SiPaul,
Thank you for clarifying the situation a little more for me. I am going to put on my Director of Operations hat and describe my approach that I would take on this problem.
Firstly, check with the planning and building departments to review the development of the offending property. Hopefully there is a generic text which relates to drainage and/or sump pump flow on the surface. Some communities prohibit overland flows and specify that the sump must pump directly into the storm system, however, most are silent. Also check to see whether there were any building or development conditions specific to the sump pump or landscaping.
Secondly, I would check with the planning/building departments to ascertain the municipality's policys and statutes relative to overland drainage. Most will specify generic references and responsibilities. Some will be drafted after a significant event has occurred, and those tend to be the most restrictive. This may help you in resolving the problem or at least give you some teeth with respect to funding splits.
Thirdly, the drainage study could either help or hinder. If it was known to be a problem and the municipality did not adhere or move with some of the recommendations, then it may hurt you a bit. If however, the recommendations were adopted in the form of a policy or bylaw, then one or both of the land Owners may be responsible, particularly if the development occurred post-report.
Forthly, I would investigate the possibility of obtaining a small easement through the landowner's property and see if overland drainage is a possibility. One mechanism that you may wish to investigate with the municipal solicitors would be to expropriate a narrow right of way, even perhaps straddling both property lines. Can some minor curbing or ditching resolve the issue?
I hope this helps a bit.
Gene
KRS Services