Urban Hydrology

In the estimation of runoff from natural catchments, the determination of T_p, time to peak, and SPR, standard percentage runoff, contain an URBAN term. Similarly in chapter 9, the equation for <math display='block'> <mover accent='true'> <mi>Q</mi> <mo>&#x00AF;</mo> </mover> </math>$$\bar Q$$, the mean annual maximum flood for Region 6, departs from the general equation and includes an URBAN term. URBAN, it may be recalled, is the fraction of catchment in urban development. The reason for this special term’s inclusion is the need to take account of the impermeable areas of buildings’ roofs, roads, pavements, car parks, etc. in a built-up environment where there is negligible infiltration and runoff is accelerated by drainage systems of gulleys, pipes and sewers. Obviously the time to peak of a hydrograph and the percentage runoff will be decreased and increased respectively compared with a natural catchment of permeable soils and vegetation.