The Volumetric Runoff Coefficient Cv is the proportion of water that falls on the site entering the drainage system. The Wallingford Procedure Volume 4 states that:
"An extensive study of the runoff data from sewered urban catchments showed that the volume of runoff was related to the impervious area, the soil type and the catchment wetness. An approximate result may be obtained by assuming that the runoff derives from a proportion of the impervious area (paved and roof), the proportion varying according to soil type. On this basis the overall average value of Cv is about 0.75, ranging from 0.6 on catchments with rapidly-draining soils to about 0.9 on catchments with heavy soils.
These values reflect the loss of some rainfall from impervious areas through cracks and into depressions and by drainage onto pervious (unpaved) areas. Similarly, any runoff from the pervious areas onto the impervious areas is also incorporated.
The above values of Cv should therefore be used in conjunction with the total impervious area (paved and roof) intended to drain to the storm system.
Alternative methods of determining Cv which take account specific soil characteristics and regional variations in catchment wetness are described in Volume1 and 4 of the Wallingford Procedure (1981: 5)"
The fact that the Cv is not 1 for impermeable areas has surprised many people who may assume that the whole of the impermeable area drains to the storm system. But the recommended values between 0.6 and 0.9 were based on site measurements.
At first glance, there would also seem to be significant difference between the Rational Method and the Modified Rational Method:
Rational Method Design
Q (l/s) = 2.78 * I (mm/hr) * A (ha)
Modified Rational Method Design
Q (l/s) = Cv * Cr * (2.78 * I (mm/hr) * A (ha))
Where Cr =1.3 (as recommended by the Wallingford Procedures)
It was found that the Rational Method could underestimate flows depending on the shape of the catchment, in particular the ratio of the length to the width of the catchment, and hence a routing factor, Cr, was introduced. The value of the routing coefficient Cr is used to factor the shape of the time-area diagram and the peakedness of the rainfall profile. The Wallingford Procedure, Volume 1, recommends a value for Cr of 1.3. If you take the typical value of Cv as 0.75 and multiply it by 1.3 you get 0.975 which is very nearly 1 and therefore, in the default case, Cv and Cr are effectively self-cancelling and, there is no significant difference between the Rational Method and the Modified Rational Method.
The factor Cr is only used in the (UK) Modified Rational Method in Network Design.
Cv may be calculated from equations 7.3 & 7.21, Volume 1, Wallingford Procedure:
PR = 0.829 PIMP + 25.0 SOIL + 0.078 UCWI - 20.7
CV = PR/PIMP
where:
PIMP = percentage impermeability
SOIL = soil type
UCWI = antecedent wetness conditions (mm)
If PIMP, SOIL and UCWI are not known, then the mean value of 0.75 may be used with the Modified Rational Method. A higher mean value of 0.84 should be used with winter rainfall as the UCWI is likely to be higher in winter conditions.