RATIONAL METHOD

Rational Formula

The rational formula is Q = CIA. On a topographic plan of the drainage area, draw the drainage system and block off the subareas draining into the system.

Determine A, the area of each subarea in acres.

Coefficient Of Runoff

The coefficient of runoff is intended to account for the many factors which influence peak flow rate. The coefficient of runoff primarily depends on the rainfall intensity, soil type and cover, percentage of impervious area, and antecedent moisture condition.

Determine the coefficient of runoff C, for appropriate class of ground surface from the following table. If more than 1 class of ground surfaces fall in 1 tributary drainage area, use a composite coefficient of runoff value.

Practice 670 210 1150 Publication Date 20Sep95 Page 5 of 21 FLUOR DANIEL

STORM DRAINAGE

Civil Engineering

This copy is intended for use solely with Piping Design Layout Training.

For other purposes, refer to the original document available through Knowledge Online.

Coefficient of Runoff C

Roofs 1.00

Pavements

Concrete 1.00

Asphalt 1.00

Oiled Compacted Soil 0.80

Compacted Gravel 0.70

Compacted Impervious Soil 0.60

Natural Bare Soil 0.60

Uncompacted Gravel 0.50

Compacted Sand Soil 0.40

Natural Soil, Grass Cover 0.40

Uncompacted Soil 0.20

Lawns 0.20

Composite coefficient of runoff C:

A1C1+A2C2+A3C3+ −−−−AnCn A1+A2+A3+An

where

A1 A2 A3 ---- An = Areas in acres of different class of surfaces C1 C2 C3 ---- Cn = Corresponding coefficient of runoff Time Of

Concentration

If rain were to fall continuously at a constant rate and be uniformly distributed over an impervious surface, the rate of runoff from that surface would reach a maximum rate equivalent to the rate of rainfall. The time required to reach the maximum or equilibrium runoff rate is defined as the time of concentration.

The time of concentration depends upon the length of the flow path, the slope, soil cover, and the type of development.

Determine the initial time of concentration using the nomograph on Attachment 01.

Use a minimum time of concentration of 5 minutes for paved areas and a minimum time of concentration of 10 minutes for unpaved areas.

Precipitation

The various precipitation amounts during specified time periods at recording stations are analyzed using common models of probability distributions.

A number of alternative statistical distributions such as Log Pearson Type III, Pearson Type III, Two-Parameter Lognormal, Three-Parameter Lognormal, and Weibull, Type I, Extreme Value are used in flood hazard analysis.

Practice 670 210 1150 Publication Date 20Sep95 Page 6 of 21 FLUOR DANIEL

STORM DRAINAGE

Civil Engineering

This copy is intended for use solely with Piping Design Layout Training.

For other purposes, refer to the original document available through Knowledge Online.

Intensity Duration Curves

Use the intensity duration curves available from federal, state, county or local agencies for the project location. If such curves are not available, construct these curves using Weather Bureau Technical Paper Number 40 (Continental United States); 42 (Puerto Rico and Virginia Islands); 43 (Hawaiian Islands); 47 and 52 (Alaska); or NOAA Atlas, Precipitation - Frequency Atlas of the United States, published by the National Weather Service.

For constructing the curves, given only 1 or 2 points, use the following conversion factors based on 30 minutes as 1.00:

Duration in

To go from 1 curve to another, use the following factors based on the 50 year maximum rainfall as 1.000:

1 year 0.428 25 years 0.898

2 years 0.455 50 years 1.000

5 years 0.659 100 years 1.108

10 years 0.762

Rainfall intensity duration curves for more than 100 years can be constructed using rainfall data for periods of 2, 5, 10, 25, 50, and 100 years; and time periods of 20 minutes, 60 minutes, 2 hours, 3 hours, 6 hours, 12 hours; and 24 hours using the following formula:

_ _ Xji = Xi + Kj Si Xi where

j = Return period in years

i = Specific storm duration in minutes, hours or days Xji = Precipitation in inches for return period j and duration i Xi = Mean maximum annual storm for duration i

Kj = Frequency factor (in standard deviations) for a return period of j years Si = Standard deviation of maximum annual storm for duration i

For more detailed procedures using this formula, refer to "Analysis of Data," Pages 7 to 25 of Rainfall Depth Duration Frequency for California, Department of Water Resources, State of California, November 1982.

A sample set of curves is shown in the sample problems in this practice.

Practice 670 210 1150 Publication Date 20Sep95 Page 7 of 21 FLUOR DANIEL

STORM DRAINAGE

Civil Engineering

This copy is intended for use solely with Piping Design Layout Training.

For other purposes, refer to the original document available through Knowledge Online.

Using the initial time of concentration, determine "I" intensity of rainfall in inches per hour from the intensity duration curve for the plant's geographical location using the proper yearly rainfall frequency.

Compute Q = CIA.

Refer to sample problems in this practice.

Travel Time

Determine the size of the channel or pipe required to carry Q on the slope of the drain.

Determine the velocity of flow.

Measure the length of flow to the point of inflow of the next subarea downstream. Compute the time of flow for this reach and add it to the initial time of concentration for the first area to determine a new time of concentration.

Calculate Q for second subarea, using the new time of concentration and continue in similar fashion until a junction with a lateral channel is reached.

Start at the upper end of the lateral and carry its Q to the junction with the main channel.

Storm Runoff At Junction

Compute the Q at the junction.

Tributary area with longer time of concentration

Tributary area with shorter time of concentration

QA QB

TA TB

IA IB

Peak Q cfs (cubic feet per second), time of concentration in minutes, rainfall intensity in inches/hour.

Tp = Peak time of concentration at junction

If more than 2 tributary areas are contributing at 1 junction, combine 2 areas at a time and proceed similarly until tributary areas are combined.

Practice 670 210 1150 Publication Date 20Sep95 Page 8 of 21 FLUOR DANIEL

STORM DRAINAGE

Civil Engineering

This copy is intended for use solely with Piping Design Layout Training.

For other purposes, refer to the original document available through Knowledge Online.

DITCHES AND