What best explains hamzah in صَحَائِفُ?

Miscellaneous Morphological Explanations: Eight Rules for Wāw, Yāʾ, Alif and Hamzah

• Every wāw becomes hamzah
• Every alif always becomes hamzah
• All definite nouns contain hamzah
• Substitution of an extra long-vowel letter after the alif in faʿāʾil