If the Fort Langley floodplain is engineered to prevent flooding the land will sink as it did in New Orleans and has done everywhere this has been done. Floodplain land is not like farm land elsewhere as its soil is relatively loose and unpacked. When you stop or reduce flooding:

1) There is less silt deposited so there is less new soil added to the surface very year.

2) The yearly floods tend to “fluff up” the soil and decompact it. This effect is lost or reduced if flooding is reduced.

3) Tilling the land for higher value crops tends to cause compacting of the soil.

4) Tilling brings the organic matter to the surface where it decomposes which also gradually leads to loss of soil.

All these effects tend to cause the soil level to fall over time. This eventually makes the flooding worse unless even more drastic drainage measures are enacted.

These problems have certainly been seen around New Orleans as some areas have settled/subsided several feet and trying to keep these areas dry has led to massive and expensive engineering schemes which are prone to failure and which destroy the local environment.

Keeping floodplain areas in hay allows them to be productive without going down this long, expensive and damaging path.