Submerging the Metropolis

TL;DR

How will be the urban land in Hong Kong flooded when sea level continues to rise? Here are several maps simulating the flooded area when sea level rises to a given level.

Sea level rising, revisited

Sea level rising is never a new term for us. It is estimated that when we are still doing the “business-as-usual” scenario, more than 150 million people living on land will have their living place below high-tide line by 2050.

Mitigation and adaption are both essential to climate changes. Urban planners and designers have proposed several visions to adapt the expected increase amount of water running through the cities. Concepts like “resilient cities” and “sponge cities” are suggested (at least partly aimed) to absorb and prepare the issues raised from sea level rising.

Below is a line chart of the projected increase in the mean sea level in Hong Kong from the 1990s to 2100 where sea level is estimated to raise around 0.7–0.9 m in 2100. It may not sound like a large number¹, but the trend is steadily rising.

Line chart of projected increase in the mean sea level from 1990s-2100 where sea level is estimated to raise 0.7–0.9m in 2100
Line chart of projected increase in the mean sea level from the 1990s-2100 where sea level is estimated to raise 0.7–0.9m in 2100 (Source: Hong Kong’s Climate Action Plan 2030+ Chapter 7, p.73)

However, do not forget the storm surges resulted from extreme tropical cyclones, which is in turn results of extreme weather. Most Hongkonger could still remember how catastrophic Shatin River, Hang Fa Tsuen, Tsueng Kwan O and other coastal areas are flooded when Mangkhut swiped across Hong Kong. Living higher than normal water level does not equal to refrained from flooding. The highest sea level during storms is the factor to determine whether your home will be flooded by seawater or not.

It’s kind of butterfly effect — the extreme weather causes both sea level rising and extreme tropical cyclones, and the highest sea level will be larger than merely the increase in the mean sea level. Extreme cases will, of course, not happen frequently, but we have to consider it to plan for a real resilient city.

Illustration of storm surge and high waves further rise the mean sea level
Illustration of storm surge and high waves further rise the mean sea level (Photo source: https://www.hko.gov.hk/en/climate_change/proj_hk_msl.htm)

To understand the problem, we can perceive it spatially. If we could spot the potentially affected areas, we could do and think more about methods of tackling the rising water specifically for those lands. This is what policy maps aim for — pinpointing area needed for actions.

So, how will sea level rising affect Hong Kong? It’s time to throw the light on the famous Victoria Harbour between Hong Kong Island and Kowloon Peninsula.

Topography of Hong Kong urban area

To give a sense of the geographical settings of the Victoria Harbour, below is a map of it.

Basic Basemap. Nothing more, nothing less.

3D maps would be a better choice in terms of elevation. Following are two 3D elevation models of the area around Victoria Harbour, which are not unfamiliar for Hongkongers. For those befuddled with those long tiny strokes on the map, they are the elevated expressways. The one on the top left corner is Stonecutters Bridge — a cable-stayed bridge spanning across the Rambler Channel, sitting 80 m above sea level.

Top view around Victoria Harbour
Axonometric view around Victoria Harbour

As you can see from the renderings, the hilly environment forces Hongkongers to squeeze in those tidy flatlands immediately next to the coast. A significant percentage of these flatlands are reclaimed land throughout the past century.

The sea level shown in the above two renderings is the mean sea level defined by the Hong Kong Observatory. Let’s consider it as the current sea level. When we talk about the sea level will rise X meters, technically we mean the sea is going to raise X meters from the mean sea level.

Seawater Climb

Here comes one more layer for the maps — animation. The gif below presents the flooded area on the two sides of the Harbour. The animation starts at the current sea level. The sea level then rises uniformly, until it climbs 5 meters above. The inland area turned blue has the same ground level as the uprising sea level. Alternatively saying, these are the expected flooded area when seawater rises to that simulated height.

Up it goes!
Up it goes! (2nd time)

Some snapshots

Let select a few breaks and go back to static maps to see where the flooded lands are. Rising seawater by 1m does not have observable effects to the map — the urban land are still dry. The scenario is more about the same when the water climbs 2 m above mean sea level.

1m above

Flooded areas become observable when the water rises to 3 m above. Part of Central and Sheung Wan started to fill with water. Sheung Wan is a low-lying area with ground levels between 1.3–2.5 m above mean sea level², and is infamous for highly susceptible to flooding decades ago. The flooding risk is alleviated only after the construction of the Sheung Wan Stormwater Pumping Station, which is committed to relief the loading of the drainage system.

3m above

A drastic amount of land is flooded when sea level rises 4 m. This is somehow expected, as we are taught that the ground level of the urban area is about 3–4 m above current sea level³. Almost all the low-lying locations around the coastal area in Northern Hong Kong Island are flooded. On the other side of the harbour, we could find the whole Kwun Tong filled with seawater. Kowloon City, Hung Hom and districts down Nathan Road are also filled with blue. These are the areas more fragile to sea level rising.

4m above

Finally(!), when the sea reaches 5 m above current sea level, the flooded areas are further pushed from coast to inland. Kowloon Bay and inland area of Shum Shui Po are now flooded. The whole Island Line is now submerged in seawater.

5m above

How much land is flooded?

What is the changes in the total flooded urban when the sea level gradually keeps rising? The following chart gives a rough picture. The horizontal axis presents the rise from mean sea level from 0 to 10 m, while the vertical axis counts the total sq.km. of flooded area. The line is a cumulative frequency curve, which represents the total flooded area at the specified sea level. For example, the point of lying on the line at 6 m on the x-axis is around 28 sq.km. on the y-axis. This implies about 28 sq.km. of urban land around two sides of the Victoria Harbour will be covered by water when sea level rises by 4 m.

As a quick refresh to your spatial mind, Victoria Park has a size of 0.19 sq.km. (or 19 hectares). The man-made island of Hong Kong International Airport (HKG) is about 12.55 sq.km..

Changes in the total flooded area (Caveat: the height resolution original elevation model is corrected to the nearest meter. Do not consider the numbers precise and accurate. The graph aims to present the overall trend.)

The largest gap observed is the step which sea level rises from 3.75 m to 4 m above mean sea level. The total flooded area skyrocketed from around 4 sq.km. to 14 sq.km.. As mentioned above, this is the base height of the majority of the coastal areas on the two sides of the Harbour.

Hiatus

Be reminded that sea-level rising is a series of complex movements — we cannot directly assert area with a lower height than the certain sea level must be flooded. However, for a city to become resilient, we need to prepare for the worst, so as simulate the worst scenario.

On the other hand, how should we handle these natural disasters? There have been suggestions and proposals incorporating blue-green infrastructure that could help “absorb” and “retain” water. If the land is paved by materials allowing infiltration (instead of concrete), part of the water will be absorbed immediately and mitigate the effect of flooding. How? And where? These are the questions waiting to be answered.

Reference

More on sea level rising:

If you are looking for a detailed explanation of the differences between 1. Chart Datum, 2. HK Principal Datum (mPD) and 3. Mean Sea Level, check the following:

[1] 1 m roughly equals to height from floor to the door handle.

[2] Technically it is +2.64 mPD to +3.85 mPD

[3​] We have Rifleman’s Bolt (drove in a wall a little bit higher above ground) 5.435 m above principal datum (+5.435 mPD). Mean sea level 1.30 m above principal datum (+1.30 mPD).

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Urban Data Science Enthusiast | Urban Planning | GIS | Maps | Data Visualisation | kennethwong12.netlify.app

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Kenneth Wong

Kenneth Wong

Urban Data Science Enthusiast | Urban Planning | GIS | Maps | Data Visualisation | kennethwong12.netlify.app

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