Underground Detention System

[urbanresource]

Hi all. I am working on a proposed underground CMP detention system in Colorado. I have the 2 year peaks flows, via rational method, for both the existing and proposed condition, and need to provide some detention to reduce flows (so as to satisfy the City). Please provide your input on the best way to do so. I have unit hydrograph software, but it does not apply to Colorado. I have never calculated by hand (but this will have to do if there is not a quicker way). Your input would be greatly appreciated. Thanks.

 

[Ryb01]

I would assume that given your geographical area, infiltration is not an option?? You will need to size the facility and design a outlet control structure to attenuate the post development flows to pre development rates. This can be achieved with the use of orifices or weirs located at at various heights/depths within your storage unit. An option would be to use an orifice plate or series of, in a mahole structure to control your flows.

The degree of attenuation required will influence your underground storage volume.

Software that may be useful for this application may be HydroCad, SWMM, SWMHYMO, Visual OTTHYMO. I also beleive there's some simple tools available in TR-55, you'd have to look into further as I'm not too familiar with this software.

As with any underground storage unit, check with the manufacturer to ensure the unit is design for traffic loads(if under a drivable surface) and follow the manufacturer's compaction guidelines when backfilling. Typically there is some kind of quality treatment associated with underground storage units for environmental condiderations, clogging, etc. Check with the city to see what quality standards are required.

Hope this helps.

Ryb

 

[RWF7437]

" I have the 2 year peaks flows, via rational method, for both the existing and proposed condition, and need to provide some detention to reduce flows (so as to satisfy the City). "

The Rational Method does NOT provide a hydrograph so these calculated flows are useless for detention calculations. You need to find out from the City what method they use and will accept. This may be any one of several including the TR-20 Unit hydrograph Method, the Santa Barbara Urban Hydrograp, or possible some locally developed calibrated hydrograph method or even some completely arbitrary and prescriptive local method .

" I have unit hydrograph software, but it does not apply to Colorado."

Most of the methods mentioned above apply anywhere in the U.S.

Software cited by Ryb01 would be useful to you but check with the City to see what software they use or will accept. HydroCad does a good job on most things and is reasonably priced. There is also some free software in Ryob01's list.

good luck

 

[CPENG78]

Urbanresource,
The first aspects to consider are the governing agency requirements (allowable methods of calculation, allowable formulas and assumptions made) and the size of the property or should I say watershed in question. For small watersheds, the rational method will suffice. In reading your description, I get the feeling that we are not talking about a large watershed.

With this assumption in mind, you will need to verify which storms are to be analyzed for detention purposes as required by the city. On a recent project where I had to do the exact same exercise you are working on, the city asked me to look at the 5, 10, 25 and 100 yr storms. Once you know which storms are to be analyzed you will need produce synthetic hydrographs for each storm.

Each hydrograph will have the pre and post development flows plotted on the same graph with 37.5% of the volume contained in the rising limb and 62.5% of the volume in the declining limb of this hydrograph. The differential area between the two graphed flows is the volume which you will need to retain. In my case, the agency asked me to draw an additional horizontal line on the graph which represented 90% of the peak post developemnt flow. Then it was the differential area between the pre development flow curve and this horizontal line which will be the volume to detain.

Once you obtain this value or values if you are required to look at more than one storm, then you will size your metering structure depending on said values. As Ryb01 mentions, this will be in the form of a custom structure containing orifices and/or weirs.

You can obtain your time of concentration, and peak flows and plot these values on CAD to quickly obtain the differential areas (your volume), just be careful with units.

The entire process which I just described is all done by hand calcualtions (Except for use of CAD). Hopefully this helps and my explanation is not too confusing. By the way, I wish I had seen your post sooner, I have been out of work sick for more almost a week but I am back now. Good luck.

 

[civilperson]

The design storm is usually given for the area by reviewing agency. The runoff must be limited to the pre-development flow rate for the design storm.

 

[beej67]

The only way I can think of to map rational flow rates onto a hydrograph is to use the Dekalb Rational method, which was developed for use in Georgia a long time ago, and frankly isn't a particularly good or robust method to do stormwater hydrology, and isn't even accepted very often down here anymore. Its peak flows would coincide with the ones you have calculated, though.

There are a couple of relatively cheap software packages that will do a Dekalb Rational analysis for you, including (the bane of my existence) Hydraflow Hydrographs.

I do not recommend it. I recommend getting the appropriate data to use TR-55.