Report No FR0438

MARCH 1994

I OBJECTIVE

To further develop the accuracy and range of synthetic rainfall inputs for urban hydraulic and quality modelling applications including a scoping study into the feasibility of developing a spatial variability rainfall model.

II REASONS

The existing SRG model suffers substantial loss of accuracy, in respect of certain significant parameters, for events of return periods greater than one in two years. Practical application procedures are likely to demand consideration of return periods of up to at least one in ten years. Hence the model needs to be developed to meet these requirements.

Spatial variability is a major characteristic of real rainfall events that is not adequately considered in existing design procedures. A practical and effective way of incorporating spatial variability into present design practice will remove a source of uncertainty and inaccuracy.

III CONCLUSIONS

During the technical development of an improved SRG the following conclusions were drawn:

1. Modification of the Neyman-Scott Rectangular Pulses (NSRP) model (which is the basis of the SRG), in accord with meteorological observations, by allowing short duration high-intensity raincells and long-duration low-intensity raincells to occur within the same storm, provided a superior fit to historical extreme events when compared with the original NSRP model.
   
2. Validation of the modified model showed a remarkably good fit to annual maxima at the 1 and 2 hour levels of aggregation, but under-estimated the annual maximum at the 6 and 24 hour levels of aggregation. A more complex regionalisation procedure would inevitably lead to an improved fit to the extreme values, however a more extensive data set would be required representing different geographical regions in the UK.
   
3. Overall the fit to the annual maxima was good given that the regionalisation procedure was fairly simple and that the parameters of the modified NSRP model relate to physical characteristics in the precipitation field and not directly to annual maximum rainfalls.

Independent testing of the Modified SRG (MSRG) was carried out by comparing the output with historical rainfall series at four sites in the UK. The main conclusion from this work were:

1. The synthetic annual mean rainfall was accurate to within -18 and +9%.
   
2. The percentage error between event depths over a range of return periods (RPs) was generally between +/-10%. The exception to this was for the 2 year RP storms and for all storms from the Exeter site, where errors ranged from -26 to +14%. Analysis of mean intensity values indicated a tendency for over-prediction. Maximum intensity values were usually well represented by the MSRG.
   
3. Extreme storms appeared to be simulated well by the MSRG.
   
4. When the rainfall events were run through sewer models, the MSRG showed a range of both under- and over-estimated storage requirements (for specified spill frequency standards) than the historical series.
   
5. Percentage error in spill performance was considerably reduced when daily rainfall data (rather than annual mean rainfall) were used in the model regionalisation procedure.

IV RECOMMENDATIONS

It is now considered that the MSRG can be used for a range of applications, for example, pollution management and detention tank design. The following recommendations are made to increase the level of confidence that the user can have when using any rainfall series.

1. Long complete hourly rainfall historical data series, if available, will provide maximum accuracy for any modelling study. The necessary length of this series will depend on the user requirements. For pollution management studies at least 10 years sho uld be used, but if specific extreme events are required, for example up to a 10 year RP, series in excess of 20 years are necessary.
   
2. Accuracy in the MSRG model can be improved if daily rainfall data are used to help regionalise the model. Availability of this data is relatively wide spread and can be obtained from the Meteorological Office. At least 20 years of daily rainfall data should be obtained to ensure the data are representative of the site.
   
3. The longer the series generated by the MSRG, the less likely that extreme years will bias the results. To some extent this is the same for historical data. Twenty years are recommended, but no less than 10 should be considered for most applications.
   
4. The MSRG is not currently available in a supportable software package. However, it is planned it will be incorporated into the commercially available STORMPAC(6) package to allow its practical application by water industry users. The original SRG model, which is currently part of the STORMPAC package, can still be used for applications requiring rainfall with RPs of less than a 2 year RP (rather than up to a 10 year RP in the MSRG).

V RESUME OF CONTENTS

The report is divided into 3 parts:

Part 1 - Technical Development of the SRG

Part 1 (Sections 2 to 6) describes the technical development of the SRG model. The modified model is defined in Section 2 and in Section 3 the model is fitted to historical hourly data taken from one site. The model is regionalised in Section 4 and validation is described in Section 5. In Section 6 the Extreme values generated by the model are compared to other representative values. Conclusions from Part 1 are given in Section 7.

Part 2 - Additional Testing of the MSRG

In Part 2 (Sections 8 to 11) the MSRG is tested by assessing selected characteristics of the synthetic rainfall and comparing them to equivalent historical series; and by considering the accuracy of spill/storage predictions when MSRG rainfall is used in simplified sewer flow models. Section 8 introduces the testing programme and Section 9 describes the methodology adopted. The results are given in Section 10 and the conclusions that can be drawn from the Part 2 testing are given in Section 11.

Part 3 - Recommendations and Additional Developments

Part 3 contains the recommendations from the developmental and testing work (Section 12) and provides a synopsis of the scoping study looking into the viability of a spatial variability rainfall model (Section 13).

Copies of the report are available from FWR, price £35.00, less 20% to FWR Members.