退田还湖后鄱阳湖区洪水调蓄功能的多情景模拟

1998年以来退田还湖工程的实施,对鄱阳湖区洪水调蓄功能具有明显影响。本文利用高精度数字地形模型和防洪圩堤数据,基于GIS空间分析和水文模拟,对鄱阳湖区现状(2005年)和未来不同洪水情景下的洪水调蓄能力进行了模拟分析,分别计算了相应的蓄洪面积和容积。研究表明,2005年鄱阳湖洪水位曾一度超过双退圩堤进洪水位(18.50m),双退圩堤进洪后,湖泊面积实际扩大2385hm2,相应增加6907×104m3湖泊容积。为了研究退田还湖后鄱阳湖区未来洪水调蓄功能的可能变化,本文设置了20.50m、21.68m、22.59m三种洪水水位情景,分别对应万亩(666.67hm2)以下单退圩堤蓄洪、万亩以上单退圩堤蓄洪、1998年历史最高洪水位重现三种可能情景,对鄱阳湖区洪水调蓄功能的变化进行了模拟。研究表明:与2005年相比,三种洪水情景下,鄱阳湖区退田还湖后将分别增加蓄洪面积17325hm2,57198hm2和65573hm2,分别增加蓄洪容积71426×104m3,205545×104m3和349175×104m3,退田还湖工程可以有效缓解鄱阳湖区的防洪压力。

Scenario Analysis on the Flood Regulation Service of the Poyang Lake Region

Poyang Lake is the largest fresh water lake in China. Due to monsoon climate annual wet and dry cycles, consequent fluctuations in the Yangtze River levels, and a shallow bathymetry, Poyang Lake experiences large seasonal fluctuations in water levels with a range of 12 m between low to high water levels on average. It has a surface area of approximately 3 850 km² in the high-water season, but no more than 1,000 km² in low-water seasons. In the past decades, in order to enhance the total grain production to support the dense population, large area of wetland was reclaimed as farmland in the middle reaches of the Yangtze River. Many levees were built to encircle new farmland in low elevation areas adjacent to lakes and rivers, forming polders that are protected against flooding. In the year 1998, there were about 6,400 km of levees in the Poyang Lake Region (PLR), which serve to partially restrain the lake extent during the flood season. Suffered tremendous flood disaster in 1998, the importance of flood regulation service of lake wetlands was recognized by the policy makers in China. After the flood in 1998, a Returning Farmland to Lake Policy was launched by the central government of China and implemented in the middle reaches of the Yangtze River, especially in the PLR to enhance the flood regulation function. According to the implementation plan of the policy, levees of important polders were reinforced and some small polders were designated to be restored back to wetlands or flood retention areas. In practice, the levees are classified to several categories in terms of their importance, which in turn was determined by how much farmland the polder contains and whether the polder contained small or large urban areas. Based on high resolution Digital Elevation Model (DEMs), levee information derived from Landsat TM images and large scale topographic maps, the change in flood regulation function was assessed with the approaches of geo-spatial analysis, hydrological modeling and scenario analysis. The increased area and volume for flood regulation was calculated according to three potential flood level scenarios designed by the Returning Farmland to Lake Policy. It is indicated that the flood retention area of Poyang Lake will be increased by 17 325 ha, 57 198 ha and 65 573 ha, according to the 3 flood level scenarios as 20.50 m, 21.68 m, 22.59 m, respectively. Meanwhile, the flood regulation volume will be increased by 714.26×10⁶m³, 2 055.45×10⁶m³ and 3 491.75×106m3, respectively, compared to the baseline year of 2005. The flood regulation function of the PLR has thus been advanced.