How to prepare your land before the monsoons
In the wake of depleting water levels and the need for conservation, well-designed water harvesting earthworks like swales, bunds, ponds, plugs and dams become essential for any landscape. How does one start preparing one’s land to harvest the monsoons? The following notes elaborate on details of harvesting - the types and methods. This would help landowners, designers, farmers or anyone with a piece of land increase food production, raise groundwater levels, reduce irrigation needs, and enhanced ecosystem resilience.
First some ground rules for working with earthworks:
Begin with long and thoughtful observation
Always start at the top of your watershed and work your way down
Start small and simple
Maximise organic material and living ground cover - create a living sponge
Flowing/visible water is not as important as moisture
Use a bunyip/level pipe or an A-frame to ensure accuracy when marking contour lines
Always plan an overflow route, and manage the overflow as a resource
Continually reassess your system
Slow, stop, spread, infiltrate and save the flow of water
Swales
Purpose:
To intercept rainwater running down slope and infiltrate it in a localised area (usually the soil and root zone of existing or planned vegetation)
Slope:
Up to 18% (although best to keep within 15%)
Location/placement:
On any open land that is within the slope range.
Ground rules:
Keep in mind the catchment area when planning the size of the swale (size for maximum stormwater event)
Plan for overflow - ideally link with other swales
Must be on contour otherwise will breach.
Edges must be gently sloped and not a steep cut to prevent erosion and to allow animals to climb out easily.
Must be filled with organic matter to improve the porosity of the soil and therefore the rate of infiltration.
Larger twigs and branches can be used, trunks of banana trees, and any other material available.
Mulch swale and bundh immediately after digging and weigh down mulch with rocks. If there’s no mulch use rocks on the bundh.
All swales must be monitored closely for erosion and maintained (filled with mulch, desilted if necessary)
Spacing between two swales depends on the slope, the area and the permeability of the soil. The less permeable the soil and the steeper the slope the closer the swales should be together.
Planting:
Can plant the bundh with fast growing tree species, grasses and ground cover.
If in a somewhat remote area, plant the bundh with seed balls.
Swale beds can be used for seasonal planting
Boomerangs
Purpose:
Small bundhs to catch and slow sheet flow and silt at the base of trees on small, awkward, steep or restricted sites.
Slope:
Up to 25%
Location/placement:
Use in food forests and any other planting on slopes. Can be reinforced with rocks on the downslope
Ground rules:
Should be located on the down slope.
Open boomerang shape not a tight ‘U’ to reduce the chance of erosion at the corners and to increase catchment
Ensure the boomerangs are interconnected/one overflows into the next
Sheet Flow Spreader
Purpose:
Sheet flow spreader on contour.
One rock high, crescent shaped rock mulch structure that is used to slow, spread, and infiltrate sheet flow that would otherwise erosively speed up, concentrate and run off as a more destructive channel flow.
Slope:
Up to 25%
Location/placement:
Great for areas where water is carrying sediment. Also for remote areas where digging is difficult and where swales cannot be monitored.
Ground rules:
The ends of the arc point upslope to capture and focus water flow over the middle of the structure and prevent water from cutting around the ends.
Planting:
Sow with native grasses, place seed balls between rocks.
Infiltration Basin
Purpose:
Shallow planting basin (no more than 2 ft) to catch and infiltrate rainwater, runoff and/or grey water.
Slope:
Up to 5%
Location/placement:
Primarily under trees (1.5-3 times the diameter of the drip line)
Ground rules:
Bottom should be level
Works best on flat landscapes so all surrounding runoff can drain into it
Can also work on moderate slopes as a terraced basin
Size for a maximum storm event or peak surge of grey water
Planting:
Plant inside the basin, although choose species that don’t mind having their feet in standing water for a few hours at a time.
Terraces
Purpose:
A relatively flat shelf of soil built parallel to the contour of a slope
Creates level planting area to intercept direct rainfall and some runoff from slope
Help control erosion on slopes
Slope:
50%
Location/placement:
Already existing at the farm, the bundhs of many of the terraces (particularly in the upper slopes) need to be restored
Ground rules:
As they do not have depressions built into them they need to be bordered by bundhs in order to retain rainwater and accumulate organic matter. Therefore, the bundhs need to be properly maintained and well planted.
For slopes above 50% a retaining wall is needed for stabilisation
Planting:
Terrace bundhs can be planted with trees, shrubs, grasses and groundcovers that hold the soil and can make the most of the water caught around the edges.
Gully Plugs
Purpose:
A stone dam placed in a nala or gully to slow and infiltrate water.
It allows water to pass through but acts as a silt trap. This soil can then be used on the land.
Location/placement:
In nalas across the land
Choose a site where there is a break in slope to allow the accumulation of adequate water behind the plug.
Ground rules:
Start from the top and work your way down the nalla, this will prevent surges of water damaging plugs down stream.
The sides of the check dam must be higher than the centre so that water is always directed over the centre of the dam (this avoids the dam being outflanked by the flow)
Will silt up and need maintenance.
Planting:
Plant the influence zone of the plug with riparian tree species
One Rock Dam
Purpose:
Low barrier placed perpendicular to the flow of water within a drainage.
Built just one rock tall and three to six tightly packed rock courses wide
Typically not more than one foot in height
Built in the shape of a gradually sloped speed bump.
Slows but doesn’t stop water, more gentle than a gully plug.
5-6 rows of a single layer of rocks which are 8-12 inches big.
Location/placement:
In shallow nallas, in trenches beside roads and paths.
Often used to heal a deepening gully.
Ground rules:
Place in straight sections of a drainageway not on curves
Tight fit and placement of rocks more important than size
Planting:
Plant riparian species in influence zone on the edges of the drainage
Diversion Swale
Purpose:
Used to intercept, infiltrate and redirect both sheet flow and channelised water.
Gradually sloping drainageway that slowly moves water from one point to another
Linear basin with the excavated soil placed downslope to create a bundh
Slightly off contour allowing a portion of the water to soak in while moving surplus water downhill to another place (infiltrating along the way)
Valuable for transforming the concentrated fast-moving water into a valuable resource by spreading out and calming the flow.
Slope:
Up to 18%
Location/placement:
Usually near roads and culverts.
Can be used to slow water and reduce erosion on the mud roads, and direct the storm water back into the land
Ground rules:
Ensure the slope is gentle enough to guide the water but not result in erosion over time.
Planting:
Can be used to irrigate roadside vegetation (direct the water to an infiltration basin, boomerang or towards a swale)
