Diverting Stormwater Sheet Flow

[MCD917]

I have a property going through engineering at this time and we've hit a bit of a dilemma. To preface my problem, I'd first like to describe the property and situation as best I can. The property is rectangular in shape (with the narrow ends being the front/back of the property and the long ends being the side) and increases in elevation from Front to Back. Three buildings are proposed on the site. The first two buildings (beginning at the front of the property) are rectangular shaped (170’x70’), sitting one in front of the other, 30' apart with a cartway in between, and have the long sides facing toward the front/back of the property. The third building is similarly shaped (170’x80’), 30’ apart from the second building (cartway in between), and is proposed to be built at a later date. All buildings have 30’ cartways on every side of the building and all buildings are proposed as over-under buildings. Over-under as in each building is two stories and you can access the lower level on one side and the upper level on the other side of the building. So, preliminarily, two buildings will be built with a large parking lot being in place of the third building until it is time to construct the third building. We also have a curb and gutter stormwater system proposed. All stormwater will sheet flow away from the buildings and out to the curbs for collection. With that being said, the large parking lot (which is in place of the third building until it gets built) will have stormwater sheet flow across the 6-8% grade and flood out the second building. Once the third building is in place, we won’t have that problem. But in the meantime, we need to construct some type of barrier which diverts the stormwater out to the side cart-ways and to the curb gutters. We could place an 8” high curb there (constructed to be the length of the third building and built in place where the outside wall will be of the third building closest to the second building) which will divert stormwater around to the cartways, but that is too costly for our means.

We’d like to know the most cost effective way to divert the stormwater so that it does not flood out the second building. The water will be sheet flowing across that parking lot with some velocity, so the barrier has to be tall enough to stop and divert the water. Also, this will be a temporary solution until the third building is constructed, so whatever is built to divert the stormwater will be removed when the third building is built. Any ideas?

Let me know if you need a better description. Thank you.

 

[francesca]

At 6-8% cross grade, it won't be sheet flow! You'll have a shallow concentrated flow problem at best, and if your parking lot isn't graded, you can expect some erosion that you'll need to deal with.

 

[MCD917]

The parking lot will be paved.

 

[TDAA]

Create a soil diversion berm, and protect it. Perhaps some fabric and a rip-rap or stone depending on the velocities you see.

 

[francesca]

A paved parking lot is all the more reason you won't have sheet flow. You're going to get some pretty large velocities, so make sure your berm is high enough to contain it. I'd put curbs & gutters in instead to collect the water, especially if you can locate them where they'll be used in future once the third building is constructed.

Since you can afford for water to back up in your parking lot, you should be fine with an 18" pipe and for the cost of that, I'd say it's worth it to know you're not going to flood that second building.

 

[blueoak]

You might try making a burrito out of black plastic and sand. Use concrete blocks to hold it shut if it is short term. Like TDAA mentioned, if it is paved. With high velocities a Jersey barrier every once in a while should hold it.

Longer term I might price a curb using low strength concrete (about half the cost and easy to tear out than standard concrete).

I would check out the erosion control magazines for stormwater. They have a lot of ideas that you can reproduce cheaper if need be. Also Rain for Rent and some companies rent the materials.

 

[cvg]

If I understand correctly, your parking lot is only 0.3 acres? Regardless of the steepness, that is quite small and will not generate a lot of flow. For instance, I calculated using 10 inch/hour rainfall intensity and only got about 2.5 cfs. Smaller storm would result in less runoff. With such a small area to runoff, I can't imagine that the depth of sheet flow will be great, so that the velocity of flow should not be large. In addition, with such a shallow depth of flow, momentum should not be a big issue.

To minimize the drainage issues, can you grade the lot so half flows to one side and half to the other? Or can you put a cross slope on the lot so that it flows toward the gutter system? How about grading / paving a swale at the end of the lot to convey the water?

I'm not sure what you can do to divert this water that is much cheaper than a curb, unless you really want to go cheap. You could try sand bags. Or just dump a small riprap berm (which could be salvaged and reused somewhere else later) Or maybe wood railroad ties, anchored down - which could also prevent cars from driving off the lot.

 

[MCD917]

cvg: The parking lot will actually be more around .5 acres. We cannot cross grade the lot or change the grade at all for that matter. We have to keep the grade the same for the building to come. When it is time to build the third building, we will be ripping up the parking lot. We have entertained the thought of a swale at the end of the parking lot, but this parking lot will be generating revenue, so therefore we'd like to keep it as large as possible. We're afraid that a swale would be too large and take up too much room. Sand bags and/or riprap would only slow the flow of water, correct? We also entertained railroad ties, but they may warp and rot over time and render themselves useless. We propose to not build the third building for at least a year and a half.

Maybe we'll look into low strength concrete as blueoak mentioned.

 

[proletariat]

You could make a curb out of asphalt. Cheap at less than $100/ton and easy to tear out. It will probably be ugly though.

 

[MCD917]

proletariat: I failed to mention that asphalt is the way we wanted to go. My only question with asphalt is how steep can we lay asphalt without sacrificing integrity? We don't want an asphalt ramp for the water to shoot over, but if the ramp is 8" high, we probably won't have that problem, right?

 

[proletariat]

I've seen old school asphalt curb at about 0.5:1 that lasted a loooong time. Of course, nobody was running over it either. I have heard that Lowes home improvement warehouse uses asphalt curbing wherever they can to save money, so there is a legitimate example of its use.

 

[proletariat]

This is going to sound ghetto, but I would hammer some 20 penny nails down into the asphalt parking lot to give your asphalt curb something to grab onto, to prevent sliding. Then you can either cut them off with the angle grinder, or pull them out, or hammer them in when you remove the curb.

 

[cvg]

I think you should not dismiss railroad ties so quickly. A year and a half is not a long time and you could get pressure treated timbers that would last 10 years or more.

riprap if placed properly will divert most of the flow.

As far as cross grades, you will need to do something in order to move the water to one side of the lot or the other. With 1% grade, I calculated flow depth against the curb of about 3 - 4 inches and about 5 feet wide. This wouldn't interfere with parking at all.

I would not count on low strength concrete being half the cost of higher strength. But an extruded asphalt curb might be cheaper.

 

[blueoak]

Low strength concrete I am used to is 1 sack versus 4 to 6 sack so you only have the same batch and transport costs.

I think proletariat is right and asphalt is the way to go. I always forget flexible paving because I lack the expertise to do it well.

For the asphalt, could you just drop it without compaction or would that be too weak? I have seen really nice vertical curbs with asphalt.

 

[MCD917]

Does anyone know a universal price for a railroad tie or where I could look one up. I know I could try the local lumber yard, but if there's a national source for general pricing, that would be good.

 

[LHA]

Pressure treated lumber is phenominally expensive, in northeastern USA.

For 5 cfs, I can't see how a seeded earthen berm with the cheapest geosynthetic reinforcement will not suffice. But, even if not, asphalt has got to be cheaper than PT lumber.

Engineering is the practice of the art of science - Steve

 

[nutbutter]

A 0.5 acre lot is very, very small and wouldn't generate much runoff at all. We put in asphalt curb all the time in this same situation - partial development. That'd be your easiest way to go I'd imagine. You can even drive some short pieces of rebar into the existing lot to secure the temporary curb.

Where would this water go if you didn't put any barrier?

 

[MCD917]

The water would run toward the second building. However, there will be a slight inverted grade between the second building and the proposed third building, but depending on the amount of water, we don't want the bulk of the water flooding out the second building.

 

[JasonG]

Have you considered a trench drain?

 

[MCD917]

Yes I have considered a trench drain and that idea would work well, but I'm trying to keep costs down.