The City, probably like many others, has adopted the latest Code book. Most of it is irrelevant to surveying, but one engineering item is flood retention. There are formulas to calculate the necessary flood retention structures. The most recent debacle we are doing is a subdivision of townhouses and the flood retention structure is bonkers. I asked the engineer about it and his opinion was it wasn't necessary since there was a dike built downhill to do this vary thing many years ago and he lives on the downhill side of the dike. But to get the subdivision passed the big "pond" (just under 1AF) had to be built. He called it a taking and I can't disagree with him. If on the off chance that the structure is ever filled with water it will be next to townhouses, presumably young families with children, not the best combo for safety concerns.
Next to my office there is a new bank that was built last year. It sits on a 2 Ac parcel and they also had to build a large retention pond. The kicker to it is that the bank property sits above a wetland so the pond, which had to be constructed next to the wetland because of topography, will drain directly into the wetland, there is only danger of the bank property flooding from the wetland, not the other way. Sometimes I feel like people sit in an office and come up with "good ideas" that are way more harmful than helpful, but they still get implemented.
These rules about retaining storm water on site - driven by Congress via the EPA and not local agency whims - have been in effect around here for a generation. They have long since passed the constitutional smell test. Probably your city has grown to a size and density which causes certain provisions of the law to kick in. They are about replenishing ground water and thus sustaining ecosystems, not about moving surficial water downstream in the minimum possible time.
I can certainly sympathize with you and your clients position on the matter, but I've also seen how it improves the livability of our cities.
BTW- there are more ways to skin this cat than open ponds. An example is using pervious pavements to turn parking lots into ersatz retention ponds. Another involves underground vaults.
Hopefully Trump abolishes both the EPA and the ATF. Anything the EPA does can be done at the state level, imo.
I chatted with the city engineer and asked if the retention structures had anything other purpose than flood control. He said no.
We get 12-15" of rain a year, our topography is hilly and there are very few flood zone areas, so flooding isn't a big issue.
Probably 99% of flood issues are related to snow melt and these structures do nothing for that. Yes, there are other ways to "contain" the water, and designers use every tool available, but I'm seeing a square peg in round hole thinking for our situation. As far as ground water, that isn't a problem, that's a big city, overuse issue. Our municipal systems are surface water, and we have basically the gold standard for that.
Just out of curiosity how would these structures recharge our ground water?
One size fits all I guess.
Just out of curiosity how would these structures recharge our ground water?
Don't be obtuse. The water stays in retention until it soaks in. And if the snow melt on a site is absolutely getting into the site retention system - assuming things are working right.
You may not get the same inches of rain that Portland does, but when you get it, it come in heavy downpours - am I right? All the more need to retain all that silt on site and filter it through the biomedia - instead of washing it down stream and choking out the critters it reaches.
Ground water here is in a number of different flavors, the Monarch, the Carney, the Anderson are some of the ground water layers.
The retention structures don't fill up with water unless there is a huge rain event. Those might happen once a season or not at all and are usually very localized. No, we don't get moisture normally in huge downpours, although it can happen. The structures sit dry all year and if some water does get impounded in them they are designed to outflow immediately through drainage structures. The one we were surveying at yesterday has a slotted drain 6" above ground with a grated drain on top of the concrete manhole a couple of feet higher, there will only be water in it if there is a multiple" rain event and it will leave as fast as possible through the slotted drain, if it happens to make it to the grated drain it will shoot out the drain pipe into the small natural flow line. They aren't charging ground water.
Most high ground water locally gets charged with irrigation and stream flow. A whole different subject.
Yes, they are sized for the 100 year event. They don't routinely fill up. That is intentional. The ground water these things address is near surface. Not the aquifers hundreds and thousands of feet down. Not in the short term, anyway.
The major point I make in response to your initial post is that this business is a fact of life, all over. It's a PIA for the small time developer, I agree. But it does have its virtues as well. It's the price we pay for living in higher density places.
All over is an inaccruate statement. I've been at this for nearly 50 years and have never had to include this BS into my consulting work as both an engineer and land surveyor. We receive a bit less than 40 inches of rain per year on average.
All over is an inaccruate statement.
Everywhere population and density exceeds certain levels. And what you did 50 years ago - or even 25 - doesn't apply here.
It is still a foolish deterrent to development and a "taking" of sorts without compensation to the land owners. A load of BS foisted on the unknowing by those who know practically nothing about the subject, i.e., politicians and committee appointees.
Depending on the area storm retention is a good idea. You start paving and developing areas eventually people downstream will get flooded.
Depending on the area is a good way to put it.
@mightymoe Its about matching pre- and post-development discharge rates at a given design storm. How would you determine that the new impervious surfaces from your project wasn't going to have detrimental effects on properties downstream? Certainly the sizing criteria itself can be a little over the top (regulating the 1-yr storm vs the 10-yr storm for instance). Maybe your project impact is small in and of itself, but it's the cumulative increase in impervious surfaces, over time, that create downstream issues that then must be dealt with either by the impacted individual or the community at large. Never thought I'd see the day around here that folks would be advocating for people NOT taking responsibility for their impacts on their neighbors. As far as a "taking" goes, horsepucky. It's an impact fee.
Its about matching pre- and post-development discharge rates
Everyone is well aware of that. The point being is there is no flexibility in the code. Specifically with the unnecessary 1AF pit other options were given and rejected. The easy one was to add a few tenths of dirt on a dike just below the townhouses. That would have easily covered the "problem", but the code says to impound onsite. So there is now a huge pit, difficult to care for, taking space that could even be parkland or playground. Instead, it's there to "mitigate" a flood once in 100 years when much better solutions for that were available.