The claim is that floods will not affect the graves or shrines of the Auliya.
1. The grave of the martyrs of Uhud.
Abd ar-Rahman ibn Abi Sasaca the trustworthy Tabiyee said: that he had heard that Amr ibn al-Jamuh al-Ansari and Abdullah ibn Umar al-Ansari, both of the tribe of Banu Salami, had their grave uncovered by a flood. Their grave was part of what was left after the flood. They were in the same grave, and they were among those martyred at Uhud. They were dug up so that they might be moved. They were found unchanged. It was as if they had died only the day before. One of them had been wounded, and he had put his hand over his wound and had been buried like that. His hand was pulled away from his wound and released, and it returned to where it had been. It was forty-six years between Uhud and the day they were dug up.
Malik said, “There is no harm in burying two or three men in the same grave due to necessity. The oldest one is put next to the qibla.”
Maalik’s Muwatta Book 21, Hadith 50
This narration comes from a trustworthy Tabi’i who did not witness the event himself but rather reported it based on what he had heard. This indicates that during his time, people did not believe that a flood was incapable of damaging graves. The bodies of the martyrs were found completely intact Alhumdulillah.
2. Grave of Imam Ahmad.
Ibn kathir said about the year 466 h talking about extreme flood in Baghdad:
وغرقت قبور كثيرة، من ذلك قبر الخيزران، ومقبرة أحمد بن حنبل.
And many graves were flooded, among them the grave of Al-Khayzuran and the mawbarah of Ahmad ibn Hanbal.
(al Bidaya wal Nihaya the year 466 h)
3. Likewise, Imam Ahmad couldn’t save a dome over his grave even when there was no flood at all. Even the Sufi traveler Ibn Battuta said regarding his grave:
وبالقرب منها قبر الإمام أبي عبد الله أحمد بن حنبل رضي الله عنه ولا قبة عليه. ويذكر أنها بنيت على قبره مراراً فتهدمت بقدرة الله تعالى. وقبره عند أهل بغداد معظم، وأكثرهم على مذهبه.
Near it is the grave of Imam Abi Abdillah Ahmad ibn Hanbal, may Allah be pleased with him. There is no dome over it. It is mentioned that a dome was built over his grave numerous times, but it was destroyed each time by the power of Allah. His grave is greatly admired by the people of Baghdad, most of whom follow his madhhab.
[al Rihla Ibn Battuta part 1 page 220]4. Grave of the great Wali, Maruf al Karkhi.
Same way, Ibn Kathir said regarding the grave of Maruf al Karkhi who was among the Auliyah:
وفيها: احترق قبر معروف الكرخي
In this year: The grave of Maroof al Karkhi was burnt.[al Bidaya wal Nihaya vol 12 the year 459h]
4. But when Allah wills, He can save the graves as well. Ibn al Imad Hanbali attributed to Dhahabi that he said about the grave of Imam Ahmad:
قال الذهبي ومن الآيات أن مقبرة الإمام أحمد بن حنبل غرقت سوى البيت الذي فيه ضريحه فإن الماء دخل في الدهليز علو ذراع ووقف بإذن الله تعالى وبقيت البواري عليها غبار حول القبر صح هذا عندنا
Al-Dhahabi said: Among the signs (of divine grace) is that the cemetery of Imam Ahmad ibn Hanbal was flooded except for the chamber containing his tomb. The water entered the vestibule (reaching) a height of one cubit, yet it stopped by the permission of Allah the Exalted. The mats around the grave remained with dust (untouched by water). This has been authentically established according to us.
Shadharatul Dhuhab 6/65, Dhahabi said this in Siyar 11/231
Imam al-Dhahabi affirmed that this event happened by the will of Allah, without asserting that the grave could not sink under any circumstance. In fact, the creed (‘aqeedah) of Ahlus-Sunnah holds that Allah may save the bodies of the martyrs if He wills, or He may not. Far from being a degradation, this is the only true form of spiritual upliftment, as it is entirely consistent with Islamic monotheism (tawheed). It rightly elevates the Awliya to their proper station: beloved servants of Allah, entirely subject to His will, whose ultimate reward awaits them with Him in the Hereafter and in al-Barzakh.
P.S other examples can be quoted but this is enough.