Swale design

[urbanus]

I am currently playing around with water harvesting ideas for a block of land. What is a reasonable rule of thumb for swale separation on a gently sloping block? I was thinking about one for every 2 metres of fall, which would give me roughly 20-30 metres separation between swales and catchment between 1- 2 acres (4000-8000 sqm) for each. I am looking at running the overflow into into hillside dams that each then drain into the lower swale. On a 10 acre block this would give me about 1500 metres of swales with about two acres of trees planted on the ridge above the first swale. Thoughts welcome.

[gardenlen]

g'dau urbanus,

not sure i can help much? but here are my thoughts:

first once a trench is sued to direct water to another spot by my knowledge it is no longer a swale.

now 10 acres not a massive amount of ground and not sure what your plans are ie.,. grazing food animals etc.,. then you need to consider the grazing rate and how much of your land will need to alloted to grazing. my idea was that most of us need app' 5 acres for our infrastructure, gardens and food trees, depending on needs that could be expanded out to 10 acres. then you have habitat management to consider (p/c) and wind breaks etc.,.

i would however suggest with your swales and wanting to keep as much of the ground fully usable and accessable, why not simply and effectively rip along the contours? these rips can be as permanent or temporary as you like (and they don't need to be as exact as the trench/berm type), and can be redone in a different line to spread the need so to speak. ripping puts the water under the soil where you need it most and aerates. we had a very dry sandy loam to clay block and placed our rips about 6 to 8 meters apart it worked a treat.

would think that one dam would be sufficient for that sized propery capacity might be 1/2 a megalitr to 1 megalitre?? check your local regulators for catchment waters lots of restrictions on dam building nowadays and also there is likley to be a cost for permission.

len

[urbanus]

thanks. I intend to rip the contours for soaking and remineralising if necessary but have been uncertain about width of swales and separation between them as these seem to vary greatly from what I have seen. Unfortunately this area is not well explained on the Water Harvesting DVD or elsewhere on the web or books on this topic.

I don't plan on grazing the property save a couple of goats for blackberries and scrub with geese, ducks and chickens taking care of the rest. I expect probably close to 3/4 of the block, former cow paddock, will be given over to trees leaving 1/4 for dams, house, garden/orchard and infrastructure. The slope and shape of the block lends itself to smaller dams at each end, with one being about 15m higher than the other, but will think about a single larger one when talk to the contractor. I was thinking of linking the swale overflow to the lower swale by way of a channel similar to a riffle where it does not connect to a the dam. There is also the possibility to capture significant road runoff from a drain but was thinking of putting it into a separate pond to minimise contamination of other water.

[gardenlen]

ther si always trial end error all that anyone can suggest is an idea.

at least if you don't go the extent of trench swales you can always add an extra rip into the sytem, i always worked on the less rainfall the closer or more the rips. we ahd av' r/f of between 670mm & 1100mm, so found 6 to 8 meters apart worked fery well in the sany loam we had and clay as well.

yes having channels as well to direct water tha might exit the property before it has gone into a dam is also the way to go.

grazing rates depends on the type of ground you bought ie.,. scrub country or former riperian bush country, as the aquafa affects the ability of things to grow like grasses scrub country acn have anything from salty enough to grow prawns to just fresh. so talking to local farmer types may help work out your grazing rate if you plan on being sustainable in that sector over stocking will mean lots of ahnd feeding.

we channeled all road run offs into, our property as well water is what it is all about.

len

[urbanus]

thanks again. Rainfall is around 1100mm so your reference gap between contour rips is a useful tip. The property has been cleared to pasture for as long as anyone remembers so the scrub is just the area between fence and road. Soil is rich red clay loam and expected to be suited to intensive horticulture, albeit phosphorus deficient. I guess it will be trial and error with number of swales.

[Paul Cereghino]

Hmm... I like the topic... but don't have much concrete experience to offer (never stopped me yet).

I guess if your already ripping, digging and maintaining swales sounds like work, so I'd say you'd want to achieve the desired effect with as few swales as possible. So then it comes to the goal. If the goal is percolation, I bet it would matter a lot how fast your 1.1m of rain comes? Does the sky rip open and dump (more swales), or does it drizzle forever (ripping is good enough). is the goal to create moist microclimates for select species? then you'd be trying to concentrate good sources of water, and then if you are intercropping between swale/tree/hedge rows, then what spacing do you need to intercrop. Is the swale tree row about wind effects? Are you using swales to deliver water? a different issue. Form follows function... in short I'd inventory the "why" of your swales to figure out the factors that should determine spacing.

For percolation:
Distance between swales = (volume of swale * soil permeability)/(steepness * rate of rainfall)

or.. distance between swales decreases as slope and rainfall rate increases and ability of soil/swale system to absorb water decreases.

[urbanus]

Some good ideas there Paul. Rainfall occurs throughout year with maybe 50% more in winter than summer. I will try and do the math but it gives me an idea of perhaps having one large dam and pumping water into a top swale. I have not commenced work yet so still in the observation and planning stage but clearly less is good.

[bazman]

It comes down to what you want to finally do with the land, If it's a large orchard, you want to leave room to drive a tractor between the swales and give the trees enough growing room and also have places where you can cross them with out damaging them. I tend to think live stock should be kept out of the swale if you can, when things get dry you can use your swales as a quality food source which can be picked at over a fence.

It also comes down to what you can take on, do it in stages if you like that way you can see the results over time. I have a single long swale right through my block and the catchment quality is amazing, I have also linked it up into my dams wing drains and storm water run off. 25mm of hard rain will fill my swale system and put 25cm of water into my dam. I'm more than happy with that, I do have some ideas of adding another swale which catches the dams overflow.

If you can go visit a few properties that have swale systems in place, seeing them first hand is the way to go.

Good luck.

Baz

[urbanus]

thanks bazman, I followed your swale construction with interest; btw, links are dead now. How did you determine the width/depth? I like the idea of using the swale as a path. I am not intending to have livestock.

[foggyforge]

g'dau urbanus,

now 10 acres not a massive amount of ground and not sure what your plans are ie.,. grazing food animals etc.,. then you need to consider the grazing rate and how much of your land will need to alloted to grazing. my idea was that most of us need app' 5 acres for our infrastructure, gardens and food trees, depending on needs that could be expanded out to 10 acres. then you have habitat management to consider (p/c) and wind breaks etc.,
len

Hi,gardenlen-im puzzled by your assertion that-most of us need app' 5 acres for our infrastructure, gardens and food trees, depending on needs that could be expanded out to 10 acres. then you have habitat management to consider (p/c) and wind breaks etc.,
From what i heard Bill M talk on PC he suggest about 10mx10m for food needs of a family .Am i missing something -please elaborate. :?:
How will the population of the cities survive?Those who never be able to even afford 10 acres?please don't take this as a criticism ,i'm wondering if we talking about PC here?regards ,(foggyforge)Bela