The Life and Death and Consequences of Canals and Spoil Banks in Salt Marshes

R Eugene Turner,Erick M Swenson

doi:10.1007/s13157-020-01354-w

Abstract

We describe the consequence and demise of levees (spoil banks) built from dredging canals in Louisiana salt marshes using morphometric measurements made over 30 years, soil collections on the spoil bank and in the salt marshes behind, and complementary observations from other areas. These measurements were used to determine the temporal bounds of how long spoil banks last and if salt marsh soils remaining in salt marshes are affected. If the rates of changes in spoil bank morphology continue, then the estimated life time of the shrub-tree vegetation at a representative spoil bank is 81 years, the spoil bank width is 89 years, and the dredged channel will erode to the center of the spoil bank after 118 years. The soils in marshes behind the spoil bank have a higher bulk density than in reference marshes, accumulate more mineral matter per year, have lower root mass and are weaker. These observations are compatible with measurements of spoil bank width, vegetative cover and soil compaction, and the conversion from wetland to open water on a coastwide scale.

Highlights

Canals and levees have been built for many reasons for thousands of years and with diverse outcomes
The peak height of the spoil bank (Fig. 3a) declined at 1.90 cm year−1. If it continues at this rate, it will be at the salt marsh level about 88 years after it was formed
If the current changes in spoil bank morphology since 1965 continue, the estimated life time of spoil bank vegetation at the Cocodrie canal is 81 years, the emergent vegetative cover of any kind at the site will be gone after years, the spoil bank width will be unrecognizable by years, and the center of the former spoil bank will erode into the dredged canal within 118 years

Summary

Introduction

Canals and levees have been built for many reasons for thousands of years and with diverse outcomes. Extensive canal networks existed for irrigation agriculture by 3000 to 2400 B.C. in Mesopotamia and were plagued by salinization (Jacobsen and Adams 1958), wetlands were drained throughout the Roman Empire (Allen and Fulford 1990; Rippon 2000), and extensive farming systems with levees and canals were within Amazonian swamps before European colonization (Mann 2008). Parts of the Fens in England were drained by the Romans, but it was not until the seventeenth century that it was massively altered when a confluence of Crown support, large landowners, and investors built large canals (Ash 2017). Eggelsman (1976) showed that these subsidence rates were greater with nutrient availability

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