How do you calculate 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.

How do you find time of concentration in rational method?

time for the shallow concentrated flow segment is calculated by the following formula: Where: Tscf = Travel time for the shallow concentrated flow segment in minutes (min.) L = Length of the shallow concentrated flow segment in feet (ft) V = Average flow velocity in feet per second (ft/s), as shown in Figure 1.

How do you calculate overland flow?

In the first step overland flow is calculated by proportioning daily rainfall using run-off coefficient values specified in data files. Each data file is related to one run-off zone and holds twelve run-off coefficient values, one for each month of the year.

What is overland flow time?

Overland flow travel time is divided into sheet flow travel time and shallow concentrated flow travel time. The overland flow is occurring after completion of infiltration or saturation process, it is generally assumed to be turbulent even though it mostly representing laminar characteristics (Grismer, 2016).

What is overland sheet flow path length?

In urban areas, the length of overland flow will typically be less than 50 metres after which the flow will become concentrated against fence, paths or structures or intercepted by open drains. The formula shown below, known as Friend’s formula, should be used to estimate overland sheet flow times.

How can overland flow be reduced?

A suite of best management practices for reducing overland flow may include actions to increase surface roughness and canopy interception, maintain soil porosity, and otherwise disperse concentrated or fast-moving flows of water.

What does overland flow mean?

What is Local Overland Flow? Local Overland Flow is the inundation by local runoff rather than overbank discharge from a stream, river, estuary, lake or dam.

What is time of concentration used for?

Time of concentration is a concept used in hydrology to measure the response of a watershed to a rain event. It is defined as the time needed for water to flow from the most remote point in a watershed to the watershed outlet.

When do you use the Bransby Williams Formula?

The Bransby Williams Formula is one method used to estimate the time of concentration of a catchment which is used in turn to determine the critical storm duration during runoff and drainage design. A Bransby Williams Formula Calculator is included free with the CivilWeb Rainfall & Runoff Calculator Spreadsheet.

What is the formula for the time of concentration?

Bransby Williams provides a formula for this “time taken…”, what we would now call the time of concentration. In the original paper this is provided for a circular catchment as: is the average number of feet fall per 100 ft from the edge of the watershed to the outfall.

How is the Bransby Williams Formula related to kinematic wave equation?

As such it is often compared with the kinematic wave equation for overland runoff. The area of the catchment being considered. This is the average slope of the catchment, measured along the line of the natural flow from the summit of the catchment to the point being considered.

How is the Bransby Williams Formula related to the KIRPICH equation?

This is the distance on plan, measured along the line of the natural flow between the catchment summit and the point being considered. Similar to the Kirpich Equation or Kerby Equation, the Bransby Williams formula can tend to underestimate the Time of Concentration leading to conservative high design rainfall intensities.