type A soils 3

[Surcharged]

I am using the SCS method and am required to detain the 2 year thru the 50 year storm events to pre development levels. The site that I am designing is composed of very well drained soils that are in Hydrologic Group A. The site is a small residential subdivision (one street with a cul-de-sac). Using the SCS method yields pre development numbers that are less than one cfs for the 2 year storm making it nearly impossible to detain the post. If I had any bypass flow to account for I would really be in trouble. Have any of you dealt with this problem before?

 

[RWF7437]

Don't understand what you perceive to be the problem. You are being asked to detain the difference in the VOLUME of runoff between pre- and post-developed conditions. You did NOT say what storm duration you are required to use. If you are using the NRCS ( used to be called SCS ) method you are probably using a 24 hour storm and a " standard" rainfall distribution. The peak flow is of secondary importance and only controls the size of the outlet device. It does not directly control the volume of detention required. The old TR-55 has a useful graph, Figure 6-1, for estimating required storage volume. It is available on line.

Also, oddly, many reviewing agencies do NOT require you to route the design storm, or storms, through your detention basin. Doing that routing will give you a lot of clues about how your design will likely work.

Finally, doing only whatever the reviewing agency requires will not allow you to estimate confidence levels for your design. A good question to ask your self would be; " How probable is it that my pond will overflow, or otherwise fail over the life of the pond ?"

good luck

 

[RWF7437]

Also see:

hydroCad.com
Intellisolve.com
pdhonline.org Courses H102 and H127

good luck

 

[psmart]

With low curve numbers, it's not uncommon for the 2-year pre-development flow to be very low - or even zero. If zero, you could meet the no-increase requirement by providing enough storage to retain the entire volume of the 2-year post-development storm, and letting the pond drain over an extended period of time.

http://www.hydrocad.net

(not .com)

 

[Surcharged]

The problem: I must limit the post development flow to pre development flows. In order to do this I must use very small diamter orifices which are difficult to prevent clogging. I can not retain although this is a good idea because the bottom of our ponds are usually full of water due to groundwater infiltration and therfore the pond is always assummed full. I guess that I am stuck with making extremely large ponds so that the head never gets too high on the orifices.

 

[RWF7437]

" I must limit the post development flow to pre development flows"

I believe Psmart is right. This is not a problem. You can simply retain this volume, which will probably be very small. The release rate does NOT have to exactly match the pre development rate, even if that were possible. It only has to be less than the estimated predevelopemnt rate. That estimate is, at best, only "correct" within plus or minus 30%. It is more likely it is plus or minus 50% to 100%.

" In order to do this I must use very small diamter orifices which are difficult to prevent clogging."

This is true, which is why most reviewing agencies adopt a minimum size orifice. In our area of the country they use 2-inches as a minimum diameter, which is also too small to prevent clogging but that's their "rule" and if you want your project approved you go by their RULE.

"I can not retain although this is a good idea because the bottom of our ponds are usually full of water due to groundwater infiltration and therfore the pond is always assummed full."

If this is a true statement and the ponds are really "always full" then you have an impossible task. Unless you can install pumps, or line the pond to prevent groundwater from entering or can find some other very expensive way around this. It is hard to believe that you are being required to detain runoff in such an area. Can you locate the pond elsewhere ? Can you employ underground storage pipes or chambers ? Can you seek an exemption based on this set of facts? Can your periodically dewater the pond, preferably without using pumps ? Your other problems are trivial compared to this one !

good luck

 

[Surcharged]

I am sorry for the confusion. The ponds are wet ponds due to groundwater. The pond is empty from the orifice up to the top of the bank. The portion of the pond from the orifice down to the bottom of the pond is considered full. Therefore I can't use any storage below the orifice. Once water enters the pond water begins to leave through the orifice.

 

[RWF7437]

You must still answer, for yourself, not me, all the previous questions PLUS a few more:

Can you predict the level of the groundwater accurately ?
Does it vary seasonally ?
If so, within what reasonable range ?
Can you base your design on this estimate and if so, how confident can you be in that design ?

Your original post was entitled "Type A soils". Such soils are usually considered an advantage in detention or retention design and do not represent you main problem here.

Please ask yourself the other questions and visit the links provided. They will help to identify the main issues you are facing.

good luck

 

[Surcharged]

We are on the coast and to be safe you say the water table is at the orifice (it usually is). I dont know much about retention but am guessing that it is the volume between the outfall elevation and the bottom of the pond. Water enters and either infiltrates into the soil or evaporates? We just cant do that here.

I have found that Type A soils generally yield larger detention facilities due to the larger increase in curve number between the pre and post developed conditions. For example, consider a wooded site that will be converted to 1/2 acres lots.

Type A Woods - Curve Number 30
Type A 1/2 acre SD - 54
80% increase in curve number

Type C Woods - 70
Type C 1/2 SD - 80
14% increase in curve number

Let me know if I am missing something but I find Type A soils to be a disadvantage in terms of minimizing detention facilities.

 

[RWF7437]

First, what coast of what country ? This website hosts engineers from all over the world.

Second, retention is just detention with a very slow rate of discharge. Retention is used extensively in the southwest US where many of the soils are Hydrologic Group A, B and sometimes C and drain well and "fairly rapidly" if the water table is not near the surface. I work in Oregon where we're not so fortunate and sometimes even have to deal with Group D soils and high antecedent moisture conditions. Such soils permit very little infiltration, high rates of runoff and near very abstraction. Thus, most of the rainfall on such "D" soils runs off leading to both high rates of runoff ( high flows) and high volumes of runoff ( leading to larger detention volumes required ).

Third, the ratio between Curve Numbers for before and after conditions does not automatically mean a higher detention volume is required. That depends in part on the requirements of your local reviewing agency, and mostly on the amount of impervious area in the developed condition. In the developed condition the CN depends much more on ground cover ( pavement, compaction of natural soils, plant cover, roof areas, directly connected storm drainage, etc.). When developed, the soil type or Group has much less effect on the runoff.

Have you answered those questions yet ?

good luck

 

[RWF7437]

From the HydroCad help file:

"The curve number, or CN, is determined according to the soil type and ground cover. A high CN (such as 98 for pavement) indicates minimum retention [ at the point where the rain falls], while a low CN (such as 30 for certain wooded areas) indicates a large retention [at the point where the rain falls] capability."

phrases in brackets [......] added by rwf7437

 

[TerryScan]

Surcharged:

Yes, one is having a greater impact when developing land which performs best in its pre development state. Can't get much better than wooded A soils.

This is where a conservation approach to development is truly valuable. Preserving as much woodland and avoiding development of A soils areas is very desirable.

Be grateful you aren't dealing with further hurdles. At a stormwater conference, a presentation was given on research which supports modeling all A soils in pre development as D soils after any of it has been compacted by the grading process (which would have bumped your first example from CN 54 up to CN 85).

 

[Surcharged]

RWF7437
I still diagree with you (I think developing Type A soils require larger ponds due to the greater increase in curve number...in our municipalities anyway) but you did bring up some good points and questions. I am going to sit back now and see how the other posters weigh in on the debate. Thanks.

 

[TerryScan]

Surcharged,

Just curious, where are you located? (Your scenario is fairly typical for me - coastal Delaware).

 

[DarthSoilsGuy]

I would start off from scratch and make sure the 30 number represents what you've got. Did you survey it personally? A woody brush mix is just as bad to survey and can be "woody as F$*@!", especially if it has briar patches.

If there isn't enough space to make infiltration basins w/ your type A soils b/c of the water table, i wonder if wetlands are pushing you around and if you'll be able to do on-site septic.

maybe, a teardrop cul-de-sac instead of the the large paved gourd will give you xtra space for an infiltration basin and cut down on some imp.

You can also try running your post based on a composite of real numbers instead of that 54. I just had the same type problem. b/n zoning, wetlands, and layout i had a total 14% disturbed area for the site. Instead of using the generic composite number for residential 2-acre lots, i counted everything outside of the limits of construction as orig CN. it's more egr work, but if you have some idea of bldg size and the drives, why not.

Finally, the post match or less than pre should not be a mantra. if you can keep your 10-yr less than 1 cfs total and specify good erosion control practices in your permitting along with good structures and prevention materials, you should have a good argument to present.

If nothing else works, look into logging permits. Ha!

 

[Surcharged]

TerryScan
I am in coastal ga and it is flat and wet. A lot of times we use interconnected ponds that are difficult to model.

The infiltration basins may work. I know of a subdivision where they had good Type A soils and they ran perforated pipe perpendicular to the road between the lot lines. The lines dont even have an outfall - so far so good but it hasnt been put to the test yet. My site is Type A for most of the site but then drops off fast to the wetlands.

 

[TerryScan]

Surcharged,

Very similar here. The first project I worked on was 150 acres of wooded A soils with 1 ft. of fall in 1,000 ft. Since then I have done quite a few underground facilities and a number of infiltration facilites.

Regulators are pretty wary of infiltration due to a tendancy for them to fail over time...or due to less than perfect construction practices. I go to great lengths on infiltration jobs to be clear in how the facility is to be constructed (avoiding compaction and sediment clogging during construction).

Underground storage is typically an expensive option that is only viable in valuable commercial applications. I am a bit surprised it was worthwile in a residential setting.

 

[bltseattle]

Surcharged,
You might consider ways to reduce your developed site effective-imperviousness. Because you are in Type A soils, you should be able to disperse runoff from rooftops, sidewalks, etc onto adjacent pervious areas where it can soak into the ground. The TR-55 provides calculation methods that reflect this kind of runoff dispersion (you can reduce your CN for the imperv). There are Low-Impact-Development manuals and guidelines from all over the US that also discuss in detail how to implement this practice (NC State has excellent info and is in your region).

Your soils may also be well-suited to use permeable pavement (consider Percocrete for sidewalks, for example).

Not only will these practices lead to a more natural drainage regime (more infiltration, less runoff), but should lead to a smaller r/d facility as well, and could reduce your conveyance needs. On the west coast, I believe that developments with these low-impact practices generate top-dollar when houses are sold.

Example residential applications:

http://www.coastal.ca.gov/nps/lid-factsheet.pdf

To tell you the truth, I am envious of those Type A soils - you have a lot of options to reduce your runoff (compared to sites on glacial till soils that are required to match forested runoff rates and volumes).