Abstract

Continuing land subsidence can diminish the effectiveness of an existing flood mitigation system and aggravate the flood hazard. This chapter demonstrates that, through groundwater management with an effective pumping scheme, flood hazard and related flood damage in land subsidence area can be reduced. The chosen study area is in the southwest coast of Taiwan, which has long been suffering from frequent and wide-spread flooding primarily due to land subsidence induced by groundwater overpumping. Numerical investigation in the study area clearly shows that effective management of groundwater pumping can play an important role in long-term sustainable solution for controlling the spatial-temporal variability of future land subsidence, preventing the flood hazard from worsening, reducing the flood damage, and satisfying the groundwater demand.

Keywords: flood hazard, flood damage reduction, risk analysis, groundwater management, land subsidence

## 1. Introduction

In the region with scarce or highly variable surface water resource, groundwater is a vitally important source of water for sustainable development of the region. Groundwater pumping without proper control and management could result in a rapid depletion of valuable groundwater resource, which cannot be replenished in a short period of time. Furthermore, the seriousness of land subsidence can be exacerbated, which is concomitant with increased flood hazard and damage. Phien-wej et al. [1] reported that the estimated flood damage attributed to land subsidence in the 1990s amounted to \$12 million annually in Bangkok, Thailand. Nicholls et al. [2], in their assessment of the exposure of population and assets to a 1-in-100 year surge-induced flood event at 136 port cities with more than one million inhabitants, indicated that the climate change and land subsidence contribute about one-third of increased flood exposure for people and assets. The impact of land subsidence induced by excessive groundwater extraction should be carefully examined in deltaic cities, especially in those coastal areas that are under rapid development.

By using inundation models, many studies have shown that flood hazard, after a long period of land subsidence, becomes worsened in cities like Semarang [3] and Jakarta [4] of Indonesia, Shanghai of China [5], and coastal cities around Northern

Adriatic Sea [6]. All the above studies showed that land subsidence results in increased flood inundation depth and areal extent, as well as diminishing effectiveness of existing flood protection systems. Even the flood defense system is upgraded to uphold the protection level, and the flood risk will be worsening with continuing land subsidence. Therefore, an engineered flood defense infrastructure system, jointly with a proper groundwater pumping practice with an aim to reduce land subsidence, could offer a sustainable solution to flood management problems in subsidence prone areas.

The goal for land subsidence mitigation can be achieved through effective management of groundwater pumping by constraining the drawdown. A comprehensive review of groundwater management (GWM) can be found elsewhere [7–9]. The common approach for handling subsidence control in GWM is to set a preconsolidation head as the lower bound of the groundwater level to prevent inelastic soil compaction from happening [10]. However, such an approach considers only the drawdown constraint that does not explicitly relate to the magnitude of land subsidence. To circumvent such deficiency, Chang et al. [11, 12] developed a mixed integer programming model for maximizing total pumpage, subject to drawdown and land subsidence constraints. 1D consolidation equation, which simultaneously considers inelastic and elastic soil compaction, is incorporated explicitly in the subsidence constraints.

As many studies have pointed out that the flood risk in land subsidence prone areas can be reduced through proper GWM (e.g., [1, 5]), and it is rarely found that flooding is explicitly incorporated into the model formulation. Chang et al. [13] developed a groundwater pumping optimization model, in conjunction with land subsidence and inundation models, to mitigate the land subsidence effect on flood hazard in land subsidence areas and satisfy the water demand. The GWM model determines the optimal pumping scheme for (1) minimizing land subsidence, (2) preventing flood hazard from worsening in the future, and (3) satisfying groundwater demand. This chapter, on the basis of the developed optimal groundwater pumping model [13], evaluates flood damage reduction and assesses economic benefit attainable by GWM in land subsidence prone coastal areas.

### 2. Methodology

#### 2.1 Analysis framework

Figure 1 shows the framework of analysis that was applied to a study area in the coastal zone of Taiwan (see Section 3.1 for more detailed descriptions) that is experiencing severe land subsidence problem largely due to groundwater overpumping. It can be seen that the analysis framework contains two major parts in which the first part is on the left branch for predicting the cumulative land subsidence in the study area over a 10-year period (2012–2021) based on the existing groundwater usage without management. Under this scenario, the groundwater pumpage in 2012–2014 in the study area was set to the historical average value as shown in Table 1. In 2015, a newly built Hushan reservoir began its service, and the groundwater pumpage during 2015–2021 was adjusted downward according to the planned water supply amount from the reservoir. The left branch of the analysis estimates the ground surface topography in the study area caused by land subsidence after 10 years of using the existing pumping pattern without optimal GWM. Flood hazard and inundation damage in the study area at the end of 2021 are assessed accordingly.

It should be pointed out here that, because of a relatively short management period of 10 years considered in the study, the rainfall condition was assumed to be stationary in assessing flood hazard and inundation damage. The indicators of flood hazard considered include the levee freeboard along the drainage channel systems and the maximum inundation depth in the study area. The freeboard is a measure of margin of safety, which is the vertical elevation difference from the levee crown to the water surface in the drainage channel. A reduction in the freeboard is an indication of increased overtopping potential of the levee system. The maximum inundation depth can be indicative of flooding severity. From the flood inundation simulation, the effect of subsidence on the flood hazard under the existing groundwater pumping practice can be assessed. With flood damage-inundation depth

Extraction Recharge

Mailiao 80.17 107.55 3.68 27.25 0.93 Lunbei 58.48 115.89 5.43 7.46 0.35 Taisi 54.10 34.4 1.74 17.13 0.87 Dongshih 48.36 57.33 3.25 7.54 0.43 Baojhong 37.06 54.05 4.00 8.22 0.61 Tuku 49.02 56.6 3.16 6.39 0.36 Huwei 68.74 85.18 3.39 15.25 0.61 Sihhu 77.12 58.26 2.07 25.42 0.90 Yuanchang 71.59 89.01 3.41 9.93 0.38 Total 464.47 658.27 30.13 124.59 5.44

Flood Damage Reduction in Land Subsidence Areas by Groundwater Management

Intensity (mm/day)

Annual (10<sup>6</sup> m3 )

Intensity (mm/day)

The second part of the analysis is shown on the right-hand branch of Figure 1 in

relationships available, the flood inundation risk cost can be assessed.

Groundwater extraction and natural recharge for the nine townships in the study area.

Figure 1.

Table 1.

21

Township Area

(km2 )

Flow chart showing the methodological framework in the study.

DOI: http://dx.doi.org/10.5772/intechopen.80665

Annual (10<sup>6</sup> m3 )

which the GWM model is applied to find the optimal pumping scheme by

### Flood Damage Reduction in Land Subsidence Areas by Groundwater Management DOI: http://dx.doi.org/10.5772/intechopen.80665

#### Figure 1.

Adriatic Sea [6]. All the above studies showed that land subsidence results in increased flood inundation depth and areal extent, as well as diminishing effectiveness of existing flood protection systems. Even the flood defense system is upgraded to uphold the protection level, and the flood risk will be worsening with continuing land subsidence. Therefore, an engineered flood defense infrastructure system, jointly with a proper groundwater pumping practice with an aim to reduce land subsidence, could offer a sustainable solution to flood management problems in

The goal for land subsidence mitigation can be achieved through effective management of groundwater pumping by constraining the drawdown. A comprehensive review of groundwater management (GWM) can be found elsewhere [7–9]. The common approach for handling subsidence control in GWM is to set a preconsolidation head as the lower bound of the groundwater level to prevent inelastic soil compaction from happening [10]. However, such an approach considers only the drawdown constraint that does not explicitly relate to the magnitude of land subsidence. To circumvent such deficiency, Chang et al. [11, 12] developed a mixed integer programming model for maximizing total pumpage, subject to drawdown and land subsidence constraints. 1D consolidation equation, which simultaneously considers inelastic and elastic soil compaction, is incorporated

As many studies have pointed out that the flood risk in land subsidence prone areas can be reduced through proper GWM (e.g., [1, 5]), and it is rarely found that flooding is explicitly incorporated into the model formulation. Chang et al. [13] developed a groundwater pumping optimization model, in conjunction with land subsidence and inundation models, to mitigate the land subsidence effect on flood hazard in land subsidence areas and satisfy the water demand. The GWM model determines the optimal pumping scheme for (1) minimizing land subsidence, (2) preventing flood hazard from worsening in the future, and (3) satisfying groundwater demand. This chapter, on the basis of the developed optimal groundwater pumping model [13], evaluates flood damage reduction and assesses economic

Figure 1 shows the framework of analysis that was applied to a study area in the

coastal zone of Taiwan (see Section 3.1 for more detailed descriptions) that is experiencing severe land subsidence problem largely due to groundwater

and the groundwater pumpage during 2015–2021 was adjusted downward

overpumping. It can be seen that the analysis framework contains two major parts in which the first part is on the left branch for predicting the cumulative land subsidence in the study area over a 10-year period (2012–2021) based on the existing groundwater usage without management. Under this scenario, the groundwater pumpage in 2012–2014 in the study area was set to the historical average value as shown in Table 1. In 2015, a newly built Hushan reservoir began its service,

according to the planned water supply amount from the reservoir. The left branch of the analysis estimates the ground surface topography in the study area caused by land subsidence after 10 years of using the existing pumping pattern without optimal GWM. Flood hazard and inundation damage in the study area at the end of

benefit attainable by GWM in land subsidence prone coastal areas.

subsidence prone areas.

Recent Advances in Flood Risk Management

2. Methodology

2.1 Analysis framework

2021 are assessed accordingly.

20

explicitly in the subsidence constraints.

Flow chart showing the methodological framework in the study.


#### Table 1.

Groundwater extraction and natural recharge for the nine townships in the study area.

It should be pointed out here that, because of a relatively short management period of 10 years considered in the study, the rainfall condition was assumed to be stationary in assessing flood hazard and inundation damage. The indicators of flood hazard considered include the levee freeboard along the drainage channel systems and the maximum inundation depth in the study area. The freeboard is a measure of margin of safety, which is the vertical elevation difference from the levee crown to the water surface in the drainage channel. A reduction in the freeboard is an indication of increased overtopping potential of the levee system. The maximum inundation depth can be indicative of flooding severity. From the flood inundation simulation, the effect of subsidence on the flood hazard under the existing groundwater pumping practice can be assessed. With flood damage-inundation depth relationships available, the flood inundation risk cost can be assessed.

The second part of the analysis is shown on the right-hand branch of Figure 1 in which the GWM model is applied to find the optimal pumping scheme by

minimizing the land subsidence effect on flood hazard while, at the same time, satisfying the water demand. After obtaining the optimal pumping strategy, the corresponding land subsidence amounts are obtained to define the land topography in Year 2021. Under a different topography, the corresponding flood hazard indicators and inundation damage are obtained for assessing the effect of GWM.

elevation in 2012 was added by the cumulative land subsidence between 2012 and 2021 obtained by the land subsidence model under the conditions of with

Flood Damage Reduction in Land Subsidence Areas by Groundwater Management

4.Roughness coefficient: flow boundary roughness is categorized by the channel bed and overland surface. As almost all the drainage channels within the study area are man-made with gravel bottom and concrete siding, the nominal value of 0.02 for the Manning roughness coefficient was used according to Chow [16]. The roughness coefficient of the overland surface was determined by the

In this study, land subsidence is assumed to be caused by groundwater pumping.

An uncoupled model consisting of a layered 3D groundwater solver and a 1D consolidation model was used to simulate land subsidence [17]. The layered 3D groundwater solver is first used to simulate depth-averaged groundwater flow and pore pressure head change due to groundwater extraction in every layer at each time step. The vertical soil displacement during each time step is then calculated by the 1D consolidation equation. The simulation model assumes (1) isotropic soil medium, (2) linear elasticity relationship between average effective stress and average displacement following Hooke's law, and (3) vertical displacements only. These assumptions, however, ignore the presence of the preconsolidation head, which implies that a decrease in pore pressure head due to groundwater extraction will always cause normal consolidation and is unable to consider overconsolidation and rebound (i.e., elastic range). This renders overestimation of land subsidence. To simultaneously consider the inelastic/elastic behavior of land subsidence, Chang et al. [12] modified the 1D consolidation equation according to Leake [18] as

and without GWM.

DOI: http://dx.doi.org/10.5772/intechopen.80665

land use listed in Table 2.

2.2.2 Land subsidence model

Δsl,k,t ¼

Table 2.

23

αCc Δh p

8 ><

>:

l,k,t�<sup>1</sup> � <sup>Δ</sup>hl,k,t�<sup>1</sup>

Δh<sup>p</sup>

Relationship between the Nikuradse roughness coefficient kn and land use [15].

� � <sup>þ</sup> Cc <sup>Δ</sup>hl,k,t � <sup>Δ</sup><sup>h</sup>

drawdown; Cc = ρwgB/(2 μ + λ) with ρ<sup>w</sup> = density of water, g = gravitation

Land use kn Agriculture 0.8 Built-up 10 Water conservation 0.2 Amusement and rest area 3 Transportation 1 Other 0.5

p l,k,t�1 � �,Δhl,k,t <sup>≤</sup>Δ<sup>h</sup>

<sup>l</sup>,k,<sup>t</sup> <sup>¼</sup> Max <sup>Δ</sup>hl,k,t;Δh<sup>p</sup>

where Δsl,k,t = land subsidence within layer-l at control point-k during the t-th time period; Δhl,k,t = drawdowns of layer-l at control point-k at the end of the t-th time period; α (< <1) = ratio of elastic to inelastic compaction per unit increase in

αCc Δhl,k,t � Δh

p l,k,t�1 � �,Δhl,k,t <sup>&</sup>gt; <sup>Δ</sup><sup>h</sup>

l,k,t�1

p l,k,t�1

h i (2)

p <sup>l</sup>,k,t�<sup>1</sup>

(1)
