A new rainfall data system for urban hydrology in Switzerland

ATMOSPHERIC RESEARCH ELSEVIER

Atmospheric Research 42 (1996) 177-198

A new rainfall data system for urban hydrology in Switzerland Christian Eicher

a, *

,1, Vladimir Krejci b,2

a CH-3123 Belp, Switzerland b EAWAG, CH-8600 Duebendorf Switzerland

Received 4 December 1994; accepted 30 June 1995

Abstract A new approach to introduce a rainfall data system specific for urban hydrology has been initiated in Switzerland. New rainfall information from the raingauge network of the Swiss National Meteorological Institute shall become the center point of this data system. Rainfall is recorded in 10-min time steps since 1981 for more than 70 stations, of which 38 are useful for urban centres. The data system is to be set up in several steps. The objectives of Project Step 1 were: • to evaluate and select suitable raingauge equipment for urban hydrology needs, • to elaborate methods for control, validation, storage and transfer of rainfall data, and • to develop recommendations for raingauge site and installation requirements specific for urban hydrology applications. Key results of Project Step 1 as presented in this paper are the following: • proposal for selection of specific raingauge systems for different urban hydrology purposes, i.e. national/local, permanent/temporary gauge networks, • elaboration of standard guidelines • for control, validation, storage and transfer of rainfall data, • for selection of sites and installation of raingauges for urban hydrology applications, • proposal for a specific rainfall database with easy access for the urban hydrologist in Switzerland,

* P.O. Box 40930, Addis Ababa, Ethiopia. Reporting for the Rainfall Data Committee of VSA (Swiss Water Pollution Control Association): Dr.Vladimir Krejci, Pierre Grandjean, Walter Kisseleff. 2 Team of Consultants: Dr. Jules DeHeer, Christian Eicher, Rudi Gloor U'), Peter Kaufmann, Paul Meylan, Heinz Mutzner. 0169-8095/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSD1 0169-8095(95)00062-3

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proposal for replacement of existing methods for local selection of relevant intensityduration-frequency (IDF) curves and adoption of a combined Montana-Talbot IDF curve formulation for short as well as long rainfall durations.

1. Introduction

Over the last few decades, many tasks of the urban drainage engineer have changed significantly, with a shift from the traditional design of new wastewater networks and treatment plants towards the extension, maintenance and optimization of existing installations. These new tasks require new tools and approaches, as well as a new orientation of the engineer's perception, replacing the traditional Q,nax-Oriented view of the sewer network with an integrated philosophy involving the entire collection and disposal system as an entity, rather than as independent elements. This new approach requires also a new kind of information on rainfall, which acts as the dominant "loading factor" for the different processes of the urban drainage system. The existing "standard" intensity-duration-frequency (IDF) curve sets in use in Switzerland over the past 30 years no longer form an adequate basis for present and future requirements of urban drainage applications. Recognizing this situation, the Swiss Association of Wastewater Professionals VSA has, with financial support from several cantons, municipalities and drainage districts, launched a project aimed at providing updated and easily accessible rainfall data in suitable form for the present and future needs in urban drainage. A support commission appointed by VSA in 1988 developed the outline for a three-part project as well as the terms of reference for the first phase of the study, which was recently completed and will be presented in this paper. The main elements of the three-phase project are defined as follows: Project 1 - - Establish methodology for rainfall data handling - - Terms of reference and cost estimate for Project 2 Project 2 - - Prepare workbase infrastructure -Project management (organization, staff, cost control, financing) - - Terms of reference and cost estimate for Project 3 Project 3 - - Data acquisition, collection and transfer to end users This paper is a summary of the Final Report on Project 1 (VSA, 1994), prepared by the group of consulting engineers: DeHeer, Eicher, Gloor (t), Kaufmann, Meylan, Mutzner. 1.1. Extent o f Swiss urban areas

In Switzerland, urban centres and isolated cities with more than 10,000 inhabitants, as well as smaller settlements larger than 5000 inhabitants make up 74% of the population and occupy 20% of the territory, or 55% of the impervious surfaces of the country. 97% of the population lives on the plateau and in the valley areas below 800 m altitude.

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The impact of this concentration on the aquatic environment is significant, in particular where water bodies cross urban settlements. Therefore, conceptual and design errors due to the lack of adequate and updated rainfall information can have serious negative effects in urban centres. 1.2. Water protection investments in urban areas Current annual investments into wastewater collection and disposal in Switzerland are of the order of one billion Swiss Francs (CHF 1,000,000,000). Since 1960, this totals about 20 billion Swiss Francs from public funds, plus another ten billion Swiss Francs from private sources. Protecting the value of this investment as well as maintaining and operating the systems produce estimated annual costs in the order of another billion Swiss Francs. Despite these enormous sums, many tasks in urban drainage projects as well as in operating the systems are currently based on relatively poor basics when compared to other areas of daily life - - to a significant extent due to the lack of dependable local rainfall data. 1.3. Importance of rainfall data for urban drainage The significance of having available adequate and locally relevant rainfall data grows with increasing complexity of the drainage problems to be solved. Sizing and constructing new sewers for new developments is increasingly loosing importance as a consequence of the high degree of service in wastewater collection in Switzerland. On the other hand there is a growing tendency for new and technically more demanding facilities, such as the treatment and management of storm runoff, concurrent with ever-increasing effluent restrictions and requirements. Designing such facilities requires an increased degree of detail and local representativity of the rainfall data. Operation and management of the wastewater collection systems as well as their optimization will be of significantly more importance in the near future. Improving our knowledge on the role of precipitation in this context is a prerequisite for a better management of the enormous investments in our drainage and collection systems, besides being a must for improved protection of our aquatic environment (Arnell et al., 1993). 1.4. Current status of rainfall measurement in Switzerland Switzerland is cluttered with a fairly large number of diverse precipitation measurement stations, operated by federal, cantonal, municipal institutions as well as private individuals. Similar diversity exists in terms of the quality of measurements and recordings, ranging from professional maintenance and quality for the data collected by the Swiss Meteorological Agency (SMA) to virtual garbage from some local measurement efforts. Lacking a systematic inventory of the stations and their data availability, a complete overview is impossible at the present time.

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The overall financial commitment for installation and maintenance of all the different installations is substantial. Unfortunately, the usability of the existing data must be considered very limited at best, for the following reasons: • insufficient consideration for specific urban drainage-related requirements, • lack of continuity of the measurements, • control, verification and archiving of data is often not assured, • lack of appropriate data evaluation, and • inadequate quality of measurements a n d / o r data recording. 1.5. Objectives f o r Project 1

The bottom line of the Projects 1 to 3 is to provide rainfall data in a form that is readily accessible and directly usable by engineers in the urban drainage field. Primary objective for the initial Project 1 is the development of the methodical groundwork in order to reorganize and optimize, as well as to coordinate all efforts in the measurement, handling and treatment of precipitation data. For this purpose, the study has to encompass the entire path of rainfall data from the measurement down to the end user. The individual segments as well as their relationships are represented in Fig. 1, with the following key elements: measurement networks, measuring instruments, data transfer, data archiving, statistical analysis, data distribution.

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1.5.1. Literature survey

In order to establish a solid basis for the entire Project, an international literature survey was conducted during the initial steps of Project 1. A selection of the literature reviewed is given at the end of the paper. The objectives of this review were to establish current and future trends in urban drainage computations involving rainfall data. In addition to this, information was to be collected on the latest developments in the fields of precipitation measurement, data handling and treatment as well as with respect to the statistical processing of rainfall data for urban drainage applications.

2. Measurement networks

Rainfall data for urban drainage applications have to fill a number of specific requirements. The engineering applications which define them can be roughly classified as follows: • development and analysis of drainage concepts, design of installations, • generalized and detailed studies of drainage systems and their installations, • analysis of selected storm runoff events, • operation, control and optimization of drainage systems. The rainfall data requirements depend on the nature of the problems to be solved. Precise information on individual storm events, for example, would require high-density networks, with distances of 1 km or less between raingauges. For Switzerland, this would mean in the order of 6000 raingauges to cover all urbanized areas, which is beyond realism. On the other end, for statistical analysis of daily rainfall data, a measurement network with 50 km between raingauges, or 170 stations, would be sufficient to give a confidence level of 20%. Table 1 shows that the requirements for the length of record are similarly dependent on the nature of the engineering problem to be solved. The wide range of the different tasks in urban drainage engineering requires individual types of raingauge networks, as outlined in the following list and in Table 2: the National Reference Network: Permanently operated raingauge network with high quality standard, long observation period but relatively limited station density, serving as statistical base network for design and flooding protection level reference.

Table 1 Minimum length of observationperiod for different tasks in urban drainage engineering Minimum length of Engineering task observationperiod (yr) Design of sewers 20 Design of infiltration and 30 retention installations Computationof 5 CSO characteristics

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Table 2 Raingauge network type specifications Type of network Nationalreference network Application range Coverage Operation m o d e Site selection criteria

same rainfall region national permanent within or close to urban centres with 10,000 i n h .

Application of data conceptualanalysis, design Number of existing stations

38 ANETZ stations in useful locations

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upgradeENET (7 stations). extension ENET (4 stations) large-scalerainfall regions

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Local and regional networks

within hydrologicbasin local-regional permanent or temporary within catchment studied, one or several stations system control, model verification together with runoff measurements (to be newly developed) various networks (Berne, Geneva, Lucerne, Zurich, Altenrhein,...) 1st stage: 250-450 stations 2nd stage: depending on specific requirements parameter maps for (no regionalization statistical base values required) close to hydrologicbasin national permanent climatic + ralngauge stations/WWTPs > 5000 PE preliminary projects, proofing analysis

Relies on the automated national networks ANETZ and ENET, which collect climatic information for some 70 + stations at present (Hoegger et al., 1992), the National Urban Drainage-Specific Network: Permanently operated high-density raingauge network with high data resolution and good quality. This new network will require development from scratch, local and regional networks: Existing and new permanent or temporary raingauge networks with very high station density on the local level, for specific applications such as detailed storm drainage system analysis, operation and management. 2.1. Raingauge networks extension priorities

Based on the different requirements in urban drainage engineering, the study defines the following priorities for possible extensions of the existing raingauge networks as well as for modifications and improvements: 1. Improvement of national network A N E T Z / E N E T . Set up around 1980, A N E T Z has excellent data in computerized form available, but access for the private engineering sector is difficult and not well established. A suitable databank must be set up, furnishing this data in processed and readily usable, well-documented form. The currently fixed 10' time step for the rainfall data should be reduced or eliminated as soon as possible. Statistical treatment of the database in connection with previous strip-chart recordings from the same locations is feasible for most locations, 2. Improvement of local and regional raingauges. VSA is to develop and distribute standard guidelines for the installation and operation of raingauges, as well as for data acquisition and handling. Standardizing these

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procedures is a prerequisite for later data integration into a national databank, 3. Implementation of national urban drainage-specific network. This new network must be designed and built from scratch, integrating existing raingauge installations where possible and appropriate, with adapting and modification of existing equipment as required. This high-density network will take several years to realize, 4. Integration of local and regional networks into databank. Integrating raingauge data from existing installations with sufficiently high quality standard and length of record is seen as a future objective, providing access for interested users to a much larger database. Quality control and classification of all data shall provide full transparency and differentiation relative to the "National Data Standard' '. 2.2. The national reference network

The existing National Climatic Measurement Network ANETZ of the Swiss Meteorological Agency SMA has to serve a wide range of diverging requirements with its approximately 15 sensors on each station and on-line data transmission in 10-rain intervals. This background is important in order to understand why some users' expectations may not be ideally fulfilled. Precipitation is measured using heated tipping-bucket raingauges with 0.1 mm resolution. The tipping pulses are summed up over fixed 10-rain intervals and transmitted to a central computer in Zurich. The 10'-data are currently not corrected, but assembled to the official hourly and daily data published by SMA. Totalizer data from daily recorded pluviometers are used to verify and correct the hourly and daily data. The recorded but normally unpublished 10'-data is available on request for special applications. Unfortunately, the 10-rain time step is too large for several urban drainage engineering problems and can lead to significant underestimation of computer-simulated peak flows from fast-responding drainage catchments, as demonstrated by a pilot study. A second national automatic network of SMA is potentially of importance for urban drainage engineering: The Extended Meteo-Network ENET is to replace the existing old Warning Network for Extreme Wind, Thunderstorm and Frost Conditions. Not normally equipped with a pluviograph, this and other sensors can be attached to ENET stations. Data is collected and transmitted to Zurich in the same 10-rain intervals as with ANETZ. Until 1997, some 70 ENET stations will be established. Despite several disadvantages and limitations, these two networks provide at present and for the years to come by far the best available data for the majority of engineering tasks in urban drainage applications. Well-secured continuity in operation and maintenance as well as fairly high and uniform quality standards are the key advantages of ANETZ/ENET. As outlined in the introduction, drainage problems originate to a great extent from large urban centres. The study has therefore evaluated the degree of spatial coverage provided by the national networks A N E T Z / E N E T with respect to the major urbanized areas. For this purpose, every station was given a weight factor relative to its distance and altitude difference from the next urban centres, pondered additionally with the size of the urban centre.

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From this theoretical evaluation, it was concluded that 38 of the 70 + stations can be considered of significant value for urban drainage applications, since they are located within 20 km and not more than 500 m above or below the urban centre. The results of this evaluation of the existing national raingauge networks are shown on Map no. 1 (Fig. 2). The map also shows the location and relative size of the large urban centres and isolated cities within Switzerland. Another 14 urban areas with more than 10,000 inhabitants are not presently equipped with a national rainfall measurement station within the limits mentioned before. Eight of these could be served with raingauges if the nearby existing or planned ENET Extended Network stations would be equipped.

3. Digitizing strip-chart rainfall data The value and reliability of the results from most hydrologic studies involving extreme-probability problems are tightly connected to the length of record of the rainfall data-time series used. The availability of long records is, therefore, an asset to many engineering problems. For many locations in Switzerland, historical pluviograph charts exist, which when transformed to digitized form would be of great value for urban drainage engineering applications including statistics, continuous as well as single-event runoff simulations. As a starting point, priority should be given to the 38 ANETZ and ENET locations identified to be of significant value for solving drainage problems in urban centres. For 30 of those locations, pluviograph stations have been in service in the past at or near the present automatic installations. For 13 of these data records, statistical analyses have been carried out by the Swiss Institute for Forestry Research, EAFV-WSL, for time segments down to 10 min. Furthermore, for ten of the 30 mentioned stations, the HiArler and Rhein analysis in the early sixties, which forms the traditional IDF curve basis of the Swiss engineer, has covered an average of 25 data years, see Section 4.1. These stations would be of great interest for a comparative analysis of different evaluation periods. Assuming that all the proposed stations cover the full period, there would be about 500 station-years available. With a limitation to 30 data-years per station, 164 stationyears would have to be digitized. Estimated cost for full digitizing would be 1.2 million Swiss Francs. For a selection of only 10 to 20 significant extreme events per year, the digitising cost could be reduced to approximately one sixth of this figure. Considering the wide range of measurement and recording quality, the full extent of digitizable data is not known at the present time. A detailed inventory with quality analysis of all data is an important prerequisite before such a task can be undertaken. Pilot testing of digitizing different data recordings have shown that manual digitizing of strip charts using a variable-step polygon method is most economic and reliable, with least sensitivity to different individuals carrying out the work. Automatic digitizing using scanners and appropriate software has not proven successful, even with the best available chart recordings. Previous control and eventual correction of strip chart

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recordings has been found to make up for a significant and extremely demanding part of the entire work.

4. Rainfall intensity curves IDF curves have up to now been the indispensable basis for the determination of maximum flows for channel and sewer design, and they are likely to remain so for the time to come. Although new design procedures utilizing selected historical or model design storms are on the way, they are not likely to replace the Rational Formula in everyday work. For the Swiss engineer, most IDF curves are based on the work by HiSrler and Rhein covering 16 stations all over and close to Switzerland, as well as later local implementations of their work. Within Project 1, the existing IDF curves have been reviewed and compared to newer data covering longer observation periods, and recommendations are formulated as to the future form of IDF data. 4.1. The HOrler and Rhein IDF data (H6rler and Rhein, 1962) Htirler and Rhein's analysis in 1962 is based on 5 to 60 rain time segments. The 16 series comprise between 6 and 30 years of data. The IDF data is presented in the form i = K / ( T + B) which corresponds to the Talbot formula. The parameters K and B depend on the location, K additionally on the recurrence period. Parameter B was found to be reasonably constant in HiSrler and Rhein's analysis, while newer studies with longer time series show also a significant dependency on the recurrence interval for some locations. In today's view, the problems of this first systematic IDF analysis in Switzerland are the following: • data time series for some stations are too short (6 to 11 years for 4 stations), available time periods are not homogenous. The average engineer assumes that he obtains safe design figures using the established IDF data, since experience shows that flooding in urban centres is relatively rare and hardly ever occurring over large areas. The authors, however, believe that the relative freedom from flooding should not be attributed to the IDF data, but rather to the design practice and the oversized building zone assumptions. The time frame for drainage master plans of 20 to 30 yr, plus generous pipe diameter oversizing have added a significant degree of safety into many sewer systems, the extent of which has never been proven systematically, and therefore is not known. 4.2. The Swiss institute for forestry research IDF data (Geiger et al., 1991) The hydrology branch of this research institute has undertaken a large-scale analysis of rainfall data, concentrating initially on the alpine and pre-alpine regions of Switzer-

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land, where formerly little systematic rainfall data analysis has been done. Their study then extended gradually all over of the country, realizing the enormous value of a uniform analysis of all available data. The analysis is based on annual extreme value statistics, with the majority being daily and longer data segments. The distributions used are Gumbel and log-Gumbel, corresponding to common practice in general hydrology. Standard reference period is 1901 to 1970, with less or additional years being considered, depending on availability. This analysis is relatively little known among urban drainage engineers. Its different theoretical base and the specific data presentation make it also difficult to use in urban drainage practice, even where short data segments down to 10 rain have been analyzed, for the following reasons: • results are published in graphical form only, with relatively poor readability of discrete values, • the application of different distribution functions complicates the handling of transitions between the two, • interpretation and usage of the isohyetal map information is unhandy and in many situations error-prone for non-specialists, and • the presented extrapolations of daily data to hourly and shorter time segments is relatively unsafe and for some stations questionable. 4.3. Proposal f o r new I D F curves

The evaluations in Project 1 have led to the following strategy proposal for the development of a new Swiss IDF database: • application of a combined IDF curve equation Montana-Talbot, with Talbot describing the short-duration part up to around 30 rain, and Montana for the long-duration part up to three days (Fig. 3), • a Pivot point defines the transition between the two formulae. Its position is dependent on the location and the recurrence period.

RAIN INTENSITY IN L/(S'HA)

10 100 RAINFALL DURATION IN MINUTES

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Fig. 3. CombinedTalbot-Montanaintensity curve.

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This concept is similar to a proposal from HiSrler (1977), who showed that the Talbot formula underestimates intensities for longer rainfall durations, and a correction would be needed, derived from daily rainfall statistics. Further evaluation of the available statistics procedures has shown that apart from details no problems exist with the currently applied methods and techniques. The true problem is the continuous aging of the base data and consequently the statistics derived from it. Therefore, all statistics should periodically be updated and presented on a uniform basis. The following three strategies are available in order to derive new extended data series from the national network stations for the development of new updated IDF data: Strategy A: Just wait and do nothing, until by the year 2010 the automatic stations of ANETZ have 30 data years available for statistical treatment, Strategy B: Treatment of the available ANETZ-data, with a current average of less than 15 years by 1995. In relation to HiSrler and Rhein the number of stations is doubled, but the average data period is more than 5 years less, Strategy C: Digitize historical pluviograph strip-charts to extend the ANETZ records to 30 years. Strategy A - - "wait and do nothing" - - must be rejected, as it would extend the present unsatisfactory situation (which had led to the commissioning of this project!) to the year 2010. Strategy B is advantageous for its significantly better coverage of Switzerland, but the data periods are too short for many applications, bearing the risk that individual exceptional years have dominant influence on the statistics. Strategy C appears optimal from the urban drainage viewpoint. Digitizing all available historical data would not only provide reliable IDF statistics, but with the better spatial density they would give the opportunity to develop a new regionalization of the IDF parameters on the national level. This would allow the selection of intermediate IDF parameters also in areas not covered by national network stations, replacing the present simple "Zone Chart", see the next Section. Although the most expensive, Strategy C opens up the most possibilities in conjunction with the otherwise important investments required to bring the Swiss rainfall database up to modem standards. Longer time series of rainfall data are a primary resource for important and demanding engineering applications, such as continuous simulations for design and evaluation of critical retention installations.

5. Regionalizing IDF data In order to account for the different rainfall intensity characteristics all over Switzerland as determined by the H~rler and Rhein study 30 years ago, a "Zone Chart" with six different IDF parameters has been established, with the zone boundaries further based on thunderstorm frequency statistics compiled by climatic station observers. This chart became part of a Standard Sheet of the Swiss Normalization Association. The idea of this chart was to relieve the engineer from determining which of the originally analyzed sixteen stations with rather uneven placement over Switzerland would be most suitable for a given problem in a specific location.

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In practice, however, the large discontinuities along the fixed boundaries between the individual zones are difficult to explain and to handle, and newer statistics including the work of the Forestry Research Institute give rise to questions regarding the validity of this old "Zone Chart". Systematic tests to this end as part of Project 1 have confirmed that the higher station density and the longer data series of the Forestry Research Institute analysis indeed produce significant differences for some locations, while other regions show virtually the same values. Deviations exceeding 20 percent on the 5-yr, 10-min storm intensities have been found in the southern and western part of Switzerland. Within the limited scope of the tests, it is, however, not possible to explain the reasons for the observed deviations. Based on these findings, it has been recommended to review and modify the present chart and the corresponding values for at least the zones in the southern and western parts of the country. The methodology of regionalizing rainfall statistics has yet to be fully developed and tested. Within the scope of Project 1, several techniques have been explored, some of which are proposed for final consideration. The proposal suggests to use an approach different from the old concept, elaborating individual "Parameter Maps" featuring the possibility to interpolate between measuring stations. The four different maps would contain the parameters of the Talbot and Montana equations. Transformed to numerical form, the contents of the maps could also be stored in a databank and thus the parameters accessed directly by means of coordinates. With the current status of the investigations, it is not yet possible to confirm the feasibility of the suggested cartographic approach for all necessary parameters. An in-depth pilot study to this end has therefore been proposed for Project 2 in the area of Zurich, where the density of pluviographic stations is fairly high. Estimated costs for this pilot study are CHF 100,000, and for the subsequent treatment of all data within Switzerland with the same technique, another CHF 200,000 will be required.

6. Recommendations for improving the national raingauge networks The authors of the study recommend that the presently existing gaps in the degree of coverage of the national networks with respect to the requirements of urban drainage engineering are filled in, and in addition that steps are undertaken in order to reduce the size of the data time steps. In addition to this, the following additional measures should be considered in order to improve the extent and quality of available rainfall data for the urban drainage engineer: 1. digitizing of historical pluviograph records at or near ANETZ stations, in order to supplement rainfall data time series for statistical analyses for the purpose of solving design problems with greater reliability, 2. development of new IDF statistics for all stations relevant for urban drainage engineering applications, extending and supplementing the existing limited IDF database,

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3. development of a new regionalization scheme for the IDF statistic parameters to replace the existing inadequate scheme, 4. supplement seven stations of the automatic national network ENET with pluviographs and create four new stations in order to serve the remaining minor urban centres not presently equipped with national raingauges, 5. modify or supplement the raingauge data logger system of the national network stations for time-step-free event-recording of the tipping bucket impulses, 6. supplement all national network raingauge locations with additional weighing-type pluviographs in order to increase data redundancy. The most important improvement options are outlined in detail in the following Sections. 6.1. Extending the geographic coverage of ANETZ and ENET

From the viewpoint of urban drainage engineering, the national raingauge network should at least provide one station for each urban centre with 10,000 or more inhabitants. Of the 70 + stations currently in service, 31 are located ideally, and 7 more are within acceptable distance from such centres. According to the geographical analysis, 11 additional national stations would be needed to fulfill the above-mentioned standard. 7 of these 11 locations are expected to be equipped with ENET stations in the near future; the 4 additional stations should be considered for the next ENET extension phase. If, in addition to the above, smaller centres with 5000 inhabitants and up are also considered, another 10 ENET stations scheduled for the first phase of installation would have to be supplemented with raingauges. This extension has lower priority than providing raingauge service to the major urban centres mentioned before. 6.2. Improving measurement quality of ANETZ and ENET

The current equipment and recording technique of ANETZ are not optimized for the specific requirements of urban drainage engineering. The study has shown that improvements to the raingauge as well as to the data logger would be beneficial, with the following modifications in order of priority: 1. upgrading rainfall measurement with an additional continuous weighing type raingauge in order to improve measurement redundancy, 2. reduction or elimination of the data recording time step, towards tipping event-wise recording, 3. improve equipment a n d / o r data handling logic to enable differentiation between liquid and solid precipitation. Priority 1: The acknowledged good data quality and dependability of the existing national network could be further improved by increasing the measurement redundancy with a second measurement. Using a weighing-type gauge would add to the dependability, providing additional control values over intermediate periods. Priority 2: With the data recording and storage equipment currently available, time-step size should not any longer be an important consideration. Tipping-bucket

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raingauges are inherently provided for event-wise data recording - - the only technique which prevents any distortion and biasing of the measured data, leaving open all possibilities for subsequent averaging or transformation. Priority 3: The possibility of differentiating between liquid and solid precipitation, plus the reliable measurement of solid precipitation give way to significant improvements in the estimation of extreme runoff situations. Such events are not in all cases related only to extreme rainfall, but often to combinations of melting snow and rainfall on frozen ground - - conditions which are difficult to account for properly with the current lack of appropriate data. Combining the two directions of improvement for geographical coverage and data quality yields a set of options recommended by the Group of Engineers. The feasibility of these improvements depend on financial, institutional as well as technical boundary conditions, since urban hydrologists are not the only group presenting wish-lists to the Swiss Meteorologic Agency. If the proposed options create excessive technical disturbances to the existing ANETZ facilities, then the RANETZ option could become feasible. The parallel-network RANETZ is an independent system which would require its own raingauge stations, transmission network and centralized computer. Based on the existing 38 significant ANETZ stations, plus the 11 ENET locations considered for additional raingauge equipment, the estimated investment cost would be close to 1 million CHF. The high cost reduces, however, the attractiveness of this alternative. 6.3. National urban-hydrology-specific network

In order to eliminate on the medium and long-term timescale the present dependency of urban hydrology on the national networks ANETZ and ENET, as well as to overcome their limits and disadvantages, the implementation of a new national raingauge network, with specific orientation towards the needs of urban hydrology, should be considered. This proposed new network is not meant to replace, but to supplement the existing national networks managed by SMA. This becomes obvious by the fact that the proposed new network will not likely be operational until the point in time when data from ANETZ and ENET exhibit their best value in terms of the length of record available, particularly for statistical analyzes requiring long time series. The optimal density of the proposed new network would require one raingauge on a grid of 1 to 4 square kilometers, which amounts to 300 to 1200 raingauges for all the larger urban centres in Switzerland, or one raingauge per 4000 to 16,000 inhabitants. If all urban areas are to be covered, the above number of raingauges will have to be doubled. It is obvious that a raingauge network of this density will require a long time to implement and will be difficult to manage. As a first step, the following alternatives are proposed: Alternative 1: Upgrade all existing climatic and totalizer raingauge stations of SMA below 800 m altitude with automatic raingauges and dataloggers. This new network called RENETZ will total some 230 installations,

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Alternative 2: Install automatic raingauges and dataloggers on every wastewater treatment plant serving 5000 population equivalents or more. This network called SIRENE will include about 360 stations. In both cases, organization and financing are yet to be defined. Costs will be very high for both alternatives, with investments of approx. 2 million CHF for RENETZ and 3 million CHF for SIRENE, and annual costs including amortization in the order of 1.5 and 2.5 million CHF, respectively. It is obvious that financing will have to come from the user/beneficiary side in the urban hydrology sector. As for their advantages and disadvantages, RENETZ will benefit from established locations with potentially available daily recorded historical rainfall data, plus in many cases knowledgeable operators. Geographical distribution is reasonable, with the exception of the Zurich area. SIRENE is finding a technically-minded environment with lots of measuring equipment, but where rainfall recording is yet to be introduced. As wastewater treatment plants tend to be on the downstream side of urban settlements, the raingauges are not always optimally located, but the overall geographical distribution is also fairly good. Map no. 2 (Fig. 4) gives an idea of the location of the raingauge stations in the SIRENE network, based on wastewater treatment facilities serving population equivalents (PE) of 5000 to 30,000 and larger areas. Shown on the map are also the major urban centres with their relative sizes. 6.4. Regional and local raingauge networks

Besides stressing the need for a contiguous national network with uniform coverage, the study has also clearly identified the value and importance of regional and local raingauge networks, based on experiences from a number of good existing installations. Several cities and wastewater authorities in Switzerland have established their own networks, some of them exhibiting station densities that would never be reasonably possible in any national network. Most of these networks have been established as a consequence of lacking adequate data from the national raingauge stations for the solution of specific urban drainage problems. It appears important to include regional and local raingauge data into the national databank, provided that certain minimum standards are met and that the data is clearly documented and identified. The conceptual design and the implementation of local and regional raingauge networks depend very much on the specific requirements of each situation. For this reason, no attempt was made to include such principles in this study. However, the interested public as well as the authorities concerned are strongly encouraged to establish and maintain such networks as well as individual raingauge stations, since they provide important information and help to fill in existing gaps. To this end, information campaigns should be set up by the Swiss Wastewater Professionals' Association VSA, and Technical Guidelines should be developed, based on the respective elaborations in the Final Report on this Project 1. In order to standardize data handling and quality control on all levels, with the goal of introducing all worthwhile data into the national rainfall databank, the early appoint-

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ment of a special technical Advisory Commission has been recommended. This Commission is to handle the following tasks: • provide conceptual and technical advice, • provide education and training for operating personnel, • develop Technical Guidelines.

7. Raingauge station inventory Apart from a number of individual one-time attempts to collect information on the different raingauge stations still in operation or discontinued, there exists no systematical inventory in Switzerland, other than for those stations regularly operated or analyzed by the Swiss Meteorological Agency SMA. As found within the Group of Engineers in this Project 1, the individual members had knowledge of several more raingauge stations than were reported in any one of the available listings. Establishing a systematic inventory of all existing or discontinued raingauge stations of significance is a must. Announcing of all newly established stations, with a standard specification questionnaire sheet will have to be declared mandatory, as well as taking raingauge installations out of service. The inventory should be established and maintained by the Swiss Federal Service for Water Resources. Structure, procedures and financing need yet to be arranged.

8. Rainfall databank system In order to provide easy and efficient access by all users in the field of urban hydrology, rainfall data must be organized in a suitable databank. At a later stage, this rainfall databank could become part of a much larger national databank with hydrologic, hydrometric and climatologic information. In a first stage of its implementation, the databank could contain only the 10-rain data from ANETZ, which are of foremost value for urban hydrology. In that case, the administration of this databank could be handled fairly simple. Annual updating with SMA data would appear sufficient for all but special applications. 8.1. Minimum requirements f o r data input to the databank

Prior to being fed into the rainfall databank, all data has to be validated and documented as to eventual errors, corrections, or periods with missing data. Collecting data that has not been validated or with missing documentation should by all means be avoided. Primarily, data from permanently recording tipping-bucket raingauges will be fed into the databank. This includes data from the national networks ANETZ and ENET, as well as the newly to be established networks SIRENE or RENETZ. Digitized data from historical strip-charts for national network locations will also be accepted in the databank, provided that preparation and validation of the digitized information is following similarly high standards as for direct data input.

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Second, data from regional and local raingauge networks will be accepted in the databank, if they conform to the minimum standard and there is sufficient public interest to warrant their introduction. 8.2. Auailable data types

The databank shall contain primarily the rainfall data in a suitable format, i.e. date and time plus rainfall depth since the last date-time mark. For tipping-bucket gauges, this corresponds to the bucket volume, corrected for the influence of intensity. For the transferred ANETZ data, this will be the accumulated volumes per 10 min constant (or less in the future). In addition to this, the databank must supply base information on every installation, including station history and location map, etc. The databank should also provide access to extreme-event catalogues, statistical analysis data as well as regionalization base information. 8.3. Standard data format

The Group of Engineers recommends the use of CODEAU, a coded date-time-value format to be adopted for transfer and storage of rainfall data in Switzerland. CODEAU is not only a format type, but also a software-supported general procedure for handling, validation and archiving of any kind of hydrometric data, which allows flexible codification of each piece of data in accordance with its measurement, correction, transformation, or other validation and treatment. Windows graphic support gives it great flexibility for visualizing multiple time series for comparison and evaluation. CODEAU's programmable interfaces provide access to almost every third-party text format. 8.4. Access to data

The current rainfall data requirements of the urban drainage engineer go increasingly towards long continuous time series for long-term simulations. For these applications, annual updating of the data appears sufficient. Data transfer shall be by electronic information media only, i.e. computer diskettes as well as increasingly also CompactDiscs (CD-ROMs) for large data quantities plus handling software. Data transfer shall be in transparent standard ASCII text format. 8.5. Implementing the databank

In a minimum configuration, the databank will contain only the basic rainfall data, as well as catalogues of the stored events. Statistical treatment of the rainfall data as well as the regionalization of the parameters could be contracted to third parties, such as specialized private consultants or to research institutions. This concept would then require the following two separate bodies: a data supplier, handling also the databank management, and an individual or organization which handles the statistics.

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Depending on the selected alternative for the raingauge network, the following organisms could become the primary rainfall data supplier: 1. the Swiss Meteorological Agency SMA, who manage at present the national networks ANETZ and ENET as well as the conventional rainfall totalizer and climatic observation networks. The alternative RENETZ, partly automating stations of the conventional networks, would potentially also be handled by SMA; 2. state agencies and larger cities, managing their own raingauges in order to supplement the insufficient coverage of the national network; 3. regional wastewater collection agencies, in the case that the alternative network SIRENE would be realized, as well as in cases where they manage their own regional raingauge network. SMA is currently by far the single-largest supplier of rainfall data, and is likely to remain so in the future, particularly if alternative RENETZ will be realized. If the alternative SIRENE will be implemented, then the regional wastewater authorities could become over time the major supplier of rainfall data for applications in urban hydrology. SMA's current status as a Federal Agency excludes the introduction of third-party rainfall data. If SMA takes on the task of managing the national rainfall databank, it could become difficult to include other data of interest for urban hydrology at a later date. To summarize, a databank with information specifically for the urban hydrology sector could be implemented as follows: 1. as an independent databank, with only data specific to urban hydrology, 2. as part of other existing databanks with rainfall a n d / o r climatic data, 3. as part of a general hydrological databank such as managed by the Swiss Environmental Protection Agency or the Swiss Hydrological Service. Irrespective of the selected implementation path for the rainfall databank, the following basic principles and objectives should be maintained : • long-term continuity must be assured for the management of the rainfall databank system, taking into account the requirement for having long data series available for most hydrologic problems as well as in view of the important investments in urban drainage installations. For this reason, a public agency appears to be suited best for the databank administration, • rainfall measurements, data acquisition as well as control and validation have to be in the hands of a single organization; splitting up of these tasks would be detrimental for the quality of the end product, • statistical treatment of the data can be separated from the data acquisition and administration organization, to be handled on a contract-basis by third-party specialists.

9. Base recommendations for national rainfall data acquisition system

The Group of Engineers proposes the following basic steps for the improvement and extension of the existing A N E T Z / E N E T raingauge network : 1. supplement 7 important ENET stations by adding automatic pluviometers to the

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2. 3. 4. 5. 6. 7. 8. 9.

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existing measurement equipment, and install 4 additional ENET stations with pluviometer at important locations, supplement 49 ANETZ and ENET stations by adding a second raingauge with weighing totalizer and event-recording datalogger, digitize historical raingauge records at ANETZ locations or in close proximity, develop rainfall event catalogues, with emphasis on extreme events, update existing and develop additional rainfall intensity curves for all important locations, develop additional classified depth and intensity analyses for urban hydrology applications, develop new regionalization charts for the statistical parameters, establish a rainfall databank with easy, non-restricted access for the specific requirements of urban hydrology, seek a capable common organization to manage financing and operations of the national rainfall data acquisition system.

9.1. Minimum starting configuration

In consideration of the current difficult funding situation on all levels of government, the following minimum program was formulated in order to save at least the most pressing requirements from being blocked altogether by financial restrictions: 1. utilize ANETZ transmission channels currently not in use for the transfer of intermediate data time steps, hereby increasing the usable rainfall data resolution, 2. establish a reduced rainfall databank, supplying existing and future A N E T Z / E N E T measurements, with the possibility to incorporate data from third-party sources, 3. convert available A N E T Z / E N E T rainfall data into the CODEAU format, including validation and correction as needed, 4. establish VSA guidelines for rainfall measurement and data handling, organize short courses for engineers and interested organizations, 5. develop standard statistics software in order to unify treatment of rainfall data.

10. Financing As budget restrictions and empty pockets have become a major issue also in wealthy Switzerland, new ways are being sought to finance urban drainage installations, operation and maintenance of wastewater collection and treatment, as well as water protection projects. Taxing discharge loadings from wastewater treatment plants is being discussed as a way of creating an incentive to reduce the load of pollutants being discharged to receiving waters, while at the same time rising funds for projects to extend and upgrade treatment facilities. These funds replace the subsidies that have in the past come from ordinary tax money. This prospective future source of money is being considered also to finance the improvements to a national rainfall measurement and data acquisition system.

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As the implementation of this new financing concept might take several years to gain political acceptance and backing, alternative ways need to be explored. One possible idea is to charge an extra "Rain Centime" on the drinking water as part of the existing wastewater charge. Funds raised in this way could be collected by a special Foundation, administered by the wastewater treatment agencies, who are the major beneficiaries of having better rainfall data available, and who would then also control how the money is allocated.

References Arnell, V., Harremo~s, P., Jensen, M., Johansen, N.B. and Niemczynowicz, J., 1993. Review of rainfall data application for design and analysis. Geiger, H., Zeller, J. and RtJthlisberger, G., 1991. Starkniederschliige des schweizerischen Alpen- und Alpenrandgebiets. 9 volumes. EAFV-WSL, Birmensdorf. Hoegger, B., Kunz, A. and Pilet, G., 1992. ANETZ: Ausbaustand, Betrieb, Entwicklungen. Colloquium Automatische Messnetze, 22 May 1992, Federal Institute of Technology, Zurich. HiSrler, A., 1977. Die Intensit~iten von Starkregen l~ingerer Dauer ftir verschiedene Ortschaften in der Schweiz. Gas Wasser Abwasser, 57. Jahrgang, 12: 271-281. H~irler, A. and Rhein, H.-R., 1962. Die lntensit~iten der Starkregen in der Schweiz. Schweiz. Z. Hydrol., XXIV: 291-352. Verband Schweizerischer Abwasser- und Gew~isserschutzfachleute (VSA), 1994. Regendaten ftir die Siedlungsentw~tsserung, Projekt 1: Erarbeitung der Methodik zur Aufbereitung von Regendaten. Final Report. Ziirich, Dec. 1994, 214 pp.