brick, │ │ │ │ │ │ │ │ │ wood block│ 145.5│ 11.0│ .85│ 123.7│ │ │ │ 5.0│ .85 Granite │ │ │ │ │ │ │ │ │ block │ 111.4│ 8.4│ .75│ 83.6│ │ │ │ 1.0│ .75 Macadam and │ │ │ │ │ │ │ │ │ cobble │ 23.2│ 1.8│ .40│ 9.3│ 238.6│ 15.4│ .40│ 4.8│ .40 Granite and │ │ │ │ │ │ │ │ │ poor │ │ │ │ │ │ │ │ │ macadam │ │ │ │ │ │ │ │ 0.4│ .20 │ │ │ │ │ │ │ │ │ Unimproved │ │ │ │ │ │ │ │ │ yards and │ │ │ │ │ │ │ │ │ lawns: │ │ │ │ │ 692.4│ 44.7│ .15│ │ Tributary to│ │ │ │ │ │ │ │ │ paved │ │ │ │ │ │ │ │ │ gutters │ │ │ │ │ │ │ │ 57.1│ .15 Not │ │ │ │ │ │ │ │ │ tributary │ │ │ │ │ │ │ │ │ to paved │ │ │ │ │ │ │ │ │ gutters │ │ │ │ │ │ │ │ 7.9│ .10 ──────────────┼──────┼─────┼─────┼──────────┼──────┼─────┼─────┼─────┼───── Total │1324.2│100.0│ │ 1127.0│1550.7│100.0│ │100.0│ ──────────────┼──────┴─────┴─────┴──────────┼──────┴─────┴─────┼─────┴───── Impervious │ │ │ coefficient │ │ │ for the │ │ │ district │ 85.1 │ 44.4 │ 35.9 ──────────────┴─────────────────────────────┴──────────────────┴─────────── C. E. Gregory[27] states that _I_, in the expression _Q_ = _AIR_ is a function of the time of concentration or the duration of the storm. If _t_ represents the time of concentration and _T_ represents the duration of the storm, then when _T_ is less than _t_ _I_ = 0.175_t_^⅓, but when _T_ is greater than _t_, _I_ = 0.175⁄_t_(_T_^{4/3} − (_T_ − _t_)^{4/3}). Gregory condenses Kuichling’s rules with regard to the per cent run-off, as follows: 1. The per cent of rainfall discharged from any given drainage area is nearly constant for heavy rains lasting equal periods of time. 2. This per cent varies directly with the area of impervious surface.