the software will help determine the necessary size, but you will have to determine the location and spacing. If you are designing a road - then this is often required by the local drainage ordinance or policies.
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.
env21tech,
I disagree. Typically, Inlet spacing is dictated by hydraulic criteria. For instance, the allowable width of flow in a street maydictate where inlets need to be placed based on normal flow in a road section.
I have written a spreadsheet that will (with some coding for geometry) calculate the flow rate and flow width based on roadway station. I also coded my spreadsheet to compute the inlet capture rate and resulting bypass flow. This was all based on the FHWA HEC-22 manual and the rational method. I have not found software that will easily evaluate the data without constant coding (entering drainage area). Although my spreadsheet simplyfies the approach, it works very well.
HEC-22 software performs necessary calcs, but relies on you to put in the hydrology. Since every time you change the inlet spacing the catchment area changes it becomes iterative to "optimize" a design.
Spreadsheets can be set up to recompute area each time you tweak the inlet spacing, but require time to setup and adjust for each project.
If this is something you are doing for one project, consider using the HEC-22 software. If you are going to do roadway inlet spacing analysis frequently, you might as well get used to the spreadsheet approach. You can often find a starting point from your local City or DOT. Attached find a link to a sample worksheet.
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 spacing. In most cases it is convenient to locate the inlets at street intersections.
The localities will often dictate spacing, but you have to account for spread (and HGL) in the calcs.
StormCAD will do a lot of these calcs, but you have to decide where the inlets will go, and then verify that they have capacity and that the spread is not too great.
