Draining/limiting depth of lagoon to prevent flooding

[lindsaylu]

I'm hoping to find some advice for a friend on how to proceed with an infrastructure project in Guatamala. He is volunteering for a group that is hoping to limit seasonal flooding of a village on the coast of Guatamala.

The situation involves 2 lagoons. One lagoon drains into the ocean, the other cannot drain directly into the ocean and floods a village on its way along the coast to the point of entry into the ocean.

The lagoons are only a couple of kms from each other and this organisation (with the village itself) would like to make a trench that would drain water over a certain elevation into the bigger lagoon where flooding is not an issue.

What I am looking for is a starting point for this group. If you have experience in this field, what sort of information do they need, where should they start. They're hoping to get a CIDA grant (Canadian international development agency) and need to put a proposal together for them.

In your experience, is this project a legitimate possibility? I've added a link to the location where they'd like to this. It's the smaller lagoon that doesn't drain directly so they'd make a trench from that lagoon to the NW into the bigger one.

We'd love some opinions and a direction to move in if anyone has any experience with this sort of thing.

Thanks,
Lindsay

 

[cvg]

Is the grant for construction or for feasibility study? You need to determine feasibility which will take a significant amount of engineering work. Start with a topographic survey (or at least a profile along a preferred alignment) and also review the geology along the alignment. Hydrology study is needed to determine the size of the channel / spillway. You should also do some basic sediment transport / scour analysis to determine if the channel is stable or will erode. Estimate the excavation volume and any erosion protection that is needed. Then a cost estimate for construction. Also, don't forget to research environmental compliance issues. After that you could apply for funding to build it.

just looking at the Google map, I would say that it might be a challenge to do this.

 

[beej67]

Why aren't they considering draining it to the ocean?


My first impressions are that your approach is not the one I would use at all. Typically in coastal areas the water surface of detached lagoons localizes to coincide with the groundwater table, and typically the groundwater table is very low, like a few feet above sea level. Without survey information indicating otherwise, I highly suspect that the lagoon is only a foot or two above sea level.

If the lagoon is fresh water, then digging a ditch to the ocean to drain it will make it tidal, and brackish, which might not be ideal. I would think what you'd want to do is grade an overflow ditch from the lagoon to the ocean, slightly above peak high tide level, and let the lagoon spill down that to the ocean during storms.

You need some regional topo, or a couple of days site visit, to determine what the best engineering approach would be.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[lindsaylu]

Thanks for the help.

The grant would be for the construction, not the study...

Also, apparently the locals have tried in the past to make a channel directly into the ocean and it has not been a success. I will certainly give you that this might not have been an engineered approach. From what I understand, the channel silted in almost immediately.

 

[cvg]

the channel silted in because the lagoon is too low and there is no hydraulic gradient between the lagoon and the normal mean sea level. this is the same problem I see with draining to the other lagoon, only perhaps worse since it is farther away and higher than sea level.

 

[bimr]

Google maps shows an water elevation of 0 in the bigger lagoon (more or less sea level) and a water elevation of 10 ft in the smaller lagoon.

The lagoons are about 0.6 km apart with approximately 20-30 feet of soil depth to be removed if you want a channel.

 

[beej67]

What's the metadata? How does Google Maps know the elevations of lakes in Guatemala? I ask because I don't know, and because I highly doubt that the small lagoon is truly ten feet above mean sea level, particularly if previous attempts to ditch it to the sea failed.

Step one is get some survey of the lake and of the topo between the lake and A) the ocean, B) the big lagoon. You can't know what you're designing until you get that. My suggestion is to phrase your CIDA grant in three phases:

1) Feasibility Study (includes survey)
2) Design
3) Construction

If infeasible, then you don't do phase 2 or 3. If feasible, then which design you pick will affect construction costs, so include pricing alternates for Phase 3 in your grant application.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[lindsaylu]

Thanks to everyone for your input. It has been greatly appreciated and very useful!

All the best,

Lindsay

 

[bimr]

Actually, I was referring to google earth.

Google Earth uses digital elevation model (DEM) data collected by NASA's Shuttle Radar Topography Mission (SRTM). This means one can view the whole earth in three dimensions. Since November 2006, the 3D views of many mountains, including Mount Everest, have been improved by the use of supplementary DEM data to fill the gaps in SRTM coverage.

I don't know the accuracy of the topographical elevations, but the elevation in the street in front of my house is accurate. I suspect that the elevations are probably accurate within a few feet.

I would not expect somebody to design off the google elevations, but it should provide you with a preliminary idea of the scope of the problem.

 

[boo1]

how do you see the El?

 

[aepaul]

You can see the elevation just by hovering your mouse pointer over the point that you're curious about. It's displayed at the bottom of the Google Earth screen just to the right of the latitude and longitude.

Adam
Cramer Northwest, Inc
Kent, WA

 

[cvg]

by hovering in google earth pro I get anywhere from 0 - 25 feet elevation on the large lagoon. I get between 10 - 20 feet for the small one. I seriously doubt the accuracy of the elevation data on this particular google map.

 

[Phidelt218]

google earth is a tool but not very accurate, you dont know the distance between elevation shots.....I tried to export google elevations into Civil 3D and the the surface was way off. I had a steep grade and it was not shown on google earth because it had an elevation shot at the top of hill and then in ocean.....so ocean had an elevation above 0

Just be careful with assumptions from google earth