Some management possibilities

aicago (Sheaffer

et al,

1970). Some large cities such 4s Philadelphia, early on dedicated most of their flood plains to parks, or such as Los Angeles, developed extensive systems of major drainage channels in step with urban development, (Wood, 1970; Rantz, 1970). However, new suburbs have often aggravated flooding with subsequently induced damage in many major

cities located on large streams or bodies of water because their low topological elevation makes them hydrologically subservient.

the tenacious inclination of people to occupy the flood plains of urban streams, but these require changes in the activities of occupants and are therefore socio-political in charac- ter.

study of occupant motivation, hazard perception and attitude, and receptivity towards various forms of communication designed to discourage further encroachment, (James et

al.,

1971;

James, 1972).

Discharges from conventional storm drainage facilities and flood-plain -intrusion by structures both tend to aggravate flooding, and thereby jointly tend to raise the risk of flood damage. Revising storm sewerage criteria, such as by including much more in-system storage, can be an effective adjunct to flood plain management. While there is universal agreement that planning and development of drainage systems and flood plain management programma should be co-ordinated and integrated, prospects for accommodation diminish in the

fact of increasing concern over water quality in sewered areas and a contemporary neutrality or indifference on water quality matters by agencies dealing predominantly with flood plains.

It is ironical that much of the flood plain flooding problem, as well as the land runoff water quality problem, could possibly be remedied more effectively on the land feeding urban water courses.

of main channel improvements for metropolitan areas, sharp differences of opinion arise, partly or perhaps principally because implementation of the two methods commonly falls with- in different authorities (McPherson 1971a)

.

There are several non-structural means for mitigating flooding damages inevitable through The tendenq of home-owners to remain on flood plains has been demonstrated in a pilot

While the principle of the use of local detention storage is often championed in lieu

111.1.6 SOME MANAGEMENT POSSIBILITIES

In the past, drainage conduits were deliberately designed to accelerate the movement of stom, water to receiving water bodies by gravity flow. Detention storage was seldom incorporated in systems because of the preoccupation with rapid removal of surface water. However, in- system storage is being provided in a number of new systems and is being added to some exist- ing systems. Interest in in-system storage has come about because of broader attitudes to integrated facilities-planning. As noted earlier, there is widespread interest in multi- purpose drainage facilities that exploit opportunities to provide for water-based recreation, substantial increases in levels of flood protection for buildings at small extra cost, use and re-use of storm water for water supply by artificial groundwater recharge and related means, and reduced md/or controlled pollution burdens at outfalls. These all require the use of storage.

alternatives to the direct disposal of storm water runoff have been explored by Laursen

et al,

i968 .

land-use controls, such as designed ponding (Rice, 1971) and encouragement of land develop- ment site grading to increase flow distances over unpaved areas (Jones, 1971). The turf areas separating pavements have long been exploited to produce ponding of overland flow, with consequent savings in drain size and reduction of peak outflows, at airports (Hathaway, 19451, highway intersections (Forest and Avonson, 1959) and for shopping centres (Anon, 1960) I

Recreational areas have also been utilized for temporary detention storage, (Daily, 1961).

as in the basements of homes. Hence, to justify the greater use of designed storage in place of undesirable storage depends on the relative protection from flooding afforded by different systems at corresponding differences in cost. That is, 'trade-offs' between advantages and disadvantages should be resolved by choosing among various mixes of flow-acceleration and storage components. Unfortunately , the contemporary absence of a satisfactory body of hydro- logical and economic field data on urban storm drainage system floods constitutes a monumen- tal liability in the assessment of +ose floods and their associated damages.

Requirements for assessment of flood damage by storm drainage have b,een studied, and Peak flows of storm drainage facilities can be reduced by a number of structural and

Damage by storm drainage is the consequence of storing water in the wrong places, such

Some

management

possibilities

The Council on Environmental Quality (1971) estimates that the cost of alleviating pollu- tion from combined sewer overflow, other than by separation into independent storm and waste water systems, would be greater than the combined a m u n t required to eliminate existing deficiencies in waste water treatment facilities and to provide new treatment capacities to meet replacement and population growth needs between 1971 and 1974. Because twice as many people are served by separate storm sewer systems as by combined sewer systems, it is evident that the cost of alleviating pollution from storm water discharges would be even greater.

Stated another way, the cost of alleviating pollution from combined sewer overflows and storm sewer discharges would be at least twice the replacement value of all such existing sewers.

The US Government has been involved for several years in the development of measures for countering pollution from combined sewer overflows (Field & Struzeski, 1972; Office of Research and Monitoring, 1972). The ultimate solution for the problem of pollution abatement from both urban storm sewer discharges and combined sewer overflows is the treatment of such flows before their release into receiving waters. Collecting, transporting and treating all discharges/

overflows at unattenuated flow rates would require gigantic sewers, pumping stations and treatment facilities

-

all of which would be used a very few hours in a year. Therefore, practically all schemes for system-wide discharge/overflow collection and treatment incorporate some form of auxiliary storage for the purpose of reducing sudden inflows, to scale down the size of collection and treatment facilities to reasonable and manageable proportions.

On the basis of limited information, it appeared to Urt: (1972) that complete collection of combined sewer storm water would require perhaps 300 m3 of storage per hectare of drainage area.

of storage required can be reduced by taking advantage of the fact that, because of the areal and temporal variability of rainfall intensity, the proportion of main sewer capacity in use at a given time varies between sewers throughout a rain storm.

There are three basic approaches involved in the most advanced comprehensive attacks on the combined sewer overflow problem. The first utilizes exploitation of ambient storage in existing trunk sewers by manipulating add-on constrictions (movable dams or gates) in outfall sewers. The constrictions are placed in the vicinity of existing regulators, and all flows other than from rare rainfalls are released thereby at attentuated rates to existing inter- ceptors and thence to existing treatment works.

-

That is, by raising the constrictions in those main sewers where the potential storage volume is under-used at a given time during a storm, and lowering the constrictions in advance of local inundating inrushes, the flows into the interceptors can be adjusted for more efficient use of the interceptors. This is reflec- ted in a greater diversion of flows to the treatment plant by way of the interceptors and consequent reduced overflows to the receiving waters. This basic approach is exemplified at Minneapolis-St. Paul, (Anderson, 1970; Callery, 1971; Anon, 1971; Tucker, 1971; Anderson e t

al,

1972) Detroit, (Brown and Suhre, 1964; Detroit Metropolitan Water Services, 1970; Remus, 1970; gnon, 1970)

,

Seattle, (Gibbs and Alexander, 1969; Municipiality of Metropolitan Seattle, 1971; Gibbs e t

al,

1972a and b) and Cleveland (Pew e t

al,

1972).

A second basic approach incorporates new storage located at elevations well below all street sewers, in the form of tunnels or vaults, and new or auxiliary discharge/overflow treatment works. It may or may not have ancillary features such as pretreatment basins or pumped-storage power generation to offset the substantial energy requirements for eventual lifting of flood waters from underground storage to the ground surface. A bonus of this

approach is that street trunk sewers would be converted into manifolds of diversion diffusers, with the result that their effective hydraulic capacity would be raised.

is exemplified in the pioneering plan developed in Chicago (Pikarsky and Keifer, 1967; Anon, 1969b; Koelzer e t

aZ,

1969; City of Chicago e t

al,

1970; Pikarsky, 1971; University of Wisconsin, 1970; Flood Control Co-ordinating Committee , 1972) and in analogous plans under

consideration elsewhere (Parthum, 19 70)

.

The third basic approach is embodied in a plan proposed by the Department of Public Works for San Francis0 (1971b) which includes: a single, new combined flow treatment plant; a number of new detention reservoirs located immediately below the streets in the upstream portions of a majority of catchments; a number of new shore-line detention reservoirs; a deep cross-system storage and transmission tunnel; and achievement of a fully automatic operational control for the whole system.

of their complexity (McPherson, 197l-b) .

However, more recent estimates place storage requirements nearer 600 m3/ha. The amount

This basic approach

All three basic approaches incorporate plans for some degree of automatic control because Because combined sewer overflows occur very suddenly,

164

Research status and needs

any facilities provided for treatment of potential overflows must be put on-line a l m s t in- stantaneously. This means that they would have to be activated immediately any storm water flow exceeds interceptor sewer capacity. Further, such plants might be idle most of the year. The effectiveness of overflow pollution abatement, using treatment facilities designed specifically for that purpose, will therefore require some form of automatic operational control. Remote supervision would quite likely not be responsive enough. The control logic required has yet to be developed and it is possible that different metropolitan sewer systems will require their own unique logic development.

Pollution treatment of water from separate systems of storm drains faces almost the same difficulties. In one sense, requirements are more exacting because

& J .

^storm water must pass through new and special treatment facilities, there being no interceptor sewers in such systems to divert small storm occurrence flows to peremidly operated waste water treatment plants.

Storm water conduits may be incorporated in multiple-service tunnels as is the case in numerous existing utility tunnel systems in Europe, Asia and North America (Corey, 1972).

Most utility tunnel concepts (American Public Works Assn., 1971) deal with structures loca- ted fairly close to the ground surface and include possibilities for limited incorporation of storm sewers (McPherson, 1972). Ac an adjunct or alternative, consideration could be given to much deeper locations, at least for main feeders and arteries, for power transporta- tion, surface water and waste water, and for other services (Sarensen, 1971).

Lastly, possibilities exist in rainfall-deficient regions for deliberate capture of urban storm water to augment water supplies, provided the quality is suitable, a requirement that might be met by various managerial schemes (Angino e t

al,

1972). Manipulated recharge of groundwater supplies with swrplus surface water has been practised for many years in some regions (Task Group, 1963; Knapp, 1973).

111.1.7 RESEARCH STATUS AND NEEDS

'The field of urban hydrology is almost devoid of modern research investment,' said Ackermann in 1966. 'More research is needed to develop understanding of the whole storm water pollu- tion problem. This research should cover the hydrology, the hydraulics, the treatment, the effects on receiving waters, and related factors ' (National Research Council, 1766) ' . . . .

.

too few data have been collected to describe the effect of urban and suburban .'svelopment on flood runoff,' (Task Force 1969). As a consequence, little is known about the rainfall- runoff process and still less about rainfall-runoff-quality, particularly for sewered catchments. '

...

considering the huge relative investment in sewered systems and the almost complete absence of hydrologic data on sewered systems, it has been obvious for quite some time that this is the sector most woefully needing research attention,' (ASCE, 1969).

Sui table data collected with properly co-ordinated instrumentation in networks representing a variety of climatic, topographical and land-use conditions are virtually non-existent.

Further, while all metropolitan areas have subsurface systems of storm drainage, flooding of waterways is a serious problem in some metropolises but negligible or non-existent in others.

Additionally, flood amelioration is preoccupied with rainfall-runoff whereas rainfall-runoff- quality processes are of more vital concern nationally.

proposed, together with a documentation of its justification, (American Society of Engineers , 1969) but progress in implementation has been painfully slow. In anticipation of the possible future availability of new data, considerations for modelling sewered urban catchment

rainfall-runoff-quality processes were drawn up (Dawdy et

al,

1968) and considerations for characterizing rainfall time and spatial distributions in future research were explored (Thomasell, 1968). In support of the latter, existing raingauge networks were surveyed by Tucker in 1969b and 1970a. The US Geological Survey is developing a programme of data

collection and studies to serve national needs in urban hydrology (Sneider ,1969 ; Water Resources Division 1972). The Office of Water Resources Research has developed a broad programme of projected urban water resources research, (1972) including urban hydrology analyses.

the 'rational method, ' (American Society of Civil Engineers 1969) . The method yields only an estimated peak flow but none of the other attributes of a hydrograph and its many limita- tions have been reviewed elsewhere (McPherson, 1969).

A national programme for the acquisition of much needed sewered catchment data has been

The procedure used for design of storm sewers in the United States is almost exclusively A full hydrograph is needed for the

Dans le document of urbanization (Page 159-162)