Are the events of the biblical Noah plausible ?

         Raja Petra's account of the biblical Noah in his article
Sunday bible studies Noah expounds on the biblical event of
the great flood. He estimates that around 20 million or so
species of vertebrates (excluding fish) and Noah's extended
family were saved from the catastrophic flood.

        The tanakh does not mention numbers or types. It simply
mentions beasts הַבְּהֵמָ֡ה (ha-be-hemah) and birds מֵע֧וֹףo (me-o-p)
genesis 7:3-4

Why did Elohim(YHWH) destroy human kind?

ה  וַיַּרְא יְהוָה, כִּי רַבָּה רָעַת הָאָדָם בָּאָרֶץ, וְכָל-יֵצֶר מַחְשְׁבֹת לִבּוֹ, רַק רַע כָּל-הַיּוֹם.

Va-yar Elohim(YHWH) ki ra-bah ra-at ha-adam ba-a-ret ve-kal
ye-ser mah-se-bot lib-ow raq ra kal - hayovm.
Vayinahem Elohim(YHWH) ki-asah et ha-adam ba-aret vayiaseb
el lib-ow
 

 And saw, Elohim, that, great, the wickedness, of man, on the earth,
and every, imagine, of the thoughts, his heart, only, evil, all, the time.
And was sorry, Elohim, that, breath made, the man,on earth, and it
grieved, in- (his) heart. ~ Genesis 6:5

 

Noah however found grace in the eyes of the Lord

 

ח  וְנֹחַ, מָצָא חֵן בְּעֵינֵי יְהוָה

 

Elohim(YHWH)   b(e)-e-ne  hen  ma-sa  va-no-ah

 

And Noah, found, grace, in the eyes, (the) Lord  ~ Genesis 6:8

The Lord called him a righteous and a wholehearted man. Is it because
the Lord imputed this upon him, or was he righteous by his own strength ?