HGL Computations for a storm drain 2

[Quest101]

Hello,

Hoping someone can help me with hgl analysis up the pipe storm drain. I’m designing my storm drain for the 25 yr storm.

If one has the flexibility of starting the hgl computations from anywhere in the basin or pipe ( crown or half way ), which would be the best place to start ? I have been starting from the peak elevation for the 25 yr storm in the basin given to me by hydrocad and inserting that number into hydraflow, but my pipe is submerged and my water surface level is well about the crown of pipe.

It seems that because of the submergence of my pipe additional water coming through the pipes will not make it into the basin because there seems to be no head difference ( flat hgl ) to push the water from the pipes into the basin, resulting in water going up the inlets and spilling out. Does hydraflow understand that my basin pipe is submerged?


Am I understanding this correctly?
Hope you can help me

Thank you!

ps: sorry if I mistakenly added my thread as a reply to one of yours.

 

[jgailla]

Your hgl should start at the high water level in the pond.
The pipes will be submerged up to the point where the crown of the pipe is above the hgl.
We usually use a 5 year storm elevation in the pond instead of 25 year for storm pipe calcs on a site.
You have to start at the bottom of the system and work up.

 

[Xbxaddict]

I dont quite understand your problem, but I will add this maybe it will help. HGL isnt your actual water surface elevation, but a head representive only, since it includes velocity. It can exceed your top of pipe, just not your top of the inlet castings. I havent encountered a flat HGL before though, so maybe dbl. check your calcs.

 

[Surcharged]

The difference between the top of the grate and the high water elevation of the detention pond is the head that you have to work with. Hydraflow does understand this.

 

[Surcharged]

Question for jagailla:

Are you saying that you size the pipes for a 5 year storm? Or do you size the pipes for a 25 year storm but use the 5 year pond peak as the starting elevation for your HGL calcs?

 

[RWF7437]

Xbxaddict,

The HGL does NOT include the velocity head. The HGL plus velocity head (v2/2g) = the Energy Grade line ( EGL). Only at Normal depth, in an open Channel, are the HGL, the channel invert and the EGL all parallel.

Quest 101,

It is very unlikely that the peak 25 year ( 0.04% AEP ) flow would occur simultaneously with the peak pond elevation for the same probability event. Still, you may be required by some arbitrary regulation to assume so. This would certainly be conservative and might easily lead to surcharging the collection system. As Surcharge says, Hydraflow should "understand" this and give you reasonable estimates of the amount of surcharge.

good luck

 

[jgailla]

Surcharged,
The standards in most counties around here require the conveyance system to be sized for a 5 year storm. Since we size the conveyance system for a 5 year storm, we use the 5 year pond peak as the starting elevation for HGL. This is a minimum. Sometimes a client wants a lower frequency storm. RWF makes a good point that it is unlikely the pond peak will coincide with the conveyance peak.

 

[gbam]

Quest101,
Is this stormdrain the main conveyance system contributing to the pond? If not check your regulations for starting conditions. HEC-22 identifies frequencies for coincidental occurrence (Chapter 7) based on contributing drainage areas. You may want to consult HEC22.

 

[Surcharged]

RWF,

I agree with you:

"It is very unlikely that the peak 25 year ( 0.04% AEP ) flow would occur simultaneously with the peak pond elevation for the same probability event."

This begs the obvious question:

How do you determine the starting water level? I generally use the 25-year peak but am interested in seeing what others think about this.

 

[RWF7437]

Surcharged,

It may be that Gbam has provided one answer. I haven't yet had time to look up HEC-22 to check but it seems like a reasonable place to start.

A better approach might be to calculate the probability of the simultaneous peaking of both the design flow and peak water level. Then, assuming some design life for the pond ( say 50 years) one can calculate the acceptable confidence level ( say 90% ) of either the pond, or collection system surcharging during the design life. This will usually lead to a higher design flow but if the pond has sufficient freeboard it may, or may not, require a larger pond or more freeboard to prevent uncontrolled overflow.

This approach is seldom seen in regulations and appears hardly to be recognized in "the Rules".

 

[Surcharged]

RWF

I like the way you think but I wouldnt know where to start calculating probabilities. An easier answer is to use a model like SWMM that models pipes and ponds together.

 

[RWF7437]

Surcharged and Quest101,

The HEC-22 method is presented in Figure 7-3. Sadly, HEC-22 does not offer any explanation for these relationships or any supporting evidence. They may be based on experience or statistics or both but it is impossible to tell from Chapter 7 of HEC-22. Personally, I don't trust it.

For Quest101, it may be possible to model your system by taking the collection system plus the detention pond plus the receiving stream and routing the design hydrograph through it. HEC 22 does offer some suggestions for choosing the starting water surface elevation in the receiving stream. This is often the normal depth or the elevation corresponding to the top of the pipe from the pond to the stream. HEC-22 takes a very conservative approach in most calculations so this should also be a safe way to go. This routing step is almost never required by regulators but has always seemed to me to be essential if you want to have any confidence in you design.

For Surcharged, if you'd like to see an imperfect example of a probablistic approach visit PDHOnline.org and look for Course H127, Eclectic Hydrology.

good luck

 

[Quest101]

Thank you all for your responses. Only now am I able to respond. I should take a look at HEC 22.


Surcharged,

If hydraflow does understand that there is no head sufficient upstream of my basin pipe why does it not tell me or show me that water would be spilling out (flooding) out of my inlet junction just before the terminal pipe going into the basin?


RWF7437,

The ordinances I follow do not require me to start my hgl at any particular elevation, however, I wanted to design my basin and storm drain system in the best way. If there is no requirement by the jurisdiction, should one start at the crown of pipe, as a reasonable starting point? I guess the last question is for everyone.


Gbam,

Yes, the storm drain is the main conveyance to the pond. My regulations don't say anything about starting hgl.


If SWMM models both systems simultaneously, basin and storm drain, that would be the best progam to use. I guess we are not too sure as to what the best answer is.


Thank you !

 

[RWF7437]

For Quest101, it may be possible to model your system by taking the collection system plus the detention pond plus the receiving stream and routing the design hydrograph through it. HEC 22 does offer some suggestions for choosing the starting water surface elevation in the receiving stream. This is often the normal depth or the elevation corresponding to the top of the pipe from the pond to the stream. HEC-22 takes a very conservative approach in most calculations so this should also be a safe way to go. This routing step is almost never required by regulators but has always seemed to me to be essential if you want to have any confidence in you design.

As for Hydraflow specific problems please visit their website. I have always found them very responsive to questions. I wouldn't try to learn SWMM for a single, small project such as yours unless you have unlimited time and patience.

good luck

 

[Surcharged]

I use xpswmm which is more user friendly than swmm but it still took a long time to get started on it.

Hydraflow. When you run the system through it asks for a starting elevation. your choices are crown, normal or enter your own elevation. I choose "enter elevation" and type the elevation of the 25 year pond peak. If you do this and you dont have any spilling then keep going, you are ok.

 

[gbam]

Quest101,
Since the SD is the main conveyance, I would use the design storms tailwater in the pond as a starting condition to compute a steady state HGL. If you are concerned with the Steady State use Hydrocad or similar to route the hydrographs through your system. You can always start your design based on preliminary sizes and tweak as needed. Out here in AZ the standard practice is to analyze SDs using the Steady State approach. This is why we use HEC22.