Under Shapes and Forms Feng Shui, it is considered inauspicious to have a large open drain or canal behind the house.
To neutralise this and create a symbolic mountain behind the house, high walls is good.
Thus "if it cannot be seen (at ground level from inside the house), it is no longer a threat" applies.

1. Please refer to attachment.
2. In the attachment, there are three plots that are circled in RED.
3. In the attached government survey map, the circled (RED) areas shows that the plot is often undergoing re-building.
4. Often, either by coincidence or otherwise, all the three plots circled in RED have a large canal behind the home.
5. By coincidence, most likely these homes have changed hands e.g. sold by the existing owners for one reason or the other. And often, a new owner would rebuilt a home on these plot of land.
6. If you notice carefully, to the right of the canal and the three circled areas, there are few if any plots that are undergoing a rebuilt.

Quote

On 11/6/2011 3:54:09 PM, Anonymous wrote:
Under Shapes and Forms Feng
Shui, it is considered
inauspicious to have a large
open drain or canal behind the
house.To neutralise this and
create a symbolic mountain
behind the house, high walls
is good.Thus "if it cannot be
seen (at ground level from
inside the house), it is no
longer a threat" applies.

For your eg on Dunbar walk , so you saying all the houses with river at their back is not good ? 

As mentioned, it is less than ideal. But the earlier article also mentioned a fix.