Backfilling canals to mitigate wetland dredging in Louisiana coastal marshes

Published source details Neill C. & Turner R.E. (1987) Backfilling canals to mitigate wetland dredging in Louisiana coastal marshes. Environmental Management, 11, 823-836.
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This study is summarised as evidence for the following.

	Action	Category
	Backfill canals or trenches: brackish/salt marshes Action Link
	Plug/dam canals or trenches: brackish/salt marshes Action Link
	Backfill canals or trenches: freshwater marshes Action Link
	Plug/dam canals or trenches: freshwater marshes Action Link

Backfill canals or trenches: brackish/salt marshes

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).

(Summarised by: Nigel Taylor)
Plug/dam canals or trenches: brackish/salt marshes

A replicated, site comparison study in 1983–1984 of 27 backfilled canals of varying salinity in Louisiana, USA (Neill & Turner 1987) reported that plugged canals had greater coverage of emergent marsh vegetation than open canals after 6–60 months, and were more likely to contain submerged vegetation. Statistical significance was not assessed. Considering 17 canals in brackish and saline marshes, emergent vegetation coverage was 7% within plugged canals (vs <1% in open canals) and 53% on the former spoil areas alongside plugged canals (vs 36% alongside open canals). Considering 27 canals of varying salinity (but mostly brackish or saline), 92% of plugged canals contained floating or submerged aquatic vegetation (vs 43% of open canals). Methods: In 1983 and 1984, vegetation was surveyed in up to 13 canals plugged with earth or seashell dams at one end, and 7–14 canals that were not (or no longer) plugged. Coverage of emergent and floating vegetation was estimated from aerial photographs. Submerged vegetation was identified in ground surveys. All canals, originally dug by the oil and gas industry, had been backfilled with adjacent spoil between 1979 and 1984. This study selected canals from the same master set of 33 used in (2).

(Summarised by: Nigel Taylor)
Backfill canals or trenches: freshwater marshes

A replicated study in 1983–1984 of five backfilled canals in freshwater marshes in Louisiana, USA (Neill & Turner 1987) reported that they all developed some coverage of marsh vegetation, but mainly alongside rather than within the channels. After 6–60 months, emergent marsh vegetation coverage was 27% in former spoil areas alongside the channels, on average (range 20–62% for individual canals) but only 6% within the channels, on average (range <1–26% for individual canals). The study suggests that some of the variation between canals was related to the quality of the backfilling/skill of the dredge operator. Methods: The area of marsh vegetation alongside and within five backfilled freshwater canals was estimated from aerial photographs taken in 1983 and 1984. 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 0.4–1.4 m. Three of the canals had also been plugged at one end with earth or shell dams. Four canals in this study were also studied in (2).

(Summarised by: Nigel Taylor)
Plug/dam canals or trenches: freshwater marshes

A replicated, site comparison study in 1983–1984 of five backfilled canals in freshwater marshes in Louisiana, USA (Neill & Turner 1987) reported that emergent marsh vegetation coverage was greater within plugged than open canals, but that coverage was similar on the adjacent former spoil areas. Statistical significance was not assessed. After 6–60 months, emergent vegetation coverage was 15% within plugged canals (vs <1% in open canals) and 35% on the former spoil areas alongside plugged canals (vs 35% alongside open canals). Methods: In 1983 and 1984, vegetation was surveyed in three freshwater canals that had been plugged with earth or seashell dams at one end, and two canals that had not been plugged. Coverage of emergent marsh vegetation was estimated from aerial photographs. All canals, originally dug by the oil and gas industry, had been backfilled with adjacent spoil between 1979 and 1984.

(Summarised by: Nigel Taylor)