1. Rational Method:

    a). The intensity of the rainfall is constant and is applied to the entire watershed
    b). The runoff coefficient remains constant throughout the storm event
    c). The frequency of the peak flow is equal to the frequency of the rainfall intensity

   Rational Formula:

   Q = 10 C.i.A

     Q - peak runoff, m³/hr
     C - runoff coefficient
     i - rainfall intensity, mm/hr
     A - Tributary area, hectares (10⁴ m²)

   runoff Coefficient:

     Concrete pavement = 1.0
     Asphalt = 1.0
     Lawns = 0.2
     Uncompacted soil = 0.2
     Natural soil, grass cover = 0.4
     Compacted sand soil = 0.4
     Uncompacted gravel = 0.5
     Natural bare soil = 0.6
     Compacted gravel = 0.7
     Oiled compacted soil = 0.8
     Compacted impervious soil = 0.6

     Composite runoff coefficient:

    C = (A₁.C₁+ A₂.C₂ + ...) / (A₁+A₂ + ...)

2. time of Concentration

   This is the time it takes for water to flow from the most remote point in the watershed to the point of discharge

  Intial time of concentration for sheet flow:  t_c = 7 n^0.6 . L^0.6 / ( i^0.4 . s^0.3)
     t_c - the time of concentration, minutes
     n - Manning resistance coefficient
     L - the distance from the upper end of the plane to the point of interest in meters
     i - rainfall intensity, mm/hr
     s - slope of the surface

   Minimum time of concentration: for paved areas, 5 minutes; for unpaved area, 10 minutes

   Manning's Resistance Coefficient (n):

     Concrete,asphalt = 0.01-0.013
     Sand = 0.01-0.016
     Lawns = 0.2-0.3
     Bare clay = 0.012-0.033
     Gravel = 0.012-0.03


3. Intensity-Duration-Frequency Curve(IDF):

   i = a / (t + b)

     i - intensity of rainfall, mm/hr
     t - duration of rainfall, hr

     for example:
    i₁ = 72mm/hr, t₁ = 0.25 hr,
    i₂ = 3 mm/hr, t₂ = 24 hr;
    then, a =72.2, b = 0.753;
    If t = 1 hr, then, i = 72.2 /(1+0.753) = 41 mm/hr


4. Travel Time:

  Time of flow in one reach + Initial time of concentration of the area


5. Storm Runoff at Junctions

   For tributary area with longer time of concentration, Q₁, T₁, i₁
   For tributary area with shorter time of concentration, Q₂, T₂, i₂

    If T₁ = T₂,     Q = Q₁ + Q₂,     T = T₁
    If T₁ > T₂,     Q = Q₁ + Q₂.i₁/i₂,     T = T₁
    If T₁ < T₂,     Q = Q₂ + Q₁.i₂/i₁,     T = T₂