Flooding in Delta and Coastal Cities

Jan T.L. Yap

CKNet Indonesia

General ^{1, 2) ,3)}

1. More than half of the world’s population lives in urban areas, especially in delta cities. By the mid of this century, the majority of the world’s population will live in cities n or near deltas, estuaries or coastal zones, resulting in even more people living in highly exposed and vulnerable areas to the changing global environment. It is estimated that more than two thirds of the world’s largest cities will be vulnerable to rising sea levels and climate change, with millions being exposed to the risk of extreme floods and storms. It is also expected that the frequency, intensity and duration of extreme precipitation events will increase, as well as the frequency and duration of droughts. 
2. Socio-economic trends further amplify the possible consequences of future floods, as more people move toward urban areas and continuous investments are being made in ports and harbours, industrial centres and financial business in flood-prone areas. 
3. At the same time, many delta cities suffer from severe land subsidence, because of uncontrolled groundwater extraction for water supply purposes. As a consequence of these urban developments, trends and projections for land subsidence and climate change, the vulnerability of delta cities is expected to increase in the decades to come. 
4. The choices made today will influence vulnerability to climate risk in the future. It is important to link adaptation measures to ongoing investments in infrastructure and spatial planning, and to draw up detailed estimates of the benefits of adaptation. In this way, adaptation becomes a challenge rather than a threat, and climate adaptation may initiate opportunities and innovations for investors and spatial planners.
5.       Sea level rise is a natural phenomenon, and historical measurements in several cities which could be attributed mainly to regional subsidence of the earth’s crust, which is still slowly readjusting to the melting of ice sheets since the end of the last Ice Age. Land subsidence in Rotterdam and New York accounts for 3-4 mm per year, mainly due to post-glacial geological processes. Much higher subsidence rates occur, for example parts of North Jakarta are sinking at a rate of 4 to 10 cm per year due to groundwater extraction.
6.       Important elements of Flood Vulnerability are

a.      Probability of a flood occurring,

b.      Possible consequences of a flood in terms of casualties, direct economic damage and intangible damage, and

c.      The adaptive capacity of a city or system following the event through evacuation, recovery, financial aid and insurance relief options.

Estimates of flood risk and vulnerability can be further disaggregated into vulnerability to coastal floods, vulnerability to river floods, and vulnerability to extreme rainfall. In all these cases the impact can be very high with numerous casualties and much damage to property.

Delta and Coastal Cities in Developing Countries

Population Growth and Land Use

7.    Population growth in many large cities in developing countries is putting huge pressure on the urban environment. Traffic is gridlocked, air quality is at crisis point and city drains and rivers are choked with human waste and garbage. Poor sanitation also creates serious health threats. Population pressure converted a large part of the city’s small lakes and green areas into residential or commercial areas, leading to severe reductions on retention capacity and increases in peak flow discharges. Some of these large cities at present have less than 10% of city area that can be defined as open and green areas, while such area should cover minimal 30%, according to many government regulations.

8.      As the built-up area expands, its water retention capacity for direct rainfall reduces while overflow of the rivers and streams flowing through the cities increases runoff from the upper watersheds due to uncontrolled deforestation and overbuilding in these watersheds. The upper watershed runoff has increasing sediment loads due to erosion and landslides from the deforested build-up areas and, in turn, this exacerbates the damage of flooding downstream. In some coastal cities, such as Jakarta, large areas of the city are below average sea levels. It is clear that floods coinciding with high tide are further exacerbated by back-up of the flow of rivers and drains.

Land subsidence due to uncontrolled groundwater extractions

9.  Rapid urbanization along with severe uncontrolled and over-extraction of groundwater in areas not connected to the municipality water supply distribution system leads to continuous subsidence of the ground surface. Over pumping of the shallow and deep aquifers underlying the area causes land subsidence that, in turn, exacerbates local flooding due to poor and impeded internal drainage and reduction of outlet capacity. Failure to address groundwater abstraction controls could exacerbate local flooding and traffic disruption from normal rainfall in the medium term and require expensive “pumped polder” systems and large outlet infrastructure over large areas.

10.   A recent study revealed that the coastal area in north Jakarta has reached a very critical level in terms of land subsidence. Recent subsidence measurements indicate that a 2.5 cm/year subsidence rate was too conservative and recommends that the value should be considered much higher. Most experts assume a rate of 7.5 – 10 cm/year, but the latest figures show that locally subsidence rates may reach 15 – 25 cm/year. This will bring the northern parts of Jakarta some 4 to 5 metres below sea level in the 15 – 20 years to come. When subsidence is not stopped, by 2100 north Jakarta will sink at least another 5-6 meters from 2010.

11.   This will leads to impeded drainage even for normal rainfall and permanent inundations from the sea even at low tides. These areas will become unsuitable for human settlements, unless a polder concept (expensive) is being applied for these areas.

Urban Development and Management

12.   Many coastal cities are prone to flooding due to their unfavourably low location on the coast. Some are located within river basins of several rivers transporting large amounts of water. Over the years the water contains more and more silt and sediments, while the peak flows becoming higher and skewed. Most of these changes in flow pattern are due to developments in the upstream parts of the river basins. For the past decade, climate change has probably also contributed to a change in river flows due to higher rainfall intensity and thus higher peak flows.

13. These changes in rainfall pattern and river flow characteristic together with land subsidence along the coasts created a major challenge for the drainage system of these coastal and delta cities. Because of land subsidence the natural outflow into the sea does not exist anymore at many sea-outfalls.  Although technically speaking engineering solutions for the flood problem may be identified, these will require enormous amounts of funding, apart from the question of their social and environmental feasibility.

14. The reduction of open space and green area also affected the drainage flow pattern in the cities. The direct runoff within the city is much higher than originally estimated for the design of the drainage system. In addition, the drainage system itself is very much affected by land subsidence leading to low and ineffective performance of the system.

15.  Any structural solution and adjustment of the physical infrastructure to appropriate protection levels will require careful economic, financial and social trade-offs. If financial and social constraints require a lower standard, then an enforceable system of non-structural flood control measures (flood insurance, flood proofing, and flood zoning). In many coastal cities in developing countries to date these systems have been inconceivable and/or for which no appropriate urban and regional institution exists, and should be seriously considered.

16. The issue of sustainable of any design standard is a major problem where no agency accepts or is allocated operational responsibility for maintenance and/or inadequate Operation and Maintenance (O&M) funding is provided. Maintenance neglect and lack of a sustainable fiscal framework for O&M is as large a cause of flooding through under-capacity as any physical or land-use control reason.

17.   Spatial Plans of almost all large coastal cities in developing countries underscore the problems of limited water supply, annual flooding, inadequate garbage and sewage management, and the continuing challenge of providing enough decent and affordable housing for low income families. A lack of open space and massive air and water pollution problems topped the list of some spatial plans of most metropolitan cities. Air and water pollution, including industrial waste, are cited as related environmental deficiencies.

18. The rapid loss of the open spaces and green areas means that even mid storm events result in excess water that cannot be naturally absorbed or retained. In some cities a system of key flood gates exists in some of the urban area canals and retention ponds. However, these suffer from blockage and may not be properly operated in a regional manner as they were designed for a smaller city and are not operationally adapted to the present land-use or present urban flood hydrology. Recent storms in the last decade thus brought out the consequences of a failure to follow a logical urban development zoning plan in a natural floodway area. Certainly, a more integrated operation of the floodwater regulation network is required, especially one that reflects a consensus between upstream and downstream communities.

Institutions and Management Issues

      19.  Most of the coastal and delta cities flood studies, listed the following institutional root causes of flooding              in these areas:

·    Lack of enforcement of regulations on groundwater abstraction.  Some municipalities prohibit groundwater abstraction without a license.  In practice, this regulation is not enforced.

·          Lack of enforcement of spatial plans and building regulations.  Regulation of buildings according to a spatial plan (based on floodplain management) is not carried out or enforced.  Furthermore, new housing developments within and around the coastal cities have not been regulated by a spatial plan designed to ensure retention of adequate green areas that would have stored and absorbed normal flood runoff.  The increased paving resulting from extensive build-up of housing and roads further reduces soil capacity to store rainfall and exacerbates runoff within the city.  The lack of enforcement of building regulations also contributes the land subsidence, riverbank encroachment, and the rapid disappearance of city lakes.

·          Limited coverage of solid waste collection services.  Many municipalities in developing countries collect less than 40% of solid waste that is generated within its boundaries.  In the absence of alternative options for waste disposal, the remainder is discarded in uncontrolled dumpsites or into the city’s canals and lakes, thereby clogging floodways and drains.

·          Insufficient funding for operations and maintenance.  Actual O&M budgets of the Municipality Public Works Departments are substantially lower than budgets required to properly maintain the infrastructure (for example, in 2002, NEDECO estimated that spending by Jakarta Municipality on routine maintenance was less than 10% of the required amount).

·          Limited technical expertise.  The organizations responsible for flood control systems in these cities lack the technical expertise to manage these systems according to the standards needed for a large size metropolitan area.  Many do not have structural systems for annual inspections of flood control systems, flood preparation drills, or collection of flood data (such as post-flood mapping).  In addition, flood warning and disaster management systems are not well developed or effectively operated.

·          Lack of enforcement of forest law and regulations.  After many years of illegal logging, most forests in many river catchments have disappeared. In some cases a planned reforestation program exists, which would mitigate erosion along the river and its tributaries, but has not been implemented accordingly.

·          Insufficient funding for investments in new flood control infrastructure.  Because of budget constraint, national and sub-national governments spend available funds almost exclusively to operations and maintenance.

·          Lack of coordination between authorities responsible for water resources management.  No platform exists for the coordination of the planning, operation and maintenance of the entire flood control system among the various national and sub-national governments responsible for water resources management (including activities closely related thereto, such as solid waste management).

·          Lack of incentives for interregional coordination.  Provinces, districts and cities in upstream areas do not have financial or other incentives to mitigate floods that mainly affect citizens outside their jurisdictions.

·          Absence of political leadership to address the above issues in integrated manner.  This is perhaps the most important constraint to the mitigation of coastal and delta cities’ annual floods.

      20.   Although many of these issues have been given more attention in the various project initiatives in the                   past, many of these institutional arrangements that has been included or initiated ceased to exist after                 termination of the projects. These initiated institutional arrangements and systems need further                           development, facilitation and guidance after the project has ended, to become a sustainable institutional               instrument and platform for collaboration and coordination.

References:

1)     Connecting Delta Cities, 2010, City of Rotterdam, Arcadis, Vrije Universiteit Amsterdam, Rotterdam Climate Initiative

2)     First UCCRN Report on Climate Change in Cities, Forthcoming, Rosenzweig, C., Solecki, W. and Hammer S.

3)     Ranking Port Cities with High Exposure and Vulnerability to Climate Extremes Exposure Estimates, OECD Working Paper No 1, 19/11/2008.