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Chapter 20. Analysis of storage for the regulation of surface water in the Huang-Huai-Hai plain for south-to-north water transfer
Hebei Institute of Geography
Henan Institute of Geography
STORAGE is absolutely necessary for any south-to-north water transfer. It would serve to reduce the coverage of the main canal, economize on project investment, make better use of the transferred water, raise the rate of utilization of water resources and ensure water use in the north during the arid season without affecting the upper reaches in the south.
ANALYSIS OF SURFACE STORAGE CAPACITY
There are three main forms of storage of water resources: (1) surface reservoirs and ponds; (2) reticulated reservoirs on the plain formed by natural channels, deep trench waterway networks, ponds, lowlands and shallow lakes; and (3) underground reservoirs. This chapter deals with the second category (see Figure l).
River channel storage is carried out by constructing gates in suitable reaches to regulate water. As long as they ensure safe discharge of flood waters and drain excess surface water, these plains river gates fully retain surface water and replenish the groundwater for irrigation development.
On-site investigations and observations of the eastern Henan plain indicate that a gate built on a large river (with a drainage area greater than 3,000 km²) allows the channel to be used as a reservoir which can store about 10 to 50 x 106 m³ at a time, capable of irrigating 20,000 to 27,000 ha, with a maximum backwater length of 40 to 50 km, a lateral seepage distance of 3 to 5 km from both banks and a possible recharge of groundwater of approximately 20 x l 06 m³. A gate constructed on a medium river course (with a drainage area of 500 to 3,000 km²) can store 3 to 8 x 106 m³ at a time, capable of irrigating 7,000 to 13,000 ha, with a backwater length of 15 to 25 km, a lateral seepage of 2 to 4 km from both banks and a groundwater recharge of about l to 3 x 106 m³. A gate on a small river (with a drainage area smaller than 500 km²) can store 0.5 to 2.0 x 106 m³ at a time, capable of irrigating 700 to 1,300 ha, with a backwater length of about 10 km, a lateral seepage of 1 to 2 km from both banks and a possible groundwater recharge of 200,000 to 800,000 m³. It is clear that river gates play a very significant role in retaining surface water and replenishing the groundwater. According to preliminary estimates, the existing gate and embankment facilities along the principal rivers of the east Henan Plain have a surface water storage capacity of 300 to 400 x 106 m³ or more.
Figure 1. System of Water Regulation and Storage in the Proposed Water Transfer Region
The Huaibei district of Anhui Province is located in the middle and lower reaches of the Huai He, where there are numerous tributaries and quite a bit of channel storage capacity. According to incomplete statistics, over 20 gates with a total storage capacity in excess of 500 x 106 m³ have been constructed on the Shaying, Guo, Quan, Tuo, Sui, New Cihuai and New Bian He.
Preliminary figures indicate a total storage capacity of about 2 km³ in the river courses in four sectors south of the Huang He. These are eastern Henan, northern Anhui, northern Jiangsu and the ancient courses of the Huang He.
One method of storage in the Hai He Plain north of the Huang He is to construct gates on some of the key drainage courses. One of these is the Wuqiao gate which was built in October 1972 on a fork of the flow of the New Zhangwei He. This gate can utilize the main river channel to retain some of the water at the end of the flood season to provide for irrigation and other uses. This measure has played a definite role in rapidly bringing the farmland along both banks under irrigation. The water retaining depth of the channel is about 6 to 7 m with a storage of over 10 X 106 m³, enough to irrigate over 3,300 ha.
Storage should be combined with the deepening of the river channels for the drainage of excess surface water and the removal of salts (e.g. the Nanpai He has been deepened by 7 m). According to preliminary calculations, if the river channels on the Hai He Plain are enlarged to meet the standards for draining excess surface water from 250 mm within three days of rainfall, these channels can also be used to store 600 to 700 x 106 m³. Table 1 provides a breakdown of storage capacity for specific rivers.
Deep Trench Waterway Network Storage
Some areas on the Huang-Huai-Hai Plain have already developed networks of deep trenches to a considerable extent. The bottom depth of these networks is 1 to 3 m lower than groundwater during the dry season. This provides the trenches with quite a large capacity for storing surface water, in most cases 50,000 to 100,000 m³/km² but sometimes even greater. Lianchi Commune in Shenqiu County, eastern Henan, has dug over 50 trenches involving 3.6 x 106 m³ of earthwork and having a total storage capacity of 2.7 x 106 m³, which can supply the irrigation needs of the entire commune for 15 to 20 days.
Because of its depth, a trench network has not only a large storage capacity but also very good effects in draining excess surface water and controlling salinity. A medium-sized trench (4 m deep, 3 to 4 m wide) has a cross-section sufficient to drain a 1 0-year flood, while a large trench can reach a 5 to 10-year drainage standard. This greatly exceeds the drainage capability of ordinary river channels.
In addition to storage and drainage, deep trenches also serve as a recharge network for underground reservoirs. They can effectively and beneficially connect precipitation, surface water, groundwater and water imported from other catchments.
Table 1. Water Storage Capacity of River Channels on the Hai He Plain
|New Zhangwei He||0.14|
|Old Zhang He||0.01|
|Old Yan He||0.03|
|Old Sha He||0.01|
|Main branch bf Gang He||0.01|
Ponds provide another of the main forms of storage on the plain. They require little investment, have quick returns, are not restricted in surface area, can be built on their own by the local populace and are easy to popularize. Their ubiquity may be seen in the popular saying that "if there is a village there is bound to be a pond".
Ponds can be connected with rivers, deep trench networks and shallow lakes to form a "melons-on-a-vine" type water system which can both store and drain. They can also be combined with back-irrigation projects to serve as an important path for return recharge of the groundwater and for incorporating surface and groundwater storage.
The enormous storage potential of ponds is shown in data from Zhoukou Prefecture in eastern Henan, where there are ponds and lowlands with a surface area of 20,000 ha in a total area of 11,637 km². If they were rebuilt and all excavated to a depth of 4 m, they could store more than 400 X 106 m³ of water (calculated on the basis of a storage depth of 2 m).
Rainfall runoff observations from the Huxi (west of the lakes) area in Jiangsu show an average of four runoffs during a year's flood season. If 2 ha of ponds were sited in each square kilometre of the catchment area, 80,000 to 90,000 m³ could be stored.
Ponds are widely used for storage throughout the Hai He Plain and there is a wealth of experience with them there. Preliminary investigations indicate that the total storage capacity of ponds is about 150 X 106 m³ in Tianjin, 210 x 106 m³ in Yundong (east of the Grand Canal) and 200 x 106 m³ in Heilonggang.
Preliminary estimates are that the present storage capacity of ponds in the Huang-Huai-Hai Plain is about 1.26 km³ (Table 2).
Table 2. Pond Storage Capacity
|Region||Tianjin||Yundong||Heilonggang||E. Henan||N. Anhui||N Jiangsu||Total|
|Storage capacity (km³)||0.15||0.21||0.20||0.25||0.30||0.15||1.26|
Storage in Lowlands and Shallow Lakes
There are few reservoirs or lakes north of the Huang He and only 1.0+ km³ can be stored in river channels and ponds, far from satisfying storage requirements. Therefore the lowlands and shallow lakes, which store as much as 1.69 km³, constitute the mode of storage with the most sizeable capacity in the Hai He Plain. The largest of these, with individual capacities of roughly 200 x 106 m³ or more, are Beidagang, Tuanpowa, Baiyangdian and Qianqingwa.
Among those with less than 100 X 106 m³ are Xiaonanhai, Yongnianwa, Dalangdian, Nandagang and Yangcheng (Figure 2). Lowlands and shallow lakes occupy 7-10 per cent of the surface area of the Hai He Plain, so their development potential is quite large.
Surface storage on the Huang-Huai-Hai Plain has an extremely important role to play in the proposed south-to-north water transfers. First, it can provide full storage for locally discarded water. Second, it can collect northward transferred water during the non-irrigation season for use during the irrigation season when water use is large and concentrated and there is a seasonal shortage of water sources. Third, by evening out the import flow in the river channels as much as possible, engineering costs can be reduced by reducing the number of dredging, pumping and transporting projects.
ENVIRONMENTAL EFFECTS OF SURFACE STORAGE AND THE PRINCIPLES OF PROJECT PLANNING
We feel that in the future it will be necessary to hold to the following principles and measures in planning surface storage in order to avoid the unfavourable environmental effects of excess surface water and salinity.
Combined Drainage, Storage and Irrigation, with Drainage as the Most Fundamental
The most important measure in artificially regulating and controlling the watersalt movement and balance in a region is drainage. Thus when the feasibility of various types of surface storage are considered, one of the most important criteria is to see whether or not they raise the standards of draining floods and accumulated surface water. This is particularly important in the flat Huang-HuaiHai Plain with its concentrated rainstorms. Excellent results have been achieved in practice when drainage, storage and irrigation have been combined in the planning and design of surface storage projects, with drainage as the most basic function.
Combine Wells and Canals to Regulate Groundwater Depth at the Appropriate Times
Surface storage on the plain will inevitably lead to an increase in downward and lateral seepage, causing the water table to rise. There are two measures to control the rising of groundwater-digging drainage ditches and using wells for both irrigation and drainage. In many regions of the HuangHuai-Hai Plain there are numerous difficulties in digging relatively deep drainage ditches due to the sandiness of the soil. A more effective measure is to combine wells and canals on the basis of a sound drainage system. Well irrigation can facilitate drainage (through well irrigation and drainage) while effectively raising the utilization rate of water resources. Some irrigation districts affected by the proposed northward water transfer are suited to the joint utilization of wells and canals.
Figure 2. Sketch Map Showing the Distribution of the Main River Courses and Depressions in the Hai He Plain
Deal with Each Area in Keeping with Local Conditions
The success or failure of storage on the plain depends on strictly controlling the height of storage. Because of regional variations in natural conditions from south to north and from west to east on the broad expanse of the Huang-Huai-Hai Plain, the forms of storage and the appropriate height of storage are dissimilar. For example, in that part of the plain lying south of a line connecting Zhoukou and Su counties, precipitation is greater, leaching is stronger and soil salinity and mineralization of the groundwater are both quite low, so there is little danger of soil salinization. Water may be stored in this area at any time except during the flood season. To the north of this line, however, soil salinity and groundwater mineralization are relatively high and the height of surface storage should be strictly maintained below the critical depth of salt accumulation in the soil. In those areas where soil salinity is relatively serious and underground saline water is found throughout, river channels for storage diversion should be separated strictly from the trenches for saline drainage to allow free and unimpeded drainage.
The main environmental effects to be studied in making use of lowlands and shallow lakes to store water are how to prevent the rise in surrounding water table and how to protect the enclosing dikes from storm waves. There is indeed quite a bit of experience in these areas throughout the Hai He Plain, where seepage interception ditches which are normally 2.5 to 3.5 m deep are dug around the lowlands and shallow lakes. Water flows in these ditches must drain unimpeded (generally by pumping). Several kilometres outside the interception ditches, a certain density of shafts is provided to control the water table via the method of joint irrigation and drainage with wells. In order to protect the safety of the dikes and prevent storm waves from eroding the embankments around the lowlands and shallow lakes, strip fields, dikes and surrounding embankments may be built and reeds and trees planted within the lowlands.
Establish Storage Systems and Strengthen Scientific Management
Water is one of the most active elements in the ecological environment. Strengthening the management and control of water is a central link in the comprehensive control of drought, flooding and salinity. Poor management led to a very large portion of the problems which appeared in the past, such as excessive high storage levels and imbalances between storage and drainage. Henceforth we must establish and strengthen storage management systems so that the various kinds of storage can be operated scientifically and environmental effects can be improved continuously.
(1) It will be especially important to do a good job of water storage after the south-to-north transfer project has gone into operation, in order to reduce the coverage of the main canal, economize on the project and raise the rate of water utilization.
(2) Investigations, observations and experiments carried out over a number of years have shown that, in addition to reservoirs and lakes, natural river channels, deep trench waterway networks, ponds and lowland shallow lakes are effective modes of storage on the Huang-Huai-Hai Plain. These types of storage have a large capacity, require little investment and yield benefits quickly.
(3) Water storage has a very large impact on the natural environment. In the. unique ecosystem of the Huang-Huai-Hai Plain, drought, flooding, alkalization and salinization are important natural factors affecting agricultural production. Therefore, when drawing up the principles and measures for storage project planning, we must give consideration to environmental effects in the immediate surroundings and strive to achieve the goal of having useful storage without flooding or salinization.
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