Vegetation control keeps plants from blocking and damaging a channel: clear all trees within 3 m of the channel and inspect it annually, because roots crack linings and weeds slow the water. Weeds raise the roughness coefficient from about 0.022 for clean earth to 0.027 for short grass, which cuts the flow at the same depth by about 19% (0.022 ÷ 0.027 = 0.81).
Part of Aqueducts and Channels
Vegetation control is the routine of keeping roots out of linings and weeds out of the flow. Plants grow wherever a channel gives them water, and they damage it in two ways: by force and by drag.
Roots Crack The Lining, And Weeds Slow The Water
Roots. Roots seek water, and a channel is a steady supply. They enter cracks and joints, widen them as they thicken, and lift linings. The aqueducts page lists tree roots near the channel as a mistake that cracks the lining and blocks flow, and prescribes clearing all trees within 3 m of the channel and inspecting annually.
Weeds. Plants growing in the water add roughness and take up part of the cross-section. Using Manning’s equation, flow at the same depth is proportional to 1 ÷ n. Typical values for earth channels (Chow, 1959) are about 0.022 for clean, weathered earth and 0.027 for short grass with few weeds. The flow ratio is 0.022 ÷ 0.027 = 0.81, a drop of 19%. The worst weedy case (0.033) gives 0.022 ÷ 0.033 = 0.67, a drop of a third. To carry the same flow, the water must rise, and with only 10-15 cm of freeboard that can mean overtopping (Channel Cross Section).
Plants on the banks above the water line are another matter: grass on the bank holds the soil against rain erosion, so remove the roots that threaten the channel and keep the grass.
What You Need
- Hand tools: a sickle or scythe, a hoe, a spade, a saw and an axe
- A way to dispose of cuttings well away from the channel
- A schedule: annual inspection, and a clearing in the growing season
- A boundary that stops new trees being planted or left within 3 m
Method: Clear, Cut, Check, Repeat
Step 1: Clear all trees within 3 m of the channel, cutting them down and pulling out or killing the stumps. A cut stump often resprouts.
Step 2: Cut weeds in and beside the channel before they seed, and take the cuttings out. Cuttings left in the water block gates, screens and pipes downstream.
Step 3: Pull weeds from the bed, roots and all, working from the downstream end.
Step 4: Inspect annually, looking at joints, cracks and lining for roots.
Step 5: Repair damaged lining after removing the roots.
Worked Example
The standard channel (30 cm bottom, 1:1 sides, 20 cm of water, 0.2% slope) carries about 48 L/s with clean earth at n = 0.022. With short grass at n = 0.027 it carries 48 × 0.81 = 39 L/s, a loss of 9 L/s (19%). Weed-choked at n = 0.033 it carries about 32 L/s, a loss of a third.
Common Mistakes
- Cutting weeds and leaving them in the water. They jam gates and pipes.
- Cutting a tree and leaving the stump. It resprouts and the roots stay.
- Using weedkiller beside a drinking-water or irrigation channel. The chemical enters the water. Cut and pull by hand.
- Planting trees along the bank for shade. Roots follow the water.
- Letting grass grow in the bed. It slows the water.
Safety
Sharp tools and hidden hazards
Scythes, sickles and axes cut deeply: keep others well clear and wear sturdy boots and gloves. Snakes and biting insects live in long grass beside water; look before reaching in. Bank edges are undercut and soft; do not stand at the very edge.
What’s Next
- Learn next: Sediment Management — the other routine maintenance
- Combine with: Clay Lining — roots crack clay too