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Pumping Stormwater??? 1

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nutbutter

Civil/Environmental
Joined
Sep 1, 2005
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US
Hello,
I'm a land development engineer and I was wondering if anyone had any advice/ideas about a site I may be working on.

The site is in a small city and I'm not exactly certain of the size at this point (my info at this point is very limited). The site is currently farmland and it sits several feet below the adjacent highway. I don't think that there is any drainage path for this parcel of farmland. All rain that falls on the site must either infiltrate or evaporate since the site is basically a large basin.

The developer has an adjacent site that sits above grade where he could get most of his fill material, however, it's been determined that the required amount of fill is about 1,000,000 cu yd.

Considering that staggering amount of earthwork, I am trying to think outside of the box on this one. Would it be feasible to develop the site without filling it and try to pump the stormwater somehow? I have no experience with pumping stormwater and my initial thoughts are that it wouldn't be feasible, however, if it is feasible in the slightest then I would like to investigate it further to avoid the earthwork.

Sorry I don't have more information at this point. The developer is related to me and just mentioned something. I'm just going on memory and I don't have much information. I would guess that the site is probably 40-80 acres. The site generally lies 6-10' below the adjacent highway and developments. Also, I'm not exactly certain where the stormwater could be pumped.

Any thoughts would be appreciated.

Peace,
Stoddardvilla
 
You could size a basin to detain the entire design storm volume, and then pump it out at the allowable rate. Instead of an outlet structure with orificia and weirs and such, you would just have a pump. Of course, what would the allowable post rate be if the pre-development rate is zero?!
 
Thanks for your help. That's kind of a long the lines of what I was thinking.

In KC metro area we have an allowable release rate of 1.3 cfs/developed acre in some municipalities.

This job is in rural Iowa though in a small city/large town. I don't think the city engineer there has much experience.

I guess without more details there's really no way to determine if it would be more cost effective than the earthwork. Operation and Maintenance of the pumps would be a continuing cost, but I don't really have an idea what O&M costs for a couple of pumps would be per year.

 
I am involved with a similar project where pumping was the only feasible option. The gravity discharge option was 42-inch pipe and DEEP excavation. A pair of 8-inch pumps (Flygt CS-3201) discharge into an 18-inch discharge pipe, flow about 5,000 GPM. We took the approach outlined by Proletariat, and sized the system to meet the required discharge rate that would meet the freeboard requirement for the pond berm. Luckily for us we have an emergency spillway route that goes by gravity - sounds like you may not. We set the rim of the wet well at the elevation corresponding to the required storage volume if it were a traditional discharge riser structure, and gave it a dewatering hole at the proper elevation for the dry storage (this is in Virginia, by the way).

The pumps are expensive, around $20k each, just for the pumps. Then there are controls, a wet well, bringing 3-phase power in, emergency power generator, etc. You can take a SWAG at your power consumption by figuring the HP needed to move X amount of water against a head of H feet, and figure 60-70% efficiency of the pump & motor.

Hope this helps.
 
Pumping should be a last resort. Can a pipe be open cut or bored under the Highway? In any case, you can only discharge your stormwater where it was going hisorically and "in an quantity and manner that does no more harm than formerly," or similar legalspeak according to Iowa drainage law. Otherwise you'll have to obtain an offsite drainage easement.
 
A company named park Environmental makes pumps that can pump storm water as well as sewer water. I'm working on a job right now in Lockhart Texas that is extremely flat and I'm having to pump our a large truck dock area.
 
I agree with ccor that pumping should be your last resort. if you do install a pump station, you will still have a discharge line which will require a suitable outfall. it will also likely require an easement which can be quite expensive these days and could take a while to acquire. you also need to consider the added risk you will be taking (think Katrina) If the pumps fail for some reason, you could suffer considerable flood damage. Especially if you have multiple storms which fill up the basin and then cause flooding during the subsequent storm.
 
I am probably stating the obvious, but you have to consider how to best ensure a reliable power supply for the pumps, because in many regions the largest storms - yielding the most stormwater to pump- coincide with extremely high winds and power outages that could affect your pumping station.
 
I guess of course quite macro examples of situations perhaps arguably some similar to this are Holland and of course areas of New Orleans etc.(and of course the systems in general when properly designed "work", at least it appears barring catastrophic events).
 
The initial cost (piping, pump station, easements, back-up power) is not all there is to consider. Who will be paying for the operaion and maintenance, power costs, and the eventual replacement of the equipment? The City or County may not be willing to take over this site, which will leave it to the owner or to an association.
 
Maury has the idea.

I do various municipal engineering duties, and I would never recommend approval of this plan - and since it is a stormwater issue, the Planning Commission wouldn't if I didn't - unless there were an iron-clad O&M Agreement signed, notarized and Recorded as a Deed Restriction. The signer(s) would have to be on the hook for perpetual O&M of the pumping system. The implications to the market value of the developed lot may (or may not) be more restrictive than the first costs of design and installation.

I'd map that out with the client, while (not after) discussing the first costs.

Women and cats will do as they please; men and dogs should just learn to live with that - Steve
 
Thanks for all of the good advice. As I mentioned earlier, O&M costs were a concern.

I've spoken with the developer and he told me that the city has already informed him that he will have to fill the site. They would NOT approve it otherwise, so, Iha you hit the nail on the head.

The site is 118 acres and it's going to require roughly 1,000,000 cu. yd. to fil. It's really a horrible site, however, the developer also owns a large parcel of land across the street where he can get much of his fill. I'm not sure on the exact quantities he'll be able to pull because I don't have that site topo'd. The town may allow us to run graders right across the street and not even bother with hauling the fill across the street. The town is pretty excited about this development because they're getting a casino nearby and this will be great commercial land right out on the edge of town.

Thanks for all of your help.

Peace,
Stoddard
 
Search for a lower elevation drainageway within a reasonable radius of your site,(10,000 ft or less). Dig part of your site deeper for borrow material and to find permeable layer for aquifer recharge. Do a evaporation study to see if you can store peaks through the droughts.
 
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