how do I build a 7 to 9 inch high water dam? In wet weather, water gets into my house. I need to create a berm of earth that will be grass covered, and it should also have a water barrier to keep the soil from washing away. How can I build such a dam? It will be about 40 feet long.
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The answer to your question is to not build a dam, but instead prevent this scenario from occurring. Probably the biggest issue I can think of with a dam is water on the wrong side of it - you've essentially built a mini-pool against your foundation and this will just make the problem worse.
The ground surface around your house should be sloped away from the foundation. If this is not the case, you can build as large of a dam as you want and water will eventually find its way in. If you go look at a new development, you will see that all of the lots slope towards each other and in the middle there is low point that serves as a drainage path that will lead towards the street in order to direct rainwater away.
There are a couple things you can do here. The first and probably most important is to make sure that your gutters are free of debris and properly collect water from your roof. When it rains, is there a sheet of water pouring down from your roof? This is a sure sign that your gutters are blocked. Assuming your gutters are not blocked, you need to identify where they discharge water to. In older homes, the gutter downspouts might be hooked directly to a storm sewer, but in newer areas they will usually discharge water on your property. You should ensure that they discharge water far enough way from your foundation. If they do not, you can add an extension to satisfy this. This also relates to grading as I discussed above.
If this alone does not solve the problem you might consider a drainage cannel to direct the water away from your house. This might be as simple as a gravel-filled trench, or a bit more elaborate with an embedded drain tile (pipe with holes in it), wrapped with landscaping fabric (to keep dirt and debris out) that is in a gravel filled ditch. A gravel-only ditch might eventually clog up from roots and debris. Of course you need to ensure that the slope of this ditch is away from your foundation.
A ditch will probably be required if you have either highly compacted soil or soil with a high clay content that doesn't readily absorb water. If water pools on your lot with light rain, this might be the case.