Groundwater rise due to development

[REB1955]

In a review of geotechnical recommendations the state geologic society has requested that I model slope stability with an increased groundwater level due to development. Is there any guidlines for estimating the impact of development on the groundwater level.

 

[LCruiser]

I was under the impression that groundwater fell due to increased runoff over a development. What makes them think it might rise?

 

[oldestguy]

Sounds screwy to figure a rise.

Pumping from municipal wells and increased use of paving and roofs sure doesn't fit with a rise of ground water. It drops.

I have seen some effects like this where the development changed the surface water flow, but only in rare cases. For instance a side hill with new roads and the ditches carrying the surface water laterally. Even then, ground water was not raised significantly.

 

[LCruiser]

Are they trying to find a way to prevent a development from happening? If so you could ensure the ground water infiltration wouldn't increase. Is there a lake in the design? Line it with clay...

 

[epongra2]

Ok, I'll play the role of the devil's advocate here.

What about water perching within the compacted fill placed during mass grading? I see this all the time. Combine this with the amount of landscaping water used here in Southern California and we see elevated water issues all the time. The piezometers in the fill will have perched water, while piezometers placed in natural materials twenty or thirty feet away will be completely dry.

Depending on which study you read and/or believe, here in Southern California the amount of landscape watering done in certain areas over the course of the year is equivalent to one to two seasons of average rainfall!

 

[dgillette]

Any septic tanks or leaky swimming pools or stormwater-detention facilities that might be constructed upslope?

Eastern Washington has a lot of loess bluffs that are stable until heavy irrigation occurs near the top. (If you ever fly into Pasco WA (Hanford), you can see the center-pivot irrigation and the slides along the Columbia.) epongra2's concern about the lawn watering may be right on.

 

[TDAA]

Epongra2 has a point. Here in CO, the ground water will typically rise as people add 30 inches of irrigation to the 16 inches of typical rainfall. To assume pumping of a municipal well is going to decrease the water level is incorrect. Colorado Springs gets its water from surficial sources. Most of the water is from the other side of the continental divide as well, so diverting it does little to groundwater here.

Based on our climatic conditions, we assume a rise of 3-5 feet in the wetter seasons and irrigation. This can be very site specific though, based on subsurface conditions and topography.

 

[BigH]

Sort of off the main topic but what is the "state geological society" doing in this scenario? Is it in fact the state government whose geological/geotechnical section has asked for this - and why are they involved if it isn't state land or state project? A geological society sort of implies, to me, like the Canadian Geotechical Society, etc. - they have no 'power' to recommend etc. - at least as far as I can see. In other words, are they putting their nose into something that isn't their business.
As far as groundwater rise or fall, that, really, is a matter of the stratigraphy that you have at site. Normally, development means a lot of paved over space - hence the probability that the groudwater would fall, not rise. Points are well taken that in some conditions as with irrigation and watering for landscaping, this might cause a rise - but it seems to me that for the water to rise in slopes - does that mean that your development is at the top of slopes (as in Washington - Seattle or Vancouver area)??
Anyway the points by others are well considered - I am just interested in how a "geological society" has become involved.

 

[TDAA]

In Colorado, they tend to review preliminary geotechnical information, and geologic hazards. I am not sure when they are required to do that though, since others in my office do most of the prelims, and it is the developers responsibility to submit. Their review is similar to a building department review, but they take these since they are more knowledgeable in the geotechnical / geological points of view.

 

[BigH]

TDAA - who pays them to review? Is it really a "learned" society or ?

 

[TDAA]

Sorry, what I was refering to here is the Colorado Geological Survey. They are a state agency, paid by me, the taxpayer. I am not sure if they charge developers aditional fees to review documents. Below is a link to their web site:

http://geosurvey.state.co.us/