Assessment of land with a slight slope

Hello everyone,

we are currently looking at a plot of land and still have some concerns about
the slope present here. It concerns plots No. 5 and 6 – we would take 5 completely
and 6 approximately up to the level of the turning circle. So we can still combine parts of the plots here. That would then be
a total of 1100 sqm. Behind the plot, lying even lower, is the RRB on the right and oaks are to be planted there.
The heights are also indicated here.

Looking at the heights, I currently have some worries and am not quite sure
if this is the right plot for our building project.

There should be about 200 sqm of living space in a staggered storey with a double garage next to it. So we will have to compensate for about 1.5m height difference along
the length of the building.

The groundwater level is approximately at -1.2 m. The entire lower area behind the plot was, this winter when there was a lot of rain here in S-H,
under water for quite a long time. Whether it reached the plot, I cannot assess unfortunately. I was not on site at that
time.

I now see 3 acute problem areas from my point of view:

1) The water coming "from above" from the terrain, i.e., from plots 3 and 4 and even higher up – which are at least approximately 2 m above us.
A friend of mine says that a jet deflector in the form of a small wall at the plot boundary would be sufficient from his point of view –
a 40-50 cm high small wall would then deflect flash floods during extreme heavy rain events
around the plot and the house. Is that sufficient from your perspective or is that too naive?

2) In the same village this year, a house without a cellar apparently became wet in some interior rooms due to the high groundwater,
because the foundation stood in the groundwater and the water was drawn up and got into the house through the floor. So far,
I only knew such things from cellars. The architect mentioned a drainage that could then direct the water into the lower-lying areas,
but specifically the question here is, what if the lower area can no longer absorb anything? Then drainage would not help anymore, right?
Alternatively, the foundations or the floor slab could also be made with waterproof concrete (WU concrete), but that would significantly increase the costs.

I am a layman, sorry for the question – how would you handle this issue given the heights and the location?

and last but not least

3) Is it possible to exclude that with progressing precipitation amounts parts of the plot could actually remain permanently under
water? I have images in my mind where the paddocks along the rivers here get flooded and remain underwater for weeks.
That is, of course, not nice either. Rationally, I tell myself that the entire area down there is "huge" and lies about 2 m lower than our plot,
so there is actually enough space down there, and if those 2m were completely underwater, that would be billions of liters.

A soil survey specifically for the plot is still to be done, but one has already been done by the road builder, which I will also attach, the soil condition does not seem very problematic according to the architect.

How would you deal with the location of such a plot, do you see any further problems that I haven’t even considered yet?

Am I too cautious? I have a cellar that was equipped with a black tank in 1998; it has never really caused problems, but it
basically stands in the groundwater and I have to pump water away from a sump all day because no one told us back then
that there are also white tanks. These are mistakes that simply cannot be undone, and I naturally want to prevent something similar with the last house I will build in my life.

Looking forward to suggestions, assessments and opinions.

Regards

P.S.: I know that in contrast to other federal states this is of course not an extreme slope, but probably just a plot with relatively steep terrain in interpretation.
But in view of the climatic changes, I am already thinking a lot about it today.


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You probably mean that you would take 6 and the 5 up to the turning circle height...

What really happens during heavy rain, where the groundwater stands in January after a very rainy year... you can't deduce that just from the pure elevation data. It may look like that at borehole 4, but 5 meters away it can already be different again. Therefore, water can also collect completely arbitrarily in one place but drain well in another. If drilling was done in summer, you might also have completely different groundwater levels than in spring or autumn.

After the very rainy last year, the groundwater here was just below the top of the floor slab at the beginning of January. Now in May it stands about a meter lower. We still have a KG pipe from the construction sticking out in the terrace where we can observe it well. What you describe, that the field behind the property was under water for a long time, certainly suggests that drainage is generally poor in that area.

And it definitely won't get better if now 30 or 40% of the area is sealed with roads, floor slabs, paths, etc.

If you can save yourself the basement, then save it.

Hi,

first of all, thank you for your feedback.

I am a bit confused, but it's probably because I formulated it incorrectly.

I have no interest in building a basement and that was not the question. If I buy the property, it is more about assessing the risks due to the hillside location for the entire house without a basement regarding the amount of water coming from "above" (sky + street + properties that are higher up). That amount is increasing, and I don't see any proper place down there where the water could stay if it rains for several weeks, as happened this year.

I actually went there after a cloudburst, and of course now there is water on the field (there is no road / development yet), which is not surprising. But the lower left corner of "our" property is completely underwater. Now – after a cloudburst – that would be the garden area, i.e., ornamental lawn, etc. Clearly – there is no sewage system yet, etc. But in principle, the amount of rain that would fall on the lawn would ultimately rather increase by what would then come from the sealed surfaces, right? If the water can already not "infiltrate" there now, it won’t be able to do so once the sewage system etc. is built; that doesn't affect it, or am I wrong about that?

Best regards

Hello,

for the reduction of surface water, it will surely be crucial how large the sewer system / gullies are planned to be / how much the connection to the main lines will be utilized.
The roundabout is apparently also planned as a "collection point," the paved gutters should help with the transport of the water.
Are there any specifications regarding the rainwater (infiltration or discharge)?

The 50-60cm wall should also be compatible with the garage; in case of doubt, the water will simply find its way.
It is also hard to say how much the "Koppel sink" will fill up when the sealing / directed discharge across the street increases.

But the plan already looks as if this Koppel is supposed to be the planned "flood area." Plot 6 would then basically be a part of it.

Maybe it will work differently, and the expansion of the sewer system at the left end of "Street B" (gully) alleviates the normal case situation of the Koppel, since it can discharge there into the sewer. Then your part of the property would potentially no longer stand in water, unless there are really strong, persistent rains that overload the sewer system in general.

The best thing would be – if possible – to discuss these concerns with the development company.

I can tell you from my own experience in a residential area with poor infiltration that the sewer system is of little use. It only drains the streets, i.e. the sealed surface. An exception would be if you are allowed to discharge your rainwater, but that is not exactly cheap either. Infiltration problems are usually worsened rather than improved by the sealing and compaction of the soil, as is common in residential areas.

Thank you for the feedback, I thought so and my gut feeling did not deceive me.

We will consider how to proceed with it now.

Your assessments definitely help us move forward.