Unreliable Stormwater Outlet 2

[SJcivil]

I have a situation where the existing stormwater disappears into a fractured rock bed in the side of a mountain and disappears. Locals have stated that there may be a cave underneath. Either way it is impossible to determine what its capacity is other than the fact that it has been working up until now. We are installing stormsewer on an adjacent roadway and this will increase the stormwater flow. There are expensive options to transport this water to a nearby lake by stormsewer but are not feasible. Is the only solution to continue to use this outlet and provide stormwater ponds if it cannot handle the flow. A pumping station was briefly entertained but the owner did not want it. Anyone encounter anything similar to this situation. A few nearby houses could be affected if the water surcharges without a holding area.

Thanks
SJcivil

 

[RWF7437]

Go test it with actual known flows and develop a rating curve. Can't be done with smoke and mirrors.

good luck

 

[sam74]

You would have to treat it like a sinkhole in my area. Make sure there is enough available volume to retain the 100 year 24 hour storm event without discharging or overtopping.

You can't be sure that whatever it is will always take water in.

 

[RWF7437]

From Fema website

http://www.fema.gov/mit/tsd/fq_term.htm#frequt3

What is a "100-year flood"?
A 100-year flood is a flood that has a 1-percent chance of being equaled or exceeded in any given year. A base flood may also be referred to as a 100-year storm and the area inundated during the base flood is sometimes called the 100-year floodplain.

What does the "100-year flood" mean?
The term "100-year flood" is misleading. It is not the flood that will occur once every 100 years. Rather, it is the flood elevation that has a 1- percent chance of being equaled or exceeded each year. Thus, the 100-year flood could occur more than once in a relatively short period of time. The 100-year flood, which is the standard used by most Federal and state agencies, is used by the National Flood Insurance Program (NFIP) as the standard for floodplain management and to determine the need for flood insurance. A structure located within a special flood hazard area shown on an NFIP map has a 26 percent chance of suffering flood damage during the term of a 30-year mortgage.

 

[francesca]

And further to what Sam has said, you should install a Class V injection well to stabilize it and prevent it getting clogged up. A vortex is formed when water drains down (like your plug hole) which is erosive and increased flows frequently make sinkholes unstable, leading to fall-outs adjacent or further along the fracture in the rock.

 

[RWF7437]

If only we knew what a Class V injection well is .
If only we knew what kind of rock underlies this site.
If only we knew how erodeable that rock is.
If only we knew ..............

 

[SJcivil]

Thanks for the suggestions, I am leaning to SAM74's direction, even if you test it to see what the capacity is you can't rely on it staying that way in the future. I am in the process of analyzing some software to see if I can model this catchment area to determine the required holding area. I am in Canada so I am not sure what actual data I should seek to determine the size needed. Should I try and locate the total rainfall for the worst storm on record? I have looked at the rainfall intensity curve for this area for a 100 year storm and it is 7mm/hour (about .25inches/hour) for a 24 hour duration. Is this 24 hour period enough or should I go longer, any thoughts? I have a catchment area of 10.5 hectares or 26 acres.

Thanks
SJCivil

 

[sam74]

Some links about what francesca is refering to

http://www.epa.gov/ogwdw000/uic/class5/index.html

http://www.epa.gov/ogwdw000/uic/class5/types_stormwater.html

and what we do locally around here is similar to the closure description at the end of this article except that we grout a 36" dia. perforated HDPE pipe verically into the open throat and extended to the ground surface and slightly beyond (with a cap over the end to prevent accidents or clogging). Of course this probably doesn't apply to your particular situation with a fractured rock bed.

http://www.campbell.army.mil/envdiv/EnvSubjects/Water/Water Class V UIC Management Plan.pdf

 

[RWF7437]

http://en.wikipedia.org/wiki/Meteorological_Service_of_Canada

 

[cvg]

generally, drywells (class 5 injection) cannot handle peak flood flows. We typically use about 1 cfs or less as the maximum flow rate they can handle. Perhaps with the flow discharging into fractured rocks, you could handle more. But I doubt it would handle the peak runoff from the watershed. You will need storage - perhaps the cave could be located and sized for retention storage

 

[RWF7437]

http://geology.about.com/od/canadageomaps/Canadian_Geologic_Maps.htm

 

[francesca]

I have to snicker at that Ft Campbell document, Sam. The basin where Figure 5's photo was taken is huge and somewhere near where to the right of the frame is a Class V injection well right next to another one because they didn't do the job properly the first time and it fell out next to it. You can see their riprap skirts between the trees and the end of the main riprap apron.

http://maps.google.com/maps?ll=36.634587,-87.445673&spn=0.002871,0.006775&t=h&z=18