Community- Ornamental Pond, mitigating storm spillage/overflow 4

[injunear457]

Hi guys...Firstly I am an EE so my knowledge of this kind of thing is limited.
I'll preface my question with this description of what we currently have in our community to manage storm overflow of a relatively large (approx 300'x150' by 8'?depth) ornamental fish pond.

When (severe) inclement weather occurs it appears, and I assume, that the pond's rising level is mitigated by a riser/overflow pipe connected to storm drain piping and subsequently to a smaller detention pond nearby.

The problem is that the smaller detention pond's also used for storm water (watershed) of the 'typical' surfaces, blacktop and green space and *I* believe that it is sized for the ordinary runoff rate. It always performs flawlessly until the ornamental pond overflows and then the detention pond is overwhelmed and floods private property at lower elevations by sourcing water(and fish) via the storm drain grates at street level, all of which are also connected to the aforementioned detention pond.
It appears to me, and again I'm an EE so I don't know much about this stuff, that this is 'flooding by design'. It appears to intentionally divert the larger pond overflow to lower elevations as a ‘circuit breaker’ without regard to flow rate.

My question(s) is this:

1) Is this the 'typical' hookup to mitigate overflow of an ornamental pond?

2) Am I correct in my assertion that all the methodologies used to size, volumetrically, the detention basin are 'thrown out the window' when making this extraneous riser/storm drain connection to the larger pond?

3) Any other ways that this might be implemented (I think pumps and controls) would be better to divert the overflow to a different detention pond a short distant away under controlled flow rate to share the burden.

I might of course follow up with a couple more questions later...

Oh, and thanks in advance!

 

[injunear457]

Well, I got everything you said Lincoln, and then some.....

They (the HOA) did not get the permits and compliance certification(s) needed for the latest modification. The last permit pulled was for the original pond construction in 1996. There's no record of any other permit since.

I asked if it could still be in process and the answer was no given the time involved. They were also supposed to contact 'Compliance' to touch base and notify them of pending work ( a little fuzzy on this though).

They did none of this......The best I can surmise is that they contracted Coastal Engineering for advice and possibly design of the restored shoreline but likely used a GC to intall materials in accordance with what Coastal said.
This is all speculation on my part as they will not even talk to me and refer me to their attorney....

I think I have adequate evidence that they at a minimum modified the DRA (pond) without approvals and at the most did so that places myself and my neighbor at increased risk.

 

[cvg]

around here, detention basins are required to be in a separate parcel and/or an easement. This allows easier access if the flood control district, city or county needs to enter the property and perform "maintenance". This because HOA's historically have not always been very responsible for maintaining drainage facilities. For certain facilities (such as floodwalls), they are also required to prepare an Operation and Maintenance Plan, stamped by an Engineer and that gets filed with the flood control district also. That plan requires annual inspections by a registered engineer. On a recent project they also required funds placed in escrow to be used as necessary for future maintenance.

 

[injunear457]

cvg says:
they are also required to prepare an Operation and Maintenance Plan, stamped by an Engineer and that gets filed with the flood control district also.

By operation do you mean instructions on levels to operate and instructions not to exceed, and so forth?

As an aside, I did verify that the frequency is supposed to be 100 year interval.

 

[cvg]

yes, that is what I mean. very typical requirement for floodwalls, dams, gates and spillways, levees and especially multi use ponds where it is essential that the owner knows what is required for proper operation.

 

[cvg]

this might be a little extreme for a small urban fish pond but will give the idea:

http://dnrc.mt.gov/wrd/water_op/dam_safety/dam_owners/example_o-m.pdf

 

[injunear457]

I know it's a dumb question to you guys but I need a concise definition of what exactly is a 100 year plan.

Does this mean that the frequency of storms, worst case, that would produce a failure of the storm drainage system, by whatever means, is defective and below design expectations?

Would a flood frequesncy of 4~6 years constitute a failed system?

 

[cvg]

100-year design will be able to handle a 100-year rainfall. The chance of a 100-year rainfall occuring or being exceeded is 1% in any given year. The detention basin, channels, storm drains and any other drainage facilities designed for the site must be able to capture and safely convey the entire 100-year rainfall on the site. Most agencies also require attenation of these flow rates and volumes by construction of retention basins, sediment basins, dry wells, grassy swales etc. Any offsite stormwater which flows through the site must also be conveyed without changing the flow rate, volume or location of the flows. What does this mean to downstream properties? It means that there is theoretically no increase in flood risk caused by any upstream development. However, it does not mean that the upstream property owner reduces your risk. That is up to your developer to properly design your site to manage stormwater.

yes, 4-6 years would not be acceptable anywhere that I know of
100-year recurrence interval rainfall is used for 100-year design and a 100-year design should not fail in a 100-year storm.

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

 

[LincolnPE]

"This argument stems from their insistence that we (my neighbor and I) simply need to suck it up and put up with recurrent flooding (every 3~5 years) from their operation of an ornamental fish pond."

Just a couple curious questions about these 3 to 6 yr events:

1. Are any structures being inundated with surface water on your property?
2. If not, what is the elevation difference between the flood water surface and the lowest floor of your home (i.e., depending on your home's construction: top of slab on grade, crawl space lowest floor, or basement floor?

 

[injunear457]

Well, I don't know the elevation but it probably would be impossible to flood the first floor of our homes. I've had an elevation survey done for purposes of flood insurance and I recieved a LOMA (essentially a map ammendment to indicate the home is at a level above the flood plain) and the surveyor told me that there's no way, dues to elevations that I could ever flood.

We are generally concerned with sinkhole damage, foundation probelms, driveway/cars, and the never ending dead lawn (cost me $1200 to replace the last time) as sometimes it takes days for the water to recede...Oh, and the flood water blocks our entry/exit from the property.

I've been reading through all the documentation I copied (it's all on microfishe and I had to do all the work copying) and some of it is illegible but the best I can surmise is that the expected 100 year rainfall is approximately 13".

I've been searching trying to find any mention of the Fish pond's calculated self storage but no mention. I probably don't know what to look for.
I'm also curious as to how the level of the pond is set and if there was a specified depth during design...

I still need to visit the South West Florida Water Mangement District office (SWFWMD)and find out what the process is supposed to be for modifications to an existing Pond.

 

[LincolnPE]

Ok thanks, got it.

Background: In civil design there are two storm events to accommodate in subdivision design, the Minor and Major (100yr) storm events. There are certain restrictions on the Minor storm event design, such as you typically can not allow runoff to overtop the curbs, and that subsequently dictates the placement of the street inlets. The Major storm can run deeper down the road, 12"-18" deep depending on local criteria, but structures always have to be kept safe with adequate freeboard above the Major event flood levels.

Across the country the Minor storm event ranges from the 2 though 10 year events by jurisdiction. So the storms you mention are close to your Minor event.

Don't worry about researching the pond volumes and hydrology, you're not qualified, waste of your time, your layman opinion won't be considered in that area. What you can do is build your case that an unauthorized alteration was made to you subdivision's storm detention pond system and it is adversely impacting you. Maybe they changed the grading tributary to your lot also, increasing the tributary area and possibly increasing the runoff you receive.

You need to be able to honestly say, before reasonably going forward with this, "I never had these problems until after the unauthorized alterations occurred".

So build the preponderance of evidence like you have started (you mentioned no authorized permits since 1996, etc.) for arbitration with the HOA and your attorney, or if that fails, convincing the judge you have been adversely affected.

In your favor:

"the flood water blocks our entry/exit from the property"

The runoff depth within the right-of-way should not be deeper than 6" (no curb overtopping) for these order of magnitude Minor storm events.


Not in your favor:

"and the never ending dead lawn (cost me $1200 to replace the last time) as sometimes it takes days for the water to recede"

Unfortunately, this is not in your favor when considering Minor storm events. A properly grade lot should adequately drain the volume created by these storms. This reflects possible poor grading (ponding) on your lot due to original poor grading of the lot, or landscaping detrimental to the approved lot grading. i.e.,

 

[injunear457]

LincolnPE
In your favor:

"the flood water blocks our entry/exit from the property"

The runoff depth within the right-of-way should not be deeper than 6" (no curb overtopping) for these order of magnitude Minor storm events.

Thanks for that posting as it gives me some much needed perspective. The flooding, at the street level, is near 20~24" as best I could measure it from ground level near my mailbox and close to the storm drain grate.
The lawn area is covered from this same flood water but absolutely is not due to grading because the water is sourced at that point from a street storm grating and travels uphill onto to my property (against a very steep grade).

My property slopes from my garage door to the street level (where the grate is) about 2 feet over approximately 25ft. It is rather steep. The adjacent lawn areas are covered in flood water all sourced from the grate.

It's difficult to visualize this but I'll try to clarify further. The DRA is near the back of my home. The larger fish pond is even further back behind my home on the other side of a main road. The storm drain pipes are under the road to the DRA.

The street in front of my home is the lowest elevation and the street storm drain inlet (outlet in this case) is at the curb level to the left side of the end of my driveway.......

Since water seeks its own level, the DRA assumes the level of the far away pond and begins to fill the DRA. At this time the storm connection that attaches this DRA to the my street storm drain grate sources water to the street (lowest elevation)....Phew, wish we could post pictures......

P.S. I'll post a picture to my Flickr account and hopefully you can see what I mean.
I'll post the link here.

 

[injunear457]

Ok, here some links to photos....Keep in mind that the storm peak was at around 8 P.M. and it was too dark to take photos so some of these were taken the morning after and one or two a couple days later...

 

[injunear457]

Hmm.....

Hope it works this time because I don't know how to edit on this site, LOL.....

 

[LincolnPE]

"The flooding, at the street level, is near 20~24" as best I could measure it from ground level near my mailbox and close to the storm drain grate."

"The street in front of my home is the lowest elevation and the street storm drain inlet (outlet in this case) is at the curb level to the left side of the end of my driveway......."

Oh... I get it now. I should have read more closely up earlier in the thread..

You are at the low point, or one of the low points, of your subdivision, yes?..Lucky you having all the fun!

Yes, that is too deep for the Minor storm event range at the gutter flowline in front of your home...Too deep for the Major event!! Reference my post above on flow depth criteria.

So this is a problem sourcing out of the public right-of-way, adversely impacting your property. This is a driver safety issue also. This is in the public realm of things now now.

So take your pictures, and pictures of your depth measurements wearing your waders if you have them, down to your City/County engineer and file your complaint. Tell them these ponding depths are grossly far beyond the street flow depth criteria and you want them to remedy immediately.

They may have sewer maintenance issues and/or other system problems. They will need to work backward from this problem location at the storm grate in front of your home.

 

[LincolnPE]

Nice photos. I actually sent my message before seeing these.

How many inches of rainfall was storm Debby in your area?
How many times have you experienced this type of flooding over what time period?
How old is your subdivision?

May be a developer/developer's engineer issue here from what I am seeing...

Warning: This could get interesting. For example, I just finished a 5 year $42 million subdivision structural, grading and drainage repair job. The subdivision was about 5 years old.

We worked for the developer's insurance who was sued by the homeowners and HOA. The attorneys like to bring everyone to the party during the litigation process part of the project >>> The City did not really want to deal with us too much. They were worried about being brought in because they approved the plans..

I am part of an experienced team of forensic engineers that specialize in this type of work. We are civils, geotechs, structurals, geoscientists, mechanicals (and attorney colleagues)ready if you need to go that route. We not only did the repair designs, but supported the attorneys with copious analysis and court exhibits.

 

[LincolnPE]

"Vacationers were wearing ponchos instead of swimsuits at the peak of the summer season because of the tropical storm, which has drenched Florida for at least four days straight like a giant shower head set up over the state's Gulf Coast. Debby has dumped as much as 26 inches of rain in some spots.

Read more:

http://www.foxnews.com/weather/2012...-tropical-storm-debby-weakens/#ixzz2EwamM2uM"

I just quickly searched and read a little about the accounts of Debby. Probably not a good reasonable example for your case. (Hey, that's Florida? )

However, if the street flooding criteria is consistently being exceeded for Minor storm events, that would be the strength of your case.

 

[injunear457]

Don't have much time at the moment to post but the storm Debby brought us (locally) about 10"~12" in 24 hours.
The subdivision is 10 years old and the frequency of flooding is about once every 3 years (three former flood events like this).
If you look at the aerial photo you can see that the Fish pond's connected to the same DRA that my street, Mistwood Ct. is connected to. What's not apparent is that this same DRA services part of the Wexford Ave (main road) as well.

 

[LincolnPE]

Yes, their could be some original subdivision design issues going on affecting performance, like your street's elevation relative to the ponds, etc.

I alluded to this above.

 

[cvg]

My initial thought is that a) the pond discharges to a storm drain in your street which is too small (alternatively, the pond discharge is too high) or b) the pond discharges to a bubble up structure in your street

Option B would be a stupid design, but I have seen things like this done in the past...

Get the storm drain as-builts from the city and take a look at the details.

 

[LincolnPE]

See attached.