Gen 7:17–20: the waters prevailed; "fifteen cubits upward"

Says וַיְהִי הַמַּבּוּל אַרְבָּעִים יוֹם, "and the flood was forty days" on the earth (Swete adds the nights; note.textual.Gen.7.1-Gen.7.24.minor-variants); the waters וַיִּרְבּוּ, "increased," וַיִּשְׂאוּ אֶת־הַתֵּבָה, "and lifted the ark," וַתָּרָם, "and it rose." 7:18: וַיִּגְבְּרוּ הַמַּיִם, "and the waters prevailed" (BDB for the Qal "be strong, mighty," "prevail"), וַתֵּלֶךְ הַתֵּבָה עַל־פְּנֵי הַמָּיִם, "and the ark went on the face of the waters." 7:19: גָּבְרוּ מְאֹד מְאֹד, "prevailed very, very much," וַיְכֻסּוּ כָּל־הֶהָרִים הַגְּבֹהִים אֲשֶׁר־תַּחַת כָּל־הַשָּׁמָיִם, "and all the high mountains under all the heavens were covered" (BDB for the Pual "be covered"). 7:20: חֲמֵשׁ עֶשְׂרֵה אַמָּה מִלְמַעְלָה גָּבְרוּ הַמָּיִם, "fifteen cubits upward the waters prevailed," וַיְכֻסּוּ הֶהָרִים, "and the mountains were covered." Swete: δέκα πέντε πήχεις ἐπάνω ὑψώθη τὸ ὕδωρ, "the water rose fifteen cubits above." The feet and meters in the BSB's footnote are a conversion, not the Hebrew.

Implies The verb is one of strength, not height: the waters "prevail" in 7:18, 19, 20 and 24; their rising is told by other verbs in 7:17.

Uncertain What the fifteen cubits measure. (1) The depth of water over the highest mountains: ground, the clause stands between the two statements that the mountains were covered; objection, it does not name the mountains as its base. (2) A depth tied to the ark, half its thirty cubits of height (6:15): ground, the water it would need to float clear; objection, 7:20 does not name the ark (note.language.Gen.6.14-Gen.6.16.ark-dimensions-gopher). Neither reading settles the flood's extent. No lean is taken here.