Not sure what the Waller Method is, just keep in mind that for most cases it is adequate to detain about 75% of the rainfall for the 10-yr,1-hr duration.
Don't go crazy with computer modeling because the IDF correlations are just exrapolations of very limited rain gage data.
The overland travel time (time to reach the inlet) for a one acre parking lot will be about 100 ft/2fps = 50 seconds. For this reason computer models that require a minimum Tc of 5 minutes are inherently invalid.
The Rational Method was never intended to be used with Tc --this misconception was...
One guideline for inlet spacing regards accumulation of debris along gutters. Closer spacing will result in a smaller debris pile that needs to be flushed to the inlet. In other cases the spacing is dictated by the terrain -- flat terrain usually results in more PVI's and subsequently closer...
The local highway department may have guidelines for maximum inlet spacing. There is no hydraulic criteria -- just Good Engineering Practice.
Be careful using the FHA computer Models -- the hydrology may not be valid.
psmart --the five minute duration would be used for sizing storm sewers and I agree that a longer duration (e.g. 1-2 hrs) is needed to estimate detention.
I ususally assume the rainfall hydrograph = the inflow hydrograph because the site will have been saturated by the leading edge of the storm...
Lcruiser -- you are correct. I should have typed " there is no relation between rainfall intensity (what is happening in the sky) and Tc (surface water flow patterns).
There is no relation between rainfall intensity (what is happening in the sky) and Tc(what is happening on the ground). The raincloud will act the same whether it is raining on a parking lot or on a heavily wooded forest.
The water generation capability of rainclouds is available in IDF curves...
By all means use the Rational Method and IDF curves (Intensity = average intensity during the duration). Avoid software that requires a relation between Tc(what is happening on the ground) and average rainfall intensity (what is happening in the sky).The raincloud will act the same whether it...
(1)IDF data is based on raincloud precipitation and is independent of watershed travel time (what is happening on the ground).
(2)Hydro-35 precipitation data appears to be based on sampling durations of 5, 15, and 60 minute.
Average rainfall intensity = depth collected /duration of sampling...
Step 1- assume you will be detaining during the IDF 60-minute duration. The average intensity associated with longer durations is usually a drizzle.
Step 2 - rainfall depth for this duration = 60 min x average intensity. This is what you would need to detain if you had no outflow pipe. This is...
A flexible rubber flap on the bottom 12" of the garage door would allow floodwater(but bnot critters)to enter the garage, thereby equalizing the water elevations with that outside the garage.
This would prevent the garage walls acting like a dam (until they finally buckle).
Depends on the type of vegetation on the forest floor,the slope of the forest floor, and the soil codition (dry, moist, wet).
Any CN you use will be an engineering estimate.
Flip a coin. Heads = 40, tails = 60.
The Rational Method will do fine.
Use the 5-minute rainfall duration -- this provides 500-1000 acres of raincloud size. Average rainfall intensity for the cloud is given in IDF curves.
If your site is relatively flat assume the best you can do is a flow velocity of 3-4 ft/sec.
Do not go above...
