Stream/Pond Model

[psportsman]

Have an exisiting pond close to a stream. Currently the pond and stream are not hydraulicly connected. Only in the 100-Year flood event, when stream floods over pond. We want to bring an intake pipe into the pond from the stream and then install a riser in the pond and outflow back into the same stream. This same pond is also used for stormwater detention. Stream drainage area is 30.5 square miles. Area to pond is about 3.9 acres and was oringinally designed using TR-55. Need to figure out how to model this thing for the 2 thru 100-Year events.Any ideas on how to model all this? What software/methodolgy to use?

 

[sam74]

Is whichever municipality going to allow you to do this?

Is there an existing outlet riser for the detention pond?

How would you compare pre vs post when you are adding to the pond from an outside source?

Logic makes me think if you are any where near a populated civilized area they will not allow this from a review standpoint.

I think it might possibly could be modeled using Pond Pack interconnected pond modeling depending on exactly what you want to do. I do believe that you would have to alter your pond size and outlet structures.

If you had a small diameter pipe (I would think 2" to 4" max) coming from the creek you could model this as an upstream pond. Model the creek to determine what elevation the creek is at for these various storm events using say HEC-RAS if it has not already been modeled and made publicly available by FEMA. Set this as a constant head for your upstream ponds outlet for various design storms.

Your downstream pond would have the 3.9 ac as inflow as well as the upstream pond (creek) outlet. Then you would likely have to design your riser structure, overflow spillway, and likely alter pond volume. I just don't see the rational of determining what your predevelopment numbers are going to be so you can have that or less as the postdevelopment numbers.

And I'm pretty sure you can't have two riser stuctures exiting a pond in Pond Pack. Although I'm a few years removed from using it or any other of the latest softwares.

 

[froude]

Sounds like a pretty complex situation. What is the reason for connecting the stream and pond? I frequently do what Sam74 has implied and use a combination of PondPack/HEC-RAS and rating curves to do analyses, but I can't say I've done anything similar to what you are doing.

It sounds like you may be able to use EPA-SWMM (free from the EPA; several proprietary SWMM based models are also available) for your analysis. It will make your modeling effort cleaner because your hydrology/hydraulics for both systems will be in the same model. You will be able to specify your channel characteristics, pond characteristics, and all outfall characteristics in detail. I think the program will be well-suited to handle the routing.

From the various threads in this forum, I think HydroCAD may work for this analysis also. But I am not familiar with that program. Maybe others who are will respond to this post and give you more help!

Good luck.

 

[psportsman]

Thanks Sam74 and Froude...Sam74, there currently is no outlet structure on the pond..Past design just indicated the pond had enough available storage above it's normal pool elevation to hold the design storms without overtopping...So the predevelopment flow from the pond was always zero before...For this design, the allowable outflow from the pond could be equal to the stream inflow...In order account for this inflow in the pond hydrograph, we would simply add the cfs associated with the inflow to each timestep of the hydrograph. We have info. for the 100 year storm through FEMA...Was able to figure out flows for lower storms through USGS streamstats..Trouble is the 100-year generated from that regression analysis doesn't match the FEMA one...Municipality is aware of this..Not a huge populated area...

Froude, yes it is very complex. The whole purpose here is to provide a more constant source of freshwater to the pond..The pond does not have enough base groundwater flow in the drier summer months and becomes stagnat...Owner wishes to add water from stream to help this..

We already prepared an analysis that considered the stream inflow to the pond to be equal to the intake pipe capacity (only considering manning's equation. We added that flow to each hydrogrpah timestep and routed the pond using the tailwater from the normal stream elevation we had determined. The thought is the stream will be at normal elevation at the begining of each storm and by the time it would peak the peak in the pond would have already occurred. Now of course the stream is also rising at the same time and even though may not be at peak at the same time as teh pond, it will still have an impact...Tough to prove though..The site is in floodplain and we know from that the pond is flooded by the stream in the 100-year event..So pipe connection or not....Equalization occurs....Given that fact, we know the site wont be any worse for the lower storm events...Problem is still trying to model things from the detention aspect...

I'm trying to put an analysis together now using Hydraflow.....Any othe thoughts?

 

[beej67]

Wait, back up.

Your existing site has a 100 year retention pond with no outlet control device that caught and held the full 100 year runoff from 3.9 acres of development, even though it was adjacent to a stream?

So your existing site has zero discharge?


A) Are you sure about that?
B) Where is this project at?
C) How does the existing pond draw down to empty? Groundwater infiltration only?
C2) If so, why connect it to the stream at all? Isn't it currently groundwater fed to the stream elevation via water table connectivity?

In most areas I've worked in, your post development discharge can't exceed your predevelopment discharge, which in your case is .. zero?

Do you have some maps or something?


All that aside, I would use SWMM (EPA-SWMM or XP-SWMM) to model what you've got going on. You can do a stream flow model for the overall watershed, complete with HEC-RAS style cross sections, put your current watershed and pond online, and watch the tailwater spill into your pond when you route the storms. I would NOT use Hydraflow, and I'd have an honest heart to heart with Peter Smart before using HydroCAD, although HydroCAD might handle it if you made some very bland assumptions about the geometry in your stream channel. If I was stuck using Hydraflow, I'd do what Francesca suggested, and route a hydrograph through HEC-RAS to get a variable tailwater to work with, presuming Hydraflow can even handle that, which I'm not sure it can. I haven't used it in years.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[Ryb01]

I think sam74 hit on a key point which may be the "be all-end all" for this project....This being "Is whichever municipality going to allow you to do this?" There is a component to this project that has not been discussed yet.

Unless I'm missing something here, the proposed configuration you are speaking of will be creating an online pond. I have not heard of this being approved anywhere within the past 10-years. The reasons for this type of pond not getting approvals is the water temperature increase when flow is directly interchanged with the pond water and also the loss of base flow within the creek. I believe that a maximum of 10% (plus or minus) of baseflow can be diverted. It is one thing to discharge runoff to a creek through a pond but it is another thing to divert creek water into a pond. A number of golf courses and private home owners run into this issue quite frequently.

Before getting too wrapped up in the hydraulics, I would consult your local municipality to see if this is allowed.