Design Storm and not flooding proposed houses, Opinions please 6

[proletariat]

I've got a site of about 150 acres which recieves an offsite flow of 233cfs during the 25yr storm. Observation by the town engineer, and other surveyors/engineers in the area reveals that this flow is grossly overestimated. However, none of them have been around to see a 100 year storm either.

My quandry is, Should I design a culvert and swale system to convey the 25yr storm of 233cfs without flooding adjacent homes, which is agreed by all to be an overestimation, and which is also technically allowed by ordinance? Using the 25yr storm under normal circumstances (where the flow is actually what the models show) would seem like a breach of the engineering standard of designing to the 100yr storm to protect life and property.

 

[bltseattle]

While the "town engineer, and other surveyors/engineers in the area" have an opinion, I bet they are not putting their engineering stamp on the design or hydraulic analyses. You have to make assumptions and judgements that you believe are adequately conservative to protect properties and meet code, because you (or your firm) will be the engineer of record and could be liable for flooding as a result of under-sized conveyances.

I would investigate further as to why the other parties believe flow is overestimated, and see if there are adjustements that should be made to the hydrologic model. For example, maybe the land cover is misrepresented (case could be made you should be designing for the basin's future build-out land cover). Or, maybe there is a bottleneck upstream - but this could be corrected in the future.... Unless the other parties can give you some engineering info to base your design on, I would base your design on the most accurate information that YOU have available - future homeowners won't thank you, but they won't curse the engineer during the big storms as much, either!

 

[RWF7437]

First, the 25 year storm has nothing to do with 25 years. It is the storm which has 4% chance of being equaled or exceeded in any one year. The 100 year stoerm has a 1% chance of being equaled or exceeded in any one year. It has a 24% chance of being exceeded in the life of a 30 year mortgage. It has a 64% chance of being exceeded in 100 years.

Second, there is no "engineering standard" requiring anyone to design for the 1% AEP storm. The choice of the 1% storm for flood studies was a political compromise made at the time the National Flood Insurance Program was created.

Third, designing for the 1% storm will NOT ensure that a property will not flood. It might ensure that the homeowner's flood insurance will pay, if he has such insurance.

Fourth, how do you, or others, know that 233 cfs is an overestimate ?

 

[cvg]

designing for the 100-year storm won't necessarily protect life and property - it simply reduces the risk. If the code requires design for the 25-year flood, then that should be your "minimum design". If your client will allow it, then recommend a design for the 50- or 100-year. And make sure that your analysis correctly estimates the offsite flow (regardless of the return interval) and that your recommended culvert and swale system can handle that flow.

As stated before, if you sign the drainage report or plans, you will be named in the lawsuit if their is any flooding. Even if you have done everything by the book, you will still have to defend yourself. Make sure you have done it right and you will sleep better at night.

 

[proletariat]

We obtained the FEMA flood study for the creek that runs through the site, which the rumored 233cfs offsite flow is tributary to. Our site is bounded by roads with bridges. The bridge at the upstream property line is listed as passing 50cfs less than the bridge at the downstream property line during the 100year (or 1%) flood. This means that FEMA attributes only 50cfs to our property and all property tributary to ours. This, for starters, is why we think it's a gross overestimation.

Secondly, there is an 18" culvert that is supposed to pass the 233cfs offsite flow. Information second-hand via the town engineer, from the homeowner living at the outlet of that culvert would indicate that no water has ever come through during any storm during at least the past 20 years.

However, the 25yr storm isn't that much different from the 100yr storm as far as order of magnitude goes. So, we are going to suck it up and accomodate the 100 year. Instead of accomodating the Ohio River, we'll be accomodating the Mississippi.

Now it will just be an issue of embarrasment when our grandiose stormwater conveyance channel is dry 365 days out of the year.

Thanks for your input.

 

[francesca]

Here's what I would do:

1. Calculate the offsite flow myself.
2. Design for the 25-year storm.
3. Check that the 100-year storm doesn't extend beyond the drainage easement.
4. If it does, design for the 100-year storm.

It's acceptable to have water standing in someone's yard for a few hours in a big (25, 50, 100-year storm), but not to have a large storm rip up their back yard because the ditch turns into a gushing channel.

If the 100-year storm exceeds the drainage easement on the ditch, you'll have to plat it and that will have implications for the property value.

Anecdotal evidence is useful when it's stuff like "the creek flows over the road in a big storm." It's useless when it's stuff like "I've never seen that creek burst it's banks and I've lived here fifty years." If no water has ever come out of the culvert, then it's not in the channel or it's blocked or the homeowner has never been outside while it's raining.

When was the FEMA study done? FEMA Studies in the town I worked in were published in 1984. A lot has changed in 20 years. Also, one creek that was published as having "no significant flooding" ended up having enough flooding (from a HEC-RAS study) to make us redesign the entire subdivision eliminating a road and about 15 lots.

I wouldn't set finished floor elevations without doing my own HEC-RAS study or getting a copy of the FEMA study if it's available. Remember that you have a duty to your client not to over-design and cost him money on supersized structures, but you also have a duty to your company not to cause them lawsuits years down the road and you have a duty to yourself not to lose your license.

 

[redneckengr]

two additional points:

1) FEMA generally uses regional regression equations and I'll bet you put a ittle more effort into your hydrology. Personally I have noticed that regional regression equations don't work for small waterseds like this one!

2)Is your design to pass the 100 yr or to prevent flooding of houses during the event. You may be able to design the culvert to pass the 25 yr and still provide protection for the 100 yr during peak flows.

 

[proletariat]

A little more info just to follow up:

Hurricane Ivan moved through the area in 1994. This produced widespread floods in the 25yr range. There was localized flooding (not necessarily at my site) over the 500yr range. I'm going to guess that my site saw at least a 10yr storm. Assuming that the homeowner went outside during the event to look at the culvert next to his driveway, I can say from his observation that there is a high likelyhood of no water passing through the culvert during the event. All of this is interesting, but now irrelevant.

At this point, none of us wants to stick our collective neck out on the basis of anecdotal evidence contrary to accepted stormwater models. So, we are going to make a case to the client that this channel may very well stay dry all the time, but our firm as well as his is sufficiently protected from liability if we design to this standard. We'll see how they take it.

 

[proletariat]

To answer redneckengr,

We could design the road to overtop in the 100yr storm, but as you know, density is everything in residential development. When we're done here, you will be able to hear water rushing through the channel while standing in the basement of adjacent houses. (Maybe I'm hyperbolizing just a little bit) If we flood the road, we flood basements.

 

[cvg]

It is possible to have only a 50 cfs peak flow increase in the river if you account for the fact that the hydrograph for the river is much larger and longer than the one from your small contributing area. The two hydrographs peaks don't coincide, so your small area doesn't increase the peak flow in the river very much. However, for your design, you need to worry about the full peak flow coming off and flowing through your site. As far as the 18 inch culvert goes, that most certainly wasn't designed for either 50 cfs or 233 cfs and probably just intended to handle nuisance flows.

 

[proletariat]

cvg,

That's a very good point. I do know that we are near the headwaters of this spring-fed stream running through the property, so that would yield more similar Tc lengths between my property's watershed, and the stream's watershed. Beyond pure speculation though, I can't nail anything down that is usefull.

Your point will be very useful in debunking the FEMA evidence, and building a more convincing case for presentation to our client.

star for you.