Chapter 21. Using ancient channels to regulate water through storage: the example of the Hebei plain

Wu Chen and Wu Jinxiang
Hebei Institute of Geography

THE WATER of the south-to-north transfer proposals may be regulated in part through underground storage (for surface storage, see Yu Fenglan and Wang Wenkai, Chapter 20). For example, there are many ancient channels buried at a depth of 30 to 50 m under the plains of Hebei Province whose sand and gravel layers can provide ample storage capacity for groundwater. In recent years, the authors have engaged in investigations, prospecting and experiments on these channels and have formed the following views.

THE SAND AND GRAVEL LAYERS OF THE ANCIENT CHANNELS PROVIDE AMPLE GROUNDWATER STORAGE CAPACITY

The regulation and storage of groundwater is carried out by means of underground reservoirs. These are porous underground rock layers and water storing formations, either natural or artificial, from which extra groundwater is extracted (to empty the reservoir) or artificially supplied (to fill the reservoir) in order to convert atmospheric rainfall into surface water and groundwater thus making total use of all water resources (Wed and Wu, 1979).

The Hebei plain south of Beijing and Tianjin was formed during the late Pleistocene period primarily by the repeated inundations and silting of the Huang He, Zhang He, Hutuo He and Yongding He when they breached their banks and changed channels. This process left behind many ancient channels. The sand and gravel layers of these channels provide ample groundwater storage capacity for the following reasons:

(1) The ancient channels have ideal rock strata for storage. According to prospecting data, the water-bearing rock characteristics of these channels are highly porous gravel, coarse-medium sand (in the alluvial fan area), mediumfine sand, fine silt (in the river channel zones) and silt (in the delta). Their base is at a depth of 30 to 50 m. The sand layer is generally 10 to 30 m thick with a water yield of 10 to 20 per cent. The soil layer is 20 m thick with a water yield of 5 to 10 per cent. The total storage capacity is 1.5 to 3.0 x 10⁶ m³, and the storage capacity of a 6 m fluctuation in the water table is 0.3 to 0.5 x 10⁶ m³.

There are certain water storage formations in the water-bearing rock layers of the ancient channels. They have a 2 to 5 m thick clay base with few cracks which are shaped like shallow dishes (main channels of the ancient rivers). This provides an ideal reservoir bottom. Where the sand layer pinches along both sides, the clay changes rapidly to alternating layers of subclay and subsand. This is a relatively good reservoir bank. There are no good water-blocking formations along the ancient channels however. In the narrow stretches of the ancient channels, especially near the estuary, the building of underground dams can be considered. But gradient development requirements rule them out in the alluvial plain because of the slow flow of groundwater in the high water table period before extraction (0.2 m per 24 furs, according to flow tests at the Nangong underground reservoir).

(2) There are an average of 8 to 10 extractive wells per km² on the alluvial fan of the Hebei plain with an average yield of about 50 m³/hr. On the alluvial plain, 70 to 80 per cent of the extractive wells are located on the ancient channels, averaging one well per 7 to 8 ha with an average yield of 20 to 50 m³/hr.
(3) The ancient channels have a relatively convenient source of recharge, as they commonly serve as flood channels or aqueducts.
(4) The ancient channels have good paths of infiltration. This is because: (a) On the surface they are usually sandy soil or sandy loam. Some are even sand dunes or river beaches; (b) Most of the ancient channels are interlinked with present rivers or canals, which is beneficial to lateral recharge of the groundwater. According to the data of the Bureau of Geology, Hebei Province, the rate of infiltration in medium sand is 12 to 18 m/day; fine sand, 6 to 10 m/day; and silt, 4 to 6 m/day. The lateral area affected by seepage of the Shijin main canal, which is linked together with the ancient Hutuo channel, can reach up to 4 km on each side.
(5) The ancient channels have the necessary drainage outlets. Generally speaking, in the Hebei plain the high land of the ancient river beds alternates with the low land between the ancient rivers. In the high land of the ancient beds, there is often a difference in geomorphic types between the ancient channels with banded uplands and those with trough depressions. The bottom land of areas between the ancient channels are large drainage channels (such as the Laoyan He). The ancient channels of the trough depressions are small drainage channels such as the Dongsha He.

ESTIMATION OF THE STORAGE CAPACITY OF THE ANCIENT CHANNELS OF THE HEBEI PLAIN

Underground reservoirs storing water in the sand and gravel of the ancient channels are distributed throughout the Hebei plain south of Beijing and Tianjin (Figure 1). They have three geomorphological types: the alluvial fan, which is the best, followed by river channel belts and deltas.

We must proceed in a planned and measured fashion in our exploitation and utilization of this underground storage capacity, and first develop the better reservoirs. Therefore, we have relied primarily on the lithological characteristics and the infiltration paths to select those underground reservoirs of the alluvial fan and river channel types which have the largest capacities and best sections for infiltration so as to be the most suitable storage sites. Their regulation storage capacities have been estimated (Figure 2).

There are 41 of these reservoirs with a total surface area of 22,367 km² and a regulating capacity of 12.726 km³ between 2 and 8 m. Of this, 4.243 km³ is natural storage capacity which can be supplied through atmospheric precipitation (i.e., the capacity when the water level varies between 2 and 4 m which can be recharged through the atmospheric precipitation in an average year). The remaining 8.483 km³ is artificial storage capacity (i.e., that capacity within a variation of 4 to 8 m which requires artificial recharge).

Of these 41 most suitable storage sites, 24 are within the irrigation districts of the Middle Route of the proposed northward transfer project. These have a total surface area of 19,988 km², a natural storage capacity of 3.909 km³ and an artificial storage capacity of 7.816 km³. Twelve sites are within East Route irrigation districts, with a total area of 1,643 km², a natural storage capacity of 0.23 km³ and an artificial capacity of 0.461 km³. The remaining five reservoirs lie between the Ziya He and the Zhulong He and have a total area of 736 km², a natural storage capacity of 0.104 km³ and an artificial capacity of 0.206 km³.

Cones of depression have already appeared in the first aquifer of areas along the front and lateral edges of the alluvial fan. When these cones are in high water tables, they extend over 1,330 km² and the water table is 9 to 12.43 m deep, with a storage capacity of 0.259 km³. When the water table is low, the surface area is 2,676 km², the water table is 11.78 to 18.49 m deep, and the storage capacity is 0.589 km³. This capacity urgently needs to be recharged.

SUGGESTIONS ON PROPOSALS FOR UNDERGROUND STORAGE IN THE ANCIENT CHANNELS

We advocate that all of the sand and gravel layers of the alluvial fans and the sand layers of the ancient channels be used as shallow well extraction districts. Large amounts of shallow freshwater should be extracted to evacuate the storage capacity so that an underground reservoir may be constructed through artificial recharge. This will not only alleviate the problems of drought, flooding and salinity, but also simultaneously make thorough use of our water resources.

But there are some critical technical problems in using underground reservoirs in the Hebei plain for storage. These are:

(1) how to speed up the rate of infiltration, especially that of flood waters (or major amounts of transferred water);
(2) how to enlarge the storage capacity; that is, how to gradually enlarge the freshwater storage of the ancient channels while reducing the saltwater storage between channels.

To speed up the rate of infiltration, we propose:

(1) that we thoroughly utilize ancient river channels on the surface and the sandy land of the present river courses to store water for infiltration recharge into the groundwater. There are roughly 500 km² of this kind of surface in the Hebei plain. If half of that area were used for surface storage, at a rate of infiltration of 0.086 m³/day per m² (Gaocheng recharge data), 21.5 x 10⁶ m³ per day could be infiltrated into the groundwater;
(2) that the dry river beds of the piedmont be used to block and store flood waters temporarily and supply the underground reservoir of the alluvial fan with surface water via the sand and gravel groundwater layer. The hydrogeological team of Beijing Municipality has used water storage in the Daning reservoir in the valley of the Xiaoqing He to recharge the underground reservoir of the piedmont alluvial fan at a rate of 20,162 m³/day (Beijing Hydrogeological and Engineering Geology Team, 1980).

To enlarge the freshwater storage capacity, it is our opinion that the methods of "staircase storage", "layered storage" and "saline/fresh storage" should be employed.

"Staircase storage" makes thorough use of the "connected nature" of the sand and gravel layers of the three kinds of underground storage to develop the underground reservoirs in the ancient alluvial fans. It serves to increase the lateral supply from the piedmont surface reservoirs; to develop the underground reservoirs in the ancient channel belts in order to increase the lateral supply from the underground reservoirs in the ancient alluvial fans; and to develop the underground reservoirs in the ancient delta to increase the lateral supply by the underground reservoirs in the ancient alluvial fans. Finally the sand and gravel layers of the ancient channels provide a large storage capacity.

"Layered storage" thoroughly utilizes the "succession trait" of the surface and buried ancient channels to use the storage of water on the sandy land of the surface ancient channels to recharge the soil layer reservoir near the plant roots; and to use the soil layer reservoir to recharge the sand and gravel aquifers of the ancient channels. Finally, it turns an area several metres deep (according to foreign data, up to 20 m deep) into a large storage capacity.

"Saline/fresh storage" means to place primary emphasis upon canal irrigation (including both direct irrigation of the fields and groundwater recharge) and well drainage (to pump water from the ancient channels in order to irrigate the land between the channels) in the ancient river channel zones. In the areas between the ancient channels, primary stress is on well irrigation and canal drainage (of underground salt water and excess surface water) (Figure 3).

After a number of years of operating these methods, the three types of underground reservoirs and their vicinities will become a large freshwater storage area.

CONCLUSIONS

Through the above analysis, we have arrived at the following conclusions:

(1) The proposed diversion of water from south to north requires mass quantities of water to be transported over a long distance for large-scale irrigation. It will therefore be necessary to regulate the water through storage.
(2) There are numerous ancient river channels on the surface of the Hebei plain and 30 to 50 m deep. The sand and gravel layers of these ancient channels provide good paths of infiltration and ideal storage space for groundwater. We should make thorough use of this large capacity.
(3) Most of this capacity is in alluvial fans, where the paths of infiltration are also best. The river channel belts have the next best conditions. Under present circumstances, the underground storage capacity of the deltas is without practical significance for storing water.
(4) Storage capacity is greater in the irrigation districts of the Middle Route, as are infiltration conditions. Capacity is less in East Route irrigation districts and infiltration conditions are more deficient.

References

Beijing Hydrogeological and Engineering Geological Team, 1980, "Experimental research on artificial groundwater recharge," Hydrogeology and Engineering Geology, No. 1.

Wei Yongchun and Wu Jun. 1979, The Artificial Groundwarer Recharge and Underground Reservoirs. Water Conservancy and Electric Power Press.