Abstract
Levees are normally the last barrier for defending flood water and storm surges in low-lying coastal cities. Levees in a large delta plain were usually constructed in different time and criteria and have been changing with age as well. Fast and quantitative assessment of levee stability is critical but faces many challenges. This study designs a scoring approach to quickly assess levee stability and overtopping threats with geometric parameters from airborne Light Detection and Ranging (LiDAR). An automated procedure is developed to extract levees geometric parameters from 0.5 m grid LiDAR elevation, such as crown height, width and landside slope. The surveyed levee is seated in the Hengmen waterway in the Pearl River Delta, Southern China. Results show that the stability index using the assessment scores is higher than and superior to the common qualified rates adopted in previous studies. The qualified rate is defined as the count percentage that each parameter meets the designed criteria, while the assessment score proposed in this study assigns different credits to those below/above the designed criteria. The continuous crown heights provide detailed information on levee overtopping threats. The crown heights of levee A and B are above the designed elevation and the flood stage (4.5 m) in a 200-year return period. The crown heights of levee C, D and E are generally lower than 4.5 m and vary in a large range on different sections. The middle section of levee E for the harbor and dock area has front elevation slightly below the flood stage (3.54 m) in a 20-year return period. Moreover, the high precision LiDAR altimetry data reveal various morphological modifications in all levees, such as natural subsidence and artificial modifications, which greatly reduce levees safety and are severe threats to the community. The procedures and assessment approach developed in this study can be easily applied for levees fast assessment in the entire Pearl River Delta and somewhere else, thus offer a suitable mitigation suggestion ahead of levee failure or overtopping.
Highlights
Levees play a crucial role in defending flood water in the low-lying coast
The primary objective of this study is to develop a procedure to automatically extract levees geometric parameters from airborne Light Detection and Ranging (LiDAR) data, and to design a scoring approach to assess their flood defense capacity, i.e., levee stability and overtopping risk, according to levees geometric parameters, the construction code, the sea level rise rate, and the designed water levels at several flood frequencies
This study proposes a scoring approach to assess levee stability and overtopping threats based on the criteria of levee geometric parameters and the extreme water levels at four flood stages or frequencies of 1:20, 1:50, 1:100, and 1:200 years
Summary
Levees play a crucial role in defending flood water in the low-lying coast. Coastal cities often face flood threats caused by river fresh flood and ocean storm surges. The rising sea level under the context of global warming aggravates the flooding risk of coastal cities, such as the low-lying Pearl. River Delta of Southern China [1]. Hydraulic engineering measures such as sea walls and levees can. Water 2020, 12, 403 effectively resist flood shocks, while they are usually the last barrier to protect the lives and properties of local residents [2,3,4]. Living behind a levee faces unique flood risks since levees are designed to reduce the impact of a flood event at certain scale [5]
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