Backfill canals or trenches: brackish/salt marshes
Overall effectiveness category Likely to be beneficial
Number of studies: 4
Background information and definitions
Backfilling involves returning dredged or excavated material to a canal (e.g. dug for boat traffic) or trench (e.g. dug for pipelines). Sometimes additional soil or sediment is brought in if there is not enough excavated material left. In theory, backfilling restores more natural wetland conditions: the water depth in the canal and the height of adjacent spoil heaps are both reduced. The whole area then has more natural water levels and may support desirable marsh or swamp vegetation. Backfilling a canal will usually prevent boats from using it too. The success of this action may depend heavily on the skill of the operator, e.g. their ability to create the desired water/soil elevations and avoid overcompacting the fill material. Turner et al. (1994) estimated that backfilling canals in Louisiana cost US$1.20/m3 (US$1.98 corrected to 2017).
For this action, as throughout the synopsis, we have only summarized results that are solely or predominantly related to the specified habitat. For example, the results in Turner et al. (1994) combine data from approximately 80% brackish or salt marshes and 20% freshwater marshes – so they have not been summarized as evidence for freshwater marshes.
Evidence summarized for this action relates to effects on vegetation within or immediately adjacent to canals or trenches, dug as or associated with service corridors.
Related actions: Plug/dam canals or trenches; Raise water level to restore degraded marshes; Raise water level to restore/create marshes from other land uses; Fill/block ditches not associated with service corridors; Remove surface soil/sediment.
Turner R.E., Lee J.M. & Neill C. (1994) Backfilling canals to restore wetlands: empirical results in coastal Louisiana. Wetlands Ecology and Management, 3, 63–78.
Supporting evidence from individual studies
A replicated study in 1983–1984 of 31 backfilled canals of varying salinity in Louisiana, USA (Neill & Turner 1987) reported that all but one developed some coverage of marsh vegetation within 6–60 months, and that aquatic vegetation was present in most. Considering only the 26 in brackish and saline canals, emergent marsh vegetation coverage was 53% in former spoil areas alongside the channels, on average (range 0–99% for individual canals) but <1% within the channels, on average (range 0–40% for individual canals). The study suggests that some of the variation between canals could be related to the quality of the backfilling/skill of the dredge operator. Of 27 canals of varying salinity (but mostly brackish or saline), submerged or floating aquatic vegetation was present in 18. Methods: The area of marsh vegetation alongside and within 31 backfilled canals in brackish and salt marshes was estimated from aerial photographs taken in 1983 and 1984. Submerged vegetation was identified in ground surveys. The canals, originally dug by the oil and gas industry, had been backfilled with adjacent spoil between 1979 and 1984. This reduced their water depth to 0.1–1.8 m. Eighteen of the canals had also been plugged at one end with earth or shell dams. This study selected canals from the same master set of 33 used in (3) and (4).Study and other actions tested
A paired, site comparison study in 1984 of a backfilled canal crossing predominantly brackish and saline marshes in Louisiana, USA (Abernethy & Gosselink 1988) reported that it developed coverage of emergent vegetation over four years, but that this remained lower than in natural marshes. Statistical significance was not assessed. In 65 of 83 sampled sections, emergent vegetation coverage was lower within the backfilled canal than in adjacent undisturbed marsh (data not clearly reported). Vegetation coverage in the backfilled canal varied with canal width, excavation method, substrate and coverage in the adjacent marsh (factors which were themselves correlated). The backfilled canal contained 2–10 submerged plant species, depending on salinity, with submerged vegetation present at 10–59% of sampling points (data not reported for undisturbed marsh). Methods: In 1979–1980, a canal dug for an oil pipeline was immediately but incompletely backfilled with spoil. The canal predominantly crossed brackish and saline marshes (94% of study area); data for freshwater marshes were combined with weakly brackish marshes. In August 1984, vegetation was surveyed in 83 sections of the canal (each 0.62 km long) and natural marsh adjacent to each section. Emergent vegetation coverage was estimated from aerial photographs. Submerged vegetation was sampled with a rake at 20 points/section.Study and other actions tested
A replicated study in 1983–1990 of 30 backfilled canals predominantly in brackish and saline marshes in Louisiana, USA (Turner et al. 1994) reported that they developed some coverage of marsh vegetation, but mainly alongside rather than within the channels. Between 6 and 60 months after backfilling, coverage of emergent marsh vegetation was 47% on former spoil areas alongside the channels, but only 5% within the channels. Upland vegetation occurred alongside the channels, with 28% coverage, in patches where spoil had not been completely levelled. Similar coverage was recorded 6–11 years after backfilling (marsh alongside canal: 51%; marsh within canal: 5%; upland vegetation alongside canal: 26%; statistical significance of changes not assessed). Methods: The area of marsh vegetation alongside and within 30 backfilled canals was estimated from aerial photographs taken in 1983, 1984 and 1990. This study selected canals from the same master set of 33 used in (1) and (4). The canals were originally dug by the oil and gas industry. They were backfilled with adjacent spoil between 1979 and 1984, reducing the water depth. Some canals were also plugged at one end with earth or shell dams. The study does not separate results for freshwater, brackish and saline marshes, but most canals (approximately 80%) were in brackish or saline marshes.Study and other actions tested
A replicated, site comparison study in 2000–2004 of 30 backfilled canals of varying salinity in Louisiana, USA (Baustian & Turner 2006) reported that they all developed some coverage of marsh vegetation after 20–25 years, but found that they had higher plant species richness than adjacent natural marsh. Considering only the 25 brackish and saline canals, emergent marsh vegetation coverage was 65% in former spoil areas alongside the channels, on average (range 5–95% for individual canals) but only 1% within the channels, on average (range 0–100% for individual canals). The study suggests that marsh vegetation coverage on spoil banks was related to how much of the spoil bank was actually levelled to marsh elevations. For 22 canals of varying salinity (but mostly brackish or saline), plant species richness was greater alongside backfilled canals (11 species/6 m2) than in nearby natural marsh (6 species/6 m2). Remnant spoil banks supported some upland species. Methods: The area of marsh vegetation alongside and within 30 canals was estimated from aerial photographs and field surveys in 2000 and 2004. Plant species were recorded alongside 22 canals (six 1-m2 quadrats/canal) and in nearby natural marsh (six 1-m2 quadrats/site). The canals, originally dug by the oil and gas industry, had been backfilled with adjacent spoil between 1979 and 1984. Between 5 and 100% of the spoil heaps alongside each canal were levelled, and the canals were made shallower (but not filled completely). Some canals were plugged at one end with earth or shell dams. This study selected canals from the same master set of 33 used in (1) and (3).Study and other actions tested