Flooding Streets to Control Storm Water 6

[TBCEng07]

Our City fathers have come up with the idea of flooding the streets to help eleviate basement flooding. The area of concern has combined storm and sanitary sewers, so when the sewer becomes overloaded it floods into the adjacent homes.

I am sure that I am going to be asked to study the feasibility of this effort.

Has anyone else had any experience or guidance for me?

I admit this approach sounds counter-intuitive. I mean the purpose of storm sewers is to convey water from the streets. I think the real answer is to install new sanitary sewers and separate the systems.

 

[dicksewerrat]

you have the right idea. But your leadres are a bit short sighted. Flooding the streets with combined flows is a very bad idea. Engineers have been working for centuries to get rid of cholera, typhoid and a few other illnesses brought on by untreated sewage. How do they intend to clean up the mess? Ask the people in New orleans about the aroma? Granted the street may be better than the basemant but not a lot.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[RWF7437]

"Our City fathers have come up with the idea of flooding the streets to help eleviate basement flooding. The area of concern has combined storm and sanitary sewers, so when the sewer becomes overloaded it floods into the adjacent homes."

Are you sure the flooding is caused by "overflows" ? Are these homes protected from sanitary and stormwater backflows by backwater valves ?

Allowing streets to flood with stormwater ( not sewage or combined flows ) is not a big problem, by itself. Indeed, it is unavoidable but allowing sanitary sewage to flood into open areas accessible to the public is a public health hazard neither you or the City Fathers can escape legal liability for. Their job and yours is preventing or dealing with such conditions, NOT creating them.

good luck

 

[PELS]

I have seen this concept in use but not with combined sewers. There are areas in California where the land is so flat, projects have nowhere to drain to. So the streets are designed with extremely flat grades (0.0015-0.0025). The idea is to keep the flows in the streets as long as possible without flooding buildings and use the streets as a conveyance facility during heavy storms. The flows are then directed to parks or open space areas with large basins that allow the discharge to percolate and/or evaporate.

If you were to implement this concept with an existing system, you should first separate the flows (as you stated) and install separate systems. Untreated sewage should never be allowed to be exposed to the public.

 

[francesca]

A vehicle is considered a "total loss" if water reaches the mid-point of the tire.

http://www.autoextra.com/vehiclefloodtips

That leaves about 12" of storage before the city is on the receiving end of law suits from insurance companies if there are parked vehicles in these deliberately-flooded streets.

 

[RWF7437]

In the interest of accuracy,

"At what point is the car not repairable?
The reality is, if the water has reached that midpoint on your tire, you most likely want it to be declared a 'total loss'. Once an automobile has been flooded, the entire electrical system becomes questionable - and many critical systems are located on the bottom of the car. Water has a way of ruining electronic components, especially for vehicles equipped with a computer controlled engine management system. Again, do not start the car and owners should find an ASE certified technician to determine further damage. Vehicles that have been completely under water should be destroyed. If the car has been sitting in saltwater - consider it a total loss. Saltwater destroys paint, rubber, electrical wiring, metal and all upholstery.

 

[tsgrue]

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Where are you located? If the stormwater has come in contact with the sewage, then it is all considered sewage. The intentional flooding you describe is not allowed under federal law and I would think all US states.

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tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

http://hhwq.blogspot.com

 

[PELS]

Just for clarification, I agree that the intentional flooding of "sewage" is not acceptable anywhere in the US, but the use of streets as a storm drainage conveyance facility is a concept that has been in use in various parts of the world, including here in the US.

 

[TBCEng07]

The basic thinking is not to release sanitary sewage into the streets, instead the catch basin covers will be replaced with sort of cover (not a cheaply modification i imagine. therefore there would not be any sewage going into the street. basically the street would operate as a extended detention basin with an orificed outlet. I am nervioso about the time it is going to take us to design each street since there will not be any standard design.

holding more than 12-inches of water should not be possible given the fact that the curb section is 6" high and the R/W has a pretty gradual slope.

there is opposition to installing backflow valves. i think this is a more appropriate (and safer during freezing weather) solution. homeowners with flooding problems should not be finishing basements and should have standpipes installed during rains.

 

[RWF7437]

How on Earth are you going to separate sanitary flow from storm flows if the sewers are, as you say, combined and overflowing now ??????

Stop, run, get out of this town.

 

[msquared48]

With the possibility of bathrooms with toilets, showers, sinks, and washing machines in the basements, any house with a basement would need a backflow valve (duckbill valve or something similar) on the sewer line to prevent the sewer from backing up into the basements anyway. It's all hydraulically linked!!!!!!!!!!!!!!!!!!!!!

Separate the systems.

Mike McCann
McCann Engineering

 

[TBCEng07]

The way this would work is the water in the street would be soley storm water. The sewer main might only be flowing half full with sanitary flow and whatever volume of storm water we permit into the pipe through orifices.

Skokie, IL appararently has done something like this already.

http://www.skokie.org/public/presentation/index.htm

My concern is with this freezing. We are in the midwest also.

 

[semo]

Adding stormwater to your sewer system is NOT a good idea. EPA will tell you so once they find out it is happening. The idea is to eliminate I/I (Inflow/Infiltration) not add to it.

Your wastewater treatment plant would have to be sized to handle the additional flows, loads, and whatever constituents are in the stormwater. This adds additional costs to capital improvements and to O&M.

Why not use some other techniques to store and/or dispose of the water. A stormwater collection system could be installed. LID techniques (rain gardens, absorption pits, etc.) could be used to reduce the runoff. Purchase some low property and install a detention basin. There are plenty of options to examine before you put the water into the sewer system.

I would examine possible alternatives and present these to the fathers and explain the inherent problems with the idea of putting the water to the sewers. Do some research and show them better, cheaper, easier solutions so that they have reason to change their minds.

 

[TBCEng07]

There will not be any storm water flow added to the sewer. The storm water portion of the combined sewer shall be reduced by placing an orifice over the catch basin outlets.

Currently the systems has automatic overflows to the receiving stream. Holding the storm water portion of the combined sewer flow on the streets would serve to cut down the peak flow.

Is there anything else we should be considering?

We have concerns that once we separate, EPA is going to require that we begin pretreatment to our storm water anyways.

 

[francesca]

I don't see the point of playing tag with the EPA. They'll catch up with you in the end, and it'll always be sooner than you think.

Where is this combined sewer going to? Directly to the river or into a treatment facility?

 

[semo]

Ok, I think I understand this a little differently now. Essentially, you are taking stormwater that is already discharging to the sewer through a combined system and slowing it down? Basically using your streets for equalization.

That is a different approach to the overloading problem; but, I still think separating it is the way to go.

Unless you are a large enough city or next to a large city and fall under the MS4 stormwater criteria, you don't currently have to treat your stormwater runoff. Even then, you don't have limits to meet. Now, that might change in a few years (who knows); but, right now you don't.

Just my thinking here; but, other things I would consider include:
What shape are the streets in?
Will ponding water soak into the subgrade and cause the pavement to fail? (ie. create maintenance problems/costs)
What kind of traffic hazard is this water/ice after a storm?
Will kids play in the standing water and be exposed to any hazardous chemicals (oils, pesticides, herbicides, bacteria from sanitary waste runoff, etc.)

There are a lot of calculations and study to do for this; but, another big consideration of mine would be: What storm event do you design for where these orifices only cause 6" of backwater? What storm event do you need to design for so that the system doesn't flood the basements? What happens when a larger storm occurs and the backwater is higher than 6" and it runs into dicksewerrat's or francesca's garage and floods his/her living room instead of his basement? Who's responsible and gets sued for their design when this happens?

 

[dicksewerrat]

Well now that we only have stormwater in the streets, we can look for places to store it. ADS makes some storage items. Large ponds are a good idea. diverting the storm water to a detention facility and then pump to some natural waterway would be a good idea. But still the best solution is a seperate storm water drainage system. O & M at the treatment plant will go down and you may end up with some nice green spaces, detnetion facilities, in the neighborhoods. Low spots thay habitually flood with only 5-10 year events are the places for the ponds, etc. No solution is cheap, so get ready for a long involved process.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[tsgrue]

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You might be interested in this poster:

Sanitary Sewer Overflow Analysis and Planning (SSOAP) Toolbox

Rainfall Dependent Inflow and Infiltration (RDII) into sanitary sewer systems has long been recognized as a source of operating problems in sewerage systems. RDII is the main cause of sanitary sewer overflows (SSOs) to basements, streets, or nearby streams. RDII can also cause serious operating problems at wastewater treatment facilities. Thus, there is a need to develop proven methodologies and computer tools to assist SSO communities in developing an optimal capital improvement program that is in line with the projected annual capital budget and provides flexibility in future improvements.

http://www.epa.gov/sciforum/2005/pdfs/ordposter/Lai_ORD15.pdf

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tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

http://hhwq.blogspot.com

 

[TBCEng07]

Yes, i think the concept of this idea is viable, however putting it into practice is probably easier said than done. Like dicksewerrat mentioned. This is not going to be a 10-20 year storm event. the problem then is what to do with the bigger storms. Our City doesnt have much ground contour allowing for low areas to store water. That is my concern.

 

[Surcharged]

You have concerns about your contours. If you use pumps to pump up to a natural waterway you can go deep with your pond. (expensive though)