In some jurisdictions if you disperse roof runoff onto pervious ground surface or with drywells it can be considered disconnected impervious area and would be modeled as pervious, and your calculated site runoff may show no difference in pre- and post-developed for the 13-acre property.
Read regs/drainage code carefully, it is either a change in impervious surface area or change in runoff rate that triggers detention requirements, typically. If your project is below those thresholds, then detention would not be required.
Here are other factors that fall under "engineering judgement", for consideration along with the regulatory requirements:
1. Are you in a "sump"/"sag" condition - if yes, use a higher factor than you would on a continuous grade.
2. You can also look at the tributary area for potential sources...
There are different ways and no obvious "best way". It sounds like you need a hydrograph for the road subbasin that has the rain gardens, and that the curb cut itself does not need to be modeled explicitly....
In municipalities that use TR-55/curve number methods, the storage of "IMPs" can be...
Get your base course down asap. Asphalt treated base is not erosion prone. Perhaps the work should be scheduled so as to not have the grading occur too far ahead of the installation of base course materials.
Also Consider:
polyacrylamide (PAM) or tackifier applied to disturbed soils to hold...
Here is a VBA function I put together for this purpose because I didn't like the built-in options. Open the VBA editor and paste this into a module. Then you can use the formula in your worksheet:
=SIGFIG(number_to_be_rounded, Number_sig_digits)
If the format of the cell that this formula...
Check if it's in the city/county or state drainage manual/guidelines. If not, you could use the TR-55 values:
Gravel (including right-of-way),
HSG A/B/C/D RCN = 76 85 89 91 respectively
The standard around here is round flow control structures.
See http://www.wsdot.wa.gov/Design/Standards/Plans.htm#SectionB
B-10.40 and B-10.60
Large projects often require custom structures, which could be cast-in-place or precast vaults with internal weirs, baffles, orifices added.
Try using lookup tables. I found this much simpler to set up than trying to use a precise formula to find d/D. You can find all the values you should need from Appendix E: Area, Wetted Perimeter and Hydraulic Radius of Partially Filled Circular Pipes in the Civil Engineering Refernce Manual...
Ideas:
For clay based liner, ask your geotech and/or look for SCS/NRCS specs/instruction for farm fire/irrigation ponds.
For geosynthetic, check with manufacturer.
Bentomat claymax is a roll-out clay liner that can self-heal, worth considering too.
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...
TSGRUE points out potential concerns - but that doesn't mean that all those factors apply to each site, and to each paving system. Many of the clogging issues can be resolved by using appropriate source controls (cover stockpiles and storage areas, provide TESC during grading and earthwork...
