Storm Intensity in Relation to Area 1

[Trackfiend]

I was asked a question this morning pertaining to the relationship between storm intensities (i), and the area to which it is being applied. Most agencies give IDF curves or curve values for their particular area in order to calculate the peak flow (Rational method). This information is given in 5, 15, 30, and 60 minute intervals with the lower time intervals having a greater intensity than the longer time periods. The question asked was:

"Isn't the intensity greater for a smaller area (10 square miles) than for a larger area (100 square miles)?" My intuitive answer was "yes", that smaller storms have greater intensities than larger storms. But when asked to expound on the answer I drew a blank. How does one relate the intensity to area?

 

[Trackfiend]

In exanding my first post, I realize that the intensity is a chosen frequency for a duration of time equal to the time of concentration. This would put the peak of the storm at the same time as it would take for all parts of the watershed to contribute to the overall runoff. In relation, the area is directly tied to the time of concentration which is directly tied to intensity. My original question, expanded, would be this, are there tables given or provided for that depict a direct relationship between two different areas and their prospective intensity values? Or would someone (myself) have to simply compute the time of concentration and then relate that back to the given intensities for that area?

 

[cvg]

I believe what you are looking for is an aerial reduction factor which is used to reduce the runoff for larger areas. Generally for areas less than a square mile and particularly for the small areas which are appropriate for using the rational method, there is no aerial reduction. For larger areas, 10 square miles and larger there are very significant reduction factors since there are very few storms that are that large.

 

[RWF7437]

http://www.forester.net/sw_0607_voodoo.html

 

[bltseattle]

This is not something I typically deal with due to small project areas and really wide storm fronts here, but I learned at a modeling class in Denver that this precise issue is addressed in the Colorado Urban Hydrograph Procedures and technical manuals from the Urban Drainage & Flood Control District. They have procedures and computer software to make lag time & intensity adjustments.

See

http://www.udfcd.org/downloads/down_technical.htm

for leads to chase this out