RATIONAL METHOD OF ESTIMATING RUNOFF

 RATIONAL METHOD OF ESTIMATING RUNOFF

Rational Method: The rational method is used around the world for peak flow estimation of small rural drainage basins and is the most widely used method for urban drainage design.  

The rational method equation is given below:

Q = kCiA

where: Q - peak flow (m3/s).

k - conversion factor equal to 0.00278 (metric). (=1/360)

C - dimensionless runoff coefficient.

i - rainfall intensity (mm/hr).

A - catchment area (ha).

The Catchment Area, A

The catchment area, A, is determined from a map which includes the drainage area of interest. 

The boundaries of the drainage area using a contour map. 

Once the boundaries are known, the area can be determined using the map scale. 

Since the area must be in acres for use in the Rational Method equation, a useful conversion factor is 43,560 ft2/acre.

The Runoff Coefficient, C

The runoff coefficient is the fraction of rainfall striking the drainage area that becomes runoff from that drainage area. It is an empirically determined constant, dependent on the nature of the drainage area surface. 

An impervious surface like a concrete parking lot will have a runoff coefficient of nearly one. 

A very tight clay soil will also have a relatively high runoff coefficient.

Sandy soil would have more infiltration and a lower runoff coefficient. 

In addition to the nature of the surface and the soil, the slope of the drainage area has an effect on the runoff coefficient. 

A steeper slope leads to a higher runoff coefficient. 

Tables showing the values for runoff coefficient for a variety of types of drainage areas in handbooks, textbooks and on the internet. 

Sample values for ready reference is shown below

The Design Rainfall Intensity, i

The design rainfall intensity is the intensity of a constant intensity design storm with the specified design return period and duration equal to the time of concentration of the drainage area.

Once the design return period and duration are determined, the design rainfall intensity can be determined from an appropriate intensity-duration-frequency graph or equation for the location of the drainage area. 

INFILTRATION & RUNOFF DEFINITIONS

 INFILTRATION & RUNOFF

Two important aspects of Hydrological Cycle are

1. Infiltration

2. Runoff

INFILTRATION

All precipitation will not become surface runoff.

Some quantity of precipitation infiltrates into the ground.

The infiltration plays a significant role on the relationship between rainfall and runoff.

If the soil is wet, less water infiltrates and more surface runoff is generated.

Similarly, if the soil is dry, more water (usually) is able to soak into the ground.

RUNOFF

The water left after infiltration flows overground as runoff

Volume of runoff depends on:

 Rainfall intensity and duration

 Type of surface (pervious or impervious)

 Area of catchment

Usually

(a) Low intensity rainfall - Mostly Infiltrates

(b) High intensity rainfall - Infiltrates and become surface runoff

Runoff depends on the catchment characteristics and can be related as

(a)Thick vegetation = low volume, slow runoff

(b) Paved area = high volume, fast runoff 

Introduction to Hydrology

 INTRODUCTION TO HYDROLOGY

Hydrological cycle Comprises of

1. Precipitation

2. Evaporation

3. Transpiration

Precipitation & Evaporation are the two most important elements of the water cycle for hydrologists.

The water cycle is

1. A closed system

2. No water being created or lost, just moved around.

3. Its a Finite resource – but renewable if quality controlled.

Components of a Hydrologic Cycle is as shown in the figure

A Typical flowchart of a Hydrologic Cycle is as shown in the figure below

Precipitation: All forms of moisture being released from the atmosphere.

Rainfall and evaporation are measured by depth (usually millimetres)

One cubic metre (m3) = 1000 litres

One millimetre depth over one hectare = 10 m3 

10cm depth over one hectare = 1000 m3 = 1 Megalitre (ML)

Rainfall and evaporation are also by rate (mm/hour,  mm/day,  mm/year)