SURFACE WATER RUNOFF .1 Introduction

The total water inflow to an urban area may consist of river flows, water withdrawn from an external source and used for different purposes in the urban society and the precipitation which falls on the watershed. Some of this precipitation is taken advantage of in urban areas such as for watering lawns and cleaning roofs and streets, and for domestic uses, thus reducing the withdrawal needs. But more often this water is disadvantageous and measures have to be taken to reduce these disadvantages to attain certain social goals.

The measures may be defined as urban drainage and flood control projects.

Urbanisation is a gradual process and somewhere in this process the drainage and floods create problems. A description by Vlachos and Flack (1974) is a setting for the subsequent discussions:

'Traditionally the rural or low-density suburban area outside the city initially develops its water supply and sewage disposal facilities on a private house-by-house basis. As the area becomes more developed and density increases, small water systems emerge. Finally, as the urban constellation emerges and population increases rapidly the limited water, sewer, and disposal systems prove inadequate and communities are forced to consider much larger permanent facilities. It is with this last stage that water management becomes a crucial element in bringing together planning and construction on a larger scale.

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On top of this, another problem is that urbanizing areas tend to develop in flood plains.'

Urbanisation thus creates a demand for all types of services, some of which are viewed as infrastructures, which should be provided by the public sector. Urban drainage and flood control projects provide a service to urban areas in three ways:

1. Flood control 2. Convenience drainage 3. Environmental sanitation

As pointed out by Jones (1967) the urban drainage system has two subsystems: a major one which accommodates the rarer more severe events, and a minor one which provides for the drainage of frequent runoff events.

5.4.2 Flood protection

The need for flood protection measures has been apparent for a long time as flood damages occur when man makes use of lands liable to inundation. Flood damages, therefore, result from the forces that encourage people to use or locate property on flood plains, (James et al.

1975). The reasons for using flood plains are well described by the same authors:

'Historically, development on the flood plains along major rivers has held locational advantages for many types of industry and commerce, and the constraints of low incomes and slow transportation have

caused people to live near their jobs. Agriculturalists were attracted to the soils made fertile by flood-deposited sediments and have in turn attracted agriculturally-oriented cities. A more important factor in attracting urban developing to the flood plain, however, has been the use of rivers for transportation and power and their attractiveness to industry and commerce as sources of water and as depositories for wastes. These factors made industrial and commercial development least expensive near rivers, job opportunities migrated toward river-front cities, and residential development followed on nearby flood plains.

Today technological advance has reduced the dependence of industry on rivers. The commercially important water wheel has long since gone.

Few industries rely heavily on waterborne commerce. Water can be brought from great distances, and wastes can no longer be freely dumped into rivers and forgotten. Nevertheless, these historical factors have led to large sections of many older cities being located on

flood plains and made flood plain management an important consideration in many urban redevelopment programs.

Technological advance has also made possible unbroken urban development covering hundreds and sometimes thousands of square miles. No matter where such a complex is located in relation to major rivers, local runoff will periodically collect in its internal drainage system and create significant flooding. The only way for a metropolitan area to be free of flood damages is to keep every low-lying area along every stream free from buildings, to create a trellis work of open space throughout the metropolis.

Homesites are one example of an urban land use that may enter a flood plain or any other open space in the urban community. A person seeks some set of site characteristics in selecting a location for

Surface water runoff

his home. Proximity to employment was the dominant factor in the days of slow transportation, long hours, and low incomes; however, today faster transportation, more leisure time, and higher incomes lead many people to view these economic factors as no more than constraints bounding the general location and characteristics of the home site they select. Selection of a specific lot relates more to the attractiveness a location has in terms of the activities a family enjoys at home, aesthetic values, and

opportunities for social relationships with compatible neighbours.

A particular flood-plain resident may or may not be aware of the hazard. Some living groups recognize the hazard at the time they move on to the flood plain. They choose the site because they believe the advantages to exceed the cost and are willing to pay a flood damage bill to achieve something they want. Others enter ignorant of the hazard. After each major flood, some from both groups still see too many advantages in a site to want to leave, but others from both groups re-evaluate the advantages and costs and seek another location.'

Thus, in earlier years, attention was paid mainly to remedial works, ie structural and non-structural measures to reduce the inconveniences of settling in a hazard area.

Structural measures included such projects as reservoirs, channelization, and levees, and non-structural measures included flood warning and flood insurance.

In recent years, there has been a shift to preventive activities, the objective of which is to reduce, or minimize, the occurrence of situations requiring remedial works. The

tools may be proper land use planning, regulations, information, education, etc.

As pointed out earlier, every flood-control project provides a service for the improve-ment of living conditions in the urban area. As a service it provides for specific needs;

in this case, the need for flood damage mitigation and protection. But, urban flood-control projects must compete with other urban projects for funding from the limited public purse.

Thus it is important to be able to describe and enunciate all the benefits that these projects provide so that they can compete for funding. (Grigg et al. 1975).

The benefits from a flood control project may be classified according to the following, see also Grigg (1975):

1. Tangible (a) Direct

Reduced flood damage to public and private facilities Reduced probability of loss of life

Land value enhancement 2. Intangible

(b) Reduced inconvenience Increased sense of security Improved aesthetic conditions

Methods for evaluating the benefits mentioned are not far advanced. Attention has been focused mainly on the potential reduction in direct flood damages. This is probably due to the visibility of flood damages after severe floods and to the availability of data for

quantifying direct benefits. However, according to Grigg et al. (1975), the damages fall into five categories:

1. Direct damages, which affect structures and their contents, public facilities such as roads and utilities, and vehicles.

These damages are experienced mostly by flood plain occupants.

2. Indirect damages, which include the value of lost business and services, the cost of alleviating hardship, safeguarding health, re-routing traffic, delays, etc. Indirect damages are usually taken as percentages of the direct damages.

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3. Secondary damages, which may occur when the economic loss caused by flooding extends further than the immediate area of flooding. Secondary benefits are now generally considered to be outside the scope of flood-control project evaluation because of their complex nature (or considered offset by secondary costs).

4. Intangible damages, which include the reduction in environmental quality and aesthetic values. It is not at present feasible to estimate the monetary values of intangible damages and the corrresponding benefits, but they should be considered as an important part of the analysis for project selection as many of the social goals, which are served by flood protection measures, fall under this heading.

5. Uncertainty damages, which consist mainly of the hardship suffered by occupants of flood hazard areas because of the ever-present uncertainty of when the next flood will occur and how serious it will be.

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

1. Hydrological information to characterize 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.

4. Information on affected public institutions and agencies to assess support for the structural alternatives and receptivity to the non-structural possibilities.

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

It may be worthwhile to consider some of the above points, especially those dealing with socio-economic aspects. A benefit-cost appraisal of major flood prevention proposals may well be beneficial, and techniques have been illustrated by Jones (1971a, 1971b) and Local Government Operational Research Unit (1972, 1973) .

Important information includes community factors such as the recognition of flooding as a community problem, environmental concern within a community, and the community's philosophy of public versus private responsibility. The information needed also

includes individual factors such as sympathy for programme goals, willingness to conform to regulation, individuals' philosophy of public versus, private responsibility, and perceived personal benefits and losses.

Basic information about flood-plain land-use is a factor in the total land-use pattern of the community and the well-being of the residents is affected by this land-use pattern.

5.4.3 Storm drainage

The minor urban drainage system can be seen as an environmental management service provided by the community. The main benefits of this service may be classified as follows:

1. Tangible

Reduced damage to property Reduction in traffic delays

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Reduced costs of cleaning and maintenance 2. Intangible

Reduced inconvenience

Alleviation of health hazards Improved aesthetic conditions.

According to Grigg (1975), the benefits of minor drainage projects cannot, as a rule, be evaluated by using (traditional) benefit-cost analysis because benefits for minor systems are normally those associated with convenience, aesthetics, etc. For the time being, minor projects must be designed by setting standards and criteria and then finding acceptable minimum cost solutions for meeting the standards. This approach is currently relied on in most environmental fields.

The same conclusion is reached by Green, King and Bowden (1975) who emphasize that a service is provided for the public by the drainage system. Fundamentally, it is the service itself that is desirable. This service does have a value, but the value is set arbitrarily by value judgements made by, or on behalf of, the community. These judgements are defined by prescribed standards and criteria.

Also, according to Grigg et al (1975) one of the difficulties inherent in considering intangible costs and benefits in evaluation of small drainage projects is that the cost of analysis may be excessive.

Green, King and Bowden (1975) suggest that preferences could be expressed for or against storm-sewer services that are better or worse than a defined norm. In the presentation of a drainage plan, various levels of effectiveness could be considered instead of considering only one choice (the norm) and one price.

The objectives and design of urban drainage systems have been challenged in recent years. McPherson (1974) writes:

'Historically, urban settlements have been drained by underground systems of sewers that were intentionally designed to remove storm water as rapidly as possible from occupied areas. Substantial departures from that tradition are required by new national priorities : enhancement of urban environments; conservation of water resources; and reduction in water pollution.

The greatest public concern will increasingly be on the quality of water.

This concern is intimately related to acknowledged imperatives of aesthetic enhancement, expansion of recreational opportunities and more extensive availability of waterfronts for public uses. Runoff is a carrier of wastes, either as harvested for water supplies and converted to water-borne sewage or as an urban ground-surface wash. Thus, public health considerations can transcend or temper economic considerations. In addition, comprehensive approaches for managing water pollution problems that require that other water uses, planning, and guiding sound development, also be considered. For example, utilization of the 'blue-green' development concept, which employs ponds with open space, for stormwater detention and recreation, can enhance urban property values and decrease property depreciation rates, thereby increasing long-term local government revenues. On the other hand, peak drainage runoff rates can be reduced by means of proper land-development design. The guiding principle is to reduce the liabilities and increase the assets of urban runoff.'

Thus, to obtain the benefits listed above, and also in order to reduce the costs, there seems to be a need to depart from the traditional approaches. It has been shown that it is possible to use storage reservoirs, both surface ponds and underground reservoirs built into the piped system, to reduce the peak flows and thus obtain a more efficient drainage system, see for example, Rice (1971) and Lager (1974).

An example of the use of ponds in the drainage system is in the new town of Melun-Senart in France. As well as the advantages of smaller peak runoff it is claimed that the ponds

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Other considerations

serve as sanitation sites as the ponds fit into the original ecological system, (Melun-Senart 1974). In Australia, Bonham (1975) has described a drainage system in the suburb of Beagle Bay. The system performs completely without piping. Instead grass zones are established along streets and other impervious areas where the stormwater is dispersed as widely as possible through the grass. Bonham also claims that the grass filtration will remove most of the suspended sediments and also the nutrient enrichment of the storm water. Bonham believes that the use of roads and paved areas at grade with the surrounding grassy lawns is greatly to be preferred to the conventional systems in order to increase the environmental benefits and reduce the system costs of new suburban drainage systems.

According to Field and Lager (1975) storm runoff is a significant source of pollution, 'typically' having large solids concentrations, large BOD concentrations, and bacterial contamination greater than the concentrations considered safe for water contact activities.

Further health hazards, such as nutrients, heavy metals, and pesticides, may also be present.

The conclusion of this section is that the evaluation of minor urban drainage systems is laborious and it seems to be impossible to price the benefits of urban drainage projects.

Therefore the practice today is to use standards and design the system to this standard at minimum cost. The evaluation problems are then moved to the organization that has to set the standards: the problem of relating the social factors to the service provided still remains.