As a rule this is unlawful. There are a couple of exceptions that do no seem to apply. The common enemy doctrine is the main exception and I do not see that it would apply. Be sure to express your objection in writing. If you do nothing you risk waiving your rights.
Since 1896, the law regarding surface water in Washington State has been governed by the “common enemy doctrine.” In its original form, the common enemy doctrine allowed landowners to dispose of unwanted surface water in any desired manner, without liability for any resulting damage to adjoining property. In other words, surface water was viewed as a common enemy against which any property owner could defend himself.
However, in order to avoid harsh results, the common enemy doctrine has been relaxed by our Washington State Supreme Court in recent decades. There are now several judicially created exceptions to the common enemy doctrine. First, a landowner cannot inhibit the flow of a natural watercourse or drainway. Second, a landowner may not artificially collect water and then channel it onto his neighbors’ land. Third, a landowner owner must exercise due care in improving his land to minimize unnecessary surface water impacts upon adjacent lands.
Under the circumstances that you have described, it is quite possible that your neighbor is artificially collecting water and channeling it onto your property in a manner different that its natural flow. Likewise, your neighbor may also have failed to exercise due care with the manner in which he designed and dug the new ditches. Accordingly, you may have a claim for trespass, nuisance, waste, damage to land, or other similar causes of action under Washington law against your neighbor.
This answer is provided for informational purposes only and should not be relied upon or construed as legal advice or legal opinion. The answer is based upon limited facts and general principles of law, both of which are subject to change and, therefore, may materially affect any answer given. You should contact an attorney to obtain advice with respect to your particular issue or problem. This answer does not create an attorney-client relationship, nor does it create any kind of legal relationship, duty, or privilege. This attorney is licensed only in the State of Washington.
Under the common enemy law, one can divert surface waters from their property, even at the detriment of their neighbor, however they cannot channel it. So it sounds unlawful to me. There may be more facts to review but I would seek advice from an attorney.