Dive into the pond project

Introduction

My roof rainwater catchment tanks can store 10.000L, which is enough for the house’s supply. However, an edible forest is also part of my dream and requires a lot more water, about four times more. To save the costs and the view of four 10.000 liter tanks, I started thinking about a pond. This post will introduce you to my pond project, all DIY and off-grid.

First, I will walk through some of my personal requirements for the pond, then I will discuss my search for the optimal location, and finally I will show my considerations for the size and depth together with the materials and tools necessary to install the pond.

Defining pond goals

The pond should

1. store at least 40.000L of rainwater.

2. stay clean enough as not to clog the water pump.

3. hold water to irrigate in summer.

4. irrigate as many plants as possible using gravity, i.e. without the use of a water pump.

Finding a place

The pond goals were then formulated as these key questions to help me decide where to plant the pond.

• Where can I accumulate the most water?

• Is there a good spot uphill to install the filter?

• What is the highest possible place for the pond?

• Where will it enjoy the most sun and wind protection (to minimize evaporation)?

• Is the earth stable enough to hold the water?

A spot where water naturally flows from the land into the creek seemed to be a good place to intercept rainwater and fill a pond. In the same channel there is room for the filter. The designated place is also higher than most of my trees and plants. There is some shade provided by native trees, but I will have to use some sort of sun protection until they grow taller. It is also situated on a north slope where the summer dominant south winds do not hit as hard. This way I hope to minimize the evaporation in summer. Finally, the pond will be next to a creek, which means that I will make some extra efforts to stabilize the "dam" in between. The soil is mostly clay which doesn't let water through easily, making it a stable foundation for the pond.

Photo 1: pond location.

Determining dimensions

Pond

Sizing the pond has been a bit of an iterative process as there is a limited choice of liner sizes (at least there was a limit here in Chile). I went back and forward between pond sizes and liner sizes. For 40.000L = 40m3 I found a 7mx13m liner to cover a pond shape that would approximate 8m in length, 3m in width and 2m in depth. Although some cubic meters would have to be sacrificed for shaping, the storage capacity would still be around 40m3 (the theoretical capacity is 8x3x2=48m3). My considerations for having a rather deep and narrow pond instead of a shallow and broad pond is that less surface area means less evaporation. Moreover, it facilitates aeration using an air pump, but that I will discuss in another blog post on pond accessories.

Filter

Then, I also needed to calculate the size and depth of the filter going into the pond. This filter is necessary if you are capturing rainwater from the land surface. (If you are collecting rainwater from your roof I would suggest using the filters I have used in the gutters for bigger biomass.) The filter for rainwater collection from land surface consists of three compartments side by side, see drawing 1 below. The two outside compartments are filled with small river rocks and separated from the middle compartment by large river rocks that form two dams. The water collected on the land surface is channeled by ditches into these two compartments and filtered by the river rocks from leaves, branches etc. Then it flows into the middle compartment that does not hold any rocks and is about half a meter lower than the other two. This compartment serves as a pool where the mud that might come in with the water sinks down, like a Turkish coffee! A tube is fixed through the membrane just below higher limit of the liner leading the clean water from the filter into the pond.

Liner

Calculate the cubic square of your pond and add half a meter for the liner around the border. You will have some liner extra because your pond will not be a perfect square in the end. I added two meters to the length of my liner to have a 2x7 meters patch for the filter.

Shape

Drawing: pond and filter design, where the light areas are the bottom and the dark areas are the back side of the pond and the filter. A 55mm tube is connecting both, guiding filtered water into the pond. Another 55mm tube may be used as the overflow.

Installation of the pond and filter

The excavator cleared the way first. It made a squared hole of about half a meter narrower and shorter than supposed size of the pond. This is to account for the sloped edges of the pond.

Then we I organized a Chilean ‘minga’ , where your friends come over to help you

out while you cook them up a big lunch. We started digging away ourselves using shovels and crowbars.

Drawing 1 shows the shape of the pond, which is designed to improve water circulation. To fasten the liner, a small ditch was made on the border of the bond. The ditch keeps the rocks that keep the liner in its place from falling into the pond, see photo 3. Finally, an overflow should be designed to control the pond from flooding. I just lowered part the pond border for this, but it is safer to use a tube (I've drawn one in the pond sketch), because the dirt may shift and block the overflow over time. The surplus of my pond water flows into the creek.

Photo 2: shaping the pond hole.

Next, the liner was fitted over the pond, but it did not reach well over the borders. We had to make the pond a bit shallower by filling it in with the dirt from the borders. After that, it fitted fine with about 30cm tucked in over the edges.

In my special case, I needed to stabilize the dam of dirt between the creek and pond. I used geotextile reaching from under the liner over the other side into the creek (photo 4). This material lets water through while holding dirt in its place. We fastened the liner and geotextile with the big river rocks. Finally, we bordered the whole pond with small river rocks to filter the water that flows in from the sides of the pond.

Photo 3: fastening the liner over the hole.

Photo 4: use of geotextile to stabilize critical pond borders; on the dam, right side of the picture, underneath the small river rocks and high border of the pond, lower side of the picture, still uncovered.

The space for the filter was shaped the same way; first with the excavator and then by hand. It consists of two shallow compartments on the sides and one deep compartment in the middle, see Drawing X. Then the liner was put in, secured with big stones around the edges as well as between the compartments, of which the two small compartments were filled with small river rocks. Finally, the tube was pushed through the liner using a heat gun. I practiced this first with a piece of left over liner, because the plastic will easily melt if you overexpose it to the heat, which may cause a leak.

Photo 5: filter for rainwater harvesting from the land's surface.

Some pond accounting and conclusion

In my case, the pond saved me a good amount of money. It's costs of materials and man hours were about a fourth of what I would have paid for four 10.000L tanks. (If you would like to see a spreadsheet of the costs, send me an email!) Also, I find it looks more attractive than an array of tanks. It has been way more work than expected, specifically the shaping and rock cover, but it will be worth it when I get to water my trees and plants throughout the dry Chilean summer. Even more so now that every year the climate will get more extreme as a result of global warming!

Thank you for reading this post and please leave me a comment or send me an email for any questions, recommendations or consultations. I will post another blog in spring that will talk about installing and planting pond accessories for irrigation, aeration and evaporation reduction.

Sarah Viento – sustainable systems