Peak Flow Rate - pre vs post 4

[geosavvy]

Currently our cities' Storm Water regulations dictate that the post-development peak flow rate must not exceed the pre-development peak flow rate. However, the regulations do not take into account the nature of the flow.

What I mean to say is that you can have a pre-development condition where runoff sheet flows accross a 500-ft property line, then take that entire flow post-development and discharge it through a single pipe onto the same property (as long as your overall peak flow rate is not increased).

Depending on the configuration of the property down stream, this change in flow sometimes causes flooding problems that did not ocurr prior to the development.

Another issue is when the subject property sheds runoff in multiple directions, but after development the basins are changed such that a particular adjacent property could getting more runoff than pre-development. In other words, an adjacent property that previouly 'caught' 30% of a sites runoff, could be getting 75% of the runoff after development simply based on restructuring the basins. As long as the overall peak flow rate of the site does not exceed pre-development conditions, this is ok to do (according to our storm water regulations).

I was curious if any of you deal with similar Storm Water management issues. Have any of you run accross regulations that pose more "creative" restrictions on how water is to be discharged onto adjacent property (othe than simply not exceeding the pre-developed conditions).

Thanks.

 

[cvg]

besides these regulations, you need to be aware of any increased risk or liability you are creating that your client or eventual homeowners may eventually have to deal with. Otherwise, be ready to pay the piper when the lawyers come knocking at the door later on...

 

[rmadrid]

Things to keep in mind is that your point of discharge should be the same as it was before development. If the existing flows was sheet flow then you should have multiple discharge point so that you are not concentrating the flows at one point. It is the norm not to increase the pre development flows and if your post development flows are greater then a basin is required. I have seen in certain occasions that the post development is less than the pre development and this is usually due to the hydrological soil charateristics where you have soils class C or D. The bottom line is to keep post discharge flow the same as the pre development flows. Thou shall not flood thy neighbor or I will be liable.

 

[francesca]

Where I worked previously, the regs also were based on peak flow. We had a site that we constructed a detention basin in a draw and met the peak flow condition, but in a big storm, a deep ditch was carved through the downstream neighbor's pristine lawn. We think the outflow pipe was blocked and the basin overtopped (exceeding the pre-development flow rate), but checking our calcs we discovered that we had indeed increased the velocity too and the law suit went against our client on that basis.

 

[RWF7437]

Be aware, as others have alluded to, that in addition to the "regulations" and "standards", there are laws. Drainage law varies considerably from State to State in the U.S. You need to know what the laws are in your State. Under the law, in my State of Oregon, you could not do what you describe because you cannot appreciably increase the hazard to downstream properties by changing the location, velocity or amount of runoff from your site unto your neighbors'.

good luck

 

[psmart]

Your comment illustrates a major shortcoming in many of the peak-flow regulations: By limiting the peak and allowing an increase in volume, the peak must get wider, so there will be more flooding when combined with other downstream flows. This is the hidden catch with using detention basins to manage developed flows.

Some authories (NY State DEC, I believe) address this by requiring you not to exceed the predevelopment peak at a downstream point that has ten times the contributing area as your site. (Yes, this means you must do an analysis for the watershed ten times the size of your site!)

Without these tougher regulations, we will continue to see an increase in downstream flooding. This will require more creative stormwater solutions, such as greater use of on-site infiltration, and sequencing of peak flows with other sites.

 

[RWF7437]

psmart,

You said: "Some authories (NY State DEC, I believe) address this by requiring you not to exceed the predevelopment peak at a downstream point that has ten times the contributing area as your site. (Yes, this means you must do an analysis for the watershed ten times the size of your site!)"

Did you mean at least ten times the size of your contributing area, rather than your site ?

What is the logic behind the number ten ? Could it just as easily be 5 ? Or 20?

Is this "rule" based on some NY law ?
Is this rule applied to "local detention" and to regional detention ?

Sorry Geosavvy. Don't mean to hijack your post.

 

[Surcharged]

The question that comes to my mind is how would one recreate sheet flow? Construct a detention basin the length of the property with a weir that runs the length? I would also research the adjoining property to see if it was going to be developed soon. That could have an impact on your decision on what type of drainage to use.

 

[geosavvy]

I looked into that in the past. There are "spreader" devices that discharge water in a sheetflow manner. Also creating a very broad earthen weir is what some people do.

I should clarify that I work for the municipality, not a local private firm. I need to do some research on local runoff, or peak-flow, laws. Sadly, as much as I have learned about storm water management in one year, I feel like I know twice as much as some of the local engineers. Of course, more likely, they know plenty about storm water management and design, yet choose to take advangte of loopholes in the regs in order to save their client a buck or two.

We have hired outside consultante to create an all-encompassing engineering design manual. The idea is to extract all design regulations and ordinances and put them in one place to clean up the design and review process. At the same time we are looking to update any outdate standards and even add standards and regs that we may be lacking.

I feel that our storm water management manual needs somewhat of an overhaul. The issue regarding local detention versus regional is one that I want ot explore as well. We have never required someone to analyze the effects of a development further down stream than where the flow exits the property (at least not since I have been here).

It should be obvious to everyone that you can't double the runoff volume and simply use a small local detention pond to change the shape of the hydrograph and "call it good". At some point you have doubled (or more) the total runoff volume of the entire city and your pretty little hydrographs exiting the small ponds all over town are overlapping at various points all over the place causing flooding problems. Yet there's no one to blame because everyone "didn't increase the flow" any when they developed their site.

I appreciate the input so far. I am wondering if anyone could point me to a city or county that requires more extreme measure such as "no increase in total runoff volume" or "no increase in runoff velocity", etc.

I have read some similar requirements to the "10x your basin size" rule. So basically you are required to check pre vs post to a point downstream where the basin size contributing to that point is 10x the size of basin contributing the the discharge point of your site?
This must be a pain in the butt to follow. Not only would trial and error be required to find that point downstream, but the analysis could include half the city if you are starting off with a large basin. Interesting though.

 

[psmart]

RWF7437: Sorry to be thin on the NYS details. That's all I recall from a seminar a couple of years ago. I'll let you know if I can get specifics. Perhaps someone who practices in NY will know more.

 

[civilman72]

From my discussions with attorneys who specialize in drainage law (for Colorado), they like to use the terms of "equal quantity and quality" when determining an individual landowners responsibility of how to properly convey storm runoff to their neighbors at historic conditions. The word to key on is quality. I would quantify "quality" as not only comparing the differences between sheet flow vs. concentrated flow, but also "clean" vs. "dirty" water. Insuring that water quality is met is something that has not been mentioned yet in this thread, but certainly can open your client up for a lawsuit just as quickly as increasing the quantity or type of flow, especially in this day-and-age.

Your challenge as the municipality will be to try to quantify how you write regs to make the design engineer check "quality." The easiest solution is that you make it the responsibility of the design engineer to verify that the quality has been met and leave it up them to determine how to accomplish this. Considering how quickly acceptable means of water quality controls are being implemented, it may be difficult to create a set of well-developed standards that are able to keep up with the technology, especially if your department does not have the expertise or manpower.

The fact of the matter is that once you develop a site it is very difficult to emulate historic flows exactly. Almost all developed sites create concentrated flow patterns, and truly bringing these back to sheet flow conditions (when necessary to match historic) is unrealistic. Considering the large variation that each individual site may have including: soil types, topography, existing downstream drainage infrastructure, etc., they must be addressed on a case-by-case basis. So, it's even more important that engineering judgment is utilized when designing drainage infrastructure, especially when there is never only one "right" solution.

Something you may find interesting, ironically enough here in Colorado, due to a general lack of water and in areas dominated by farming communities, it is just as important that you do not decrease the amount of water that you send to your neighbor, which can just as easily lead to a lawsuit as increasing flows. This can make detention/retention/water quality pond designs quite a challenge.

 

[RWF7437]

http://www.gradingandexcavation.com/sw_0607_voodoo.html

 

[psmart]

The NY State "Downstream Analysis" I was referring to can be found in chapter 4.8 at

http://www.dec.ny.gov/chemical/29072.html

From the introduction:

Overbank, and extreme flood requirements may be waived based on the results of a downstream analysis. In addition, such an analysis for overbank and extreme flood control is recommended for larger sites (i.e., greater than 50 acres) to size facilities in the context of a larger watershed. The analysis will help ensure that storage provided at a site is appropriate when combined with upstream and downstream flows. For example, detention at a site may in some instances exacerbate flooding problems within a watershed.

Downstream analysis can be conducted using the 10% rule. That is, the analysis should extend from the point of discharge downstream to the point on the stream where the site represents 10% of the total drainage area. For example, the analysis points for a 10-acre would include points on the stream from the points of discharge to the nearest downstream point with a drainage area of 100 acres.

A separate draft document clarifies the definition of the site area:

Downstream analysis applies to the contributing areas downstream from the site to the point where the site’s contributing area (including off-site drainage) represents 10% of the total drainage area.