1. 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

2. 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

3. 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

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

5. 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