Watershed 10/70 Rule

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Watershed Review Board Cases (Includes all Agenda Notes), Watershed Review board Cases

Planning Board Notes (10/4/05) Related to 10/70
    Revised Ordinance: Attachment 1

Planning Board Notes (9/6/05) Related to 10/70
    Attachment 1 - List of ordinances in other counties
        PDF Version
        MS Word Version with active links

How the 10/70 rule would apply to the WS-IV Protected Area in Chatham County

Streamlines Articles:

10/70 Development Option of the Watershed Protection Program (Vol. 6, No. 1)
Limiting Impervious Surfaces Protects Water Quality (Vol. 1, No. 5)
Impervious Implications (Vo. 6, No. 3)
Selected Major Topics
Back Issues

Stormwater Fact Sheet:

Stormwater Problems & Impacts: Why All The Fuss? (Fact Sheet No. 1)
Back Issues

Article:

Impervious Surfaces and Water Quality: A Review of Current Literature and Its Implications for Water Planning (Journal of Planning Literature)

Web Links:

Water Supply Watershed Protection Program under the NC Division of Water Quality (NCDWQ)
Watershed Classification Table (Note: This table is the minimum established by NCDWQ; Chatham County has additional watershed classifications above this minimum standard)
Watershed Academy Web (US Environmental Protection Agency)

Ordinances & Maps:

Chatham County Watershed Protection Ordinance
NCDWQ: Model Watershed Protection Ordinance ( Footnotes on pg. 7 discuss 10/70 rule)
Chatham County Watershed Protection Map

How the 10/70 Rules Works

Under the current Watershed Protection Ordinance standards the amount of impervious surface allowed in the WS-IV Protected Area (WS-IV PA) for non-residential projects is 36% (24% if curb and gutter is used). A total of 134,380 acres of land within the County's jurisdiction is designated as WS-IV PA.

The total amount of impervious area currently allowed in the WS-IV PA = 36% x 134,380 or 48,376 acres @ 36%.

Using a 5/70 rule, 95% of the total acreage could be 36% impervious, or 95% x 134,380 x 36% = 45,958 acres @ 36%

5% of the total acreage could be 70% impervious, or 5% x 134,380 x 70% = 4,703 acres @ 70%

Total impervious acreage = 50,661 acres using a 5/70 rule.

Using the 10/70 rule, 90% of the total acreage could be 36% impervious, or 90% x 134,380 x 36% = 43,539acres @ 36%

10% of the total acreage could be 70% impervious, or 10% x 134,380 x 70% = 9,407 acres @ 70%

Total impervious acreage = 52,946 using a 10/70 rule.

So, the 5/70 rule would allow an additional 2,285 impervious and the 10/70 rule would allow an additional 4,570 impervious acres above the current 36% standard. This assumes full development of all eligible acreage.

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