유역관리 효율화를 위한 불투수면 지표 개발과 적용(1) = The Development of Imperviousness Index for Effective Watershed Management

다국어 초록 (Multilingual Abstract)

Urbanization of watershed exerts bad influence on circulation of water, shapes of rivers, or water quality, etc. for which urban rivers suffer from flood, dry stream phenomenon, or deterioration of water quality. Especially, watersheds adjacent to the cities receiving constant development pressure are badly affected by disordered development. Nevertheless, policies to cope with these problems are rather insufficient. Although approaches are emerging recently in Korea to deal with these problems in the aspect of watershed management, practical methodological studies still run short. So it is deemed desirable to clarify the factors affecting the water circulations or conditions of rivers in the cities, and utilize them as management indexes in the course of watershed management. Watershed management indexes include impervious surface, population density, and urban land utilization, etc., among which impervious surface index is the most adequate to be used as a method of watershed management. Impervious surface index could be the most useful index for watershed management not only because it is efficient in managing watershed, but also because management is possible through the introduction of various watershed management techniques such as reduction of impervious surface or increase of permeability ratio. Impervious surface consists of two major factors, the rooftops under which we live, work, and shop, and the transportation system connecting the rooftops. In most cases impervious area related to the transportation system exceeds those related to the rooftops(house, building, etc.) For example, impervious area related to the transportation in the independent house complex, apartment complex or commercial area along the watershed of Han River occupies 63∼70% of total impervious area. As it has been proven by lots of researches, impervious surface exerts influence on the scale of debouchment into the river, water temperature, water quality, shape of rivers, and biological diversity, etc. Debouchment is increased in proportion to the increase of impervious surface. Increased debouchment causes decrease of underground water percolation which again causes less base flux, which after all accelerates dry stream phenomenon. Except the land easily affected by soil quality or degree of slope, debouchment coefficient is closely related with impervious surface. Amount of total runoff generated in the parking lot of 500m2 (runoff coefficient: Rv=0.95) is as good as approximately 16 times the amount of those generated in the wild grassy place (Rv=0.06). Further, when the runoff increases, it is natural that a river reacts to increase its cross-sectional dimensions in order to accommodate it. This reaction is proceed either by widening the banks or by shaving the riverbed, or both of them. In the stage where the riverbed is unstable, the banks are eroded or the habitats are disturbed. The increase of impervious surface also affects the quality of water. In the impervious surface, contaminated materials are piled up which are floating in the air, discharged by vehicles, or generated by other sources. These are carried by surface runoff, then are washed off when it rains. As to the relation between impervious surface and water temperature, it is turned out that the water temperature of a river is affected by the temperature of local atmosphere. During the summer season, the level of increase of water temperature in the urban watershed is greater than in the rural watershed. It is discovered that water temperature is directly related to the ratio of impervious surface, and increase of impervious surface exerts negative impact on aquatic ecosystem, damaging the diversity of aquatic insects and fishes, and causing change in the constitution of species. There are two methods of calculating impervious surface, the one to calculate total impervious area (TIA) along the watershed, and the other to calculate effective impervious area (EIA) which is impervious surface linked with water system out of the whole impervious surface. Nevertheless EIA is more accurate, it is common to use TIA as watershed management index due to various restrictions attendant upon the former. In this study, we have reviewed the relation between the quality of rivers and impervious surfaces using both water quality analyses data and land utilization data at 23 spots along the 5 rivers, Kyungan-cheon, Tan-cheon, Joongrang-cheon, Anyang-cheon, and Bukhan-gang. The result of the regression analysis of water temperatures and impervious surface of the rivers showed F=48.88 (p=0.0001), R2=0.83, and CV=6.010. Impervious surface coefficient was 0.76832, from which you could see that the increase of impervious surface ratio has a positive(+) effect on water temperature. The result of the regression analysis of BOD and impervious surfaces showed F=86.54 (p=0.0001), R2=0.80, CV=34.47, and impervious surface coefficient = 0.34638. In the case of COD, the result of the same regression analysis showed F=46.82 (p=0.0001), R2=0.8240, CV=27.48430, and impervious surface coefficient = 0.35936. You could also see that the increase of impervious surface ratios have a positive effects on both BOD and COD. The fact that CV values are relatively high in the cases of BOD and COD means that, although impervious surface ratio, an independent variable, exerts influence on BOD of the river, it would be more explanatory if independent variables other than impervious surface ratios are introduced to the regression formula. The result of the regression analysis of T-N and impervious surface showed F=28.352 (p=0.0001), R2=0.7393, and CV=0.7132. Impervious surface coefficient in this case was 0.35, which means the increase of impervious surface ratio exerts positive effect on T-N. It is difficult to maintain quality and quantity of water at the state of those before exploitation solely by means of existing water quality management devices such as sewage or rainwater disposal arrangements or road sweeping. Effective management would be possible only when we introduce watershed management system linked with land utilization such as impervious surface management, together with the installation and operation of aforementioned arrangements. They say even relatively low impervious surface ratio (10∼15%) in the classification of watershed based on impervious surface ratio could work unfavorably to the quality of a river and cause change in the ecosystem. Once impervious surface ratio exceeds 25%, a river would become ecologically unsustainable in the aspects of stability, water quality, or biological diversity. This shows that watershed management, or land utilization management along the watershed, is an important factor in the managing quality of a river. It also means that it is necessary to apply impervious surface index in the watershed management projects as a technique to protect quality of rivers in the future. In the city planning or land utilization planning so far, they used to administer and regulate just buildings, or just considered establishment ratio of sewer system. From now on, it is deemed advisable to manage impervious surface in order to maintain sound river/watershed quality. Impervious surface, which is an index of development activity, is a useful index which could be adopted in classifying urban rivers and managing individual watershed.