1. Introduction
Due to climate change, flood disasters caused by localized storm waters are increasing. Meteorological and environmental experts are expecting that global warming has been caused by increase in greenhouse gas concentration in the atmosphere, and natural disasters due to this phenomenon will persist in the future. Recent urban floods, which can be classified as technological disasters triggered by natural hazards have been caused by rapid urbanization and heavy rainfall in industrialized areas (European Commission, 2003). Development of underground complexes in citiy has continued in the direction of big and multi-functional spaces and demand for such spaces is predicted to increase. Flooding of underground spaces in densely populated may cause not only property damage but also massive loss of lives due to closed space features.
While many past studies have been done on flood risk assessments and measures against urban inland deluge, not many studies have been carried out to set up on measures against inundation in underground spaces. Dutta et al. (2003) showed a general object-oriented model that can simulate flow behavior in different components of subway station under land flood conditions. Lee and Han (2007) carried out a numerical flooding analysis for a subway station from inland flood by a link-node system technique. An experimental approach and numerical simulation model based on a storage pond model were developed by Toda (2007). Han (2012) proposed a method of flood risk assessment and a 1D-2D linked numerical model for analyzing the inundation behavior for surface flow in underground spaces.
Big scale rainwater storage facilities are evaluated as one of representative infrastructures for mitigation effectively surface inundation. Underground drainage tunnels are well known worldwide, such as the ‘Tunnel and Reservoir Plan’ system, Chicago, USA; Kanda-river underground regulating reservoir, about 4.5 km in length with 540,000 m3, Tokyo, Japan; the ‘Metropolitan Area Outer Underground Discharge Channel’ system, about 6.3 km with 670,000 m3, Saitama-ken, Japan; and the ‘Stormwater Management and Road Tunnel (SMART)’ system, about 9.7 km with 1,000,000 m3, Kuala Lumpur, Malaysia; etc. Seoul is considering a project to construct deep underground drainage tunnels in flood-prone areas to prevent frequent urban inundation damage.
In this study, the evaluation and analysis of flooding risk factors based on the concept of AHP (Saaty, 1980, 1990) are performed in order to obtain more effective countermeasures against urban inundation problems. Simulation models are applied to estimate the effects of mitigation for flood disaster for the installation of underground rainwater storage facilities. An improved construction method is provided to install multi - placed underground structures with considerations for limitations such as the rise of land-price and the limitation of urban space. An experimental approach is performed to verify the applicability and functionality of the proposed reinforced system, to set up advanced technology with structural stability.
2. Urban Flooding
2.1 AHP
The vulnerability and priority estimation of underground flooding was performed by systematically analyzing a series of parameters through AHP methodology. The AHP provides a comprehensive and rational framework for structuring factors, which are in a hierarchy of sub-factors that have a structural level for evaluating alternatives related to the overall goal of the inundation risk. A flowchart of priority selection based on this approach is shown in Fig. 1(a). The AHP allows and enables the assessment of the relative importance of multiple criteria and alternatives. AHP has three basic underlying concepts (Saaty, 1980, 1990). The weights were synthesized from top to bottom according to the results of every single rank of every hierarchy as shown in Fig. 1(b).
The factors were weighted from the responses of 60 experts to questionnaire. The total preference weights were gained by the following equation (Eq. (1)):
W = [eigenvalue] × [weights of alternatives] (1)
In this study, the AHP approach was used with four criteria to estimate the importance of factors as criteria with four alternatives relatively related to the flooding of underground structures as shown Table 1.
Table 1
Criteria & Alternatives of AHP
Brief descriptions of weights on the influence of factors to evaluate underground inundation at each hierarchy are given in Table 1.
The results shows that ‘Local precipitation condition’ factor with a rating of 0.274, is the most preferred, followed by ‘Drainage system of facility’ on the second ranked. ‘Drainage system of facility’ on a structure can be assessed as one of the very important flooding factors, considering that the ‘Local precipitation condition’ is an uncontrollable, meteorological factor. At the third-level assessment of influence factors, ‘Rainwater drainage tunnel, storage cavern’ is the most important factor (priority = 0.367) for the criterion of ‘Drainage system of facility.’ The table represents that ‘The amount of rainfall, rainfall intensity’ and ‘Capacity of drainage system in a site’ have the highest value for each criterion of ‘Local precipitation condition’ and ‘Local topography and site condition,’ respectively.
2.2 Simulation of Urban Flooding
Simulation is carried out to evaluate the mitigation effect of flooding in cases when detention facilities are installed. Discharge with and without storage facilities are simulated for a well- planned study area Centem City in Busan, Korea, using SWMM. The SWMM, developed by the US-EPA, is an effectively comprehensive model for urban runoff and combined sewer systems considering plausible worst case scenarios. The simulation is performed to identify the intensity of rainfall that could cause water entry into an underground space as well as flooding and other factors. Due to the influence of heavy rainwater in the study area have been very severe with historical flood recorded of 2003, 2008 and 2011. Study area is close to the river, as shown in Fig. 2. It also has many buildings and underground spaces in densely populated lowland areas.
The simulation was modeled with the following scenario which has a series of eight cases that depend on the storage volume of facilities. The basic scenario, S_1, was applied to the analysis under the condition of an 18,200 m3 existing storage facility (S0) installed in the lower basin. Additional scenarios based on the scenario S_1 were set according to the number of sites and locations of a new storage facility 720 m3 in volume, as shown in Fig. 3. After setting the appropriate point to install an underground storage facilities, reasonable locations and sizes of the storage facilities were identified by applying several rainfall patterns. Fig. 2 shows the time series rainfall applied to each scenario with 100 year return periods and 24 hour duration.
Fig. 3 shows the drainage system in the site for case study and the mitigation effect of inundation before and after installing rainwater detention facilities through the simulation. In the result of simulation with storage facilities installed, the peak flow and total runoff quantity decreased, and the number of flooded areas was also reduced from 21 to 3 overall, as shown in Fig. 3. The simulation shows that the storage system can control and reduce urban runoff.
3. Urban Underground Storage
When constructing underground storage structures in urban areas, underground have space constraints and are interrupted by all sorts of utilities because the use of land and buildings is highly developed. Therefore, multi-placed underground spaces are required when building under conditions of urban spatial limitations linked to the safety of existing facilities. In this study, a reinforced-pillar technology studied (Seo et al., 2012) is introduced to secure the stability of multi-placed caverns for underground storage, as shown in Fig. 4. The studied pillar system has a practical advantage of providing the strength of in-situ ground, as a time and cost-effective structure, in contrast to the concrete walls that have mostly been adopted. The proposed technology utilize most of the strength that the in-situ ground can exert by applying strongly grouting and lateral confined pressure and endures not only stress concentration but also the risk of failure at the side wall, decreasing the reduction of the minimum principal stress when the following tunnel is excavated. The pre-stress and confined pressure have a more active reinforcing effect of internal pressure than the passive confining effect of shotcrete generally used on the surfaces of tunnels. The overall conceptual diagram of the reinforcing pillar structure is shown in Fig. 4(b). The improved pillar system is composed of primary components: tie-rods installed through the pillar, by applying grouting and pre-stressing to the tie-rods; Steel fiber reinforced shotcrete (SFRS) with wire-mesh; steel ribs; and panels connected to both ends of the tie-rod on the pillar wall.
In research, Seo et al. (2012) simulated processes of the construction of multi-placed caverns in order to evaluate the function of a pillar built in urban underground spaces, as expressed by Fig. 5. Analysis was conducted on the change of the maximum and minimum principal stresses in the pillar structure in the sequence of the construction phases. The numerical results showed the applicability of the reinforced pillar as an effective supporting system for the underground structures.
4. Conclusion
Urban floods due to climate change are caused by rapid urbanization and torrential rainfall. The city area has densely populated regions in which urban infrastructures such as commercial and transportation facilities are concentrated. Therefore, more proactive prevention technologies with high-level function against the flood disaster are needed. The AHP method was adopted to examine the vulnerability of the inland and underground inundation by analyzing a series of inundation influence factors, and rainwater drainage tunnels and storage caverns were assessed as an most important alternative measure against urban floods.
To evaluate the mitigation effect of inundation, simulations were performed using SWMM which considers the discharge with rainwater storage facilities. The results showed that the peak flow decreased, and the number of flooded sites was reduced. This means the rainwater storage system has the ability to reduce the discharge of runoff and to mitigate urban flooding.
Multi-site underground tunnels by reinforcing pillars studied for the possibility of installing urban storage tunnels have been limited in consideration of construction and economic feasibility, so it is considered possible to install underground storage structures even in overcrowded cities. It has been shown that the pillar system has a practical advantage of exerting the strength of in-situ ground, as a time and cost- saving structure, without relying on pre-cast concrete structure.
This study will contribute to the establishment of appropriate and comprehensive measures against urban flood disasters.
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