more heavy rains in the catchment, the gates can be opened again to store as more clean water as possible for the next drought. Thus, at the end of wet season, the lake is full of good quality

*3) Water shortage*. Every year, after the flood season, this basin has a long dry period, and the basin often suffers the water shortage problem in this period. For example, in 2011, this

deposit in the lake, therefore the reduction of sedimentation rate will be

Fig. 6. Simplified river-lake system for the Dong-Ting Lake

the lake can be extended.

Channels

Rivers

Yangtzi River

water.

region was struck by severe drought, affecting residents and agriculture, and more than a million people did not have adequate supplies of water. About 709,000 hectares of farmland in the basin had been hit and no water for irrigation. The central region of the lake had become a vast grassland. On May, 2010, the water surface area was only 780km2, about 30% of normal water surface area.

But 10 months ago, in July, 2010, this lake suffered the flood disaster with the highest water level reading 32.9m above the sea level, and it had to rely on the upstream reservoir's regulation to mitigate the flood disaster. Thus, it is obvious that, without control, the lake cannot play a dominant role for droughts as the water has lost quickly after the previous flood. 2011 droughts can be avoided if the SPP strategy was applied and the floodwater in 2010 was impounded by the inner levees and gates shown in Fig. 6.
