Direct Rehabilitation of Alluvial Gullies at the Local Scale

Direct gully rehabilitation of alluvial gullies on a site-by-site basis is applicable to strategic priority sites with significant human interest (roads, fences, dams, buildings, yards, key riparian paddocks, key waterholes, biodiversity hot spots, and/or cultural sites) where benefits to intervention outweigh the costs. Direct intervention is also applicable to young highly-active gullies in early develop stages, where timely intervention is justified to prevent or slow future extreme erosion.

There are many bio-geo-engineering options available for direct intervention and rehabilitation of gullies, however most have not been well tested for alluvial gullies in northern Australia. Experimental trials in this report contribute to that research deficit, but ongoing research is needed to fully develop best management practices (BMPs). To reiterate, there are three main approaches to reduce the advance of existing gullies, which generally should be used in combination.

1.      Reduce water runoff into and through gullies.

2.      Stabilise gully headcuts and sidewalls with vegetation and physical structures.

3.      Reduce the gully channel slope and increase roughness using structures and vegetation.

Water Diversion/Retention Banks above Gully Heads

Direct mechanical or engineering intervention to reduce water runoff patterns and volumes can be warranted in situations where the hillslope catchment is highly disturbed (e.g., agriculture fields) and where excess water runoff cannot be managed solely by increasing perennial vegetation cover (grass, trees, shrubs). Numerous types of water retention and diversion structures can be built, including contour banks for water retention, farm dams, and earthen water diversion banks immediately above gully headcuts, depending on the situation. Some structures such as contour banks are not appropriate for intact native grass woodlands. Others such as farm dams would be difficult to install and ineffective above lengthy fronts of alluvial gully head scarp. Water diversion banks above alluvial gully headcuts can reduce gully headcut erosion. However, they are also prone to failure by piping in dispersible sodic soils and can divert the erosion problem from one gully to another nearby making the erosion worse (Plate 11).

Plate 11

Draft BMPs for Diversion/Retention Banks above Gully Heads

  • Construct contour berms in cleared paddocks only, or along roads and fences.
  • Divert excess runoff frequently and toward safe disposal areas armoured with rock. Avoid transferring the gully problem from one location to another.
  • For water diversion banks above gully heads in native grass woodlands, avoid disturbing native vegetation and dispersible or sodic soils where possible.
  • Use caution when using dispersive or sodic soil for diversion bank construction due to risk of soil piping and increased gully erosion.
  • Consider importing non-sodic soil or angular gravel from stable sites for diversion bank construction, rather than disturbing local dispersive or sodic soil.
  • Place banks far enough back from gully scarps to avoid damage by future gully retreat.
  • Use field observations of water flow paths and detailed topographic data (LiDAR) for bank design.
  • Use diversion banks in conjunction with gully revegetation, slope battering, gully grade control and cattle exclusion.

Increasing Perennial Grass Cover on Gully Slopes 

Perennial grass cover is a key factor in stabilizing river banks, floodplain hollows and gullies. Reducing cattle grazing pressure along river frontage will help improve grass cover within and around potential and existing gully areas. However, once gully erosion has initiated and exposed nutrient poor sub-soils, natural revegetation can be slow. Experiments using native and exotic grasses to revegetate alluvial gully surfaces in the Normanby catchment have had mixed results. Grass seed sown directly on gully scarps, side walls, and excavated sub soils had poor germination success. Adding straw mulch and grass seed did not improve germination. Using a hydromulch mix (grass, gypsum, fertiliser, paper, bagasse, tackifier) on raw gully scarps in the wet season resulted in partial vegetation cover, concentrated in gully bottoms where moisture collected and sediment was trapped (Plate 12). More proactive revegetation efforts in association with soil amendments, slope battering, and grade control can be used to speed up vegetation recovery and promote gully stabilization.

Plate 12

Draft BMPs for Increasing Perennial Grass Cover on Gully Slopes

  • Fence cattle off from large areas of alluvial gullies and river frontage well back from high banks to allow perennial grass to grow, seed, recover, and compete with weeds.
  • Manage fire and weeds in river frontage to maximise perennial grass.
  • Hand or aerial seed grass vegetation in the wet season after initial heavy rains loosen the soils and provide prolonged soil moisture.
  • Focus grass revegetation efforts on gully toe-slopes and bottoms, in addition to areas immediately above gully scarps. Scarifying scalds and scarps may be needed for improved germination, but could temporarily increase erosion. 
  • Revegetating gully scarps will have poor to moderate success unless intensive rehabilitation efforts are applied (e.g., slope battering, organic and chemical soil amendments, grass revegetation, cattle exclusion).

Intensive Rehabilitation of Gully Slopes

Both intact and battered alluvial gully slopes can be difficult to revegetate with perennial grass species due to the harsh nature of sodic, hard-setting soils with low nutrients and water availability. Importing non-sodic topsoil or capping sodic soils with rock and/or geotextile fabric – then revegetating with grass – can provide long-term stability. However, rock or good top-soil is not always available.Experimental results of intensive gully rehabilitation in the Normanby catchment indicate that physically regrading gullies, adding compost and gypsum, and revegetating with grass can reduce soil erosion compared to control sites with no soil amendments (Plate 13). In contrast, re-grading gullies with machinery without soil amendments or where there is poor establishment of grass can actually increase gully erosion above previous levels. Using a full suite of treatments, erosion was reduced from >26.6 tonnes/year to 6.7 tonnes/year on a gully site that was battered and shaped into a hollow, amended with gypsum and hydromulch, and treated with wood grade control structures and a water diversion bank. This highlights that gully erosion can be reduced but not fully stopped even under intensive rehabilitation of a 0.2 ha gully for $6000.

Plate 13

Draft BMPs for Intensive Rehabilitation of Gully Slopes

  • Do not batter (regrade) sodic gully slopes without revegetating and amending the soil, or capping with rock; they will re-erode into rills and gullies, accelerating erosion.
  • Where available, cap battered gullies with non-sodic topsoil or imported rock, then revegetate with perennial grass.
  • For rehabilitation of gully slopes in situ without imported soil or rock, batter (regrade) the gully into a stable shape and amend sodic soils with organic matter (compost or mulch), gypsum (CaSO4), fertiliser, and perennial grass seed.
  • A combination of hydromulch for short-term erosion reduction and compost for long-term grass growth will provide the greatest erosion reduction benefit. 
  • Install additional water diversion and grade control structures to reduce water runoff energy and local slopes.
  • Fence gullies to exclude cattle grazing from rehabilitation areas. 

Grade Control and Headcut Drop Structures

Grade control and drop structures are applicable to narrow semi-confined gullies where overland flow dominates, but only some sections of unconfined alluvial gullies such as finger headcuts. Grade control structures (check dams) can prevent incision, reduce slope, dissipate energy, and trap sediment. Drop structures (weirs, chutes, flumes, etc.) installed at headcuts (scarps) can safely pass water runoff over the scarp edge and stop the gully headcut from advancing. These grade and drop structures can be made out of rock, gabion, concrete, sand bags, steel sheets or mesh, treated lumber, large woody debris (LWD), woven brush and/or live vegetation. Proper attention to engineering design, structure frequency, geomorphic processes, the stage or time of intervention, and long-term maintenance are essential for minimizing failure. Structures should be embedded into the underlying bed and adjacent banks to avoid undermining or outflanking, especially in dispersible soils. In the Normanby catchment, the placement of drop and grade structures is required at many young, very active headcuts advancing up floodplain hollows in river frontage areas in order to prevent major future erosion (Plate 14).

Plate 14

Draft BMPs for Grade Control and Headcut Drop Structures

  • Grade control and drop structure are most applicable in young, linear gullies within semi-confined channels or valleys.
  • In large alluvial gully complexes, they are applicable to narrow finger headcuts and narrow outlets channels, but not wide gully bottoms or long scarp fronts. 
  • Key (embed) grade control structures into bed and bank to prevent undermining and outflanking.
  • Use a size mixture of angular rock (or wood) to lock structures in place.
  • Avoid using loosely placed large boulders subject to piping through or around voids.
  • With drop structures at headcuts, use lateral and vertical cut-off walls to prevent water tunnelling around structures in dispersible soil. Install water seep holes through structures to prevent water pressure building up behind structures.
  • Maintain structures over time to ensure their longevity and functionality.
  • Minimise collateral damage from machine works on highly erodible soils.
  • Revegetate construction areas using perennial grass seed or hydromulch.
  • Do not use tyres or dump trash in gullies in an attempt at stabilization; this material will promote scour and accelerate erosion.

Road Erosion: Prevention and Repair on Dispersive or Sodic Soils

Improper road and fence location, construction and especially maintenance are major causes of gully initiation and acceleration on cattle station properties. They should be a major focus for intervention to reduce cumulative sediment yields. Avoiding highly erodible dispersive or sodic soils along river frontage and minimizing the number of crossings through hollows, creeks, and rivers where gullies initiate is key to prevention. For existing roads on dispersive or sodic soils, managing excess water runoff concentrating down roads and fences is key to erosion control. Large water diversion banks (whoa boys) installed frequently (every 10-25 m) down steep erodible slopes can reduce water concentration and gully erosion (Plate 15). However banks and road slopes must also be armoured with angular gravel or rock to prevent rilling and gullying of underlying sodic soils.  Continuously re-grading basic roads (Plate 9) or constructing large formed roads with mitre drains will accelerate gully erosion on fragile sodic soils. Rather, localised road erosion issues on sodic soils should be patched up each year following well-considered road maintenance plans and procedures.

Plate 15

Draft BMPs for Road Erosion Prevention and Repair on Dispersible or Sodic Soils

  • Avoid and minimise road crossings though hollows, gullies, stream channels, and steep banks in dispersible or sodic soil.
  • Don’t grade down roads year after year resulting in entrenched roads without drainage.
  • Don’t re-grade or fill in gullies on roads on steep banks each year just to gain immediate access. Address the actual cause of the gully erosion and manage water runoff.
  • Avoid continuously re-routing the road around erosion problem and ignoring them. Address the cause of the problem (excess water runoff).
  • When road abandonment is needed, address major water drainage issues with diversion banks and re-seed with grass. Construct the new road to improved erosion control standards.
  • Bring past windrows from deep grading back onto the road surface, and use to crown the road surface and/or construct water diversion banks.
  • Reinstate the natural water flow direction (off the road) whenever possible.
  • On sodic soils, avoid constructing large formed roads with table and mitre drains, as disturbing fragile soils will accelerate gully erosion. Trying to overpower erosion problems with major machinery intervention can accelerate gully erosion on sodic soils.   
  • Divert surface water runoff early along flow paths to prevent concentrated flow.
  • Install frequent, large water diversion banks (whoa boys) on top of the existing soil surface every 10 to 25m on highly erodible slopes >5%.
  • Construct high (>0.5m) and wide (5-10m) diversion banks to ensure long-term functionality and drivability.
  • Avoid diverting water into old gullies, toward creek/river banks and into hollows susceptible to gullying.
  • Where space is available on stable soils, dig a silt pond with sill outlet for diverting water into. Use the material to construct the bank (whoa boy).
  • Import angular gravel from stable local sources to armour steep road slopes and diversion banks at stream crossing approaches. Angular gravel/rock is preferred over river gravel, but both are better than native dispersible or sodic soil.
  • Seed disturbed areas with appropriate perennial grass species. 
  • Develop annual road maintenance plans and procedures appropriate for dispersible or sodic soils. Annually patch up problem erosion areas and repair drainage structures. Allocate road maintenance budgets to proactively address problem areas, rather than let them develop into larger erosion problems that cost more in the long run.
  • Require grader/bulldozer drivers to attend workshops on erosion control.

Fence Erosion: Prevention and Repair on Dispersible or Sodic Soils

Fence lines can concentrate water and accelerate gully erosion when improperly placed, constructed, or maintained, in addition to when they are graded as fire breaks, used as roads, and cut by cattle tracks (pads). Proper fence placement around erodible soils can minimise future erosion and prolong the life of the fence. Best management practices to reduce gully erosion along fences include avoiding soil disturbance, using live trees as fence posts on steep banks, minimizing tree clearing and grass grading, installing water diversion banks armoured with gravel (Plate 16), and using prescribed fire, herbicides and/or slashers for fire breaks and vegetation management.

Plate 16

Draft BMPs for Fence Line Erosion Prevention and Repair on Dispersible or Sodic Soils

  • Avoid and minimise fence line crossings though hollows, gullies, stream channels, and steep banks in dispersible or sodic soil.
  • Do not repeatedly grade fence lines as fire breaks and road access, as this will accelerate erosion.
  • Bring past windrows from deep grading back onto the fence line surface to avoid concentrating water, and use material to patch erosion areas and/or construct water diversion banks.
  • Install frequent, large water diversion banks (whoa boys) every 10 to 25 m depending on slope; manage surface water runoff to prevent concentrated flow.
  • Construct high (>0.5m) and wide (5-10m) diversion banks to ensure long-term functionality and prevent future machine operators from grading through them.
  • On steep slopes in erodible soil, armour water diversion banks with non-sodic soil/gravel to prevent cutting by rills and gullies. Use caution to not accelerate erosion during construction on steep erodible slopes.
  • Minimise the amount of tree and grass vegetation cleared and graded during fence installation and maintenance.
  • For fences down steep banks at crossings, use existing live trees as fence posts (tree-to-tree) to avoid the need for tree clearing and soil disturbance. 
  • Use good fire management and variable early-dry season burning to control undesired fires.
  • If fencing must be used for fire breaks, use slashing and herbicides rather than grading fence lines and accelerating erosion. Maintain good grass cover in erosion sensitive areas.

Develop annual fence maintenance plans and procedures appropriate for dispersible or sodic soils. Annually patch up erosion hotspots, repair water diversion structures, and hand repair fences on steep banks and stream crossings. Require grader drivers to attend workshops on erosion control.