Rational Hydrology Method: True or Planar area?
Rational Hydrology Method: True or Planar area?
(OP)
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
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





RE: Rational Hydrology Method: True or Planar area?
RE: Rational Hydrology Method: True or Planar area?
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.
RE: Rational Hydrology Method: True or Planar area?
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
RE: Rational Hydrology Method: True or Planar area?
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.
RE: Rational Hydrology Method: True or Planar area?
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.
RE: Rational Hydrology Method: True or Planar area?
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).
RE: Rational Hydrology Method: True or Planar area?
RE: Rational Hydrology Method: True or Planar area?