|Stormwater Drainage and Land Reclamation for Urban Development (HABITAT, 1991, 94 p.)|
The importance of an adequate system for maintenance of the drainage system cannot be overemphasised if the system is to operate at its design capacity. If rubbish and silt are allowed to accumulate in channels and rivers, these facilities will not be capable of satisfactorily fulfilling their primary function - the removal of stormwater run-off. If the inlets into the drainage system from roads are not regularly cleared, they will become blocked and stormwater will be prevented from entering drains, so causing local flooding. The objective of a maintenance programme should, therefore, be to keep the drainage system operating dependably, at its design capacity, without breakdowns.
Protection of the capital investment in the drainage system necessitates a planned programme of inspection and routine clearing. Periodic, thorough and competent inspection while cleaning will reveal points at which damage begins to take place. Adequate financial and labour resources must be allocated to ensure successful drainage operation and maintenance. It is therefore necessary for drainage authorities, usually the local government, to ensure that maintenance is accorded a sufficiently high priority in the allocation of funds in annual budgets.
The objectives of carrying out the maintenance programme should be to:
(a) Keep the system operating at design standard at all times;
(b) Obtain the longest life and greatest use of the systems facilities by providing adequate maintenance and timely repairs;
(c) Achieve the foregoing two objectives at the lowest possible cost.
The history of drainage is full of examples of the difficulties which can occur when maintenance is neglected. Even the best constructed system will eventually fail if adequate maintenance is not undertaken.
Maintenance activity should begin the day the system is placed in operation or, under some circumstances, prior to completion of a system and before the system is placed into operation. Keeping maintenance work current on all facilities in a system is the keystone to any successful drainage enterprise.
There are two primary concepts of maintenance of public property, and most maintenance operations can be readily classified into one of these categories:
(a) Maintenance by necessity;
(b) Preventive maintenance.
Maintenance by necessity refers to the practice of fixing it when it breaks down. Under this approach cleaning of drains is confined to the minimum necessary to satisfy complaints. Likewise, repair work is undertaken only when a condition becomes so bad that it must be corrected or repaired to restore service or for safety reasons. Unfortunately, this approach is all too common in rapidly growing cities where the funds for routine maintenance are inadequate.
Preventive maintenance is represented by a systematic programme of inspection, cleaning and repair that reduces breakdowns and complaints to a minimum. Preventive maintenance not only pays dividends in economical operation; a smooth working system also means uninterrupted removal of water at lower cost with reduced risk of damage from flooding as a result of design storms.
Preventive maintenance also has other distinct advantages:
(a) It can be scheduled and performed on a regular basis at times that least disrupt other operations functions;
(b) Pre-ordered parts can be made more readily available; they may not be so readily available under emergency conditions;
(c) Work can be carried out during normal working hours with less emphasis on extension of hours and weekend work;
(d) More experienced personnel can be used if the work is scheduled; they may not be available in emergencies;
(e) Special tasks of preventive maintenance may be contracted out to reduce the need to carry a specialist workforce.
All structures and facilities are subject to deterioration in varying degrees over time. Regular inspection therefore is necessary to detect and correct potentially unsafe or unsatisfactory conditions as they develop. Cracks in concrete or masonry, general erosion behind structures or settlement of an embankment can result in major failures if the cause is not identified and corrected or repaired without delay. Many problems that develop may not be of such a serious nature. In channels, the control of weeds, storm erosion of banks, seepage of water through banks and base, silting and accumulation of debris or solid waste may be less serious than structural failures, but they still require regular attention if efficiency of the system is to be maintained. Frequently these latter problems are more time-consuming and costly over the years and are more frequently neglected since they accumulate slowly.
The use of appropriate materials and methods for repair or replacement is important. This can include adequate attention to the quality of the concrete aggregate and cements used, and the characteristics of protective coatings used to meet environmental requirements. Staff responsible for drainage system operation and maintenance should always be alert to the development of new materials and products and their possible adoption for the solution of maintenance problems.
The application of the agreed maintenance procedures requires close supervision and the development of a range of skills in the maintenance workforce.
To accomplish the required maintenance, questions arise concerning the use and protection of materials, the need for lines of communication and access, and the personnel requirements to accomplish the work. Regular inspections and records of complaints will identify any components in the drainage systems which are not meeting the design criteria. If it is found that a structure or facility does not perform the purpose for which it was designed, the designer should be advised. In this regard, experienced operation and maintenance personnel can be of key assistance if they are given the opportunity to review system designs before a facility is constructed. In carrying out such reviews, they should consider designs and construction that will require the least maintenance consistent with budgetary constraints.
Obtaining the longest life and greatest use of drainage facilities can best be accomplished by providing good maintenance and a programme of systematic improvements and replacements. In many instances, it is hard to determine the point at which good maintenance ends and replacement begins. Good maintenance, for example, may necessitate the replacement of a gate leaf, later a gate shaft, and perhaps later still the lifting device itself.
As drainage works advance in age, a programme to replace worn out and obsolete structures is necessary to extend the useful life of the system. The essential requirement is to have a maintenance plan and programme.
The key to good maintenance is frequent inspection. Inspection followed by proper care of channels, structures and mechanical equipment will avoid major maintenance at a later date. Proper maintenance demands close and continuous examination of system facilities by experienced personnel, followed by timely repair and replacement programmes.
The timing of some recurring maintenance needs can sometimes be predicted. However, in general maintenance needs can only be determined by careful on-site observation. It is therefore necessary to arrange for periodic inspection of all system facilities at regular intervals. The results of these inspections must be recorded so that the performance can be tracked. With the advent of computer technology and the development of smart software, this recording of information in databases for easy access and management reporting can expose trends in the extent of the maintenance effort required by various sections of the drainage system. This information system can then be used to direct the available maintenance funds to those areas.
As untreated or uncorrected minor maintenance needs can grow rapidly into major and costly maintenance problems, inspections should be made frequently. The interval between subsequent inspections may then be determined as performance dictates.
Three procedures which have been developed for the implementation of an inspection programme are:
(i) Initial inspection. When construction is essentially completed, a formal inspection of all work is made and any uncompleted items and deficiencies listed. This defines the work remaining to be accomplished and provides an opportunity for all concerned to be better informed on the condition of the system at the time control is taken over. The inspection should be made by a team representing the design, construction and operation and maintenance agencies.
(ii) Operation and maintenance instructions and criteria. A detailed manual for operation and maintenance of major structures covering all important features of the operation and maintenance of the structure should be prepared. The designers' operating criteria should be published to cover the technical operations of equipment and structures and the requirements for maintenance. This helps to ensure that the facilities will be operated as the designers intended, thus avoiding damage and extending the life of the facility.
(iii) Periodic review of maintenance. These reviews should be in the form of a thorough inspection of all facilities. The inspection team should include an engineer from the design organization, a senior representative from the operating agency, and key maintenance personnel. The principal purposes of the review are to verify the safety of the structure; to determine the level of maintenance and conditions that might cause failure of operation; to note the extent of deterioration as a basis for planning maintenance, repair or rehabilitation work; and to obtain operating experience data for improvement of future design, construction, maintenance and operation practices.
Reports prepared for all review of maintenance inspection should include the current findings, comparisons with previous inspections, and a summary of conclusions and recommendations. To provide guidance in planning and performing repairs, they should also include adequate photographs and drawings to illustrate conditions found. It is suggested that recommended repairs and operating procedures be grouped into categories according to the importance of problems involved. These might be:
Category 1: recommendations involving matters of great importance which must be acted upon within a prescribed period;
Category 2: recommendations covering a wide range of important problems that should be solved;
Category 3: recommendations for modification of the inspection, operation and maintenance procedures.
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