X.X. Lu

Application of Remote Sensing in Flood Management with Special Reference to Monsoon Asia: A Review

The conventional means to record hydrological parameters of aflood often fail to record an extreme event. Remote sensingtechnology along with geographic information system (GIS)has become the key tool for flood monitoring in recent years.Development in this field has evolved from optical to radarremote sensing, which has provided all weather capabilitycompared to the optical sensors for the purpose of flood mapping.The central focus in this field revolves around delineation of floodzones and preparation of flood hazard maps for the vulnerable areas.In this exercise flood depth is considered crucial for flood hazardmapping and a digital elevation model (DEM) is considered to bethe most effective means to estimate flood depth from remotelysensed or hydrological data. In a flat terrain accuracy of floodestimation depends primarily on the resolution of the DEM. Riverflooding in the developing countries of monsoon Asia is very acutebecause of their heavy dependence on agriculture but any floodestimation or hazard mapping attempt in this region is handicappedby poor availability of high resolution DEMs. This paper presents areview of application of remote sensing and GIS in flood managementwith particular focus on the developing countries of Asia.

Sediment delivery to the Three Gorges: 2: Local response

Abstract Information on soil erosion and sediment yields is very limited in the immediate vicinity of the Three Gorges Project (TGP), although the inceptisols developed on Triassic shales and sandstones in this area are known to be highly susceptible to erosion. Estimated sediment yields for a small reservoir catchment (Yiwanshui) in Changshou County, downstream of Chongqing, are high (around 3500 t km−2 a−1) and similar to the data from 38 reservoir investigations in Sichuan basin, but exceeding the yields estimated in the feasibility studies for the TGP. The high sediment yield reflects severe soil erosion due to cultivation aligned with slope on steep slopes, which is very common throughout Sichuan Basin and particularly in the TGP area. The total sediment delivered to the Yangtze from the TGP area has been estimated, using small reservoir data and sediment load measured at hydrological stations. These are comparable with previous estimates but there is much temporal variability in sediment supply from different parts of the Three Gorges region, suggesting the importance of local impact on sediment flux. Resettlement of agricultural activity around the Three Gorges Reservoir is likely to further enhance soil erosion. Temporal changes in sediment yields are reconstructed by using caesium-137 (137Cs) dating techniques. They indicate significant changes resulting from human activity and rainfall variability during the past four decades. Comparing reservoir sedimentation against erosion estimates on catchment soils, derived from measurements, indicates that 84% of eroded soil is delivered to the reservoir and the remainder is deposited in valley floor paddy fields. It is estimated that over 60% of sediment is contributed from arable land. The trap efficiency of small catchment reservoirs, such as Yiwanshui, may be an important control on sediment delivery to the main channel of the Upper Yangtze.

Sediment delivery to the three gorges:: 1. Catchment controls

Abstract The paper examines sediment yield and its response to catchment disturbance and environmental variables in the Upper Yangtze basin, where the attention of environmentalists has been drawn to the Three Gorges Project (TGP). Information about the source and conveyance of sediment from the catchment area to the Three Gorges Reservoir has implications for management strategies. Methodologies for establishing the relationships between land cover, climatic and topographic variables with sediment yield are introduced. The analysis uses a sediment load data set, containing 250 stations with up to 30 years of measurement, a 1×1 km resolution land cover database and variables extracted from various geodatabases. The mean sediment load delivered from the Yangtze upstream of Chongqing is 318 Mt a−1, but the contribution from the Jialing tributary is higher in terms of specific sediment yield at 928 t km−2 a−1. Long-term sediment yield at Yichang has not exhibited an upward trend despite the evidence for increased soil erosion within the basin. Examination of sediment response to catchment disturbance and spatial variability in relation to controlling variables has been undertaken in an attempt to predict future sedimentation impacts. Time series analysis illustrates that significant increases in sediment yield have occurred over about 8% of the catchment area while about 3% have experienced decreasing sediment yields. The latter are associated with major reservoir schemes on the tributaries of the Yangtze. When the spatial pattern of sediment yields within the basin is analysed, “natural” climatic and topographic factors explain most of the variability in the relatively sparsely populated western part of the Upper Yangtze basin, but do not afford very good prediction in the more populated eastern part. Incorporation of land cover information does not provide additional explanation of spatial variability. Examination of the response of sediment delivery to catchment disturbance and environmental variables provides an illustration which may have some lessons for the management of the sedimentation problem in the Three Gorges Reservoir and a basis for modelling future changes in sediment delivery.

Estimating erosion rates on sloping agricultural land in the Yangtze Three Gorges, China, from caesium-137 measurements

The paper describes the application of caesium-137 measurements for estimating soil erosion rates in a small catchment in the Three Gorges region of the Yangtze River, China. The construction of the Three Gorges Dam has drawn attention to the impact of erosion and sedimentation, but there are relatively few quantitative estimates of sediment transfer for this area. The suitability of the fallout radionuclide, caesium-137, for the rapid appraisal of soil redistribu- tion in the steep and dissected terraced landscape of the Three Gorges is investigated here. Previous applications of the technique in Chinese agricultural environments have indicated the difficulty of obtaining reliable baseline fallout estimation. The integration of monthly rainfall data with a model of global strontium-90 fallout is developed to provide an independent estimate of baseline fallout, which is consistent with field measurements. The method also enables a mass balance model of caesium-137 mobility to be calibrated. Mean annual soil loss during the last four decades is estimated at 4500 t km y2 yr y1 . Erosion rates are strongly related to field slope angles but highly variable spatially. q 2000 Elsevier Science B.V. All rights reserved.

Estimation of suspended sediment concentrations using Terra MODIS: an example from the Lower Yangtze River, China.

Traditional measurements of suspended sediment concentrations (SSC) through in-situ sampling in rivers are expensive and time-consuming to perform. Thus, these methods cannot provide continuous SSC records. Although remote sensing has been used for SSC estimation, little research has been undertaken on inland rivers, especially for highly turbid rivers like the Yangtze. Previous studies have proposed Landsat TM/ETM+ Band 4 as a spectral SSC indicator for the Yangtze, but its limitation on temporal resolution is insufficient for the study of dynamic changes of sediment. This paper presents a method of estimating SSC of the Yangtze at Jiujiang using time-series satellite data of high temporal resolution Terra MODIS. It was found that differences in water reflectance between Band 2 and Band 5 could provide relatively accurate SSC estimation even when in-situ atmospheric conditions were unknown. After cross-validation, mean absolute relative error (ARE) and relative root mean square error (RRMSE) were relatively low (i.e., 25.5% and 36.5%, respectively). This empirical relationship was successfully applied to the estimation of SSC at Datong in the Lower Yangtze River, although the SSC values were generally underestimated. This study suggests that Terra MODIS could be used to estimate SSC in large turbid rivers, although some influencing factors require further study to improve the accuracy of SSC estimation.

A quantitative assessment of human impacts on decrease in sediment flux from major Chinese rivers entering the western Pacific Ocean

Major rivers with high sediment or water discharge act as natural integrators of surficial processes, including human activities within their drainage basins, and they are also the primary sources of terrestrial materials entering the ocean. The river-derived materials flux entering the coastal oceans, however, has been strongly affected by anthropogenic activities. Recent studies related to human impacts on river sediment flux have mainly focused on qualitative descriptions. Here we present a quantitative assessment of human impacts on decrease in sediment flux from nine major Chinese rivers entering the western Pacific Ocean, including Changjiang (Yangtze), Huanghe (Yellow), Zhujiang (Pearl), Songhuajiang, Liaohe, Haihe, Huaihe, Qiantangjiang, and Minjiang. During 1959-2007, dams and reservoirs, soil and water conservation programs, water consumption, as well as sand mining decreased the amount of sediment delivered to the ocean by 28, 11.5, 7.5 and 3 gigatons (Gt), respectively. If combined (50 Gt for the period 1959-2007), this reduction was close to the total decreased sediment flux (43 Gt) measured from these nine major rivers over the same period. Besides, the temporal variations in water and sediment fluxes into the ocean from these rivers generally during 1953-2007 were presented. These results are useful for further studies on Chinese and even global river-derived material flux to the ocean and associated ecological risks. Citation: Chu, Z. X., S. K. Zhai, X. X. Lu, J. P. Liu, J. X. Xu, and K. H. Xu (2009), A quantitative assessment of human impacts on decrease in sediment flux from major Chinese rivers entering the western Pacific Ocean, Geophys. Res. Lett., 36, L19603, doi:10.1029/2009GL039513.

The Irrawaddy River Sediment Flux to the Indian Ocean: The Original Nineteenth-Century Data Revisited

The Irrawaddy (Ayeyarwady) River of Myanmar is ranked as having the fifth‐largest suspended load and the fourth‐highest total dissolved load of the world’s rivers, and the combined Irrawaddy and Salween (Thanlwin) system is regarded as contributing 20% of the total flux of material from the Himalayan‐Tibetan orogen. The estimates for the Irrawaddy are taken from published quotations of a nineteenth‐century data set, and there are no available published data for the Myanmar reaches of the Salween. Apart from our own field studies in 2005 and 2006, no recent research documenting the sediment load of these important large rivers has been conducted, although their contribution to biogeochemical cycles and ocean geochemistry is clearly significant. We present a reanalysis of the Irrawaddy data from the original 550‐page report of Gordon covering 10 yr of discharge (1869–1879) and 1 yr of sediment concentration measurements (1877–1878). We describe Gordon’s methodologies, evaluate his measurements and calculations and the adjustments he made to his data set, and present our revised interpretation of nineteenth‐century discharge and sediment load with an estimate of uncertainty. The 10‐yr average of annual suspended sediment load currently cited in the literature is assessed as being underestimated by 27% on the basis of our sediment rating curve of the nineteenth‐century data. On the basis of our sampling of suspended load, the nineteenth‐century concentrations are interpreted to be missing about 18% of their total mass, which is the proportion of sediment recovered by a 0.45‐μm filter. The new annual Irrawaddy suspended sediment load is \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Recent lake sedimentation in the middle and lower Yangtze basin inferred from 137Cs and 210Pb measurements

364\pm 60