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630 mm of rain was recorded. Rainfall patterns were compared with two other gauges within a kilometre's radius; these patterns appeared consistent, and no significant events appear to have been missed. To measure runoff and hydraulic performance, combined recording level and velocity gauges (Montec DETEC 3510 Surveyloggers) were installed at the outlet and approximately halfway along the main drain. This equipment recorded levels using pressure transducers, and velocities using ultrasound. |
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Surface Run-Off Modelling. |
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A version of the US Soil Conservation Service TR-55 model (SCS, 1975) incorporated in the SPIDA package was used to model rainfall-runoff relationships. While the more sophisticated 'Wallingford Procedure' also built into SPIDA is more suitable in many applications, it is based on field research in UK catchments under hydrological conditions distinctly different from those encountered in India. The modified SCS method is simpler and easier to calibrate than more sophisticated models, and is also more widely known outside the UK. Two parameters are particularly important in the method for calculating a runoff hydrograph from rainfall over a surface: storage and routing coefficients. For each type of cover, users must specify an 'S' or storage value to compute a 'curve number' which determines the proportion of rainfall converted to runoff. Based on field surveys and observation, four types of cover were identified, and the fraction of the catchment made up by each was estimated. Appropriate 'S' values were estimated during early stages of model calibration, by comparison of predicted and observed |
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| Table 14.1 Types of cover and SCS curve numbers | | Type of cover | | | | | Roof areas | | | | | Paved area | | | | | Open ground | | | | | Muddy ground | | | |
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