Evolution Of The Irrawaddy Delta In Myanmar And The Impacts Of Human Activities During The Past Half Century

Deltas are one of the most densely populated areas and many deltas in the world are showing signs of serious degradation due to human activities and the effects of climate change.Researches on delta evolution have been undertaken globally to understand the evolution characteristics of many large deltas.However,despite being one of the largest tropical deltas,the Irrawaddy Delta has received relatively little attention from the research community.To reduce this information gap,this article aims to conduct a comprehensive assessment of the evolution of the Irrawaddy Delta over the past half century by using remote sensing images and existed literature and analyze the impact of human activities.Firstly,this paper adopted the method of thresholding annual maximum NDVI to extract shoreline to avoid the impact of vegetation growth and tide level based on Google Earth engine platform and Landsat series images,so as to monitor the temporal and spatial evolution characteristics of shoreline,channel and geomorphology of the Irrawaddy Delta from 1974 to 2018.This paper further comprehensively collected data on land use changes(forest loss,land mining,aquaculture),dam construction and sediment extraction across the Irrawaddy River Basin and the delta,and combined with the features of landform changes derived from remote sensing and analyzed the effects of human activities on the evolution of the Irrawaddy Delta in detail.And this paper summarized the current evolution model of the Irrawaddy Delta.The main conclusions include the following aspects:(1)In the past 45 years,the Irrawaddy Delta overall accretion of the delta's shoreline was at a rate of 10.4 m/year,but approximately 42%of the shoreline was subjected to erosion during the past four decades.Shoreline evolution has obvious spatial characteristics.The western coast is dominated by erosion,and the eastern part is dominated by accretion.The rate of change from west to east coast gradually increased.About 31%of the western coasts showed increasing erosion,whereas 52%of the eastern coasts showed increasing accretion.With the help of the average shoreline change rate,we divided the Irrawaddy Delta into four regions,from west to east,they were region 1,2,3 and 4,of which region 2 is the most active.(2)The overall area change rate of change of coastal and inland rivers in each period is accretion,with the net accretion area of 194.0 km~2.Among them,the channel bar maintains an accretion rate of nearly 1 km~2/a,accounts for 11%of the overall.The changes on channel bank are the most active,but the gap between erosion and accretion is small and the accretion area accounts for only 5%of the overall.The development of estuary bar near bifurcations is stable,and the accretion accounts for 8%.The delta front maintains a stable accretion rate,contributing 76%of the overall.The analysis results show that Irrawaddy,Bogal and Toe are the three most active channels in the delta.(3)Remote sensing observations reveal that the Irrawaddy river channel and its coastal geomorphology have changed significantly.Since 1974,river channel between the first and second bifurcations has increased in length,width,sinuous index,and braiding index to varying degrees.The characteristics of changes in local areas should be analyzed based on the geographical location.The most obvious change is at the bifurcations of the channels.The derivative direction of the coastal bars and spits are consistent with the prevailing monsoon wind direction in the Irrawaddy Delta.(4)The comprehensive analysis shows that,although the Irrawaddy Delta is still in a state of sedimentation equilibrium,human activities such as deforestation,dam construction,and sediment extraction have already had a profound impact on the Irrawaddy Basin and its delta.Considering that Myanmar is in an era of rapid population and economic growth,human activities are expected to increase.At the same time,rising sea level has led to increasingly severe seawater erosion throughout the delta and enhanced ocean dynamics.Future climate change and potential land subsidence will inevitably put additional pressure on the sustainable development of the Irraeaddy Delta.Therefore,the equilibrium state of the delta is likely to be disturbed in the near future.