Evaluation of the effect of land use/cover change on flood characteristics using an integrated approach coupling land and flood analysis

Land use/cover change (LUCC) is one of the crucial factors influencing the hydrological process, thus the flood characteristics in time and space. Therefore the evaluation of the change of flood characteristics implies an integrated analysis of LUCC and hydraulic simulation. In this study, the effect of LUCC on flood is examined based on an approach composed of three parts: (1) reproduction of spatially explicit LUCC; (2) application of a 2D hydraulic modelling for flood simulation; (3) demonstration of results for Beijing. The approach is applied to a flood-prone area in Beijing. The results show that 8% and 21% of the study area experienced LUCC during 1991–2001 and 2001–2011, respectively, and these changes greatly influenced the characteristics of the 20-year flood, i.e., (1) the flood zone is doubled during 1991–2001 and about four-fold during 2001–2011; (2) the water depth is increased for most of the study area; and (3) the flow velocity becomes faster. It indicates that flooding still exists within Beijing and is even more dangerous than 40 years ago and suggests that actual land use pattern and existing flood protection works should be re-evaluated regarding the flood characteristics change due to LUCC.


INTRODUCTION
The temporal and spatial pattern of floods is attributed to sev- Most of China's cities have greatly expanded during the last decades and this increasing trend is foreseen for the future. In the areas of these cities, major changes have been observed in the land-intensive sectors like housing, road building, as well as crop production, grazing, forestry and mining. Simultaneously, more disastrous urban floods have also been observed in these cities in recent years, for example, the floods in June 2011 and July 2012 in Beijing in northern China, that in May 2010 in Guangzhou city in southern China, and that in July 2010 in Anqing city in eastern China. There is no doubt that LUCC has altered the flooding characteristics in these cities, but due to the complexity of the processes involved, the magnitude of their effect on flood characteristics and the spatial and temporal variation of these effects are still highly uncertain. This paper is a follow-up to the earlier work of the authors (Wang & Yang ) who have examined the effect of land use change on floods with various frequencies and pointed out that a 20-year flood can be affected the most. This is why the paper especially focuses on such a flood.
The work presented in this paper focuses on three main questions, always accounting for the cross-cutting issues between land and water sciences: 1. Which kind of land use changes have been observed in the past, and what was their spatial distribution in the landscape? 2. Which flood characteristics (e.g., flood zone, water depth and flow velocity) were affected by LUCC and how can they be quantified?
3. What is the related significance for urban development and flood protection?
We demonstrate the advantages of a coupling approach representing land and water in a common framework to study LUCC consequences to floods. Furthermore, we discuss new insights about urban development generated from this coupled analysis. The investigation does not address the influences of infiltration and hydraulic infrastructures.

STUDY AREA AND MATERIAL
Beijing, the capital of the People's Republic of China, is governed as a direct-controlled municipality under the national government with 16 urban, suburban and rural districts. It  in association with economic boom. Additionally, the remote sensing data are available and have identical accuracy for these analytical years.
Grid cell generation and determination of altitude and land use/cover The remote sensing images at 30 × 30 m pixels are the multiple band TM images at 1:50,000 scale and are available at Geospatial Data Cloud (http://www.gscloud.cn/). Correspondingly, the study area was described with a uniform grid containing 6,654 × 9,038 grid cells. The altitude at the central place of a grid cell is taken to represent the grid cell's altitude. From the same data source, the altitude data of ASTER GDEM

(Advanced Spaceborne Thermal Emission and Reflection
Radiometer Global Digital Elevation Model) were downloaded and used for defining grid cell slope. The produced altitude for each grid cell is shown in Figure 1. It is found that the slopes of the study area are gentle. In fact, 70% of the contributing area has a slope comprising between 1 and 3%. In order to get land use/cover information for each grid cell, the multiple band TM images were processed with Environment for Visualizing Images software at grid cell basis and a geometric adjustment to the images of 1991 and 2001 was made referring to the images of 2011 with the binary quadratic polynomial method, and the nearest neighbour method was applied for resampling. The processing was accepted if the verification shows that the adjustment error is less than half a pixel. Based on the accepted images, the land use/cover was interpreted for each grid cell by the supervised classification method and maximum likelihood classification method. The Normalized Difference Vegetation Index was adopted for classifying vegetation.  Table 1. It is seen that these coefficients range from 0.016 to 0.15 with a difference of more than eight times with the greatest for forest and the smallest for urban area. Each grid cell was assigned a Manning's roughness coefficient dependent on its land use/cover type.

RESULTS AND DISCUSSION
LUCC During the last decades, considerable LUCC has occurred all over China. In general, the cultivated land and forest, which characterized most areas around cities, decreased due to increasing urbanization and deforestration. A similar behaviour occurred in the study area. In order to investigate the land use change that occurred in the Chaobai River basin, historical land use maps for the analytical years were produced for the study area, which are comparable since they were derived by means of photo interpretation of remote sensing images carried out with same procedure as described in the previous section. As some of the 16 original land use/cover types adopted by land use analysts possess similar hydraulic features, such as roughness to water flow, they were merged into seven major types for hydraulic analysis, namely, urbanized area, bare land,   For comparison, this area (hereafter refer to as 'comparative area') is especially focused on in the following analysis.

Water depth mapping
The water depth can also be calculated for each grid cell using the approach, and the water depths averaged in each grid cell over the comparative area for the three analytical years are mapped in Figure 6. It is observed that water depth gets deeper and deeper during the 20 years and the grid cells in and adjacent to the river channels and urbanized areas are characterized as high water depth.
Statistics of water depth, as shown in Figure 7, were done by considering four ranges of water depth, i.e.,   Downloaded from https://iwaponline.com/hr/article-pdf/47/6/1161/367604/nh0471161.pdf" /><meta name="description" content="Land use/cover change (LUCC) is one of the crucial factors influencing the hydrological process, thu by guest      The results from this study provide further evidence that the change of land use pattern, i.e., transition of less impervious land use type to an impervious one, can adversely affect flood peak and flood propagation, leading to a larger flood zone, higher water depth and greater flash response.
• The study suggests that the land use pattern and flood protection works should be re-evaluated regarding the change in flood characteristics due to LUCC and their trade-offs should be identified and predicted while planning for urban development.