Rational Hydrology Method: True or Planar area? 1

[Riversidean]

Right now I am trying to recall the rational hydrology method. Would one use true area when performing a calculation? or planar area?

I am thinking planar because I believe rain comes down in a flat sheet, at least theoretically for these purposes. Any advice would be appreciated.

Thanks,
Riversidean

 

[civilperson]

The difference between the two methods of measuring or calculating area is less than the accuracy of any hydrology method.

 

[Riversidean]

For clarification you are saying that the difference does not change the end result of the hydrology method? Or that the method is not accurate to begin with?

This would include deep canyons and/or mountain channels where planar areas and true areas differ a large amount in comparison to a valley? I know that in the rational study method there are several ways to perform calculations depending on the terrain.

 

[RWF7437]

"For clarification you are saying that the difference does not change the end result of the hydrology method? Or that the method is not accurate to begin with?"

The Rational Method is very limited and can be no more "accurate" than the rainfall data used in the calculation. That data is typically +/- 30% or more. The Rational Method should only be used for small areas. FEMA allows it to be used for areas as large as 1 square mile but most engineers I know would not use it for anything larger than 10 to 50 acres ( 0.02 to 0.08 sq. mi.).

"I know that in the rational study method there are several ways to perform calculations depending on the terrain. "

I've never heard of calculating Q=CIA by any method other than multiplication, regardless of the terrain. What are the other methods you refer to and where did you learn of them ?

If at all possible use some method other than the Rational Method for larger areas. All methods available depend on statistical data and few are calibrated. Errors of plus or minus 30% to 100% are common .

good luck

 

[Riversidean]

I am sorry for not being clear. I was refering to C with it being the coefficient of runoff and I being the Rainfall intensity (inches/hour) coresponding to the time of concentration. This method does not account for things such as slope, flow path, soils and etc? I guess I am refering to variables then of state. I am only working with the Rational method for right now since my area is roughly 30 acres otherwise I might try the Synthetic Unit Hydrograph method if my area was larger than 300 acres.


I learned the information from the Riverside County Flood Control and Water Convservation District Hydrology Manual.
I think I am going to pick up a copy of the Hydraulic Refence Manual as I have been hearing great things of it as of late.

 

[RWF7437]

"I learned the information from the Riverside County Flood Control and Water Convservation District Hydrology Manual."

Good luck.

The RCHM costs $74.
It has not been updated since 1978. ( even though updated rainfall data is available )
It uses a unit hydrograph method similar to, but much more complicated than, for example:
The SCS Unit Hydrograph
The Santa Barbara Urban Hydrograph
The LA County Modified Rational Method Hydrograph

Every town in RC applies the RCHM in a different way and insists their way is Gospel.

No City, or the County, has ever invested in any calibration effort.

Other than that, it is a great place to do hydrology.

 

[rday]

I think civilperson is saying the margin of error in planer vs true area is smaller than the margin of error inherent in the rational method.

Rational is just a conservative estimate of maximum flow. Even the equation is not dimensionally consistant. You get a CFS flow using a dimensionless constant (coefficient of runoff), a rainfall intensity in inches per hour and an area in acres.

Terrain effects are included via C (soil type, hardscape vegetative cover) and the time to concentration calculation (flow routing).

 

[cvg]

The RCHM can be downloaded for free - but it is 318 MB so it takes a while...

 

[chicopee]

To the original question use planar; however check references due to limitations of formula.