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Residential flooding from upstream development.

Residential flooding from upstream development.

Residential flooding from upstream development.


I'm not an expert in water management. I'm an electrical engineer.
I'd like to pick your brains and ask for advice on my situation. I
appreciate your time in advance.

My problem is flooding. I live along a trout stream in northwestern PA, there's a bridge in my driveway. I live in a valley. A few miles upstream there is a lot of development taking place. The elevation drop is 400' from the development area to my home. I believe one problem area is a detention pond for the mall.
I've only had major flooding 3 times in the past 25 years (when getting about 4 inches of rain in a short time). There was a flooding a couple months ago then it was 10 years before that. Basically my flooding occurs when down trees and debris upstream, dam up under my bridge. After a flooding, the creek bed is clean for a while. However I believe it will get worse due to heavy development that's been taking place. Not far from the mall there has been a hospital put in, a high school upgrade, new hotel and strip mall and more to come as well as a road widened to 3 lanes.

My main question concerns the mall detention pond. This was built in the 80's. I did some investigating on it and even after heavy rains you don't see water collecting in it. There's a standpipe 30" in diameter that is only 4' high. Although the pond is huge, (over an acre with 30' side slopes), this standpipe is at the low end of the pond and it doesn't take long before it is draining the pond, at which time the upper end (of the bottom of the pond) is still dry.

I've been to the township meetings to see if they could get the mall to raise the pipe and got nowhere. The township also assures me all new development has to go by certain regulations to take into account water runoff. I feel I'm being ignored because it's just me and a handful of neighbors that have to put up with this problem.

I have pics of the mall standpipe/pond I can post if need be.

I don’t know what to do and am seeking any tips, input or advice?
Do I have to hire an expert to determine if that pond is functioning correctly and if new development is causing me heartache?

RE: Residential flooding from upstream development.

Firstly, a 4" rain isn't a lot by "flood" standards.  In Middle TN, 4" of rain would have to fall in 45 minutes to be considered a "100-year storm," and a 100-year storm is the typical design storm for a detention basin.  What I'm saying is that your reference for a flood condition is much less than can be expected.  Your home should be built above the expected 100-year elevation.

Raising the elevation of the standpipe in the mall detention basin would actually be counter-productive.  The pipe as it is designed, outlets the water at a controlled rate, which for a 30" pipe (weir length = 2 x pi x 2.5 ft / 2 = 7.8 ft) is really not particularly fast.  If they raised the stand pipe, what would happen is that the pond would fill up more, leaving less storage once the pond starts to drain.  The pond would be overtopped, resulting in essentially uncontrolled release from the pond, at a much faster rate than the less-than 8-ft weir is releasing it at.

All of the new development that you talk about should also have detention basins, also designed for the 100-year flood.

Here's where it gets tricky.  The rate at which water is released must be the same or less than the rate at which it left the site in the pre-developed conditions.  Unless these ponds have infiltration ponds attached, however, the volume is greater.  What this means is that the flow rate, while the peak is the same for each individual parcel, stays higher than average (dry) conditions for a longer period of time.  Cumulative effects (symultaneous timing of the release of the (near) peak flow rate from each individual detention basin) could mean that the peak flow passing your house is greater.  

The caveat to this is that you would notice this more in the frequent storm events, and unless you either have been scientifically monitoring the flow past your house or want to pay for an expensive drainage study, you can't prove this.

You can do some quick-and-dirty calculations: use a topo map to estimate the drainage area upstream of your property; find your local USGS Regression Equations and apply the urban equation to your drainage area; this gives you a peak flow (100-year).  Toss that into Manning's equation (estimating the geometry of your creek or culvert and using tables to estimate your roughness coefficinet ("n")) and you'll get a flow depth.  If this depth is higher than your culvert, or comes up to within a foot of your house's finished floor elevation, then you can toss that on the desk of your local drainage people and maybe convince them that they need to do a study. (Of course your culvert only needs to pass the 10-year or 25-year storm event...)

RE: Residential flooding from upstream development.

Until recent times, all stormwater management for new construction focused on control of larger storm events (25, 50, and 100 year storms) and more-or-less ignored the smaller storm events.  Unfortunately, this line of though failed to recognize that 90% of all yearly precitipation events result in less than a 2 year storm event.  In other words, it seems to me that the problems lies within the detention basin(s) for the upstream shopping center, and any other facilities constructed prior to 1997 or so.

I agree with francesca in that you need to evaluate the contributing drainage area, determine the runoff characteristics, and determine how the swale/stream/culvert under your driveway react during storm events.  BTW, a 4" storm event over 24 hours is approximately equal to a 5-year storm event, with a 20% chance of ocurring in a given year.  Do you have any friends or friends of friends that work within the CivEng industry?  (PennDOT, private practice, etc.)

If the outcome of the hydrologic analysis of your driveway crossing indicates a significant problem, you can petition the local municipality to require a reconfiguration of the mall detention basin to control the more frequent (smaller return period) storm events.  If you have no luck with the municipality, I recommend contacting your regional DEP office and possibly even DCNR.

RE: Residential flooding from upstream development.

I've had some additional thoughts.

Unfortunately, simply reconfiguring the mall detention basin to detain the smaller storm events for longer time periods would simply result in less runoff reaching your property, but for a longer time span.  It's a Catch-22 scenario.

The real problem is that a reduced release rate (less flow per unit of time) by your property will result in less flooding and a lower water surface elevation, but the longer time span will create more channel erosion and result in greater debris accumulation at your culvert.

The only true solution is for the upstream developments to reduce their release rates, and to reduce their volume runoff through other practices (infiltration, capture/reuse, retention).

BTW, I'm outside Philly and see problems like this very often.

RE: Residential flooding from upstream development.

Thanks for replies.
I’ve been to the municipality to address the mall. The municipality claimed no responsibility; they said it is the malls responsibility. They agreed to talk to the mall manager about their pond.
After that the mall manager had a structural engineer inspect their pond and deemed it functional. However the mall agreed to modify the outflow standpipe voluntarily. What they did was cover up slots that were cut along the base of the 30” standpipe by wrapping a band of sheet metal around the base of the 30” standpipe. This is not a watertight seal but thought it would slow the water some. In retrospect I believe they could of left those slots asis and just raise the overflow height and that would have been a better improvement. At any rate, I took pictures of it before and during a storm and even with an inch of rain the water was up to the top of the standpipe. I showed the pics to the municipality and they again agreed to talk with the mall manager. However they only talked with them on the phone and didn’t present the pics, it’s like they don’t care. And the mall manager refused to do anything further to their pond and said the residents would have to hire an engineer to do a study to prove there’s a problem in order for them to do anything further to their pond.
Don’t really know anyone in the CivEng industry.
After the flood hit 2 months ago I was lucky my bridge held up. The length of the bridge is about 28’. The actual opening under it is 25’ x 4’. The height of 4’ use to be 7’ however after past flooding it has filled in.
There is a state road that runs parallel to the creek just on the upstream side of my bridge, when the flood happens water ends up flooding the road. After the last flood dep came in and told penndot what they were allowed to do to the creek. They told them to only dredge out 6” of creekbed and put it along the roadside of the creek and leave my property side alone. Dep claims my lower yard is a natural flood plain for this creek. I was pretty unhappy with dep although penndot ended up dredging out more than 6”, which was redickuless.
When the dep guy was down he said our township needs to adopt act 167 which would force the township to fix the flooding problems. I don’t understand why dep can’t do the study if need be.
I know for a fact the high school doesn’t have any water management. How can they get away with this? Is it because it’s state property?

Don’t want to ramble on any more. You suggested I “determine how the swale/stream/culvert under my driveway react during storm events”
If you haven’t already, can you walk me thought this or point me to a website explaining how to do this. Also should hire someone to do this for a professional opinion, or is it pretty cut and dry?
One more thing, how do I found out who my local drainage people are?

I appreciate any further comments or help.

RE: Residential flooding from upstream development.

From a flow perspective, another thing that you might look at for some piece of mind is what ordinances that the current development has to follow. It sounds like you find the current frequency of flooding (once per 12.5 years?) grudgingly acceptable.
What current and future increases in impervious area will do to you really depends of what rules have to be followed. In my home town, (south of Pgh.) the rules were "do what you like" until about a year ago. Then they passed what I refer to as the "Party's Over Ordinance." In a nutshell, peak Q post = peak Q pre for round numbered storms 2 yr through 100-yr and 25-yr 24-hr volume pre = 10-yr 24-hr volume post. That's hardly oppressive by many jurisdictional standards, but a hard blow to developers use to "do what you like." The second requirement is pretty good as it practically requires "infiltration" of a big volume of the water. Yadda yadda, the point is that how much increase in flow you see if any sort of depends on how strictly regulated development is.
That being said, you mention that debris blocking your opening is the main problem for you. I don't think all the stormwater ordinances in the world are going to keep brush and trees out of the stream. The only things that solve that are either a bigger bridge, or a sucker to build a smaller bridge just upstream.
Rereading your post I see that you asked for advice. Unfortunately, I don't have any good advice beyond being curious and making sure that people upstream are following the rules, and lobbying for local and county ordinances to be improved. You are in a tough situation. In Western PA there are droves of people who are too close to the water and get wet every other year. When they get wet every year due to increased runoff, it's hard for the people upstream who live high and dry to comprehend what the difference is between the former and later situations. In other words, your experience with elected officials is common. It is probably worth having a civil engineer look at your situation. When someone comes in with your story, I can usually tell if it merits serious (i.e. expensive) investigation after a fairly brief (i.e. inexpensive) look at the site and watershed. I would expect it to be similar for your local engineer.
Good Luck.

RE: Residential flooding from upstream development.

Out of desperation I have given the debris issue some thought and considered going upstream, and running some steel cable across and above the creek at an angle tying it to large trees hopefully at the flooding height. Hoping to deflect some of the floating debris off to the side trapping it behind large trees.
Does this sound reasonable or will it just end up building up to the point of snapping the cable and all coming down at once?
Also what's the ballpark cost for a civil engineer to look at my situation?

RE: Residential flooding from upstream development.

Although I realize you suspect the mall has caused your problems, it is likely that the entire watershed upstream of your bridge is much larger in area than the mall.  So, even if the mall has increased the discharge from it's property - it's very likely that this may only account for a fraction of any increased runoff impacting your bridge.  You can hire an engineer, but expect that engineer to look well beyond the mall property to all the developed properties upstream which combined may be causing your problem.  In addition, you need to investigate the design of your bridge - which it appears may be designed for about a 10-year storm.  With a 10-year design bridge located in your driveway, you should be very happy you have been flooded only 3 times in 25 years. You may be better off flood proofing your house, putting in the debris barrier cable or other type of trash rack or possibly putting in a larger bridge.

RE: Residential flooding from upstream development.

I'm a little confused as to what conveys the water across your property -- is it a grassy swale or a bona fide stream that has water in it most of the time, grey soils (due to anoxic conditions from being under water) and critters.  If it's a bona fide stream, there isn't a lot you can do to touch it.  It it's a ditch/swale, then check your plat, because you are probably responsible for maintaining it and that means that you have to dig it back down to being 7' deep at your bridge.

Driveway culverts don't really have "design" criteria; road culverts are designed for a 10-year or 15-year storm, depending on the municipality.  If your bridge is designed for a 10-year storm, then as already pointed out, it flooding the road three times in 25 years is its design behavior.

The last paragraph of my previous post gives you things to Google to do the rudimentary hydraulic analysis.  If you're feeling really game, you can take the flow you get and have a crack at modeling your bridge in HEC-RAS.  It's free software put out by the Corps of Engineers and the Hyraulic Reference Manual is all you need to figure out how to use it. (By the time you're done you might even consider a career change!)smarty

RE: Residential flooding from upstream development.

I would have to agree with cvg about the mall. Whatever can fit through a 30" pipe is a drop in the bucket compared to your 200 sf opening. As far a dredging goes, I have on only rare instances seen any positive effect from dredging, regardless of depth. The problem is that most dredging involves digging only the bottom out of the stream. This is worthless, because during the peak of the flood, that stuff is already gone, and will be replaced by new cobbles/gravel/silt as the case may be as the water slows and drops what it is carrying. I have seen some effective dredging that involved a total transformation of the channel to a much wider and efficient shape. But I don't see anyone ever doing that again as few people would stand for the total destruction of the stream.
In any case I also don't think that cleaning out under the bridge would be a huge difference. That stuff was put there by the stream, and it will be removed and replaced in the next flood.
If you're feeling lucky, go ahead and try the cable trash rack, and report back. I've always wanted to know what something like that would do. Intuitively, if it were designed for a reasonable differential water pressure (the "backwater") and drag on the debris field, and were anchored to something heavy (I don't know if a tree qualifies) it should work. One down side is the mess that you have to clean up after each flood, but you already had that. Another negative is that I'm pretty sure that you need a waterway encroachment permit from the DEP.
There are a lot of municipalities that haven't adopted DEP's model ordinance. I guess that's why the high school, for example, expanded without any stormwater management. I have to have a guy who keeps me abreast of what's adopted where, but I think that without enough earth disturbance to need the NPDES permit there wouldn't have been anything else that they were required to do. Does that sound right, guys?

RE: Residential flooding from upstream development.

My only worry with the trash rack is that when it's full of trees, etc., it could cause water to back up onto a neighbor's property, leaving the OP foul of a lawsuit.

RE: Residential flooding from upstream development.

francesca: Since the idea is to reduce backwater, if the rack is working properly water should back up less onto the upstream neighbor. In real life your scenario is a possibility, since all bets are off with debris problems. This would be an interesting design problem, although I would be nervous as hell stamping the drawing.
Perhaps a more elequent solution is needed. How's the prison population? Can we get the sherif to have the inmates clear all the dead and dying wood from the channel? Don't laugh (well maybe laugh a little). It has been done, although that era might be over.
But seriously, d r, I think your problems might go beyond what discussion here can solve, and there is a real possibility that you're just screwed. If you don't know an engineer to call, ask a neighboring municipality that has stricter stormwater management who reviews plans for them. Or if you happen to be talking to someone who is building a pond, ask who their engineer is.

RE: Residential flooding from upstream development.

“what conveys the water across your property”… it’s a bona fide stream, trout stream, even stocked by the fish commission. Sometimes in the year it’s ankle deep in spots. But it never dries up.

I agree the mall is a drop in the bucket, but I believe that drop could easily be removed with a modification to an outdated detention pond.

I can see why municipalities don’t adopt dep’s ordinances, if they did, no one could afford to develop.

Sounds like some have heard of the cable trash idea. I’ll plan to go with it but should be several years before a true test on it to report back.
Upstream neighbors are not an issue. I’m the sucker who is the furthest up. The neighbors downstream live on the correct side of the creek except one other downstream guy with a bridge. Which he happened to loose it (toppled one side of foundation) in the last flood. But now has it repaired.

RE: Residential flooding from upstream development.

the cable trashrack idea will work, depending upon what type of trash you expect it to collect.  We recently designed a cable log boom upstream of a spillway on a dam.  The cable is anchored on each side with concrete deadmen.  It uses corrugated metal pipes filled with styrofoam so that it can float on the lake.  It prevents logs (and boats) from entering the spillway.  So far it is working well.

RE: Residential flooding from upstream development.

Seeing as it's a real stream, there isn't anything you can legally do to touch it.  If it were a ditch, you may have been liable for its maintenance, which would've meant you were responsible for clearing the debris yourself.  This could still be the case; read your plat carefully.  

Your cable trash bucket idea is well and good, but make sure that it isn't considered modification of the stream in any way.  If it ends up creating a beaver dam that bursts and causes damage downstream, you could be liable, and maybe even fined for not having a permit.  It's worth a call to your state environmental department before you do anything to discuss it with them. They can't advise you on engineering matters, but they can tell you if what you propose is within the bounds of the law/permitting system. If you give your concerns a water quality spin, they may even join your side in "the cause".

Changing the outlet structure of the mall's detention pond is well and good for smaller storm events, but if the pond isn't symultaneously increased in volume, you could be courting disaster when "the big one" (100-year storm event) comes and the pond overtops letting the full peak storm volume go over the top undetained. Unless the mall has added impervious square footage since the pond was built, you could be out of luck here.  

RE: Residential flooding from upstream development.

I am not a fan of cable trash racks over streams.  A cable system is excellent on a dam or reservoir where you have rising water and are trying to keep stuff on one side.  I have seen them rip out after they plug up a good size debris dam and send a surge down the river.

I would read over FHWA reports on rack ideas.  Some excellent stream stuff.


The other item is just on planning on overland flow. It may be good to keep a pretty good area lower than your house so when your bridge plugs the water goes somewhere.

I have something similar at my house.  My big time plans are flood insurance and enjoying the view.  Eventually I will build a rock wall or timber crib planters on that side of the house.

RE: Residential flooding from upstream development.

It may be worth checking your local governemnt's regulations regarding stormwater management. I think most cities, or counties, have storm water manuals that dictate how flood routing and detention should be designed.

I like the format of the Georgia stormwater manual. The have specific verbiage in their manual that dictates how far downstream of the developement the deisner has to analyze.  In other words, a designer can not simply design a system to control peak flows off of the single deveopment, rather he has to make sure that it does not cause flooding issued further downstream as flood hydrographs begin to convolute at varlous points. The manula explains this nicely near the end of section 2.1.

Here's a link to section 2.1 for reference:

As far as legal issues... i've heard a lawyers use the term "legal trespass" when referring to an upstream development causing flooding on a downstream site. Of course, proving your case may be difficult if you don't have solid analysis performed for preexisting conditions and current condition, as well as lots of photographs :)

If you get some backup for your case, you may consider trying to convince the city or county to construct a regional detention facility to handle the overall increased runoff in your area.  I'm not sure if these things can be petitioned, but it may not hurt to try... especially if there is land available upstream for such a facility.

In learning more and more about stormwater runoff, I have realized that detention ponds don't solve all problems associated with runoff due to development.  There's more to it than just 'peak flow rate'.  If you develop lare areas of town over the course of several years, the overall increased volume of runoff is making its way downstream eventually (regardless of how big or small your orifice is).... and the problem is that hardly anyone analyzes the effects of their runoff beyond the limits of the immediate propery they are developeing. When all of the runoff from all of the various developments meet up down stream at various times... flooding can ensue... despite the reduced peak flow rates at each source.

Flooding a few times in 25 years isn't too bad for a ditch draining a large basin. That doesnt mean that upstream developemnts haven't affected the magnitude or frequency of the flodding that you experience.

RE: Residential flooding from upstream development.

To digress, I like that Georgia manual. That chapter is a lot better written than most of what I see up here. I think their "10%" thing is more rational than other methods to address the downstream issue. Some jurisdictions require maybe a 20% decrease in peak outflow, or maybe a certain volume per impervious acre of extended detention that isn't counted toward detention in the models. That's all good, but the specific instruction to investigate the downstream effect is lacking in many places.
I think blueoak has the right ideas. HEC-9 has good direction (I forgot it existed until you posted that link) if you are ambitious and want to build something. But the ultimate solution is to have the space in your yard for overbank flow and a house that can take it.
BTW, dandydrandy asked what civil consulting time costs for something like this. I my area, the first quick look to see if the situation is even worth investigating would be a couple hundred or so, or just done free as a community service. A study of an entire watershed, recommendations of possible solutions, securing permits, public meetings, and perhaps court time would go to five figures pretty quickly.

RE: Residential flooding from upstream development.

Maybe someone else has commented on this and I missed it:  you have a vested interest in making sure that any culverts downstream of your property that could potentially increase the backup of water are also being kept clear of debris, whether the culverts are on your property or your neighbors.

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