How do you calculate rainfall intensity in Excel?

The Excel formulas in the spreadsheet are set up to calculate the constants, a and b, in the equation i = a/( d + b ), by linear regression of 1/i vs d. Then the design rainfall intensity is calculated with the equation i = a/( d + b ) using the calculated values for a and b.

How do you calculate rainfall intensity?

Formula Of Rainfall Intensity The intensity of rainfall (p) in mm/hr is given by the following general formula: p = 254a / (t+b) mm/hr Where, t = Duration of storm in minutes, a and b are constants. a = 30 and b = 10 when t is 5 to 20 minutes.

How do you calculate rainfall intensity from time of concentration?

Equation 4.1 Q = I *A *0.00278 Where Q = peak discharge (m3/s) I = rainfall intensity (mm/hr) A = area (ha). of concentration and ARI. Equation 4.1 can be applied to any ‘catchment’ if it is assumed that all of the rainfall resulted in runoff.

What is Q CiA?

The Rational method predicts the peak runoff according to the formula: Q=CiA, where C is a runoff coefficient, i is the rainfall intensity, and A is the subcatchment area. This formula is applicable to US or metric evaluation, as long as consistent units are employed.

What is rainfall frequency?

The probability distribution specifying the exceedance probability of different rainfall depths for a given duration (such as 1 hour, or a 24-hour day). The exceedance frequency is often reported as a return period (in years), which is the reciprocal of the annual exceedance frequency.

What is design rainfall intensity?

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

How do you convert rainfall depth to intensity?

In order to have the average rainfall intensity in m/s corresponding to that interval of time, you have to divide the rainfall depth for the number of seconds in the time interval or for mm/s you multiply for 1000/21600. If you want the intensity in m/h you divide for 6 hours.

What is intensity of rainfall?

The intensity of rainfall is a measure of the amount of rain that falls over time. The intensity of rain is measured in the height of the water layer covering the ground in a period of time. It means that if the rain stays where it falls, it would form a layer of a certain height.

What is the formula for time of concentration?

There are many methods available to estimate the time of concentration including the Kirpich formula, Kerby formula, NRCS Velocity Method, and NRCS Lag Method. The NRCS Velocity and Lag methods are two of the most commonly used methods for determining time of concentration and are described below.

What is the minimum time of concentration?

Minimum Time of Concentration Use a minimum tc value of 5 minutes for urbanized areas and a minimum tc value of 10 minutes for areas that are not considered urban. Use minimum values even when calculations result in a lesser time of concentration.

What is SCS method?

The SCS curve number method is a simiple, widely used and efficient method for determining the approxient amount of runoff from a rainfall even in a particular area. The curve number is based on the area’s hydrologic soil group, land use , treatment and hydrologic condition. …

What is C in rational equation?

The Rational method runoff coefficient (c) is a function of the soil type and drainage basin slope. The duration is usually equivalent to the time of concentration of the drainage area. The storm frequency is typically stated by local authorities depending on the impact of the development.

In summary, rainfall intensity is calculated by dividing the amount of rain accumulated by the period of time during which this happens. Rainfall intensity is to rainfall amount what speed is to the distance traveled in a mechanical analogue.

How to calculate the annual rainfall depth?

Arithmetic Mean: When the area of the basin is less than 500 km 2 this method implies summing up of all the rainfall values from all the raingauging stations

How to calculate runoff coefficient?

How to Calculate Runoff Coefficient. Step 1. The Rational Method is used for areas less than 50 acres. If the area you are assessing is more than 50 acres, you will have to use a Step 2. Step 3. Step 4. Using a Runoff Coefficient Table .

What is rainfall intensity?

The intensity of rainfall is a measure of the amount of rain that falls over time. The intensity of rain is measured in the height of the water layer covering the ground in a period of time. The intensity of rain is measured in the height of the water layer covering the ground in a period of time. It means that if the rain stays where it falls, it would form a layer of a certain height.