Short answer. You cannot simply let them decay. Article 515 of the Civil Code obliges a landowner with defensive water works to either make the necessary repairs themselves or allow affected neighboring owners to come onto the land and do so. Letting them fall into disrepair while neighbors suffer is not an option the statute permits.
What the law says
shall be obliged, at his election, either to make the necessary repairs or construction himself, or to permit them to be done, without damage to him, by the owners of the lands which suffer or are clearly exposed to suffer injury
Civil Code, Article 515 — Duty To Maintain Defensive Works. Read the full provision →
The duty Article 515 imposes on you
Article 515 of the Civil Code applies when land contains defensive works that hold back or redirect water, or when a change in a watercourse makes it necessary to reconstruct such works. In either situation, the landowner is obliged to act. The statute gives the owner a choice of form — repair the works yourself, or allow the neighboring owners who are harmed (or clearly exposed to being harmed) to come onto your land and do the work themselves. What the statute does not allow is to do nothing while the works deteriorate and neighboring land floods.
What 'at his election' means: two paths, no third option
The phrase at his election gives you control over which path you take, not whether you take one. If you choose to repair or reconstruct the works yourself, you bear the cost and responsibility of doing so. If you decline to do it personally, you must permit them to be done, without damage to him, by the owners of the lands which suffer or are clearly exposed to suffer injury. The neighbors enter, carry out the necessary work, and leave — and you may not obstruct that access. The statute ensures that the burden of maintaining protective infrastructure does not fall entirely on those downstream who cannot reach the problem.
When the obligation applies: existing works and changed watercourses
The article covers two situations. The first is straightforward: defensive works already exist on your land and need repair to continue functioning. The second arises when a watercourse changes its course — a channel shifts, a river bends — and works that once existed in one place must now be reconstructed in a different location on your land. In both cases the obligation is the same. The article does not require that flooding have already occurred; the threat of injury — being clearly exposed to harm — is enough to trigger neighboring owners' right to enter and repair.
What neighbors are and are not entitled to do
Neighbors who are harmed or threatened may enter your land and perform the necessary repair or construction, but only to the extent required by the water works. The statute adds the qualifier without damage to him — the entry and work must not cause unnecessary harm to the landowner's property beyond what the repair itself requires. This is a mutual constraint: you cannot obstruct them, and they cannot cause you unrelated damage. The right of entry is limited to the specific purpose of making the defensive works functional again.