The proceeds arising from the collection of assessments levied for improvements authorized by this part and part 2 of this chapter and chapter 32 of this title shall be and constitute a separate and distinct fund. Such fund, together with its accumulations, is hereby pledged for the payment of the bonds and interest coupons issued for the improvement or improvements from the assessments of which the fund arises, and shall be applied exclusively to the payment of the bonds and coupons.