design of detention storage, for evaluation of pollutant burdens, Sor design of storm water pollution abatement facilities, for designing local pratection works
,
such as pumping stationsfor passing local drainage flows over levees and dikes, and as inputs for design of stream arid river development works. Also, quantification of the effects of urbanization on the hydrolo- gical regime is dependent in many cases upon the availability of sewer outlet hydrographs.
Further, as urban water management problems become increasingly acute , the need Lfor multiple- use of water becomes more evident. In exchanging one use for another, for example by using storm water as a source of water supply, knowledge of the time-histories of flows and water qualities is essential for reliable design of transfer facilities.
over more than a very few seasons. Although rainfall-runoff had been measured, only a few of the catchments had rainfall-runoff-quality measurements made at outfalls and not insystem;
data reliability was often suspect because of known instrument errors, lack of observations synchronization, and crude signal resolution (Tucker
,
1969c) . Proportionally more rainfall- runoff data were available for partially sewered areas (Tucker, 1970b) but the amount was nevertheless pitifully small. Since then, field research sponsored by the Environmental Protection Agency (Fieldet al,
1972) and others has extended the data somewhat (Stall &Tierstriep, 1972).
ments using flumes or weirs rather than stage-gauges for determining discharge.
numerical values to model component parameters. Reliability of judgment is improved when parameter values are based upon fits to observed events for the catchment being modelled, and is maximized when parameter values can be generalized for a number of catchments in terms of physical features such as degree of imperviousness, type of land use and channel density.
The usual purpose of modelling is to project performance for future events, For urban catch- ments, this usually involves physical changes over time such as revised land use, new deten- tion storage and modified land management practices. Independently from any consideration of catchment changes, recorded precipitation must be adapted in some way to represent expected future precipitation. Thus
,
projections of runoff-quality events inevitably require the application of some degree of judgment. The principal reason for testing or calibrating a m d e l with observed data is to enhance confidence in its use, and this approach of goingfrom the present to the future is employed in all water resource computations involving risk and uncertainty.
of infiltration, depression storage, etc from total rainfall to resolve the amount and pattern of rainfall excess, which is the input from which an equal volume of direct runoff is genera- ted by models of one kind or another.
modelled for sewered and partially sewered catchments (Tucker, 1970). A project completed since then (Water Quality Office, 1971; Chen & Shubinski, 1972; Lager et
al,
1971) has developed the only existing model that readily accommodates water quality parameters. The model was developed as an assessment technique for comparing alternative solutions by means of a comprehensive computer programme capable of 'representing urban storm water runoff pheno- mena, both quantity and quality, from the onset of precipitation on the basin, through collec- tion, conveyance (both combined and separate systems) , storage, and treatment systems topoints downstream from outfalls which are significantly affected by storm discharges, ' (Lager, 1969). An improved version of the model has been tested for a 1 5 0 0 ha San Francisco sewered catchment (Roesner e t
al,
1973). These and any other models purportedly developed for sewer application suf€er substantially from a severe shortage of data for their calibration, veri- fication and, particularly, for their realistic application to ungauged catchments.generalized unit hydrograph parameters (Eagleson, 1963; Espey 8, Winslow, 1968) in analysis of areawide storm sewer performance.
for synthetic triangular unit hydrographs applicable to small non-urban catchments throughout the country (Soil Conservation Service, 1971)
.
Local synthetic triangular unit hydrograph parameters have been developed for the San Francisco Bay Region using observed data for non-urbanized catchments and empirical indications for urbanized catchments for which suitable data did not exist (Rentz, 1971).
planning for the Denver metropolitan area, Colorado (Wright-McLaughlin Engineers
,
1969 ; By 1969, only a very few sewered catchments had been gauged and records seldom extendedHowever, not much new data has been acquired on completely sewered catch- All hydrological models require some degree of subjective judgment in the assignment of
The inherent difficulty with any runoff model in the necessarily subjective separation
Nine projects were identified in 1970 where the urban rainfall-runoff process was being
In default of suitable data, some astute planners have applied necessarily weakly-founded The Soil Conservation Service has developed parameters
A unit hydrograph has been defined for use in drainage
c
166
Other deveZopments
and
considerationsClayco&, 1970) and again for estimating flows from local drainage in conjunction with river protection works (Hydrologic Engineering Centre, 19 70)
.
A sewered catchment runoff generation technique developed in the United Kingdom has been tested using some US data (Stall & Terstriep 1972; Terstriep & Stall, 1971).
A dearth of historical runoff records on urban streams, compounded by the rapid changes that have taken place in urban watersheds, has thwarted effective use of existing simulation models for planning.
A mathematical watershed simulation model that operates with continuous, multi-seasonal hydrologic data was used in a study of the effects of rainfall data time-interval and rain-
gauge density on simulated hydrographs compared with observed hydrographs for four sewered catchments (Crawford, 1971) but experimentation was severely hampered by the almost total absence of appropriate data.
the runoff volumes measured at the outlets of four sewered urban catchments (Miller & Viessmen, 1972)
.
A recently developed model requires as computational input the ratio of direct runoff volume to the volume of rainfall subsequent to the beginning of direct runoff; and the asso- ciated duration of rainfall excess (Rao @-¿
al,
1972). For predictive application these two factors must be guessed where no runoff records are available or generalized in some way in the unique instances where some runoff data do exist.flows in an interceptor sewer of a combined system (Harris, 1970) and was verified in part using unsteady flow laboratory test findings (Pinkayan, 1972).
An empirical total rainfall-total runoff relation has been developed that approximates
An elegant mathematical model has been developed for routing intercepted storm water
III. 1.8 OTHER DEVELOPMENTS AND CONSIDERATIONS
The office of Water Resources Research has prepared an extensive bibliography on urban water planning (Mangan and Swenion, 1972). Many major streams pass through several metropolitan
areas with consequent complications for runoff analysis
,
a subject considered in detail at the International Symposium on Mathematical Models in Hydrology in Warsaw , July 19 71 , and earlier by Dawdy and Kalinin in 1970.areas and here also methods of analysis have progressed considerably in recent years, parti- cularly with regard to water quality parameters (Orlob, 1972).
flood mitigation models related to planning for river management while Bowers e t
U Z
(1972) have compiled an annotated catalogue of computer programmes which includes reference to some of the few programmes developed for urban runoff. Features of existing hydrological models for simulating rainfall-runoff of urban streams have been compared by Linsley (1971).selected papers dealing with various aspects of urban storm water runoff, (The Franklin Institute Research Laboratories, 1969, 1970 & 1971; Office of Research and Monitoring, 1972a) and the US Geological Survey has compiled over 600 abstracts of selected papers on the subject of urban hydrology (Knapp & Glasby, 1973).
tinues to accumulate (Office of Research & Monitoring, 1972b).
ships for estimating stream discharge and stage frequency for use in connection with flood- plain mapping in the Chicago metropolitan area.
flood plain mapping study that was undertaken with simulation techniques (Hydrocomp Inter- national Inc., 1971).
A flood control and drainage background study has been undertaken by San Diego County, California, as a part of its comprehensive planning programme. An operational computer programme produces preliminary costs for channel improvements , based on the outputs from two
other programmes which, for alternative land use distributions
,
calculate ' the outflow from each sub-basin for a flood with any pre-assigned recurrence interval' and compute 'the relation between water-surface elevation and flow rate' (Reimer & Franzini, 1971). Analyses yield information on the relationship between projected land use alternatives and degree offlooding, and the cost of attendant flood protection measures.
and design purposes are considered, where historical data is at issue, it appears that
Some streams are estuarine when they reach metropolitan Ferguson and Loucks (1972) have reviewed a variety of streamflow, dissolved oxygen and
The Environmental Protection Agency has sponsored the compilation of 1 600 abstracts of
Information on drainage system contaminants con- A study for the Northeastern Illinois Planning Commission developed simulation relation-
'The North Branch Chicago River was the first
When the difficulties encountered in modelling urban streamflows and stages for planning
-. -.
Other developments
and
considerationscapability for real-time operation of urban stream control structures will take some time to develop, because flows and stages must be simulated simultaneously with the occurrence of precipitation events
,
including simulation of alternative operating tactics and incorporation of mostly yet-to-be-defined management operating strategies.A monumental problem in the analysis/design of drainage systems is the choice of storms to be used. In the operating mode, any control system must not onLy respond almo5t instan- taneously to the actual occurrence of rainfall but must anticipate the probable character of subsequent time and spatial changes almost before they occur. A desirable adjunct would be an incipient storm-occurrence forecasting capability. As in design/analysis , separation of infiltration, depression storage etc from total rainfall to derive rainfall excess is neces- sarily highly subjective and limits the reliability of affected control-response components.
Despite.these reservations, the generation of a sewer hydrograph for a given storm is not all that difficult and can readily be processed using one model or another. Storm defini- tions used €or deriving river basin extremes such as 'reservoir design floods' and 'spillway design flaods are irrelevant because urban sewer systems are necessarily designed with a lesser level of protection, and are expected to be overtaxed much more frequently than major river structures whose failures could be catastrophic. From this standpoint, the mean frequencies of occurrence of flow peaks and volumes and quality constituent amounts is the issue, not the frequencies of the input rainfall, and were it possible to arrive at statis- tical series for discharge-quality independently of rainfall we could vastly simplify the design issue. More correctly
,
because of metropolitan socio-political considerations , the probability of recurrence of a given event magnitude being equalled or exceeded within the same year may be more important than a mean recurrence which requires a number of years for its average to apply (Thom, 1959).Furthermore, because there are inherent non-linearities in most methods for processing inputs for linear models, and dynamic models are non-linear by definition, the statistics of the rainfall input array may differ appreciably from the statistics of some or all of the arrays for runoff-quality characteristics. That is, attempting to assign a mean frequency of probable recurrence to a 'design storm' is meaningless because of statistical non-
homogeneity of rainfall, runoff and quality. A ~ S O , such an approach neglects the effect of prior storms on the runoff from a given storm (Linsley, 1970). Statistical definition of
collective time and spatial variations of storm rainfall has thus far defied modem analysis.
rainfall time-patterns and areal distributions , there is an interim possibility that might be considered (Jens & McPherson 1964). A few metropolitan areas have reasonably dense raingauge networks that have been in operation for five years or more (Tucker, 1969). From the data available, the time-history of rainfall on a particular catchment could be interpolated and entered into a model from which runoff-quality arrays could be generated. Confidence levels for frequencies assigned to given events in the arrays would obviously not be very high, but the results could well be adequate for differentiating relative superiortiy among alternative designs.
given rainfall space-time distribution to the sizing and deployment of conduits and storage reservoirs adequate to accommodate runoff rates and volumes at the level of protection desired or elected, with an accounting and allowances for affected water quality control facilities, such as storm water treatment plants, and reclamation facilities, such as recharge spreading basins. This is a tall order. Because responsible metropolitan planning explores alternative
futures on an areawide scale, storm sewer planning of the type needed requires coarser hand- ling, otherwise information processing requirements will be overwhelming. On the other hand, storm sewer systems are designed a catchment at a time, so tile basic distinction between planning and design is greater extensiveness for the former and greater intensiveness for the latter.
of rainfall-runoff-quality processes
,
even if still somewhat limited due to the complexity of hydrological systems and the resultant inherent empiricism of applied hydrology.modelling can never be perfect or complete: it is an analytical tool and hence is surely as much an art as a science. ' -. . . . a model system is merely a researcher's idea of how a physical system interacts and behaves, and in the case of watershed research, watershed models
are usually extremely simplified mathematical descriptions of a complex physical situation.
-. .
.
. until each internal sub-model of the overall model can be independently verified, the While it is hoped that storm research ultimately will provide statistical insights toTo sum up, storm sewer design methods of the type needed would permit proceeding from a
Hopefully, field research on urban drainage will yield greatly improved understanding Hydrological
I68