Many drainage schemes are designed by consulting engineers. However, these are not usually for low-income communities. There is therefore a danger that consulting engineers may design drainage systems for low-income areas which are inappropriate or unaffordable unless the terms of reference for their work are drafted to make it very clear what sort of solution is desired. A municipality that has already obtained funds for the construction of a drainage system will generally have a fairly clear idea of its requirements, and will usually find that consultants' work is most satisfactory when they are contracted to perform clearly specified design tasks within the framework of those needs.
However, national and international funding agencies often require that, before they agree to pay for the construction of a drainage system, it should be the subject of a feasibility study by a reputable firm of consulting engineers. In such a case the funding agency will have its own requirements and will wish to participate in drafting the terms of reference for the feasibility study. Nevertheless, the municipality or the community's representatives can suggest clauses in the terms of reference to ensure that:
- the full benefits of the proposed drainage scheme are demonstrated;
- the study is conducted in such a way as to arrive at the most cost-effective solution;
- an accurate assessment is made as early as possible of the resources that will be required for maintenance of the system;
- construction of the most urgently needed parts of the system is not delayed any more than necessary by the lengthy process of project preparation, approval and design; and
- the community is involved in key decisions.
These factors are discussed in turn.
The health consequences of inadequate drainage, described in section 1.2, indicate the health benefits which improved drainage can bestow. These can be further documented by the consultants, using existing health statistics or the results of any community health surveys that have been carried out.
In addition to the health benefits, the most significant economic benefits will stem from the prevention of damage by flood or erosion to:
- public infrastructure, particularly roads,
- private property, especially houses,
- domestic furniture and other movable property.
A money value can be put on these benefits. Private property in a low-income community can be valued at a given percentage of the minimum wage.
Other benefits can include:
· enhancement of land value,
· reduced traffic delays,
· reduced losses of income, rent, sales and production,
· reduced clean-up and maintenance costs,
· reduced emergency relief costs,
· greater sense of security,
· improved aesthetic environment, and
· more opportunities for recreation.
The technical options outlined in section 2 indicate that drainage systems need not be expensive. The design criteria explained there can be varied to give options with different costs and benefits, and the best option chosen. This applies particularly to the following choices:
- return period (section 2.2),
- open or closed drains (section 2.5),
- channel lining (section 2.6).
The terms of reference for a feasibility study should require an analysis of the costs and benefits of the possible options, so that these decisions are not taken arbitrarily.
It is difficult to calculate exactly how the many benefits of drainage will be altered by changes in design criteria to allow occasional shallow flooding. The problem can be simplified by assuming that the damage caused by a flood is proportional to a damage index D:
D = F × Q × T
F = frequency of occurrence of the flood (say, number of times in a 10-year period)
Q = quantity of water that cannot be drained away immediately (mm of rainfall)
T = time for which the flood lasts (hours).
The estimated cost C of each option can be compared with the value of D to find the option with the highest ratio of C : D.
These are discussed in section 3. A typical value for the annual cost of maintenance would be about 8% of the construction cost of the system. The feasibility study should make a more accurate estimate, including an assessment of the human resources and equipment that will be needed.
The terms of reference can authorize the consultants to proceed with detailed design of the most urgently needed components of the system (say, to a value of 10% or 20% of the total estimated value of the system) once the choice of solution has been agreed, but without the need for a new contract or new approval by the funding agency.
It is conventional in any consultancy agreement to stipulate stages at which the client's opinion or approval is to be sought. The client in this case will usually be the municipality. There is no reason why the community should not be involved in this process. As far as the terms of reference are concerned, responsibility for ensuring that the community participates in decision-making could be assigned to the client or the consultant. The latter could either follow lines laid down by the client or be asked to propose a procedure for community participation when bidding for the contract.