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Watershed 10/70 Rule

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Watershed Review Board Cases (Includes all Agenda Notes), click here 

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

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Stormwater Fact Sheet:


Web Links:

Ordinances & Maps:   

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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