Hydrological Processes and Water Management in Urban Areas (Proceedings of the Duisberg Symposium, April 1988. IAHS Publ. no. 198, 1990.

CONCEPTS OF URBAN DRAINAGE AND FLOOD PROTECTION

P. Harremoës Department of Environmental Engineering, Building 115, Technical University of Denmark 2800 Lyngby, Denmark

ABSTRACT

The significance of the introduction of modern computer technology to design, analysis, operation and control of sewer systems during rain is analyzed. With new tools come new basic concepts and new engineering criteria for performance. The most significant developments are: the capability to calculate surcharging and flooding, rather than just relating to pipe capacity performance criteria; the capability of calculating long series of rain record in order to derive proper statistics on the pollutional load on the environment; and finally the capability of dynamically controlling the system in real time in order to decrease the pollutional load by optimization of the usage of storage at rains smaller than the design rain, without increasing the risk of floods.

INTRODUCTION

The Romans were able to build and manage impressive hydraulic systems, in spite of a number of hydraulic misconceptions. These included structures for the drainage of comprehensive sewerage which was a new concept derived from the need to avoid epidemics of water borne diseases, not a structural innovation. However, the knowledge of basic hydraulics has improved significantly since the Romans. In fact, through the remainder of last century the basic concepts for the design of urban sewers developed into design procedures that stood the test of time and practice until only two decades ago, when the modern computer technology introduced entirely new possibilities. We are still grappling with the new tools in the process of establishing the lasting concepts on which to base new standards of practice for the design and operation of urban sewer systems.

THE FUNDAMENTAL PROBLEMS: FLOODS AND POLLUTION

Notwithstanding the whole spectrum of hydrological problems related to the development of the modern city it is clear from history and the present situation, that two problems supersede all others: Floods and pollution.

Most cities situated at rivers and estuaries are prone to occasional inundation by floods, most of which are caused by the external hydrological conditions of the river and estuary -rather than the urban drainage system per se. These external conditions are outside the scope of this paper, except as a downstream boundary condition, with respect to which several errors in design have been seen in practice.

The real reason for the impressive investments in sewerage in European cities in the

105

P. Harremoës

1850's was pollution: Waterborne diseases due to unhygienic conditions within the city. It took another half a century to realise the extent of the pollution generated in the environment by the concentrated discharges of organic matter. It took still another half century to realise the extent of pollution with polluting substances other than organic matter. It is becoming clear that the contribution from runoff is of significance.

The new aspect of the basic concepts is not the prevention of floods, but the new emphasis on pollution over the whole spectrum of pollutants. We are simply faced with new demands from the public for better performance of urban drainage. Fortunately, these new demands coincide with the availability of better tools for the analysis of the systems with respect to performance.

- , 1 1 1 — — , , — | — $ * •

0 200 400 600 800 1000 1200 m

Figure 1. Profile of the main sewer of a test catchment and the flooding generated by a historical rain event from the 5 September 1961, at 1217 hour.

106

Concept of urban drainage and flood protection

Return period N A year

40

20

10

5

2-

1 -

Flow equal capacity

Return period N À year

- > . 0.5

Dynamic wave :

flood

flow equal capacity

level equal top

Figure 2. Return period for each manhole in the main sewer shown in Figure 1, calculated for different models and different criteria.

CONCEPTS FOR CALCULATIONS BY HAND AND BY COMPUTER, INCLUDING CORRESPONDING CRITERIA FOR PERFORMANCE

It is interesting to analyze how demands from society for performance of engineering structures are converted into engineering practice. They are simply transformed into concepts which are strongly related to the tools available to the engineer in the analysis of performance. Frequently, the tools require basic criteria nor clearly related to the basic demand. Urban drainage is no exception from this rule.

Floods

The risks of floods in traditional design is a good example. It is well established practice to formulate the basic critérium as the return period for exceeding the capacity of pull pipe flow at an energy gradient equal to the slope of the pipe. That design period is not universally related to the risk of flood. It depends significantly on the local conditions.

Figures 1 and 2 show an example of an analysis of traditional design, Spildevandskomitéen (1982), Harremoës and Eriksen (1981). A catchment north of the city of Copenhagen was analyzed with respect to traditional design and with the best

107

P. Harremoës

available hydraulic simulation. Figure 1 shows the sewer profile and the development of water levels calculated for a historical rain with the full dynamic wave solution to the St. Venant equations. The catchment has been modelled for the 100 most intense rain events from a 47 year long rain record. Figure 2 shows the return period for different approaches to the calculation:

The rational method shows a return period for exceeding flow capacity of 1 - 2 years downstream of manhole 14, and larger upstream. The kinematic wave solution shows equal return periods for exceeding flow capacity downstream of manhole 14, but somewhat larger values above. The dynamic wave solution, with respect to exceedence of flow capacity, shows similar results downstream of manhole 14, but increased values (greater than 47 years) in manhole 6. The dynamic wave solution with respect to water level exceeding the top of the pipe. Notice the difference between the criteria: flow capacity relative to water level at top of pipe. The dynamic wave solution with respect to floods above terrain in the manholes 26 and 32, and floods of basements in manholes 5 and 6.

The only position in the sewer system of the example where there is a close relationship between the return period for flow capacity and floods is around manholes 26 and 32. The results in manholes 5 and 6 illustrates the point made: There is no universal relationship between flow capacity (greater than 47 years) and flood (3 years). The result is not even on the safe side. The conclusion is that new basic engineering criteria have to be introduced with new tools. It was established practice to base design on full pipe flow capacity, in Denmark 1 year return period for separate sewer systems and 2 years for combined systems. That practice is changing. It is now formulated as return period for actual flooding. An economical analysis has shown, Spildevandskomitéen (1985), that the optimum between annual cost of floods and amortized annual cost of constructed sewers is flat in the region of return periods for floods from 5 years to 10 years. Accordingly, the return periods for reaching flow capacity have to be increased, thus introducing safer designs. It is an interesting aspect of this, that the courts are beginning to take these facts into account. After the early 1980's, design based on the rational methods are no longer accepted in cases where floods are a real risk. Design has to be analyzed on the basis of the available tools. It is now manifest that computer models using the dynamic wave solution to simulate unsteady flow are superior to the traditional methods.

Pollution

The aquatic environment is polluted from the discharge of diluted, but untreated wastewater and rain water from combined sewer overflows. It is common practice to base the design of such structures and the related retention basins on the most crude criteria, which may not even be formulated in a fashion that is related to the actual pollution. In Denmark the design practice is supposed to be based on a dilution of 1:5 for peak hour flow. However, an analysis of actual practice has shown that the design dilution varies greatly. Other rules are related to permissible frequency of overflow, e.g. 10 times per year. Others are related to yearly rates of BOD-discharge from CSO in relation to

108

Concept of urban drainage and flood protection

discharge from the treatment plants. None of these formulations are based on concepts related to the actual pollution experienced in the receiving water, whether river, lake or the sea. Distinction has to be made with regard to the time scale of the pollution. The discharge of pathogenic bacteria is a health risk while it is there. The discharge of BOD causes oxygen depletion during the event and the discharge of acute toxicants cause fish kills instantly. Such acute effects have to be judged on the basis of extreme statistics, i.e. formulated as the return period for the event and the damage it causes.

The discharge of nutrients, heavy metals and non-biodegradable organic pollutants accumulates in the environment and causes damage when critical levels have been reached. That calls for judgement to be made on the basis of yearly discharge - to be compared to other discharges, e.g. from treatment plants. Figure 3 shows two examples: extreme statistics for the discharge of BOD and yearly overflow of phosphorus from a combined sewer overflow. These figures have to related to the effects in the receiving water, as discussed in the paper by Gujer (1987).

ALTERNATIVE APPROACHES TO COMPUTER CALCULATIONS

With the introduction of new tools for design of sewer systems follows not only new basic criteria for the performance, but also new approaches to the concept of calculation.

Alternative descriptions of rain

There are several approaches to the description of rain as input to design and analysis: a Intensity-duration-frequency curves from point measurements

- Evenly distributed rain in time - Time-variable rain

b Historical rain series from point measurements - Rain varied in time as measured

c) Rain as recorded in both time and space The basic difference between the traditional use of intensity-duration-frequency curves and the use of historical rain series is a reversion of the basic formulation of the problem. In the traditional approach the statistical properties are fixed to the rain as input to a calculation, under the assumption that the statistics on the rain are transferred to the calculated result without alteration. That would have been correct had the transfer function (the runoff model) been linear. That assumption not being correct there is an unknown inaccuracy in the approach. The use of historical rain series is based on modelling all rain events in the series (or a censored extract from the series, e.g. the 100 most intense). The statistics are produced on the ultimate effect: the flood, or the pollutional load as exemplified in Figure 3. Use of rain input distributed in both time and space is in its infancy, because it requires either a very intense system of gauges or weather radar measurements. Such systems are under development, especially for real time control.

109

P. Harremoës

kg P pr. year L.B.U

15,0-

10,0- n

5.&-XN.-..T.1: 7.

gJXliXll,1,1,1,1,1,1,1,1,1 f.

m+a1

m

m-tf

U) f- (0 M " 01 (H tHi MD Jl* (U (H IA0 f- « G * HI 0 f M MD 01

33 y e a r s of ra in record

kg BOD pr. event

Return period

Figure 3. Extreme statistics for discharge of BOD, which causes acute pollution as oxygen depletion, and statistics on annual load of phosphorus, which cause accumulative pollution as eutrophication.

110

Concept of urban drainage and flood protection

30-

20-

10 -

Runoff, mm

• • • * W >

MmSSr * *

+

+ ' + + + +

1

+

+

+

20 30

Runoff, mm

1000 Annual overflow, mm/year

100

+ rain event

liniar regression

01

Rain

40 mm

•

1 ' ' I , . . . . . .

\̂

' ' ' ' ' ' ' '

— measured

— model

- — <t>.S

1 10 100 Interceptor capacity,a, l/sec • ha

Figure 4. Runoff volume versus rain depth from a five year record. The record was subsequently used for comparison of measured runoff and annual overflow from a combined sewer overflow versus values calculated by a simple and an advanced hydrological model.

I l l

P. Harremoës

Alternative descriptions of hydrology

There are two basically different approaches to the hydrology of urban runoff: a The runoff coefficient b Detailed description of the hydrological processes The traditional approach a) has been modified to include an initial loss. With subsequent runoff estimated by a hydrological reduction factor. Figure 4 shows results from five catchments in Denmark measured over a period of 5 years, Jensen et al. (1985). The result can be described by a regression line. It is obvious from the figure that there is left a significant degree of uncertainty. The new approach b) is to decrease the uncertainty through a detailed deterministic description of the hydrology of the urban area, including: wetting loss, ponding, evaporation, infiltration, flow on the surface and in the gutter. The experience in Denmark is that it is seldom possible in practice to find available the detailed information required for the detailed hydrological description. Detailed analysis of the data shown in Figure 4 also gives rise to doubt over the real value of the detailed description. It is the experience from the data that runoff from permeable surfaces is very rare. The few data above the 1:1 line might as well be interpreted as the result of uncertainty. Detailed hydrological analysis with continuous simulation during and between rain did not provide significant explanation. Figure 4 also shows the ranked discharge in mm runoff calculated on the basis of paved area in the catchment (impermeable + semipermeable). The discharge is shown as measured, as calculated using initial loss and hydrological reduction factor and as calculated using continuous hydrological simulation. The figure also shows the annual overflow from a combined sewer overflow as a function of the critical downstream capacity of the connection to the interceptor. The same three cases are shown. There is hardly any difference to be seen. Conclusion: Surface runoff from permeable surfaces plays a smaller role than can be detected. In these cases little will be gained from detailed elaborate deterministic description.

The alternative is to accept the uncertainty and to include it in the design procedure. In the case of yearly discharge (accumulating pollutants) the statistical uncertainty in the hydrological description is of lesser importance, because it is an integrated figure. In the case of extreme events like floods and rare discharges of acute pollutants the statistical uncertainty in the hydrological description may be of great importance. However, it can be accounted for in the design:

The rain volume per event can be considered as log-normally distributed, Harremoës (1986 and 1988), and the uncertainty of the hydrological description can also be interpreted as log-normally distributed. Combining the two uncertainties leads to a correction factor to the runoff coefficient (or the hydrological reduction factor) that accounts for the combined effect of statistics. Accordingly, the runoff coefficient becomes a function of the return period.

The experiences are typical of Northern European construction practice, in the sense that little is done to encourage infiltration. In Denmark deliberate measures to infiltrate locally have been practiced on several occasions, but the practice has never become dominant. The reason is tradition. City engineers design streets, road surfaces, sidewalks and gutters according to established engineering practice. It will take a major effort to change that.

112

Concept of urban drainage and flood protection

The investment in existing systems prevents new thinking from making a real impact. The introduction in the 50's of separate sewerage for all new developments on the erroneous assumption that runoff is clean may serve as a warning. Does infiltration pollute the groundwater? There is an urgent need for thorough investigations.

The experience with respect to urban hydrology for extreme rain events referred to above, may be valid for Northern European conditions practice, but it is obviously not valid for different climates. In tropical climates with dry and rainy seasons, infiltration in the dry season and the lack of infiltration in the rainy season are critical phenomena to take into account.

The special need for appropriate construction practices in the tropics and the role of infiltration give reason to encourage local investigations in the framework of uncontrolled urban development to unprecedented megatropolis.

Alternative description of hydraulics

The basic approaches of the hydraulic description of the runoff are: The rational method The time-area approach The kinematic wave approximation The dynamic wave approximation

The rational method is still good for simple design without serious flooding problems. The dynamic wave approximation is the only approach that can take downstream conditions and surcharching into account. Only a few years ago that approach required so much computing time that it was prohibitive in most cases. That has changed dramatically and dynamic wave simulations can now be made with ease on microcomputers, e.g. the MOUSE program package, Nielsen (1987). There is no way back. Such models are becoming the engineering tools in all cases where serious flooding is at stake.

That has virtually made the kinematic wave approach obsolete, because the time saved in making the calculations does no longer justify the shortcomings of the approach.

The time-area approach still has merit because it is based on the rules of superposition, which makes calculations extremely fast. Where many calculations are required in order to produce adequate statistics the time-area approach is the viable solution. The example is calculation of annual and extreme loads from combined sewer overflows, Johansen et al. (1984). The other field, where fast calculations are imperative, is in real time control.

The relational method has its merit for simple design. However, such simple formulae and other pragmatic, sometimes purely empirical, formulae, are being used even in large scale design. There is no doubt that analysis and design will be improved significantly by the application of the most advanced models for simulation of runoff.

113

P. Harremoës

DYNAMIC CONTROL OF RUNOFF

The traditional concept of building sewer systems is to use only fixed engineering structures. In few cases do movable parts function to regulate the flow. Where such regulators have been used they have usually been designed just for local regulation of a pump, a weir or a sluice gate. In a few case, these regulators have been used in a centrally organised system for real time control of the total system. However, in grappling with the new tools the centralised control is an option with perspective. Sewer systems are designed for a certain return period. Only for events of that rareness is the capacity of the system being utilised according to potential. There is a potential benefit in controlling the system for lesser rain events than the rare design event. The potential is to utilise the available storage capacity at optimum in order to minimize the discharge of pollution from combined sewer overflows, Schilling et al. (1987).

Figure 5 illustrates the approach to control in its fully developed potential. The local regulators are decentralised units with set points, set from a central unit from which the whole system is managed. In case of rain the whole system is monitored and the regulators are controlled by an optimising procedure that keeps the overflow at a minimum, while still preventing floods. That optimization programme package is supervised by an expert system, which again is supervised by the operator. With information from a weather radar for prediction of rain the stage is set for very significant reductions of pollution discharge.

THE RUNOFF AND TREATMENT PLANT SYSTEM

The pollution discharged to the surface waters from the combined sewer system has two outlets: The combined sewer overflow and the outlet from the treatment plant.

Basin

Controller

( 7 ) Pump

Operator

Figure 5. The principle for a fully developed, centralised system for real time control of runoff.

114

Concept of urban drainage and flood protection

The traditional concern has been the discharge of organic matter. It is interesting that calculations in this respect are often made on an annual basis, x kg of BOD per year, though BOD discharge and oxygen depletion has to be evaluated on the basis of extreme statistics: x kg. of BOD for an event with an associated return period, illustrated in Figure 3. Bacterial pollution is frequently ignored from combined sewer overflows.

In Denmark removal of nutrients from wastewater is becoming mandatory, nitrogen down to 8 mg/1 and Phosphorus down to 1.5 mg/1. The contribution from runoff during rain is becoming of local significance. The conclusion is to decrease the frequency of overflow from combined sewers and to store the runoff for later treatment in the treatment plant. Here a surprising fact appears: The effect with respect to nutrients is not very big, Henze (1987). The runoff stored will invariably increase the flow through the plant. During that flow nutrient concentration in the treatment plant effluent does not reduce, rather an increase would be expected. The order will be 6 mg N/1 and 1 mg P/l. That is not far from the concentration of N and P in the overflow at dilutions in the range 1:5 - 1:10, not far from the traditional design dilution relative to average flow. Accordingly, extra storage and decreasing the frequency of overflows does not a priori decrease the total discharge from the system, but it may well do so locally.

That does not change the need for better analysis of the performance of overflows and for abatement of the pollution derived from runoff during rain. There are other pollutants derived from runoff during rain. There are other pollutants, like BOD and bacteria, that justify the analysis and the structural improvements - and ultimately the cost.

CONCLUSIONS

New tools for the design, analysis operation and control of sewer systems have become available with the introduction of computer technology. It is essential to realise that with new tools comes a demand for new criteria for performance. The traditional criteria were suited for hand calculations. The new simulation capabilities makes it possible to formulate the criteria in direct relation to the actual damaging effect: Floods and pollution. More work has to be done with respect to the fundamental objectives and the engineering concepts based on relevant criteria.

The deterministic description of the phenomena has been very rewarding with respect to a number of issues, description of rain and the simulation of unsteady flow in the sewers, including surcharging and flooding. However, with respect to description to hydrology of surface runoff and the description of pollutant runoff there appear to be limits to the benefit of pursuing even further detail in the deterministic description. The alternative is to use a simple, empirical relationship and to interpret the residual uncertainty as stochastic properties to be accounted for in the analysis of extreme events and their return periods.

The overzealous researcher, occupied with fitting his 27 parameter model to a hopeless scatter of data, may well slow down the important transfer into practice of the basically new principles.

115

P. Harremoës

REFERENCES

Gujer, W. and Krejci, V. (1987): Urban storm drainage and receiving water ecology. In: Urban Storm Water Quality, Planning and Management. Proceedings of the Fourth International Conference on Urban Storm Drainage, Lausanne, Switzerland, 31 August - 4 September 1987, pp 1-20

Harremoës, P. and Eriksen, M. (1982): Alternative application of rainfall statistics to sewer system design. In: Urban stormwater hydraulics and hydrology. Proceedings of the Second International Conference on Urban Storm Drainage, Urbana, Illinois, USA, 15-19 June 1981, pp 365-373, Water Resources Publications, Littleton, Colorado, USA.

Harremoës, P. (1986): Interaction between quantity and quality models for urban storm drainage. In: Urban Drainage Modelling. Proceedings of the International Symposium on Comparison of Urban Drainage Models with Real Catchment Data, UDM '86, Dubrovnik, Yugoslavia, Supplement, pp 1-15. Faculty of Civil Engineering, University of Belgrade, Yugoslavia.

Harremoës, P. (1988): Stochastc models for estimation of extreme pollution from urban runoff, accepted for publication in Water Research.

Nielsen, J.B. et al. )1987): Towards the fifth generation of numerical systems. In: Urban Storm Water Quality, Planning and Management. Proceedings of the Fourth International Conference on Urban Storm Drainage, Lausanne, Switzerland, 31 August - 4 September 1987, pp 291-296.

Henze, M. (1987): Stormwater handling in wastewater treatment plants. In: EWPGA-Symposium"87, 7. Europâische Abwasser- und Abfallsymposoim, Munchen, 1987, Documentation, pp. 173-194. EWPCA, St. Augustin, BRD.

Johansen, N.B., Linde-Jensen, J.J. and Harremoës, P. (1984): Computing combined system overflow based on historical rain series. In: Proceedings of the Third International Conference on Urban Storm Drainage, Goteborg, Sweden, 4-6 June, volume 3, pp 909-918. Chalmers Tekniska Hogskola, Goteborg, Sweden.

Schilling, W. et al. (1987): Real time control of urban drainage systems. The state of the art. Report from the task Group on Real Time Control of Urban Drainage Systems under The IAWPRC/IAHR Joint Committee on Urban Storm Drainage.

Spildevandskomitéen (1982), Investigation of design practice for small urban catchments (in Danish), DIF Spildevandskomité, Copenhagen, Denmark, skrift nr. 20.

Spildevandskomitéen (1985), Permissible return periods for floods in urban catchments (in Danish), DIF Spildevandskomité, Copenhagen, Denmark, skrift nr. 23.

Editors'note:

We agree that too much determinism in hydrological and pollutant modelling may be inappropriate, and some stochastic variability should be allowed, but greater detail than simple coefficients is often warranted without going to 27 parameters! Modelling observed hydrographs in complex catchments, including perhaps storage and local runoff control, is often frustrated by spatial, seasonal and temporal variability in rainfall and runoff/washoff.

116

Concept of urban drainage and flood protection

A better understanding of these variabilities based on measurements and model studies can lead to better model simulations. In particular accurate prediction of the effect of a change in catchment, condition depends on the fidelity of the determinism within the model.

117

118