Report No FR0391

Jul 1993

I BENEFITS

The WURD database has been strengthened by the addition of more rainfall-runoff events of greater duration and extremity than previously. Additional knowledge has been gained concerning the accuracy and reliability of conventional flow measuring techniques in large diameter sewer pipes.

II OBJECTIVES

The main objective of this work has been to identify additional rainfall runoff events from instrumented urban catchments in Coventry, Leicester, Derby and Oxhey. These data are then included in the national WURD database, held at the Institute of Hydrology, for possible future updates of sewerage hydraulic analysis models.

Additionally, this report compares the modern rainfall runoff response for the Oxhey catchment with data collected in the 1950's.

Oxhey also provided the opportunity to evaluate the performance of the doppler velocity and depth method of sewer flow measurement in a large diameter pipe.

III REASONS

The limitations of existing sewerage hydraulic models are apparent to many users. Many of these limitations arise from the paucity of relevant data for model development.

Over time the size and integrity of an urban catchments impervious area will change and therefore the runoff response might be expected to alter.

Doppler velocity and depth flow metering is commonly used in sewer flow surveys. Although originally this method was designed for pipes up to 0.45 m in diameter it is now commonplace in much larger pipes.

IV CONCLUSIONS

1. Twelve events from Coventry, Leicester, Derby and Oxhey have been identified and included in the WURD database. A comprehensive data set covering a period of up to 10 years is now available for any future rainfall runoff model development.
   
2. Initial results suggest that there has been no significant change in the rainfall runoff response at Oxhey over a 40 year period. However, this supposition cannot be confirmed until more data are collected.
   
3. The doppler velocity and depth flow measurement technique has proved to be unreliable in large diameter sewer pipes. At high depths the mean velocity of the flow is under represented, whilst at low flows the design of the sensor housing causes it to be over represented. The Oxhey results give an indication of the correction factor needed to obtain mean velocity from the velocities recorded with the doppler sensor.

V RECOMMENDATIONS

The data collection programmes at Coventry, Leicester and Derby are terminated in March 1994.

The data collection programme at Oxhey is continued so that:

1. a full comparison can be made between rainfall runoff data collected in the 1950's and the 1990's. To asses how the catchment has altered in this period, a new impermeable area survey should be undertaken.
   
2. the performance of the doppler velocity depth sensor can be further tested over a range of depths and flows.
   
3. more events can be identified and included in the WURD database.

Unprocessed gully meter data, held at The Institute of Hydrology, for Bracknell, Wallingford, Nottingham and Stevenage are filed in the WURD database.

VI RESUME OF CONTENTS

This report briefly describes the urban catchments at Coventry, Leicester, Derby and Oxhey and how they have been instrumented. The twelve events that have been added to the WURD database are presented graphically, together with some summary statistics.

Data have been collected at Oxhey to assess the doppler velocity and depth flow measurement technique and to investigate how runoff from the catchment may have changed since the 1950's. These data are presented graphically and some initial conclusions made.

Copies of the report are available from FWR, price £15.00, less 20% to FWR Members.