Principal Spillway Hydrograph (PSH) in HydroCAD

[LDutch]

I'm trying to model a dam in HydroCAD, and have a few questions about using the "Spillway 1-day 10-day" rainfall storm type. If I use that storm type, I should input my 10-day rainfall as my "Depth". Is that correct? Also, in my Subbasin, should I be using an adjusted curve number to account for a 10-day event? My normal (1-day) curve number is 72, and from TR-60 Table 2-3(b) is shows that the equivalent 10-day curve number would be 54. Which should I be using in my Subbasin?

Does HydroCAD automate all of the procedures that could be done by hand using NEH-4 Chapter 21? I ran through a hand calc using those procedures to develop my PSH and seem to be getting different results from my HydroCAD model. Using the hand calcs, it has you use tabular data using "serial numbers" for various Q1/Q10 and time of concentractions. For the hand calcs my Q1/Q10 is 0.88, thus I use "serial number" 8 to find the tabular data to generate my PSH. Is this Q1/Q10 the same as when HydroCAD indicates that the "Spillway 1-day 10-day" storm uses Q1/Q10 = 0.4?

I'm just sort of confused and would appreciate some feedback. Maybe I'm mixing up different procedures....who knows?

Thanks.

 

[psmart]

For any storm distribution you must enter the total rainfall depth for the entire duration. So when using a 10-day storm you would enter the 10-day depth.

As noted in the rainfall comments, the 1-day/10-day storm provided with HydroCAD is from the Minnesota Hydrology Guide 1974, Figure 6-4. and has a Q1/Q10 ratio or 0.4, meaning that 40% of the 10-day rainfall will occur during the 24-hour maximum.

Since the table is based on Q1/Q10=0.4, this would correspond to serial number 3 in chapter 21. If your ratio is different you may need to create a custom rainfall definition.

No other "magic" adjustments occur when you select a specific rainfall distribution in HydroCAD. It's just a rainfall distribution, and any necessary adjustments to the CN or other runoff parameters are up to the user.

Since my experience with modeling 1-day/10-day storms is limited, perhaps others can provide additional insights.


Peter Smart
HydroCAD Software

http://www.hydrocad.net