Applying planning principles to reduce flooding: A key concern for coastal megacities in Asia

Applying planning principles to reduce flooding: A key concern for coastal megacities in Asia

This insight article is based on a research paper done by enCity’s Managing Director, Dr. Ho Long Phi published in the “Proceedings of the 2nd International Conference on Water, Megacities and Global Change” by UNESCO (January 2022).
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Dr. Phi Ho is a well-known expert and has been involved in urban water management strategic planning for 20 years. He led numerous research and planning projects on the subject for Ho Chi Minh City, Mekong delta and other coastal provinces of Vietnam. 

More than two thirds of the worlds’ largest and fastest-growing cities are built in low-lying coastal delta regions. These locations prove strategic for cities and are often attractive due to their abundant opportunities, rich culture, comfortable climate and available natural resources. Current urbanization trends and climatic conditions have however caused them to become vulnerable to flooding, submersion and rising sea levels. This article offers insights from a detailed study of one such example, Vietnam’s economic hub, Ho Chi Minh City (HCMC), which offers lessons to assist in mitigating hazards of flooding in cities through timely prevention. 

Considered to be one of Southeast Asia’s thriving megacities, HCMC is located 50 kilometers from the coast and built on unconsolidated marshland between two rivers: Saigon River and Dong Nai River. It has a subtropical monsoon climate with an average rainfall of about 2000mm during the rainy season from May to November. Although HCMC experiences few typhoons, irregular convective rains are common. During September, HCMC experiences intense rainfall which amounts to over 100mm/h, yet rainfall varies between districts. Data from the Phu An station, located at the city center, shows that high tide levels during the monsoon period reached 1.71 masl (meters above sea level) in 2017. According to the Southern Regional Hydrometeorological Center, tide levels have increased significantly over the past years, from 1.55 masl in 2010 to 1.68 masl in 2014.

The challenges of population growth, expansion and urbanization 

Over recent decades, Vietnam’s economy has grown tremendously and along with it, there has been a rise in urban population and urban migration. From 1990 to 2020, HCMC’s population more than doubled from 4.1 million to 8.7 million. This number is estimated to be closer to 13 million if it accounts for unregistered inhabitants and daily commuters from nearby provinces. With a projected annual growth rate of 3%, the megacity struggles to develop its infrastructure to accommodate the rapid increase of its population and urban activities. 

Figure 1 – Extension of the HCMC megacity from 1900 to 2007 (VCAPS 2013 in Tranh Ngoc et al, 2015)

Along with increasing urbanization, HCMC has expanded greatly throughout history. This had a significant impact on its natural surroundings, particularly to the area’s natural water network. During the French colonization, canals intersecting the city center were filled and replaced with  tree-lined boulevards. Swamps and agricultural fields were filled to build residential areas. Currently, important extensions of the city at the North East, South East and South West regions—where new residential, commercial and industrial areas are built—are developed over former marshes and rice paddies with low elevations of 0.5-2.0 masl. HCMC is now among the top 20 megacities and 50% of its population lives in low lying, susceptible areas.

This development pattern, combined with HCMC’s weather conditions and rising tidal levels, exacerbates the problem of flooding within the city, which has become a regular and annual phenomenon. Based on a study by the Vietnam Climate Adaptation Partnership (VCAPS), flooding in HCMC can be categorized into two different spatial typologies: 1. In the districts of the urban center as a result of runoff on urban soils; and, 2. Along the riverbanks and peripheral lowlands. This is caused by the combined effect of high tide and high river flow. Both cases result directly from the effects of urbanization. 

Figure 2 – Delimitation of the flood origin in HCMC, (from VCAPS 2013; published in Vachaud et al.2018)

Urban Development and Flooding 

The study reveals that there are four direct impacts of urban development on flooding:

1. creation of a heat urban island effect increasing the rainfall intensity;

2. reduction of permeable surfaces increasing surface flow;

3. poor drainage system in terms of insufficient maintenance, lack of user awareness and poor sewage network distribution; and,

4. impedance of natural water flow to rivers due to soil subsidence.

As HCMC is one of the world’s densest cities, authorities and developers are trying to address overcrowding through the construction of high-rise buildings with air-conditioning and new city roads for 7.6 million motorbikes and 700,000 cars. As a result, heat absorption by concrete, low reflectivity of roofs and roads, and thermal emission from traffic and air-conditioning units produce urban heat island effect. 

Another consequence of HCMC’s massive urban development is the loss of permeable surfaces needed to absorb stormwater runoff. At present, 90% of the megacity’s urbanized spaces are hard surfaces and this is one of the factors leading to inner city flooding. Poor maintenance and distribution of the megacity’s drainage system is heightening the issue. The sanitary and stormwater network is now undersized and lacks proper upkeep while the open canals and creeks are mostly inoperative because of impediments brought about by informal settlements and improper deposit of solid waste. 

Lastly, HCMC is actively experiencing land subsidence due to intensive groundwater pumping and its natural geological composition. As the ground is naturally soft clay and sand layers, excessive construction causes compaction and settling. Illegal pumping of groundwater daily worsens the problem along with the decrease of the hydraulic slope from the megacity and its suburbs to the river and ocean, which hampers natural drainage. 

With HCMC’s continuing expansion into low elevation areas that are nearly at the altitude of the average water level of its rivers, and its increasing subsidence rate, flooding has become inevitable when the water level rises. Regular occurrences have raised concerns about future human and economic risks in the megacity and the whole Mekong Delta region as riverine floods are strongly influenced by rising sea levels. 

Global conditions of thermal expansion of seawater and melting  polar caps, both consequences of climate change, are some of the biggest causes of global rise in sea levels. 

For a more in-depth review of the analysis and its key findings click here (find page 70). 

A more integrated and promising approach in long-term multi-disciplinary urban planning

Currently, the local government’s approach has leaned towards short-range strategies under two major civil engineering programmes: the “Urban Drainage System Master Plan” (Plan 752) of 2001, and  the “Irrigation and Flooding Control Master Plan” (Plan 1547) of 2008. Both focus on developing and improving infrastructure; filling up low-lying areas, clearing clogged sewage systems and drainage canals, investing in equipment to pump out flooded areas and building flood sluices and dykes along rivers and coastal areas. 

The drawback of these hard solutions are that they are heavily based on past data but climate variability can be highly volatile. To combat these foreseeable hazards and losses, adaptation measures should be put in place through strong political leadership. Therefore, a more flexible and promising approach is the adaptation of long-term multi-disciplinary urban planning. This involves a diverse group of scientists, planners, engineers, economists and politicians working together in order to define the best strategies between adaptation and relocation.

Some actions that would complement this approach are listed below:

1. Limiting subsidence through the development of an efficient water distribution system. By improving the city’s supply of water, the government can issue policies to restrict or prohibit  pumping of groundwater to reduce settlement. This can also include measures for rainwater storage and aquifer recharge.

2. Decreasing urban heat island effect through more effective architecture. The addition of green roofs can lower temperatures in the city  significantly when applied at an urban scale. More importantly, the reduction of urban atmospheric temperature will also lead to a decrease in convective rainfall, and therefore to lower flood conditions.

3. Adapting policies towards expanding the role of ecosystem services. Spaces for green and blue belts can be allocated in megacities and made mandatory for new planned developments. Preserved natural areas and parks can serve as multi-functional basins for stormwater retention and tidal flow volume catchment.

It is clear that without a more strategic approach to mitigate potential disasters and subsequent impacts on cities and urban populations, uncontrolled urbanization will continue to risk decades of development progress. Not only do HCMC’s policy makers need to play a more involved role in driving integrated and sustainable planning decisions by engaging multiple sectors in economic,engineering,ecology and design processes, such an outlook must be considered in development systems globally. Aside from infrastructure-oriented solutions, action needs to be taken from a holistic perspective to successfully address the various causes of urban flooding and land subsidence in the megacity. 

As an urban solution provider, enCity focuses on research and application of blue-green infrastructure solutions as well as urban management policies and frameworks to build urban resilience in areas that are more vulnerable to climate disasters. This is enCity’s long-term commitment to develop innovative and effective solutions for minimising the impacts of natural disasters on our central provinces and elsewhere, and beyond that, to contribute to the struggle of humanity against climate change. This builds on work that we have done before, such as the creation of the MOTA framework (Motivation and Ability framework), applied to strategic urban planning in the context of climate change. In a previous conversation with Dr Phi, he suggested that a new planning approach is needed to adapt to the uncertainty.

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Not only do HCMC’s policy makers need to play a more involved role in driving integrated and sustainable planning decisions by engaging multiple sectors in economic,engineering,ecology and design processes, such an outlook must be considered in development systems globally.

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