When auditing a taxpayer, the IRS agent generally asks to provide supporting documents. If receipts or other documents may not be provided - the risk is - that the IRS agent will disallow some deductions.
If receipts are not available - ask your bank for copies of canceled checks or credit card statements with dates and amounts of the expenses.
In some situations your personal records, expense account logs might be accepted.
If you've lost tax returns - you may get a tax return transcripts or actual copies of your returns from the IRS.
Reconstructing records after a flooding may be essential for tax purposes, getting federal assistance or insurance reimbursement.
If you need help - contact IRS DIsaster Assistance Hot Line 1-866-562-5227
For specific recommendation - see IRS Publication 4758, Reconstructing Your Records - page 9 - http://www.irs.gov/pub/irs-pdf/p2194b.pdf
Let me know if you need any help.