Topic II. 6. Determination of Design Runoff Quantity: Rational Method Linear Hydrographs Prerequisites: Rectangular catchment shape along the sewer main Uniform catchment slope and land cover Uniform sewer main slope Uniform rain intensity over the entire catchment

Linear Hydrographs Basic Parameters Basic Relationship Q - runoff quantity, l/s tr - rain duration, min t0 -runoff concentration time, min qr - rain intensity, l/s.ha B - catchment width, m L - catchment length , m v - mean wastewater velocity, m/s  - runoff coefficient Basic Relationship where , or where

Kinds of Linear Hydrographs (Depending of the tr and t0 Relation) t0 > tr  Triangular hydrograph with a peak lower than the maximum one t0 = tr  Triangular hydrograph with maximum peak t0 < tr  Trapezoidal Hydrograph with a peak, equal to the maximum one

Rational Method Prerequisites: Rational Formula: Valid are all the prerequisites, associated with the linear hydrographs Maximum triangular hydrograph peak is achieved at Rational Formula:

Rational Method Determination of the Time of Concentration Time of surface concentration - ts Time of gutter concentration - tg Time of pipes concentration - tp Total time of concentration - t0