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Flow routing through multiple basins 3

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PKAST23

PKAST23

Civil/Environmental
Aug 27, 2015
21
I am working on a flood control project where we are trying to route approx. 4,000 cfs (Q100) from mountainous watershed through a series of detention basins and then through a channel at the foothills. We are designing 4 basins with orifice control outlet pipes between the berms for low flows. The basins do not have enough capacity to hold the 100 year storm volume (2500 ac-ft). 100 year peak discharge would spillover to the basin downstream and this process continues till it reaches the channel after the fourth basin. Emergency spillover berms are designed to carry the flow from one basin to the other. My questions - Do I still need basin routing analysis? How do I do that for multiple basins? Is there a program or spreadsheet out there that can do multiple basin routing? Or since the emergency spillways handle the flow rate, there is no need for basin routing analysis? Appreciate responses.
 
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how do you know the spillways handle the flow rate? you don't know what the peak flow rate is without doing an analysis. suggest HEC-1 or HEC-HMS would work and are public domain. these basins sound like dams. you might have to design the spillways for the PMF
 
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PKAST23 - Is there a chance for the flow to reverse direction (Flow Downstream to Upstream) through your low-level structures (Pipes)? If yes, then you will need Interconnected Pond Routing (ICPR) which allows for reversal of flows. Otherwise, CVG's recommendation will work just fine. Yes you need to route your hydrograph(s) to account for the storage within each basin. That way you can design your downstream channel.
 
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CVG&gbam
The spillway berm/weir are up to 200 ft long with 4:1 slopes between the basins. This can convey 4,000 cfs with no issues, I think. The basin bottoms will have 0.5%-1% slopes and the berm downstream is lower in elevation than the one upstream, so I don't think there will be an issue with reversal of flows.
I have not used HMS before but I just downloaded HMS 4.1 version. So, I can use HMS to route the hydrographs from one basin to the next for 4 times??
I am not experienced in basin routing so if I sound a little green, my apologies in advance.
Thanks.
 
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so how tall are these berms?
it will take about 4 feet of water over those spillways to get 4,000 cfs, that is going to require a tall dam. and if you intend on any sort of flood peak reduction, these dams will almost certainly be jurisdictional. assuming you are in the united states, the state will require the spillways to pass a lot more than the 100-year storm.
 
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The embankments are 15-20 feet tall and the spillway would be around 4-6 deep. You are right, about 4 feet is enough to convey around 4,000 cfs. For high frequency storms, the water would pond up and the culvert pipe would release a smaller peak allowing the ponded water to infiltrate. For 100 yr storm, water would just spillover from basin to basin into a proposed channel downstream. For this reason, I thought there is no need for basin routing from one basin to the next. Am I wrong in assuming that?

A little more background. These basins are existing. We are re-grading by making them bigger so we can detain and infiltrate as much as possible in low storms and safely convey the high flows via emergency spillways. flow enters the first basin via an existing channel, so I guess the first basin can be called a dam. I need to check on the spillway and dam restrictions you mentioned.
 
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so you have a hydraulic height of 11 - 16 feet and who knows how much impoundment. If you have over 50 acre feet of storage below the spillway crest, you will have a jurisdictional dam and will probably have to pass either the half or full PMF through the spillway. So for example as shown in the link below, your dam(s) are almost certainly jurisdictional.

example for California:

[ul]
[li]you will need to set the spillway elevations including adequate freeboard,[/li]
[li]you will need to determine the amount of storage and the peak outflow[/li]
[li]you will need to do this for the entire series of dams[/li]
[li]you will need to estimate the drain down time[/li]
[li]if the dams are jurisdictional, you will also need to route the inflow design flood hydrograph which will be greater than the 100-year in order to set the spillway elevation, dam elevation and calculate the freeboard.[/li]
[/ul]

how are you planning to do this without running the analysis?
 
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What is your goal? Are you just looking to pass the entire Q100 peak safely pond after pond, or are you looking to attenuate and lower the Q100 peak release pond after pond?
 
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The goal is to pass the entire Q100 safely pond after pond, not trying to attenuate the peak. All the basins are less than 50 ac-ft in volume and 15-20 feet high so looking at the chart in the link provided by cvg, it will not be considered a dam.
I understand the need for basin routing analysis though. I have used UH Method to calculate the peak flows and volumes for 2, 5, 10 and 100 yr storms using CIVILD program which is accepted in counties in Southern California. Can I use HMS just to do the basin routing?
 
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If you are not concerned with flow attenuation and simply need to pass 4000 cfs through each pond, then you would assume the ponds have no storage capacity available, and size the spillways accordingly to handle the 4000 cfs.

You should be doing that anyway for typical pond emergency spillway design (if the pond outlets become plugged). Every pond design should have an emergency spillway with adequate freeboard to safely pass the proposed major storm event in case the outlets become plugged.
 
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Will 4 feet of flow depth down a 4:1 slope result in an erosive velocity, headcut formation, and a breach of the spillway?
 
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Spillway berms are protected with grouted rip-rap or with ArmorFlex blocks.
 
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What country is this in?

If you're in the USA, then the watershed sizes indicated would lead me to believe you're manipulating the flows within a 100 year floodplain as defined by FEMA. If that's the case, any changes you make one way or another to any of the four dams will change the flows in the reaches, and necessitate a change in the flood maps, including notice to all the land owners along the reach / etc. It's quite an undertaking, and ordinarily wouldn't be attempted by a firm unfamiliar with general pond routing principles.

For the sake of discussion, yes you would need HMS or similar to do pond routing at each pond, but you would also need HEC-RAS to determine the floodplain widths along each reach, and also potentially to do dam breach analyses and such.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -
 
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I have used UH Method to calculate the peak flows and volumes for 2, 5, 10 and 100 yr storms using CIVILD program which is accepted in counties in Southern California. Can I use HMS just to do the basin routing?

You could use Civil 3D to do the pond routing, especially since you already created the hydrographs in Civil 3D. Just curious, what unit hydrograph method are using now?
 
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This project is in California, which lot of people think is not US anyways[dazed]. Just joking.
I have used synthetic UH method using CivilD not Civil3D. This is a popular DOS based program accepted in Riverside County and other counties in Southern California. The watershed is 3 square miles, the 100 yr 24 hr Q and V are approx. 4000 cfs and 2500 ac-ft respectively. Can I still use HMS just for basin routing?? The project is in really early stages, I am trying to gain as much knowledge as possible before we present our design to the agencies. We will be preparing profiles of the basins and channel. To show HGL on profiles, Is HEC-RAS best modeling choice? Can I model basins and embankments with spillways in RAS?
Appreciate all the responses so far.
 
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You can if your hydrology program can export a hydrograph text file. You can import the hydrograph and route through your system of basins.
 
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yes you can route with HMS
yes you can get a profile with RAS
RAS would be good for determining the floodplain if you need to do that
I'm surprised you can't route through storage basins with CivilD, according to riverside county flood control district, you can



 
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Yes, HEC-RAS is your best choice for HGL/EGL profiles, and demonstrating you are safely conveying the 4,000 cfs through this system. Takes into account backwater effects..HMS doesn't.

 
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