Benefits of subsidence control for coastal flooding in China

Land subsidence is impacting large populations in coastal Asia via relative sea-level rise (RSLR). Here we assesses these risks and possible response strategies for China, including estimates of present rates of RSLR, flood exposure and risk to 2050. In 2015, each Chinese coastal resident experienced on average RSLR of 11 to 20 mm/yr. This is 3 to 5 times higher than climate-induced SLR, reflecting that people are concentrated in subsiding locations. In 2050, assuming these subsidence rates continue, land area, population and assets exposed to the 100-year coastal flood event is 20%-39%, 17%-37% and 18%-39% higher than assuming climate change alone, respectively. Realistic subsidence control measures can avoid up to two thirds of this additional growth in exposure, with adaptation required to address the residual. This analysis emphasizes subsidence as a RSLR hazard in China that requires a broad-scale policy response, utilizing subsidence control combined with coastal adaptation.


Supplementary Methods
Supplementary Fig. 1 provides a flowchart summarising the general methodological approach as explained in the methods summary. It comprises three main steps. The first step is to develop the spatial assessment units by using a detailed coastline data and a segmentation process. The produces a detailed segmentation of China's coast, comprises 2,760 coastal segments, covering 28,966 km of coastline. This is much more detailed than the global database used by Hinkel et al (1) and Nicholls et al (2) as explained by Fang et al (3). The second step is to populate the segment database with baseline exposure parameters (in 2015) using elevation and population datasets. This creates a database structure that enables the DIVA model to run. These first two steps were completed by Fang et al (3), and the same segmentation and data is used in this analysis.
In the third step, the DIVA coastal flooding module is used to assess present and future changes of relative sea-level rise and how coastal flood risk evolves for different assumptions and scenarios. This includes considering the implementation of subsidence control in the 36 coastal cities and/or adaptation strategies via protection with dikes. The scenarios of climate-induced sea-level rise and population change and economic growth are consistent with the RCPs and SSPs as explained in the main text. The adaptation scenarios consider protection in terms of no upgrade, or upgraded protection to maintain the current standard with relative sea-level rise. This analysis adds the new data on the uplift/subsidence scenarios and hence the RSLR assumptions, as well as considering subsidence control scenarios based on official government documents. This makes the analysis much more realistic of what is happening in coastal China today. To analyse the effects of subsidence on relative sea-level rise in China, we combine data on four components of relative sea-level change, which are described in more detail in   Table 2).
These four components are independent and hence can simply be summed for each segment.

Glacial Isostatic Adjustment (GIA)
Local sea-level change due to glacial isostatic adjustment caused by ice loading and unloading are taken from the ICE-6G_C (VM5a) model (4). This gridded dataset is projected to the DIVA coastal segments by assigning the average sea-level change value over all intersected grid cells to the relevant segment.

Tectonic subsidence/uplift
Tectonic subsidence/uplift was derived and verified from multiple references, mainly

Delta subsidence
Natural compaction of deltaic sediments involves natural changes in the void space within sedimentary layers (for example dewatering, grain-packing realignment and organic matter oxidation) and is typically ≤3 mm/yr (9). For the four main delta plains in China, delta subsidence varies from 1-5 mm/yr (10)(11)(12). For the Liaohe River Delta, where there is little or no data for natural delta subsidence, a minimum value of subsidence is assumed in all cases at 1 mm/yr, following the estimates of Mechel et al. (13). The delta extent is linked to the DIVA segments. Supplementary Table 1 summarises the deltas considered in the analysis and the subsidence values used.   Supplementary Fig. 3). These estimates are designed to represent average subsidence values across the whole subsiding area within each city.
Supplementary  Table 3). We take these targets as plausible scenarios of subsidence control.      This file contains the observational subsidence values, measurements for subsidence records of coastal China basin obtained through a literature review.
The source data and the R code used to produce the numbers, tables and figures are available from https://doi.org/10.5281/zenodo.6969115 (20).