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Topic II. 6. Determination of Design Runoff Quantity: Rational Method

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.

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Topic II. 6. Determination of Design Runoff Quantity: Rational Method

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  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 • 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. Linear Hydrographs 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: • 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

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