Research on Optimal Scale for Extraction of Relief Amplitude in Loess Geological Hazards Assessment Factors: a Case Study of Mizhi County, Yulin City
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摘要:
目前地质灾害相关评价工作中,部分研究人员对于“地形起伏度”(也称相对坡高)选取最佳窗口单元进行提取存在着一定程度的随意性和主观性,致使取得的地形起伏度参数与研究区实际情况相比存在一定的误差。后续地灾评价中,无论采用以栅格为基础的信息量模型还是现今普遍流行的各类机器学习方法,其评价因子本身的误差甚至错误会导致评价结果可靠性降低。笔者基于ArcGIS平台,利用陕西省榆林市米脂县分辨率为2 m的DEM数据,采用均值变点分析法,通过两轮分析,数量分别为10×10和1×1的矩形窗口逐渐逼近研究区地形起伏度的最佳统计单元,计算出该县地形起伏度为0~256.60 m,最佳统计单元为59×59的窗格,栅格单元边长为2 m,提取窗格边长为118 m,对应提取面积为13924 m2。随后依据陕北黄土地区历史滑坡及崩塌的易发坡高统计将米脂县地形起伏度等分为<20 m、20~40 m、40~60 m、60~80 m、>80 m等5个区间,受原始地形条件及削坡建房、建厂等综合影响,40~80 m为灾害隐患发育的主要区间,灾害隐患点占比为88.60%。结合米脂县地质灾害隐患点信息量值和灾害点密度对比曲线,结果显示二者有很好的相关性,体现了地形起伏度统计单元选取和区间划分的合理性。本研究所采用的高精度DEM数据的计算及分析结果,首先避免了目视寻找拐点的弊端,其次在黄土高原地区千沟万壑的地貌条件中能够满足数字地形分析与精细化地质灾害调查的需求,可为黄土高原区地质灾害评价防治及黄河中上游流域的水土流失治理与生态环境保护提供一定的技术支撑。
Abstract:In the current evaluation of geological hazards, there is a certain degree of arbitrariness and subjectivity in selecting the best window unit scale for the extraction of relief amplitude (also named relative slope height). The certain error exists as compared the obtained relief amplitude parameters with the actual situation in the study area. During the disaster assessment, using grid-based information models or various popular machine learning methods can lead to a reduction in the reliability of the evaluation results due to the errors of the evaluation factors. In the current study, we use DEM data (2 m resolution) of Mizhi County, adopt 10×10, 1×1 rectangular windows for the relief amplitude extraction based on ArcGIS platform, and use the mean change-point analysis to calculate the relief amplitude of Mizhi County from 0–256.60 m, and the best statistical cell is 59×59 with the grid length of 2 m, and the side length of the extraction window is 118m and square is
13924 m2. Afterwards, according to the statistics on the slope height easily inducing historical landslides and collapses in the loess region of northern Shaanxi Province, the relief amplitude in Mizhi County is divided into five intervals: <20 m, 20–40 m, 40–60 m, 60–80 m and >80 m. Due to the comprehensive influence of the original terrain conditions, slope cutting and building of houses, factories, etc., 40–80 m is the main interval for the development of disaster hazards, with a proportion of 88.60% of disaster hazard points.The comparison curves of information value and hazard point density of each interval were made, which demonstrates the reasonableness of the selection of the statistical unit and the division of the interval of the relief amplitude. The calculation and analysis results of high-precision DEM data adopted first avoid the drawbacks of visually searching for inflection points, and secondly meet the needs of digital terrain analysis and refined geological hazard investigation in the mountainous terrains of the Loess Plateau region. The method and the results can provide technical support for the evaluation and prevention of geological hazards in the Loess Plateau area and the management of soil erosion and ecological environment protection in the middle and upper reaches of the Yellow River basin. -
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表 1 窗口数量、面积与平均地形起伏度关系统计表
Table 1 Statistics on the number and area of rectangular windows with the average relief amplitude
窗口
数量(个)窗口面积
(m2)平均地形
起伏度(m)窗口
数量(个)窗口面积
(m2)平均地形
起伏度(m)窗口
数量(个)窗口面积
(m2)平均地形
起伏度(m)10 400 12.2912 180 129600 98.6925 350 490000 124.6421 20 1600 24.2089 190 144400 100.8045 360 518400 125.7695 30 3600 34.3080 200 160000 102.8036 370 547600 126.8710 40 6400 42.8923 210 176400 104.7012 380 577600 127.9472 50 10000 50.2548 220 193600 106.5074 390 608400 128.9990 60 14400 56.6414 230 211600 108.2303 400 640000 130.0282 70 19600 62.2494 240 230400 109.8784 410 672400 131.0365 80 25600 67.2303 250 250000 111.4593 420 705600 132.0243 90 32400 71.6998 260 270400 112.9787 430 739600 132.9916 100 40000 75.7451 270 291600 114.4425 440 774400 133.9396 110 48400 79.4344 280 313600 115.8560 450 810000 134.8693 120 57600 82.8203 290 336400 117.2234 460 846400 135.7813 130 67600 85.9471 300 360000 118.5485 470 883600 136.6766 140 78400 88.8487 310 384400 119.8355 480 921600 137.5567 150 90000 91.5523 320 409600 121.0860 490 960400 138.4219 160 102400 94.0813 330 435600 122.3018 500 1000000 139.2725 170 115600 96.4561 340 462400 123.4868 表 2 矩形窗口均值变点法统计结果表(步距为10)
Table 2 Statistics results of the mean-change-point method for rectangular window
窗口
数量(个)∆S
(S-Si)窗口
数量(个)∆S
(S-Si)窗口
数量(个)∆S
(S-Si)窗口
数量(个)∆S
(S-Si)10 4.2608 140 6.5372 270 3.8843 400 1.5723 20 6.0529 150 6.3306 280 3.6935 410 1.4080 30 6.9466 160 6.1217 290 3.5051 420 1.2453 40 7.4034 170 5.9119 300 3.3190 430 1.0843 50 7.6176 180 5.7019 310 3.1351 440 0.9249 60 7.6860 190 5.4927 320 2.9534 450 0.7670 70 7.6621 200 5.2846 330 2.7739 460 0.6107 80 7.5773 210 5.0781 340 2.5964 470 0.4559 90 7.4514 220 4.8735 350 2.4210 480 0.3025 100 7.2973 230 4.6711 360 2.2475 490 0.1506 110 7.1236 240 4.4708 370 2.0760 500 120 6.9364 250 4.2729 380 1.9063 130 6.7399 260 4.0774 390 1.7384 表 3 矩形窗口均值变点法统计结果表(步距为1)
Table 3 Statistics results of the mean-change-point method for rectangular window
窗口
数量(个)窗口
面积(m2)∆S
(S-Si)窗口
数量(个)窗口
面积(m2)∆S
(S-Si)窗口
数量(个)窗口
面积(m2)∆S
(S-Si)50 10000 0.0139 57 12996 0.0642 64 16384 0.0498 51 10404 0.0259 58 13456 0.0656 65 16900 0.0436 52 10816 0.0362 59 13924 0.0657 66 17424 0.0365 53 11236 0.0448 60 14400 0.0647 67 17956 0.0286 54 11664 0.0519 61 14884 0.0626 68 18496 0.0198 55 12100 0.0574 62 15376 0.0593 69 19044 0.0103 56 12544 0.0615 63 15876 0.0551 70 19600 表 4 地形起伏度分级的地质灾害信息量值
Table 4 The information value base on classification of relief amplitude
地形起伏
度分级分级面积
(km2)灾害点
数量(个)灾害点
百分比分级灾害
点密度信息量I ≤20 m 45.80 20 0.46% 0.4367 − 2.1489 20~40 m 134.80 115 2.62% 0.8531 − 1.4792 40~60 m 494.51 1658 37.82% 3.3528 − 0.1105 60~80 m 407.80 2226 50.78% 5.4586 0.3769 >80 m 87.82 365 8.33% 4.1561 0.1043 -
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