Effective Drainage Line Treatment for Erosion Control

When heavy rainfall hits a watershed, water doesn’t just soak into the soil – it moves. It gathers in small channels, picks up speed, and carries soil particles with it as it flows downhill. Over time, this concentrated runoff carves out gullies, deposits sediment in streams, and leaves farmland degraded and unproductive. Drainage line treatment addresses this problem directly by placing engineered structures within these water channels to slow the flow, drop sediment, and allow vegetation to reclaim the land. At the heart of this approach are check dams – one of the most proven tools in watershed management – along with a range of permanent spillway structures designed for larger, more severe erosion challenges.

What is drainage line treatment? 🔗

Drainage lines are the natural pathways along which surface runoff flows – rills, gullies, swales, ditches, and channels that develop in a watershed. When left untreated, these pathways become progressively more eroded as water velocity increases with gradient and volume. Drainage line treatment is the systematic use of physical structures along these channels to interrupt that process. According to the Food and Agriculture Organization (FAO), check dams constructed across the gully bed stop both channel and lateral erosion by reducing the original gradient, which in turn diminishes water velocity and the erosive force of runoff.

The treatment is applied at different scales depending on the size of the drainage area and the severity of erosion. Small, actively eroding channels call for temporary, low-cost structures. Larger, deeply incised gullies require permanent engineering structures built to last decades. Selecting the right structure for the right site is the defining challenge of drainage line treatment.

The role of check dams in runoff management 🔗

Check dams are small structures placed perpendicular to the direction of flow in a drainage channel. Their core function is simple but effective: they interrupt flow, flatten the effective channel gradient, reduce water velocity, and cause sediment to deposit upstream. As the Minnesota Pollution Control Agency explains, slowing flow velocities allows check dams to serve multiple functions – reducing channel scour and erosion, enhancing sediment trapping, and providing greater water quality treatment through extended detention.

A critical design principle is that check dams are nearly always deployed in a series, not as isolated structures. The toe (base) of the upstream dam should ideally be at the same elevation as the top of the downstream dam. This configuration ensures that the channel gradient between dams is effectively neutralized, eliminating the erosive energy of the water between structures. The center of each dam should be lower than its edges by at least 15 cm, creating a weir effect that directs flow through the center and prevents water from flanking the structure and eroding the banks.

Spacing and sizing considerations 🔗

The MPCA Stormwater Manual notes that check dams are most appropriate for channels with slopes greater than 10 percent on soils prone to erosion such as sandy or silty soils, and are generally limited to drainage areas of 10 acres (about 4 hectares) or less. For larger catchments or steeper gradients, permanent structures become necessary. The FAO recommends that the number of check dams required for a given gully section be calculated based on the total vertical drop across the section, the average effective height of each structure, and the compensation gradient – the minimum slope at which water continues to flow after the gully has been treated.

Temporary check dam structures 🔗

Temporary check dams are used in smaller channels and early-stage gullies where the immediate goal is to slow runoff, deposit sediment, and allow vegetation to establish. According to the FAO Watershed Management Manual, temporary check dams typically have a lifespan of three to eight years – long enough for tree seedlings, shrubs, and grass cuttings planted in the gully bed to establish a permanent vegetative cover that takes over erosion control duties.

Brushwood check dams 🔗

Brushwood check dams are among the most accessible and economical temporary structures available. They are constructed from wooden posts driven into the channel bed and banks, with cut branches and brush woven or laid between them. ICAR’s online course material on gully control describes that these dams can be used where the rate of runoff is less than 0.5 m³/sec, and that live posts of willow or poplar (8-10 cm diameter) are preferred so that the structure itself may take root and add biological reinforcement over time. The main purpose of brushwood dams is to trap fine sediment and stabilize small gully heads up to one metre in depth.

A few construction specifics matter for brushwood dams to work well. The gully sides should be shaped to a 1:1 slope, and the bed excavated 15 cm deep before the brushwood is laid – with the cut ends (butt ends) facing upstream. The ends of the structure should extend at least 30 cm into the channel banks to prevent the flow from outflanking the dam. Brushwood check dams are strictly temporary and should not be relied upon where concentrated runoff from roads or cultivated fields creates persistent, high-velocity flows.

Loose stone check dams 🔗

Loose stone check dams use relatively small rocks stacked across the gully channel. Unlike brushwood structures, they can be used in all climatic regions and are suitable for both incipient gullies and small branch gullies within a larger gully network. According to FAO gully treatment guidelines, these dams apply to channels no longer than 100 metres with catchment areas of two hectares or less. Their maximum effective height is 1.0 metre, their foundation depth should be at least 0.5 metres, and the downstream face should slope at approximately 20 percent to resist scour.

The twin objectives of loose stone check dams are to control erosion along the gully bed and to stop headward (waterfall) erosion by stabilizing gully heads. Because the rocks are permeable, water seeps through the structure rather than over it during low-flow events, which makes these dams effective at promoting infiltration and groundwater recharge. The Sustainable Sanitation and Water Management platform notes that this permeability also makes loose stone dams less disruptive to natural ecosystems than fully sealed structures, as water and fine sediment can still pass through.

Other temporary options 🔗

Beyond brushwood and loose stone, several other materials serve as temporary check dams in specific contexts. Woven-wire check dams are suited to channels with moderate slopes (less than 10 percent), small drainage areas, and catchments where flood flows do not typically carry large rocks. Log check dams – built from 4- to 6-inch diameter timber embedded at least 45 cm into the channel bed – offer a longer lifespan than brushwood when non-sprouting timber is used. Fiber rolls and sediment-retention wattles made from straw, coir, or wood fiber are increasingly used on construction sites where rapid installation and biodegradable materials are priorities. Straw bales and silt fences, however, should never be used as check dams, as concentrated flows quickly erode these materials.

Permanent structures for larger gullies 🔗

Where gullies are large, drainage areas are extensive, or flow volumes exceed what temporary structures can safely handle, permanent gully control structures are required. As the Springer textbook on Soil and Water Conservation Structures Design explains, these structures – the drop spillway, drop-inlet spillway, and chute spillway – are preferred in medium to large gullies with significantly high flows, and are typically built from masonry or reinforced concrete. Each is designed not only to convey the peak design discharge safely but also to dissipate the kinetic energy of that flow within the structure itself, protecting both the structure and the downstream channel from erosion.

Drop spillway 🔗

The drop spillway (also called the straight drop spillway) is a weir structure in which flow passes through a rectangular weir opening, drops vertically onto a concrete apron below, and then passes into the downstream channel. It is one of the most common permanent gully control structures, placed at intervals along the gully bed to act as a series of control points. The apron dissipates the kinetic energy of the falling water through a hydraulic jump, protecting the channel bed from scour. According to Soil Management India, the drop spillway is most suitable where the total head drop does not exceed about 3 metres. Its advantages include structural stability, a non-clogging rectangular weir, low maintenance cost, and ease of construction standardization.

Drop-inlet spillway 🔗

The drop-inlet spillway consists of a pipe conduit passing through an earthfill embankment, connected to a suitable inlet on the upstream side and an outlet on the downstream side. It is particularly efficient for controlling gully heads where the total drop exceeds 3 metres. Its most important feature is the earthfill embankment, which creates upstream storage – making the drop-inlet spillway the only one of the three types that serves a dual purpose: gully stabilization and flood control. Stored water can be released in a controlled manner or used for irrigation and other on-farm purposes. The main limitation is susceptibility to clogging by debris at the inlet, which requires regular inspection and maintenance.

Chute spillway 🔗

A chute spillway is a steeply sloping open channel that conveys water from a higher elevation to the downstream channel at supercritical velocity. It is typically installed at the gully head where the drop is large – usually 5 to 6 metres or more – and where the primary concern is preventing headward erosion from waterfall action enlarging the gully further. The inlet design is similar to that of a drop structure; capacity is controlled at the inlet section and computed using standard weir formulas. The outlet incorporates a stilling basin – commonly the Saint Anthony Falls (SAF) stilling basin design – to dissipate energy and prevent downstream scour. Chute spillways generally require less concrete than drop-inlet structures for the same capacity, but they do not provide upstream storage and require careful foundation compaction to guard against undermining by seepage or burrowing animals.

Choosing the right structure for the site 🔗

No single structure suits every drainage line. The decision depends on four key site factors: channel slope, drainage area size, severity and depth of existing erosion, and availability of construction materials. Temporary brushwood and loose stone structures are cost-effective entry points for stabilizing small gullies and buying time for vegetation to establish. Once vegetation is in place, the biological cover takes over and the temporary structures can be left to degrade naturally or removed. In larger, more severely eroded systems – where flows are too high and the channel too deep for temporary measures – permanent spillway structures provide reliable, long-term grade control.

It is also worth noting that structural treatment alone is never a complete solution. The FAO emphasizes that stabilized watershed slopes – maintained through good land management, controlled grazing, and adequate vegetative cover – are the best assurance for the continued functioning of any gully control structure. If the catchment above keeps generating uncontrolled runoff, even well-built structures will eventually be overwhelmed.

Environmental and agronomic benefits beyond erosion control 🔗

Drainage line treatment delivers benefits that extend well beyond stopping soil loss. Check dams and spillways slow peak runoff, which reduces flood risk for downstream areas. Behind each structure, water ponds briefly and infiltrates into the soil, recharging groundwater and raising local water tables. Studies from Ethiopia cited by Wikipedia’s review of check dam research found that in gully segments with both check dams and vegetation, peak runoff discharge was reduced by 12 percent compared to just 5.5 percent in untreated segments. This improved soil moisture supports vegetation recovery, which in turn provides habitat, stabilizes adjacent land, and helps sequester carbon in the accumulated sediment. For farmers working marginal or sloping land, these cumulative effects can meaningfully improve soil health and crop productivity over time.

What do you think? Given that temporary structures like brushwood check dams depend heavily on the availability of local materials and labour, how feasible is their adoption in regions where those resources are scarce? And with permanent spillway structures requiring significant capital investment, what financing models or policy support would make drainage line treatment more accessible to smallholder farmers in erosion-prone watersheds?

References

1. https://www.fao.org/4/ad082e/ad082e02.htm
2. https://en.wikipedia.org/wiki/Check_dam
3. https://stormwater.pca.state.mn.us/check_dams_for_stormwater_swales
4. https://stormwater.pca.state.mn.us/sediment_control_practices_check_dams_ditch_checks_ditch_dikes
5. http://ecoursesonline.iasri.res.in/mod/page/view.php?id=1932
6. https://www.fao.org/4/ad082e/ad082e03.htm
7. https://sswm.info/sswm-solutions-bop-markets/improving-water-and-sanitation-services-provided-public-institutions-0/check-dams-&-gully-plugs
8. https://link.springer.com/chapter/10.1007/978-981-19-8665-9_7
9. https://www.soilmanagementindia.com/soil-erosion/gully-erosion/permanent-structures-for-gully-control-4-types-soil-management/15073