Implementing a Rainwater Sustainable Control in Sahel-Feedback from “Niayes” Zon

【前言】Implementing a Rainwater Sustainable Control in Sahel-Feedback from “Niayes” Zon由文秘帮小编整理而成，但愿对你的学习工作带来帮助。1. Introduction?? As a new vision, “Sustainable Development” is becoming an essential concept in the world. For its implementation, recent studies aim at defining new paradigms focused on a holistic approach. This is the goal of the present study...

Sylvestre Dasylva

Laboratory of Physical Geography, CNRS, Meudon 92195, France

Received: May 25, 2011 / Accepted: July 4, 2011 / Published: January 20, 2012.

Abstract: Dakar area landscape is mainly characterized by the existence of the “niaye”-or agricultural depressions located in the sand dunes system. In these depressions, the top-table of quaternary sand groundwater reaches or overflows the soil surface. Because of a recent groundwater level drop, some of the niayes have become runoff-gathering sites. This water ensures various economical, environmental and social services, i.e. urban agriculture, drinking water supply of Dakar and formation of ecosystems supporting biodiversity. Given that rainwater infiltration is the only natural “input”, conception and implemention of rainwater management PPP(Politics, Plan and Program) must necessarily take into account this interrelation. Nowadays, two acute problems are observed in the Dakar area. On the one hand, the niayes are threatened by a hydrological drying process due to the insufficiency of rainwater refill. On the other hand, the dramatic social pressure on the environment has led to the urbanization of these bottom-lands. Furthermore, because of the pluviometry decrease, a worsening of the nuisances (floods and malaria) related to surface waters occurs. Consequently, an appropriate runoff management should integrate both the “risk” and “resource” dimensions, thus allowing the protection of natural resources and a secure living environment. According to the limits of “classical” solutions, this paper provides approach elements for building a sustainability plan focusing on emergent concerns, which would control rainwater in urbanized zones.

Key words: Rainwater control, emergent issues, natural resources protection, living environment security, participative governance.

1. Introduction??

As a new vision, “Sustainable Development” is becoming an essential concept in the world. For its implementation, recent studies aim at defining new paradigms focused on a holistic approach. This is the goal of the present study which concerns the “niayes”, depressions located in the sand dunes system. The main characteristic of these lands is that the groundwater top-table reaches or overflows the soil surface. Functionally, this water ensures various economical, environmental and social services.

In water shortage and uncontrolled urbanization contexts, this paper gives reference elements allowing to design and to implement a rational control of rainwater in urbanized zones. Principally, evaluation parameters identifying emerging territorial issues (i.e. technical, environmental, economic and societal) are indicated, through focusing on urban process, by groundwater dynamics, technical practices of farming and draining of rainwater, and institutional management. Multifunctional benefits concern the protection of natural resources, the designation of territories resilient to hydrological hazards, and the development of participatory governance.

2. Data of Problem Assessment

During the last decade in Sahel, rains have tendency to cause flood provoking different kinds of damages. In this context, we focused on the wetland features of the niayes, the impact of rainfall variability on functional ecosystems, and the need to adapt rainwater management practices to the recent drying trend.

2.1 Features of the “Niayes”/Urban Agriculture & Biodiversity

(1) In the Dakar area (sub-Saharan Africa), the niayes are bottom-lands devoted to traditional farming practices, specialized in vegetable crops (cabbages, carrots, salads, onions and tomatoes), and in fruit production (mainly mangos, papaws, oranges, mandarins). Urban and peri-urban agriculture is in constant development, and now providing about 60%—some 39.000 tons annually—of Dakar’s consumption in vegetables [1]. Because of climatic and human pressures, the reliability of water supply has become an essential concern. Even more, exponential population growth leads to a continuous increase of food demand, meaning the necessity of adjust the offering by mobilizing new water resources;

(2) A recent research in Senegalese north coastal zone by Dasylva et al. [2] confirms after that wet vegetation is maintained, in spite of Sahelian climate and human pressure, so that Niayes represent an important biological reservoir. According to Ndiaye[3]: (1) 20% of Senegalese flora (approximately 419 species) have thereto been identified; (2) 13 of the 31 endemic species of the country are located in these ecosystems. Beside, a report of IUCN (International Union for Conservation of Nature) wetlands network about fauna diversity [4] indicates that 133 species of birds have been indexed in “Grande Niaye” (Dakar), including 40 endemic, 25 migratory and 51 nesting. Moreover, these sites are the remaining “green lungs”of the capital, giving them an important role as a recreation and tourism feature.

2.2 The Water Control Issue

Due to the double pressure of rainfall variability and of human activities (agricultural and industrial pumping), the niayes are threatened on the long term by the groundwater levels dropping. As stressed by a common document by AFDB (African Development Bank), FAO (Food and Agriculture Organization) et al.[5] stating that the present and future water shortage in Africa is caused more by a lack of water storage systems than by a lack of resource. As example, the evacuation of rainwater by the Dakar urban drainage systems towards sea or evaporation basins represents a loss of resource. In this context, it is strange that the actions undertaken to mitigate the water-resource declining process in the Dakar area do not address the recuperation of urban runoff.

In the resource scarcity context, the rainwater management strategy that we target promotes the‘resource’ aspect (i.e. the valorization of runoff) in the practices. By generating additional waters, the goal is to create or support agricultural activity and/or to contribute to the maintenance or restoration of functional ecosystems like the niayes, or any other wetland in general.

According to projections, by 2020 more than half the population of Africa will live in urban areas. Given an industrial expansion not successful in reducing urban poverty, Smith et al. [6] argue that is essential to integrate urban agriculture into planning projects. Referring to the preeminence of this activity in Africa, this prospect could constitute an essential mean to fight poverty and to promote economic growth.

By relating these challenges to rainwater uses, we believe that the design and the implementation of a sustainable plan in urbanized zones imply the evaluation of several parameters, including the“resource”, “risk” and “governance” aspects.

3. Assessment Elements for Highlighting Socio Economic and Environmental Issues

In the Dakar dune sands, what are the analysis elements highlighting socio-economic and environmental emerging concerns (water constraints and beneficial use) related to a rational rainwater control? Sustainability of actions is guaranteed by taking into account the diversity of parameters controlling the living environment security and the protection of functional ecosystems. Questions raised below are often observed in Sahelian zone, and have been profoundly discussed by Dasylva in a published handbook [7] and forthcoming1.

3.1 Urban Pressure: Impacts Symbolized by a Progressive Disappearance of Wetlands and an Aggravation of Floods

The Senegal capital, Dakar (2.9 million inhabitants in an area of 550 km2), exerts a strong attraction on rural populations. The urban growth has been so fast that six town-planning plans followed each other between 1862 and 1986 (Dasylva [8]). Nowadays, the rate of urbanization reaches 100% in many neighborhoods of the city.

This urban pressure is responsible for the occupation of niayes and other flooding sites, overall by poorest people. This urbanization process is difficultly controlled by the government; as the land tenure is always dependent on traditional usages. In most case, the ‘irregular’ districts correspond to the sites where floods arise with more acuity because of the lack of equipments capable of preventing or fighting the surface waters rise. Moreover, land parceling is characterized by an irregular subdivision causing many obstacles for setting ‘classical’infrastructures, and for achieving successful response to disaster emergency. All these concerns increase the vulnerability of natural habitats and of human populations. The Japanese study office of JICA (Japan International Cooperation Agency) [9] identified in 1989, 45 flooding sites in Dakar and Pikine: we decided to review later the technical constraints related to the sustainable control of these sites.

3.2 Climate Peril

(1) An insufficient renewal of wetlands water reserves

The consulting firm “Beture-Setame” [10] demonstrated that rainwater infiltrations form the main refill factor of the “niayes” groundwater, which means that the evolution of this parameter has a decisive effect on the resource behavior. Belonging to the Sahel climatic zone, the main characteristic of precipitations at Dakar is a high variability. After 1968, the annual precipitation amount exhibits a strong fall. The annual average between these “two”periods decreased from 574 to 342 mm, i.e. a 40% deficit according to Dasylva and Cosandey et al. [11]. Consequently, the relevant information to look for is the effect of annual rainfall variability on groundwater behavior. With annual precipitation amount trend nowadays characterized by an evolution toward a“normal” situation, i.e., we would expect a “win”since the trend would have a positive impact on water resources.

In order to assess the relationship between annual rainfall amount and water resource evolution, we take the QSG (Quaternary Sands Groundwater) occupying the niayes as an example. Its behavior was analyzed by Dasylva et al. [12] according to three precipitation scenarii, representing an average, dry and wet annual pluviometry. The main results obtained by the author concerning the static level balance are: 1/for a 529 mm yearly precipitation (close to the annual average regional value), the balance is negative; 2/the annual variation is positive when the annual precipitation is about 700 mm. If we refer to the current annual precipitation amount (with the exception of year 1989), the QSG refill balance is nowadays in deficit. These results confirm the depletion trend of water reserves highlighted by Dasylva et al. [13], except in the north and south coastal zones where the groundwater level evolution seems to be stationary.

According to the role of the annual precipitation amount on the groundwater behavior and to the rainwater infiltrations being insufficient, in the current climatic context, to face at the climatic and anthropic request, we estimate that the enhancing of groundwater recharge (by a better control of rainwater infiltrating rate) is a key issue. This enhancing may be obtained through a sustainable management of runoff to be implemented in the building plan process.

(2) The weakening of traditional agricultural caused by the drop in groundwater levels

In Africa, urban traditional agriculture is a widespread practice. This activity is dominated by the use of small farming pieces of land not exceeding 1,200 m2 [14]. In Dakar area, these family farms represent nearly 70% of the structures(SNC-Lavalin/Bceom Group [15]). The groundwater is often mobilized by the farmers by use of traditional techniques designated by the vernacular term of“céanes” (i.e. excavations in the ground, reaching sometimes a 2 m depth). However, an essential performance condition is that the top-level of groundwater should not be far from the soil surface.?

With regard to water reserves depletion, the groundwater roof position is a determinant parameter for safeguarding the traditional urban agriculture in Africa. Consequently, hydrological measures are necessary to mitigate the groundwater dropping process, meaning that its depth must be maintained at a suitable level.

According to the emerging environmental and socioeconomic issues observed in the Dakar area, our paper will suggest rainwater sustainable governance elements. Before, however, the technical and political problems related to the implementation of “classical”solutions for protecting the living environment, will be discussed below, enlightening some additional parameters to be take into account.

4. Rainwater Sustainable Management Elements in Urbanized Zones

In the Dakar area, the edification of a drainage network did not lead to the eradication of flooding, hence our interest in identifying the causes of this dysfunction. In order to complete our knowledge of the issues responsible for this situation, we will review the limits of current practices technical and strategic requirements for performance of solutions.

4.1 Reflexive Return on the Organizational and Operational Issues of Current Practices

(1) The limits of the “classical” solutions

The official drainage network is essentially composed by infrastructures classified as classical solutions, i.e. linear axes, water discharge stations and reception basins. It is important to know that African engineers are strongly influenced by the “hygienist”concept, dating from the late 19th century (Act of July 10, 1894 in France). In this approach aiming the rapid evacuation of water in order to prevent nuisances in building areas, no concern is given to an economic valorization of rainwater.

The link between flood persistence and the low density of drainage network leads to a sectoral control of waters. However, we must not lose sight that there are other technological and/or environmental constraints. In Sub-Saharan Africa in general, hydrodynamic conditions and human behaviors required for a better functioning of infrastructures are not gathered. On the one hand, both the silting due to erosion and human solid waste discharges in the network are major constraints limiting operational performances. On the other hand, the runoff coefficient plays a key role in calibrating the drainage network. Given the uncontrolled urbanization process in African cities, it is very difficult to estimate correctly these parameters which are continually and rapidly changing.

In the light of these clarifications, the debate about the appropriateness of the use, solely, of the classical technologies in sub-Saharan Africa is raised. While rainwater drainage network is regularly overflowed by runoff, we think that it is necessary to adapt the technological management strategy, by promoting compensatory solutions (or “alternative” technics), in order either to strengthen the drainage network or to decrease the runoff volume.

(2) The limits of the spontaneous actions of populations

To compensate the lack of infrastructure, technical arrangements (most often implemented by populations) are taken: building either backfill or a small wall front around the concessions, or elevating the foundation of buildings. In the upstream zones, the two first actions can be efficient, the risk being mitigated in relation to the fast and easy evacuation of water on sloping areas. On the contrary, the water-gathering sites in the bottomlands are often submerged by waters. When applying the same actions as in the upstream parts of the basin, the population does not take into account the specificity of each zone and the variability of the hydrological parameter. Imitation being a general rule, protection measures may give satisfaction in particular local context. There is however an urgent need for the coordination and the adaptation of spontaneous actions. Moreover, these technical achievements tend to change or disrupt the hydrological cycle or hydrodynamic conditions. For example, the backfills induce an accumulation of rainwater in the concessions because the hydrological continuity is broken. These spontaneous measures do not eliminate the risk exposure. In the most populated districts of Dakar, another important challenge is to find appropriate technical arrangements to solve the problem of water accumulated in concessions. Because of this accumulation, the widespread use of these technics has led to a topographic leveling process, and to a spatial extension of flood impacts.

(3) The limits of the institutional management approach

ONAS (National office for sanitation of Senegal) is the official actor in charge of sanitation network. With a top-down approach, this office ensures the design, implementation and maintenance of the infrastructures. Referring to the technical constraints mentioned above, we think objectively that the solutions applied by this institutional service do not solve all problems highlighted. For example, the drainage network density remains low while the maintenance of infrastructures is irregular. Therefore, a suitable governance of rainwater will require an adaption of the current management system. To respond to the time constraint and the need of performing actions, alternative solutions require the promotion of the participatory governance of rainwater. Such governance will address more numerous stakeholders; make the rainwater management an affair for all.

Even today, waters collected by the drainage infrastructures flow out to the sea or evaporate in closed basins: both cases concern 90% of the channels of the west part of the Dakar peninsula [16]. We consider that these destinations lead to a loss of resource. In today’s water shortage context in the Sahel region, it is urgent to develop a rainwater control through appropriate actions based on clean and sober harvesting methods, and aiming to avert a loss of resource either by maximizing the recharge of groundwater reserves or by creating additional hydric resources.

We support that runoff in cities should play a key role in improving hydric reserve availabilities and for diversifying water supply sources. Differential rainwater management policies between rural and urban areas must be changed and oriented towards sustainable achievements, by integrating risk and resource dimensions, whatever the reference zone.

Before suggesting a sustainable technical approach(i.e. complementary strategy & multifunctional drainage network), a reminder of the issues reviewed is provided below.

4.2 Recapitulative of Issues/Validation Checklist of the Management Model

The diversity of challenges for controlling rationally rainwater in the Dakar area is indicated in Table 1 where we propose a thematic approach. This dashboard could be used as a base for determining a suitable multifunctional drainage network, which meanwhile would validate the relevance of an action-plan. The goal is to avoid “copy/paste”solutions, the application of which would not be adapted to the Dakar context.

4.3 Complementary Strategy of Maximizing Current Measurements Performances

In order to take into account the issues listed in Table 1, it is necessary to identify a compensatory strategy correcting the limits of current practices. In order to do so, a model describing the rainfall impacts on the natural resources and living environment, may provide the identification of the intervention parameter (Fig. 1).

In the light of this demonstration, compensatory management strategy defined in the dune sands zones is the following:

A COMPLEMENTARY STRATEGY = implementing AND DEVELOPPING TECHNOLOGIES FAVOURING INFILTRATION

The challenge is to modify the decline and deficit in soil permeability by increasing the infiltrating rainwater rate feeding the groundwater. The goal is to maximize the refill (by additional inputs), and to reduce considerably runoff rate (by adapting the capacities and performance of drainage network in place). Note: regarding financial considerations, feedbacks have shown that the use of low-cost technologies (Smart solutions) does not increase globally the capital budget. On the contrary, it contributes to reduce significantly the global cost of sanitation facilities (Deutsch [17]).

Table 1 Short list of complementary issues related to rainwater in Dakar area [7].

Fig. 1 Rainfall impact on natural resources and living environment (Dasylva [7]).

QUALITATIVE CONTROL OF WATERS

For obtaining good quality waters for human use, i.e. respecting regulations, houses are relevant intervention sites. The rainwater is immediately supported at the source, before a contact with contaminated sites existing along the path of waters. The success of such action requires to infiltrate rainwater from artificial surfaces(covered ground).

For decreasing the quantity of waters infiltrating from polluted sites, process must be carried out upstream of waste storage sites and sewage spill areas.

In case of insufficient sand cover to serve as natural filter, it is necessary to include in the devices a filtration mechanism of contaminants.

After the clarification of issues and conditions related to rainwater sustainable governance in urbanized zones, we examine below the kind of infrastructure which should concretely be implemented, with respect to the diverse problems faced?

4.4 Elements of a Rainwater Sustainable Management Plan

NB. In the niayes zone of Dakar, the success of this innovative approach implies the mobilization of additional volumes which must be sufficient enough to reverse the current hydrological trends (by stopping the deficit of groundwater recharge and the increasing of runoff volume). To achieve this target, surface waters must be controlled throughout the hydrological cycle. Concretely, infiltrating process must: (a) be initiated in the building (for capturing the waters which supplies runoff); (b) continue in the streets (for decreasing the runoff volume); (c) also proceed from sites gathering runoff (thus avoiding losses of resource due to evaporation, while reducing the stagnation time of water at the surface).

Technically, sober and robust network mixing classical technologies (channelizing excess waters) and alternative technics (achieving the complementary objectives highlighted above), must be rolled out in order to take into account all the issues necessary for the protection of natural resources and living environment (Fig. 2).

In case of flood, the increase in spatial magnitude and runoff volume requires necessarily an intervention at the start of the hydrological process, i.e. in the houses (private space). Therefore, there is a need to coordinate the spontaneous and official actions in order to optimize the effectiveness and efficiency of the drainage global network to be set up.

Fig. 2 Suggestion of a sustainable management plan applied to the “niayes” zone of Dakar.

5. Discussion: Institutional Benefits to be Expected from the Use of Mixed Infrastructures

The main innovation of this governance model is the diversification of the infrastructures, particularly when using alternative technologies. Not only, such infrastructures take into account the diversity of issues, but they are also in accordance with the response capacities of the local stakeholders. This later issue that the current institutional management system must be modified in order to promote participatory solutions. Given the acuity of rainfall insecurity in the Sahel region, the intervention of a single actor is of reduced performance in reactivity and actions. Consequently, a multifunctional network including the civil society and the users within the governance process is a priority option. The performance of actions needs indeed a suitable intervention in time and space, mostly dependant on the participation of the largest number of people concerned. The main lesson provided by the field observation is the urgency to implement a pyramidal system of governance (bottom-up approach) aiming at promoting a just sharing of tasks (Fig. 3), in order to face and solve the diversity of issues.

The challenge of the participatory governance model is focused on the change of attitudes and behavior in the society, more precisely the modification of perception and representation of rainwater management process in Africa. The goal is to build a local empowerment based on continuing involvement of stakeholders, and not on spontaneous or ephemeral participation.

Fig. 3 Participative approach for a sustainable rainwater governance (Dasylva [7]).

6. Conclusion

In the Sahel region, today is the moment to implement new paradigms for finding solutions to poverty. Changing practices means to avoid the current sectoral (or multi-sectoral) institutional approaches, mitigating in parallel food insecurity, natural resources degradation and difficulties for the access to drinking water. An appropriate strategy will be to act on a common root of the evil, i.e. the unsustainable mode of rainwater management in urbanized areas. The target of this thematic—or multi and cross sectoral approach is to mobilize additional water resources. This target can be achieved by using the high volume of runoff in these areas which constitutes a solution for facing the hydric reserves depletion and the lack of resource. This prospect constitutes the value added of the new strategy, based on the valorization of runoff (flood water) in urbanized zones. Thenceforth, sustainable governance of rainwater is a key approach, which requires attention in all environmental assessment related to food security, biodiversity, resource water depletion and hydric diseases. Consequently, this study provides diagnostic, issues and action elements allowing the construction of an appropriate management plan focalized on emergent concerns in Sahel.

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