|Stormwater Drainage and Land Reclamation for Urban Development (HABITAT, 1991, 94 p.)|
The drainage system in a city falls into a distinct hierarchy. At the lowest level is the initial drainage system, the small drains along the edges of the roads and paths, and the kerbs and channels where rain falling on the roads and buildings will slow first. These channels convey water through inlets in the kerb or road edge to the minor drainage network.
The minor network of pipes and open channels are the second level in the hierarchy where the stormwater is collected and transported to discharge into the next tier, the major drainage system network of natural streams and watercourses.
It is not possible to set one single strategy for the management of the whole drainage system which will be suitable for all cities. Much depends on the overall institutional structure in the city: is it one municipality or is there more than one municipality with a regional authority with metro-wide responsibilities for major drainage works? It is possible, however, to make some general assertions:
(a) The initial drainage system maintenance should be integrated with the maintenance of the local road network. These drains form an integral part of the road. Good drainage maintenance is an essential requirement of the protection of the road itself. The patrol gang responsible for the road maintenance should also ensure that the drains, culverts and entry points to the drains are clear, serviceable and able to function properly. In general, the public works department of the municipality would have this responsibility.
(b) Depending on the structure of the city government, the minor drainage system could also be maintained by this same public works agency. This will normally lead to efficient use of the agency's plant and equipment fleet and its labour force. In large cities this will be decentralized into districts where local depots would hold stores of materials, house equipment and provide servicing of the district's equipment. It also provides a local contact point for residents to lodge complaints. In some cities there is a division of responsibility for the construction and maintenance of the minor drainage system between the municipality and the metropolitan regional authority. For example, the regional authority might have responsibility for drains where the contributing catchment is greater in area than, say, 100 hectares. This has the advantage of locating the maintenance of these larger drains which frequently cross municipal boundaries with a single agency. This will lead to a consistent application of maintenance standards and design criteria.
(c) The major drainage system includes roads and landscape/park areas where excess stormwater from the minor system flows during heavy storms. These parts of the major drainage system are not formal drains and are used as overflows from the minor drains. Their maintenance needs careful attention so that they will properly fulfil this overflow role in times of excessive run-off, but they would fall within the ambit of the municipality for routine maintenance. The other components of the main drainage system, the watercourses, streams and rivers, are clearly regional, traversing municipal boundaries. These are not only used to carry excess stormwater but may be used for recreation, transport, irrigation or water supply. Only a metropolitan-wide organization can successfully integrate these functions with the drainage function of the watercourse. Maintenance should therefore be the responsibility of this organization. If no metropolitan agency exists, the role will most likely fall to a provincial level of government.
The following are some of the key functions related to drainage maintenance:
(a) Programming and implementation of a regular inspection and maintenance programme of the drainage system;
(b) Collection and recording of rainfall and run-off data for all catchments in the area, if this is not available through a central agency;
(c) The establishment and maintenance of records of all drains in the system and the inspection and repair programme;
(d) The training of staff and operators;
(e) The protection of the public and employees by the development of safety programmes;
(f) Public communications to increase the awareness of the public of the importance of good drainage and the community's role in achieving this;
(g) Liaison with other agencies who may be affected, or whose functions affect, the drainage system;
(h) Consultation with the drainage design and construction agencies to ensure designs incorporate details to facilitate maintenance.
The form of organization for maintenance of urban drainage systems will vary by country and the government structures established in each country. The structure shown in figure 4.1 is indicative only and is meant to show the levels of the organization and the sections which are required to provide a comprehensive maintenance programme.
The key sections are:
(a) Safety office - separate from the operations sections to ensure this aspect retains its priority and status in the organization;
(b) Operations and maintenance;
(c) Inspection, records and survey;
(d) Sub-districts in larger cities with mobile gangs;
(e) Plant maintenance;
Figure 4.1. Organization chart for drainage operations and maintenance