I live in an area with drought and flood cycles, as well as heavy clay soil. I would like to build a system, probably next spring, to catch water coming off the roof, filter it with little or no intervention and then store it until landscaping needs it. I also want to build a living willow dome that would also have a narrow bed that was wide enough for the root system that would have enough soil moisture for the willows.

I want as much of it to be made from natural materials as possible, to have as few moving parts as possible, and to be inexpensive in terms of up front costs and maintenance costs. So, I was thinking earthbag for the bed structure. I have heavy clay soil, should I use straight soil for the fill, or should I stabilize it? If I stabilize the soil, should I use concrete or bentonite clay.

Once I have the earthbag frame for the pond, I was thinking a think layer of a clay plaster. The plaster would fill the gaps between the bags and further reduce how much water seeps into the soil around and under the beds. Should it be as close to straight clay as possible? Should it have sand or pea gravel added? Should it have a stabilizer? A natural pond would likely have a plain clay floor, but there would be some water seepage. I want to minimize water seepage as much as possible.

I am thinking I should have a layer at least four to six inches thick of straw to hold water in the beds. Should it be deeper? Would clay plaster on a horizontal surface between earthbags and straw erode the way clay plaster walls can potentially do? If I plastered the outside with clay plaster, then back filled with sand, followed by straw, would that help keep the plaster on the outside from too much erosion or should I plan to either use a concrete plaster or to replace the clay plaster every few years?

I would like a secondary filter between the reed bed and the cistern letting the water perk through activated charcoal and sand. Would a cheese cloth bag filled with charcoal over the opening for the water to flow out of the reed bed into the cistern work well? The water would flow into a container filled with pea gravel, then out into the cistern.

Any ideas or suggestions on any points that might cause problems or ideas on how to make it more likely to work or to work better? I am willing to experiment as long as the initial cost can be kept fairly low.

One thread had a link to the following site on sealing ponds with clay.
http://tilz.tearfund.org/en/resources/publications/footsteps/footsteps_21-30/footsteps_25/sealing_fish_ponds/

One link a thread referred says to dig the area and compact the soil. Remove any rocks or stone.
A layer of lime that is about 3/4 to 1 1/2 inch thick. I am seeing weights rather than cubic feet. Pulverized limestone weighs about 87 pounds per cubic foot. A 1 inch deep layer would weigh 1/12 of 87 pounds per square foot or about 7 1/4 pounds of limestone per square foot of pond.
Clay, straw and water puddled in with feet sounds a lot like a cob mix, applied by hand rather than feet.
A layer of cob about 5 to 6 cm sounds like roughly 2 inches or a bit more, if my in head conversion isn't too rusty.
The site says keep area wet or cracks can form. Would having a thick layer of straw as an organic layer to retain moisture in the soil between rain falls be enough to keep the bed sealed? Is there a way to keep the bed sealed without using manure such as alternating layers of cob mix and slip covered straw?
Would cob be preferable to earthbags with clay fill for this use? Or would a layer of earthbags provide a stronger "frame" for the structure? It would be only a couple feet deep and the same wide at the top.
Would a French drain be optimal for moving water through the bed to a storage cistern? How deep should the straw layer be to hold enough water to emulate a drainage ditch that does not have any surface water but can support marsh plants?