**2.2 Reservoir removal impact classification**

Reservoir removal may reshape or even destroy rivers and coastal ecosystems, causing a series of new problems. In this section, hydrological sediment,

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*Dam Retirement and Decision-Making DOI: http://dx.doi.org/10.5772/intechopen.84392*

*2.2.1 Hydrological influence*

situation of the river.

*2.2.2 Sediment impact*

topography, water quality, ecological environment, social and economic aspects,

The construction of the dam will lead to changes in the flow rate, change frequency, duration, occurrence timing, and change rate of the hydrological situation of the river. After the dam is dismantled, the flow rate and water level of the river will change with the seasons, get rid of human intervention, and return to the natural hydrological situation, which will completely reverse the hydrological

After the dam is removed, the river will be reconnected, and the lower reaches of the river will be continuously restored. The shrinking condition of the lower reaches of the river will be alleviated, and the groundwater supply in the lower reaches of the river will be basically restored to nature. However, if the downstream channel has insufficient sediment transport capacity, sediment deposition, and riverbed elevation, the downstream section flow capacity decreases, the channel specific decline becomes slow, the flood discharge capacity weakens, and the reservoir regulation and storage protection are lost; the downstream river level is raised during

Sedimentation is one of the main causes of reservoir removal in China. After the dam is removed, the sediment in the reservoir area will move again. Sediment deposition is the result of the decrease of water flow carrying capacity which is controlled by the backwater effect and velocity of reservoir. If the operation time of reservoir dam is short or the impact of dam on sediment transport is small, the impact of removal on sediment transport is relatively small. On the contrary, the law of sediment transport will change greatly after the reservoir is scrapped. When a small radial reservoir is abandoned, the silt deposited may be mostly carried downstream by the current. After the reservoir with large capacity is scrapped,

Sediment in reservoir area is transported downstream with current, which not only increases turbidity of downstream river segment but also usually leads to sediment deposition in downstream river segment and changes topography of downstream river channel. Fine sediment may cover the original habitat, block the gap between the bed matrices, and destroy the spawning habitat of fish, resulting in the death of fish. It may also block downstream waterways and water intakes, which

When pollutants are contained in the sediment, the sediment carrying pollutants to the downstream diffusion after dam removal is bound to have a significant impact on the downstream river habitat. The content of fine sediment and the way of land use upstream are the important factors influencing the pollutant load in the reservoir. This is because fine-grained sediment has a large specific surface area and can absorb more pollutants than coarse sediment. In addition, the upper reaches of the reservoir land use mode can directly affect the reservoir sedimentation, sediment gradation, and pollutant content. Studies have shown that in the basins dominated by agricultural production, the riparian soil is eroded, and the nonpoint source pollution of the river is serious, resulting in a large amount of fine sand and rich nutrients in the silt in the reservoir area. For the watershed dominated by forest land, the sedimentation amount of reservoir is usually small, and the nutrient

and the impact of reservoir removal are classified and analyzed.

the flood period, increasing the flood risk in the downstream region.

there may still be a large amount of sediment in place.

will adversely affect human production and life.

content of sediment is low [1, 2].

topography, water quality, ecological environment, social and economic aspects, and the impact of reservoir removal are classified and analyzed.
