Dig a shallow diversion ditch, about 0.3 m deep, in an arc across the slope uphill of the spring, with its ends carried round to the sides and downhill past the box, so that storm runoff is intercepted and led round the spring instead of over it. Dig it before you build the box. On a fall of 0.5% a ditch of this size carries about 67 L/s, which is the storm runoff from roughly 1.2 ha in 50 mm/h rain, so a larger catchment needs a deeper or longer ditch.
Part of Spring Capture
An uphill diversion, also called an interceptor or cut-off ditch, is a ditch dug across the slope above a spring to catch surface water before it reaches the spring area and lead it away.
Runoff Over A Spring Washes Pollution Straight Into It
When it rains hard, water runs down the slope over everything it meets, carrying soil, dung and debris. If it reaches the spring box, the clay cap or the exposed ground round it, it brings that load with it. The Spring Capture guide’s rule is a shallow ditch, 0.3 m deep, in an arc uphill of the spring to intercept the runoff and divert it around the spring area, and its table of mistakes lists “dig the diversion ditch before building the spring box”. A ditch also keeps the excavation dry while you build.
Check That The Ditch Is Big Enough
How much water arrives. The rational method gives the peak runoff from a small catchment as Q = C × i × A ÷ 360, with Q in m³/s, i the rainfall rate in mm/h, A the catchment area in hectares (1 ha = 10,000 m²) and C a runoff coefficient between 0 and 1 for how much of the rain runs off (recalled from hydrology texts; C is an assumed value here). Take C = 0.4 for sloping pasture, i = 50 mm/h (the heavy rain the cistern overflow guide designs for) and A = 1 ha:
Q = 0.4 × 50 × 1 ÷ 360 = 0.056 m³/s = 56 L/s
What the ditch carries. Use Manning’s equation as the channel guides do, V = (1 ÷ n) × R^(2/3) × S^(1/2), with n = 0.025 for an earth channel (Channel Cross-Section). The ditch is 0.3 m deep with a 0.3 m bottom and 1:1 sides. Keeping 0.1 m of freeboard (the 10-15 cm the channel guides ask for) the water depth is 0.2 m, the fall is 0.5%, the minimum in Ditch Layout:
- Flow area A = (0.3 + 0.7) ÷ 2 × 0.2 = 0.10 m² (top width at 0.2 m is 0.3 + 2 × 0.2 = 0.7 m)
- Wetted perimeter P = 0.3 + 2 × 0.2 × √2 = 0.866 m; R = A ÷ P = 0.1155 m
- V = (1 ÷ 0.025) × 0.1155^(2/3) × 0.005^(1/2) = 40 × 0.2371 × 0.0707 = 0.67 m/s
- Q = A × V = 0.10 × 0.67 = 0.067 m³/s = 67 L/s
So the ditch carries 67 L/s, more than the 56 L/s from 1 ha: it copes with about 67 ÷ 56 = 1.2 ha at those assumptions. A bigger catchment needs a larger section, a steeper fall, or a second ditch higher up. A speed of 0.67 m/s is gentle; speeds of over 2 m/s scour most soils (Ditch Construction), so do not steepen the ditch carelessly.
What You Need
- Spades and a mattock
- A level or water tube level (Water Tube Level) and pegs
- Rock, 15-30 cm, for the discharge point (Erosion Prevention)
- Clay — for lining a leaky stretch near the spring (Clay Lining)
- Grass or turf — for the banks
Method: Peg, Dig From The Outlet, Stabilise
Step 1: Peg the arc. Set it uphill of the box and outside the gravel backfill, curving across the slope, with each end running downhill past the sides of the spring area and the planned fence.
Step 2: Fix the outlet first. Choose where the water leaves, to the side and well downhill of the spring, on ground that can take it.
Step 3: Dig from the outlet uphill, 0.3 m deep, with a fall of at least 0.5% (5 cm per 10 m), so every length drains.
Step 4: Shape and bank. Pile the spoil on the downhill side as a low bank, compacted, and seed the banks to grass.
Step 5: Protect the outlet with rock where the water leaves.
Step 6: Line a stretch near the spring with puddled clay if the soil is leaky, so the ditch does not soak water into the ground beside the spring.
Step 7: Inspect after storms and clear silt and debris.
Common Mistakes
- Building the box first. Runoff floods the excavation and fouls the spring.
- A ditch with no fall. It fills with silt and overflows.
- A ditch too close to the spring. Its water soaks into the ground beside the box.
- Discharging onto the spring path. The diverted water finds its way back.
- Letting it silt up. An uncleared ditch is a pond.
Safety
Digging on a slope
Digging a ditch across a slope undercuts the bank above you; keep faces low, do not dig deeper than 1.2 m without shoring, and keep spoil off the uphill edge. Do not work beside the ditch in storm flow.
What’s Next
- Learn next: Vegetation Management — grass and trees on the slope
- Combine with: Fencing — keeping animals from breaking the banks