When the natural levee ridges are breached by a deep canal, one of the most immediate effects 

 is partial drainage and lowering of water levels in the impounded and semi-impounded hydrologic 

 subunits between the ridges. The level of the floating marsh mat is actually lowered. A hummocky 

 topography may result where clumps of firmer substrate hold the marsh at the higher level of the 

 pre -canal, unbreached system. 



The canal provides an avenue through which bi-directional tidal movement occurs. The 

 hydroperiod of the marsh is abruptly changed. It is subject to more frequent rises and falls. 

 While mean monthly salinity values may not be greatly increased, higher highs and lower lows 

 occur. These changes in hydrology and water chemistry stress the vegetation, resulting in marsh 

 die-back. 



The inflow and outflow of the tide through gaps in the spoil bank has a pumping effect to 

 which the organic sediment is highly susceptible. The most fluid and poorly consolidated material 

 (usually under the floating marsh) is subject to early removal. Organic material exposed by marsh 

 die-back is also vulnerable to erosion. As the tidal network evolves, other biological and chemical 

 processes contribute to erosion of the organic sediments. As the ponds increase in size, wave 

 action becomes a factor in the rates of further marsh loss. 



The rate of erosion is related to energy levels. The higher the velocities and the longer the 

 duration of water movement, the greater the erosion. Thus, the greater the volume of water 

 moving in and out of a tidally invaded hydrologic unit, the greater the amount of erosion and 

 sediment transport. Once initiated, feedback causes the tidal erosion process within a hydrologic 

 subunit to accelerate. 



Highest energy conditions occur during storms (winter storms and hurricanes) when water initially 

 moves into the hydrologic subunits through canal breaches. Under severe weather conditions, 

 water levels are elevated above the surface of the marsh and above the natural levee ridges. When 

 these storm tides subside, the water ultimately drains through the gaps in the canal spoil banks and 

 the canal channel through the natural levee ridges. Under natural conditions, most of the return 

 is in the form of sheet flow, and velocities are reduced by vegetation. Under human-altered 

 landscape conditions, erosion and development of the tidal channel network are accelerated during 

 such high energy events. 



The erosive process is selective. Mineral sediment, whether directly exposed along the canal 

 cut or exposed as redeposited spoil banks, is more resistant to erosion than the organic materials. 

 Further, landforms and near-surface deposits that are composed predominantly of mineral sediment 

 make up only 10% to 15% of the surface and near-surface features that are exposed to erosion. 

 Thus, most of the sediment liberated by erosion is predominantly organic. Because the organic 

 material is subject to breakdown by biological and chemical processes, the volume available for 

 redeposition is very small. Localized redeposition of eroded organic material is evident in some 

 instances where lake-rim features, spits, and canal plugs of reworked organic materials ("coffee 

 grounds") have been observed. 



Figure 9 shows the evolution of the tidal network that originated in one hydrologic subunit 

 from a gap in the spoil bank along the pipeline canal. This figure illustrates how the drainage 

 network has expanded through time, removing the soft organic substrate by the tidal scouring 

 process and destroying the marsh fabric. 



Comparative cross sections in Figure 10 illustrate some of the major effects of the pipeline 

 canal on the marshes and organic sediments lying between the natural levee ridges. Note 



42 



