Focus on flood control in urban areas

Kurprianov (1979, 198 1 and 1988) proposes a list of the features determining the hydrological peculiarities of urbanised areas (see box 37). Kurprianov adds that “changes in the urban water runoff are primarily determined by the size of the urbanised area, by the population number, industrial enterprises available, volume of water consumption, and by the system of water use”.

According to Gladwell and Kwai Sim (1993), “it has been reported that in Venezuela, (. . .) many towns have grown rapidly, occupying in the process most of the space in the narrow mountain valleys. The streets, acting as major drains, thus concentrate the flood waters rapidly on the lower ground, much of which has been highways, buildings and other types of development. Such encroachments into the natural stream courses and plains have resulted in deaths and major property damage”.

Box 37: Hydrological particularities of urban areas

~ 1) huge water masses often imported from other basins or withdrawn from underground water storage and not drained by the river are involved into the water cycle;

2) fundamental changes in the surface and in the unsaturated zone occur. Drainage- sewerage systems are built which determine new conditions of runoff formation;

3) regime of precipitation and evaporation is changed due to a disturbance of the natural thermal regime and pollution of the air basin;

4) large impermeable areas and intensive groundwater withdrawals result in breaking natural relations between surface and subsurface water;

5) treated or partly treated water is discharged into recipient rivers at particular sites.

Takahasi (1988), referring to the situation on the Kanda River (in the heart of the metropolitan area of Tokyo) also states: “the rapid increase in population (1945-1962) lead to the expansion of urbanised areas to the lowlands adjacent to rivers and to paddy fields, with frequent flood damage since 1958 [and before flood protection measures]“. The author also insists on the extremely high value of land in urban areas. This may be an explanation among others for the difficulties encountered when designing large multipurpose flood plains and reservoirs in such areas. Yoshimoto and Suetsugi (1988) share the view that “rapid economic growth and concentration of population and properties in urban areas [induced] changes in the runoff process, including a shortening in the flood concentration time”, thus advocating that

“structural measures working on the river area cannot alone solve the urban flood problems - comprehensive flood disaster prevention measures must be designed and implemented”.

Richter and Schultz (1988) identified some of the negative influences of man’s activities (urbanisation, industrialisation, construction of roads and airports), especially in terms of aggravated floods in urban areas, on the hydrologic conditions in river basins : increase of flood peaks, reduction of time to peak, reduction of storage attenuation effects (reduction of pervious surface). As a consequence, authorities would need to quantify these changes in order to feed their mathematical or computerised models with updated data.

During the 60’s, it became evident that the solution of water management problems in cities would be very difficult and complex as it must contain both structural (“technical”) and non- structural (land-use planning, legislation, education, etc.) elements (Niemczynowicz, 1988).

One of the reasons for this complexity is that urban areas concentrate extremely different opinions regarding the (environmental) hazards related to water (for example in Sweden and in Germany, when intense discussions took place to decide whether water infiltration schemes shall be developed), as well as urban communities characterised by social, economical, cultural and political backgrounds.

Focusing on urban water management in terms of political science, Walesh (1993) introduces his much interesting article “Interaction with the public and government officials in urban water planning” by the following paragraph: “Successful urban water planning must include meaningful involvement of and interaction with the public including appointed and elected government officials. Objectives of public interaction include having water professionals earn the trust of the public, obtaining data and information from the public and ideas on potential solutions, and building broad support throughout the public resulting in rapid plan implementation (. . .) An urban planning effort that fails to include a public interaction program plans to fail”. In the same way, Gladwell and Kwai Sim, in their book

“Tropical cities: managing their water” (UNESCO- IHP and MAB, 1993) recall that “the capacity of institutions within and outside the cities to manage the effects of natural hazards”

is one of the five requirements for the satisfactory growth of a city, that had been postulated by Lugo in 1991.

b) Socio-economic and socio-cultural narticularities of urban areas

Yet, structural parameters of cities, affecting the circulation of water, do not account alone for the worsening of flooding conditions in urban areas. Cultural, as well as sociological factors, indeed contribute to the “social amplification” of disasters, and thus make it necessary for urban authorities to design and develop specific mitigation strategies that fit the urban context.

These socio-cultural factors are strongly connected with the very nature of cities;

urban areas have indeed always been places where socio-economic and socio-cultural exchanges could breed. Different populations, each with a different cultural background and a different socio-economic goal, “feed” city centres and urban areas in general, coming from the rest of the country, or of the region. There should also be reminded that these different populations and community groups, disconnected from their traditional living place, often tend to re-create small “cultural worlds” in specific areas, or districts, of the city. As a consequence, these multiple “identities” may cause inter-community solidarity among urban dwellers (for example, mutual aid and relief in the event of a disaster) to be adversely affected.

These different urban groups therefore confront authorities with a complex and changing picture of socially differing targets for the design of flood mitigation messages (e.g.:

poor people living in hazard-prone areas, recently established inhabitants, migrants, etc.). In other words, the very nature of cities induces strong differences in the public’s understanding of mitigation messages and instructions. This socio-cultural diversity also has consequences in term of economic status, income level and living conditions. For example in Fort-de-France (Martinique), in the highly seismic French Antilles, the Citron district consists in cheap houses illegally built on dangerous slopes prone to landslides in the event of an earthquake.

There is little doubt that a disaster would have a differing impact on dwellings located in Citron and on dwellings located in safer areas (houses built with better materials, or areas less likely to cause site effects). Social unrest may be triggered in urban areas where huge gaps exist between living conditions, making certain communities more vulnerable to hazards than others.

c) Ontions for narticiuatorv mitigation of urban flooding Urban Jlooding

According to James, Benke and Ragsdale (1975), referred to in Lindh (1979), the selection of an appropriate combination of measures to deal with a particular urban flooding situation requires:

1 - Hydrological information to characterise the hazard,

2 - Engineering and economic information to quantify the flood damages and to design and estimate the costs of various remedial measures,

3 - Information on the character and severity of the ecological and other environmental consequences of floods,

4 - Information on public institutions and agencies in flood-prone areas,

5 - Assessment of public support for the structural mitigation alternatives, and public receptivity to the non-structural measures,

6 - Information on how flood plain land use related to the well-being of nearby residents and to the overall objectives of the community.

We will not further mention here “technical” (both structural and non-structural) solutions developed to reduce urban vulnerability to water-related hazards, and this section will rather focus on participation-related processes, methods and tools. Yet, we may remind that one of the best options for flood mitigation remains a sound land use policy and a controlled urban development that integrates flood-related constraints.

Intensive education programs dealing with storm drainage and drainage control equipment (such as detention ponds, channels, infiltration facilities, etc.), similar to those developed for sanitary purposes, would give to recently settled urban populations a better knowledge of storm urban drainage problems and runoff dangers (Desbordes and Servat, 1988). In order to be realistic and comprehensive, such programs should be designed according to regularly updated population data, including geographic distribution of new dwellers. Such real-time monitoring of urbanisation would make it necessary for the municipality to design, if not a geographic information system (GIS), then at least an acute and interactive follow-up of the major urbanisation trends, with this in view to relying on an updated mapping of the human characteristics of the city. Such education programs should also inform dwellers on the relation existing between building characteristics and housing practices (especially in self-building situations in risk-prone areas) and runoff characteristics.

For example in 1981, the River Basin Council of the Tokyo metropolitan area published the historical inundation map for guiding land use to a proper direction and for the convenience of flood fighting and evacuation. This was favourably accepted by the citizens in the river basin in general (Yoshimoto and Suetsugi, 1988).

Flood-proof retrofitting of houses

Flood-proof retrofitting (FPR) consists of a series of methods, actively involving community members, and meant to reduce the vulnerability of existing homes and dwellings in areas prone to SZOW floods. FPR is intended to reduce the vulnerability of buildings that are not eligible for any existing flood mitigation option (e.g. older buildings in flood plains, or buildings in areas for which a cost/benefit analysis does not warrant a physical flood control structure to be built).

Bradway Laska (1991) provides a definition of FPR : “ floodproof retrofitting is the alteration of an existing structure to prevent future damage due to floods”. FPR can take many forms, and there are many factors that influence what measures will be implemented: the nature of the flooding - including its depth, velocity, and frequency - the type of construction used to build the home, the design of the home, the topography of the land on which the home is built, the person implementing the measures (homeowner or professional contractor), the amount of money available to pay for the measures, the time and money the homeowner is willing to invest in the home, as well as the nature of local construction ordinances or aesthetics styles.

This report is not dedicated to describing the technical aspects of FPR, but rather to explain to what extent FPR measures may be used by authorities as a practical and tangible way to involve the general public in local flood mitigation and flood control strategies. Yet,

before doing so, we briefly present below the four categories often used to classify FPR options (Bradway Laska, 1991) :

1 - Wet floodproofing includes those measures taken to protect utilities and home contents while allowing flood water to pass through the building ;

2 - Dry jloodproofing of a basement, as the term suggests, implies that the measures will prevent both surface and subsurface water from entering the basement through cracks, windows, or other openings;

3 - Prevention of sewer backup (by plug or stand pipe placed in floor drain, sewer check valves, etc.) when the system is overtaxed by rainwater ;

4 - Protection against surfaceJZooding : small dykes, drains, relocation of the home, etc.

We then briefly review applied research findings on community involvement in implementing FPR measures (these findings, presented in 13radway Laska 1991, are based on field surveys in the United States, and may thus not be fully relevant to contexts and practices observed in developing countries).

Homeowners generally show a readiness to implement floodproof retrofitting measures and to invest time and money in order to do so, but may not be ready to take loans with FPR as single purpose, and homeowners with flood insurance seem less willing to spend money in FPR. Moreover, this readiness requires financial and technical support from (local) government, as well as a participation of local entrepreneurs and companies related to FPR activities (carpenters, plumbers, etc.).

Several participatory techniques may be effectively used by authorities in order to foster the implementation of FPR measures in flood-prone areas : formal information and training programmes, “open houses” enabling community members to share know-how and to discuss with authorities and professionals, “mitigation tables” where inhabitants can present their views on comprehensive flood-related issues (beyond mere FPR). Most important, there is empirical evidence that the more local elected/appointed officials show concern and share time with inhabitants, the more community members are keen on implementing FPR measures. Last but not least, environmental NGOs and others should be involved in the promotion of floodproof retrofitting.

Urban policies and-flood risk

Urban policies of “city management” should also be considered as much relevant tools for flood mitigation and control.

Some years ago, in the French cities of Nimes and Marseille, cars parked in the narrow streets of flood-prone areas worked out like small dams when excess rainwater started pouring in the streets. After some hours, water pressure pushed off the cars, creating devastating flash floods waves. Recognising that inadequate car parking was a matter of increased vulnerability (disaster amplification), local authorities decided to regulate or ban parking in some areas of the city. Such non-structural measures, if properly explained to the public, including neighbouring inhabitants of the area affected by parking restrictions, may become low-cost initiatives reducing the flood risk related to water runoff.

Eventually, any initiative aimed at reducing the gaps existing between urban community groups, is likely to favour the development of social cohesion and solidarity, as

well as mutual recogtiition. Briefly speaking, we may assume that community groups of urban areas have lost part of their traditional, “natural”, “cultural space”, in which they usually practised their traditions, customs and values. As a consequence of their coming into the city, these groups tend to re-create this cultural territory, and often limit their communication with other communities and groups. This limitation is even increased by economic factors (very often do communities develop specific economic activities, or labour practices). Recently settled urban communities often propose lower labour costs, or try to develop parallel economic activities, and are thus perceived as a threat by communities already living in the urban area.

Local authorities should therefore aim at promoting inter-community activities, including games and inter-schools exchanges and twinning. The concept of city-dweller, as opposed to “non-city community member” may be used by local authorities to catalyse the fact that the city is a community in itself, likely to “adopt” community groups and form a new part of their identity. The objective is that city-dwellers, or inhabitants of urban areas, share the feeling that they belong to the same space, and that they are threatened by similar water- related hazards. Belonging to the same urban area is likely to foster communities’ involvement in city-wide flood mitigation projects and initiatives. Yet, it is most certain that specific mitigation activities, dedicated to each community or community subgroup, should be implemented. In doing so, authorities should involve community leaders, so that flood- mitigation alternatives gain in credibility, visibility and legitimacy - and reach out all community members.