Urban runoff

M. B. McPherson

American Society of Civil Engineers Marblehead, Massachusetts

U.S.A.

Hydrological effects of urbanization (Studies and reports in hydrology, 18) Paris, T h e Unesco Press, 1974

Introduction

III. 1.1 INTRODUCTION

This chapter deals with some of the principal aspects of surface runoff in urban areas of the United States of America (USA) . The reader is referred to the companion report, Chapter 6, Part II, for supplementary information related to the subject.

Flow in urban drainage conduits is principally by gravity. As in natural drainage basins, smaller sewer branches unite with larger branches, and so on, until a main sewer is reached. The smallest catchment area, in the order of a fraction of a hectare in size, is the tributary to a street inlet. For most smaller tributary areas in the upper reaches of an urban drainage system, the time required to reach peak runoff after the beginning of a storm is a matter of only a very few minutes. Hence, high-intensity, short-duration con- vectional rainfall is normally the main type of precipitation contribution to the largest runoff rates found in the majority of US metropolitan areas. However, pollution loads are also a function of land-management practices and antecedent precipitation, and maximum loads might not coincide with maximum runoff. Further, because all pollution loads are of concern, the effect of cyclonic storms cannot be ignored.

Rainfall interception by vegetation seldom has an important effect on the magnitude of urban runoff. Most soil infiltration-capacity curves approach a steady, minimum rate after one or two hours and the capacity of vegetation cover may be several times as great as for a bare soil. Antecedent precipitation can affect soil infiltration capacity but very few quan-

titative data are available for evaluating this factor.

Some of the precipitation which reaches roofs, pavements and other impervious surfaces is trapped in the many shallow depressions of varying size and depth present on practically all urban surfaces. There have been no field measurements of depression storage because of the obvious difficulties in obtaining meaningful data.

"he residual or excess precipitation remaining after infiltration and depression storage have been removed is available for detention, the storage effect of overland flow in transit.

Overland flow refers to unsteady state surface runoff across a sloping-plane surface, which occurs over extensive portions of an urban area in conjunction with storm occurrences. ûver- land flow from the land is usually collected in street gutters or ditches which in turn are drained by street inlets. Additional storage effects occur in the gutters or ditches. Con- siderable research attention has been accorded to the development of inlet hydrographs but progress has been handicapped by a scarcity of suitable field data.

Flow from the surface enters underground systems of conduits at street inlets. The volume of üetention in a conduit can effect a reduction in the peak rate of flow of an input hydrograph in the same basic way as any detention storage attenuates an inflow hydrograph, and storage routing can be applied to estimate conduit system hydrographs when estimated inlet hydrographs are used. Advances in conduit routing capability are outpacing reliable data for monitoring the suitability of techniques developed. This is not to say that the advances have not been significant but only to emphasize a disadvantage that is encountered univers ally .

A simplified description of the major components of the urban storm water disposal phy- sical subsystem is shown schematically in Figure 17 (American Society of Civil Engineers, 1969a). In a given instance, either or both of the two components indicated by dashed lines might be absent.

Whereas there is a continuum between the subsystems of water supply, water use and waste water reclamation, storm water has long been regarded as a nuisance and its subsystem has seldom been deliberately connected to the other urban water subsystems.

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 the new national priorities of enhancement of urban environments , conservation of water resources and reduction in water pol lution.

is intimately related to the acknowledged urgency of aesthetic enhancement , expansion of recreational opportunities and extraction , the availability of waterfronts for public uses.

Runoff is a carrier of wastes, either in the course of conversion from water supply to water- borne sewage or in flushing urban ground surface. Thus, public health considerations can transcend or temper economic considerations. In addition , comprehensive approaches for

Increasingly the greatest public concern will be on the quality of water. This concern

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LancZ-use changes

managing water pollution problems require that other water uses , planning, and sound develop- ment, also be considered (Advisory Committee on Inter-governmental Relations 1962)

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I.or example, utilization of the 'blue-green' development concept, which employs ponds with open space for storm water detention and recreation, can enhance the value of urban property and decrease the depreciation rates of property

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thereby increasing long-term local government revenues (Jones 1967). On the other hand, Jones also points out that peak drainage runoff rates can be reduced by proper land-development design. The guiding principle is to reduce the liabilities and increase the assets of urban runoff (Thomas & Schneider 1970).

ulation served by public waste water collection systems) is served by combined systems of sewerage (Sullivan, 1968). Overflows from combined sewers are thought to comprise a signifi- cant source of stream pollution (US Public Health Service, 1964). Combined sewers have the dual functions of removing ephemerally occurring storm water from urban surfaces and con- veying waste water on a perennial basis. Conduit size is governed by storm drainage require- ments since the capacity requirements for waste water are comparatively small. For all but the relatively few days a year that storm runoff occurs, the continuously flowing waste water is intercepted near the combined sewer outfall by means of a regulating device which diverts it to the treatment plant via an 'interceptor' sewer. Most storm flows are much too great to be accommodated by interceptors, and almost all the waste water burden is discharged through the outfall to the water course when rain storms are heavy and prolonged. At the same time, sludge and debris that have been stranded in combined sewers during relatively low rates of flow in preceding dry-weather periods are scoured from the laterals and trunk sewers of combined systems and are transported by the augmented flows and eventually dis- charged to a water course. It is estimated that, in consequence, as much as 5% of the annual flow of sewage, and 20 to 30% of the annual volume of solids, are discharged to water courses from combined systems (American Society of Civil Engineers, 196913).

that from combined sewer overflows. The US Environmental Protection Agency advised in 1971 that requirements for control of pollution from combined sewer overflows were rapidly be- coming more stringent and that control of pollution caused by urban storm water discharges was on the horizon (Cywin e t al, 1971). More recently, the US Council on Environmental Qua- lity (1971) has noted that the contribution of pollution from runoff sources is even greater than had been suspected, from both urban and non-urban sources. When abatement of pollution from storm water conveyed by separate systems of storm drains is attempted, the difficulties to be overcome may be more severe. 'ibis is because all such storm water must be passed through new and special treatment facilities, there being no interceptor sewers in separate storm water systems to divert small storm flows to perennially operated waste water treat- ment plants as in combined systems.

There is widespread intexest in multi-purpose drainage facilities that exploit oppor- tunities for water-based recreation, provide more effective protection of buildings from flooding, and allow for the use and re-use of storm water for water supply. All these re- quirements need storage facilities and special treatment plants, together with some kind of control system able to manage the sudden and brief impact of storm water. The scale of the problem is almost overwhelming: most of the larger metropolises have well over a hundred

catchment areas and cumulative drain lengths of several thousand kilometres in length. The difficulties to be overcome were compounded by the 1972 Amendments to the Federal Water Pol- lution Control Act which established a national goal of zero-pollution.

For historical reasons, about 20% of the nation's population (or, about 60% of the pop-

Pollution from storm sewer discharges (Weibal et

al,

1964) may be almost as severe as

111-1.2 LAND-USE CHANGES

Much of the land occupied by metropolitan areas has been drastically altered by urbanization, particularly in large central cities. For example, the distribution of land use in the City

of San Francisco is estimated by the City's Department of Public Works (1971a) as follows:

Res i den tia 1

Land-use chánges

public (about half recreational) 23

Vacant 8

Comercial 5

Indus trial 5

utility 3

Ins ti tutional 1

Distribution of land use between a city centre and its contiguous metropolitan area can differ apreciably. For example, the proportion of land occupied by residences is commonly higher in the suburbs. Furthermore, there is considerable variation in land use from one metropolis to another. Contributing to this variability are dissimilarities in growth rates.

Ranges in land use among seven of the largest US metropolises are given by Abrams (1956) follows :

Mass Transportation 1/8 3/10

Urban expansion absorbs an estimated 170,000-hectares of land each year in the US (Dept of State, 1971). Metropolitan suburbs will be particularly hard-pressed to expand all public facilities in the face of a doubling in residential construction expected for this decade

(Anon, 1969a). Nationally, suburban populations of metropolitan areas had surpassed those of the surrounded central cities by 1970, and this sprawling trend is expected to continue.

Of considerable importance to urban runoff management is the usual rapid decline in population density with distance from the densest centres. For example, in 1960 the urban population around the City of St. Louis was one-fifth larger than that of the City but distributed over four times as much land area (Advisory Commission on Inter-governmental Relations, 1969).

Buildings, streets and other urban land cover inhibit the access of precipitation to the soil. The presence of extensive impervious surfaces is thought to be the main reason why the total volume of direct storm runoff from urban areas is generally greater than for comparable non-urban catchments (Task Force 1969). For example, greatly increased volumes of direct runoff associated with urbanization growth have been documented for some streams in Long Island, New York (Franke and McClymnds, 1972). Although an indirect indication, evidence has been given of a correlation between the degree of imperviousness and the post-urban en-

largement of stream channels for a number of small watersheds in the Philadelphia metropoli- tan area (Hammer, 1972).

The Soil Conservation Service (1971) gives estimates of imperviousness ranges for typi- cal urban development as €allows:

Land Use % Imperviousness

Low density residential

Another estimate of imperviousness for typical urban development has been made (by Stankowski (1972)) :

Dans le document of urbanization (Page 149-154)