I'm a little confused by what you are trying to say; I'm not sure which
you want to be your cause and which to be your effect so I have written
them both ways.
1)
Environmental impacts would be caused by harbour reclamation. [cause = harbour reclamation. effect = environmental impacts]
It should be noted that the sentence, "
Harbour reclamation would cause environmental impacts,"
sounds a lot better. This is because in English your meaning is easier
to understand with CAUSE first, then EFFECT. It is how people think! ^_^
2) Both sentences are rather awkward, but technically correct.
Engineering and environmental problems must be caused by modifying the infiltration capacity.
Engineering and environmental problems have to be caused by modifying the infiltration capacity.
However a better choice would be: "The only way to cause engineering and environmental problems would be by modifying the infiltration capacity." (do you see the CAUSE/EFFECT again?) I'm a bit confused as to WHY you want to cause problems, but that is how you wrote it.