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I am talking about modeling a large watershed that contains existing developments with detention ponds. Since I do not have the information for each those nor the time to model them, what would be the best way to include that in the watershed characteristics.

What are the different approaches to including smaller non-regional detention ponds in your hydrology models? Typically, if I am doing pre and post analysis, I take the more conservative approach of assuming a pre-developed CN (for NRCS) or pre-developed impervious cover (Clark) when...

Does any have any insight and, more preferably, reading / research material discussing the difference between the two methods? Pros & Cons I should say. Thanks!

Thanks beej67. I was trying to avoid doing backwater calculations completely (setting up a spreadsheet without using macros) by just assuming that the normal depth of the culvert/channel upstream location was the HW if it is "close enough" to the actual depth... However, I have recently realized...

Flow Regime: Sub-critical & open surface flow In a scenario where one needs to do backwater calculations for a channel / culvert in subcritical flow, is assuming normal depth as the US water level acceptable? At some point the backwater calculations will converge to normal depth, but I've never...

Thanks guys. I didn't know this would devolve into argument. Its just dirt! just kidding. This was helpful. I may run the hydrology calcs with type D soils and see how it impacts it. However, this project has been ongoing and previous designs used the NRCS "pre-developed" B soils. The maps...

Is anyone familiar with this HSG Soil Type, "URLX - Urban Land". I was recently pulling HSG maps from the NRCS website and there was a large residential development with this soil type that matched the boundaries of the development. Looks like pre-developed HSG soils were type B at one point...

your TW would be determined by your downstream drainage facility at the rainfall frequency as dictated by your governing authority. In this case, IT SEEMS that it would be the WSE of your street gutter, but it is hard to tell with little information.

What are the limitations of the NRCS Lag method for determining Tc? I know there is a range for the CNs used (50 to 95) but are there any risks to using this method with very small watersheds or using this Tc in the Rational Method?

If I remember correctly, HEC-HMS uses a different delta d (hydrograph time step) than pond pack for computing the hydrograph. The "time step" that the user has the ability to change only changes the interpolated output values taken from the actual hydrograph.

I am preparing an additional section for a drainage manual that includes supplemental criteria. I would like to use images from the NRCS NEH-630 and possibly an example that they have provided. However, I am not sure on the copy right status of that document. Does any one know? I do not see any...

There are about 5 different developments with detention ponds in this drainage area. Tracking down the drainage reports for these developments and then modeling them would be difficult. I could model as developed, but wouldn't modeling with pre-existing conditions be conservative? Perhaps...

I am doing a mitigation study for a proposed development that will be outfalling into a creek. Using the SCS method to developed hydrographs for the creek and the development, I am trying to prove that the new development will not create an increase in peak flow for the creek due to the storms...

I am doing a feasibility model in WaterCAD for water service to a new residential development. My firm's SOP for fireflow is 1500 gpm per fire hydrant using two fire hydrants at the same time (totaling 3,000 gpm) for two hours while maintaining a 20 psi pressure. That standard has been passed...

I was just trying to avoid banging through the math, but it's no big deal. Thanks for your help.

gbam, I see. So it ultimately depends on the HGL. Thanks for the clarification. Is there a formula for critical flow depth of a circular pipe?

When doing partially full analysis on closed conduits, if a pipe is surpasses full flow capacity do you switch over to 100% capacity analysis or do you continue analyzing it under partial flow to 93% capacity (real max pipe capacity)? hypothetical example: I have 3 pipes with a capacity of 10...