Item 8A~.
AGENDA ITEM #8A
MARCH 13, 2006
CITY OF ATLANTIC BEACH
CITY COMMISSION STAFF REPORT
AGENDA ITEM: Request for Commission Direction with respect to revision of the
definition of Impervious Surface within Section 24-17 of Chapter 24, the Land
Development Regulations, related to the designation of swimming pools as
pervious area.
SUBMITTED BY: Rick Carper, P.E.
Public Works Director
DATE: March 6, 200fi
BACKGROUND: At the November 28th meeting, the .City Commission inquired as to the
history of the current definition for Impervious Surface, which specifically exempted
swimming pools from being counted as an impervious surface and, following
discussion of this matter, requested that this definition be modified. A draft
ordinance to revise the Impervious Surface definition to include swimming pools was
prepared by Staff. Along with the proposed definition change for pavers, this
~" requested change was referred to the Community Development Board at the
January 9th Commission meeting.
The Community Development Board considered these revisions to the definition of
Impervious Surface at their January 17th meeting. After considerable discussion by
the CD Board and comments, from the public, the CD Board recommended that the
~" definition remain unchanged with respect to the treatment of swimming pools as
pervious area, but recommended that other changes related to the use of pavers,
pervious concrete or open grid paving systems be adopted by the City Commission.
~" A major factor in the CD Board decision was the ability of swimming pools to absorb
about 6 inches of rainfall (compared to the COAB design 25 year / 24 hour storm
rainfall of 9.3 inches) before there is any runoff from the pool surface area.
~.
At the January 23'~ Commission meeting, Staff was tasked with further investigating
how to classify the water surface area of swimming pools, including the rationale of
adjacent cities that included this area in their definition of impervious area.
Additionally, Staff was tasked with studying how this proposed change in definition
would impact Core City properties if adopted by the City.
Staff inquired of St Johns County, Neptune Beach and St. Augustine Beach for the
,~„ rationale for classifying swimming pools as impervious. The two responses received
indicated it was essentially because pools have an impervious bottom and do not
allow percolation.
Attachments 1) and 2) provide a verbal and graphical summary of the impact of
changing the impervious definition to include the water surface area of swimming
pools.
The original purpose for adoption of impervious surface limits was to ensure runoff
~. did not exceed the capacity of the installed storm water collection system, effectively
March 13, 2006 regular meeting
AGENDA ITEM #8A
MARCH 13, 2006
~, rolling back flood control improvements made by the Core City Project. Drainage
studies for the Core City improvements indicated there should be no home flooding
for the 25-year storm. As discussed and recommended by the CD Board, a
standard in-ground swimming pool is able to absorb substantial amounts of rainfall
(generally 6 inches) before contributing to runoff, therefore the water surface area
should be defined as partially impervious. The City's design storm for on-site
retention purposes (defined after the impervious limit was instituted) is 9.3 inches.
As a percentage, swimming pools can contain 64.5% of the 25-year storm (6"/9.3").
For ease of review and calculation, Staff recommends pool water surface areas be
~. considered 60% pervious.
BUDGET: No budget issues.
RECOMMENDATION: Direct Staff to prepare draft ordinance Modifying Land
Development Regulations Section 24-17 to define swimming pools as 60% pervious.
ATTACHMENTS: 1) Comparison -Impact Of Treating Swimming Pools As Impervious or
Pervious in Core City
~. 2) Diagrams of Lot Coverage (max footprint, pool 100% impervious,
pool 40% impervious)
REVIEWED BY CITY MANAGER:
*~* March 13, 2006 regular meeting
gas
Impact Of Treating Swimming Pools As Impervious
AGENDA ITEM #8A
MARCH 13, 2006
Primary lots of concern in Old Atlantic Beach have an average size of 5000 SF
(50' x 100').
Impervious Surface Area coverage is limited to 50% for RS-2 zoning by LDR
24-105 (f) (1). Max impervious surface for 5000 SF lot is 2500 SF.
Maximum home footprint with existing setback requirements is 2100 SF
(35'x60').
With pool and impervious water surface area:
Average size of pools in Old Atlantic Beach is between 400 and 450 SF.
Assume an impervious pool deck of 350 SF.
Using 450 SF, 1700 SF is available for home and driveway (and separate
garage) coverage.
Assuming a 400 SF driveway (typical of recent redevelopment in Old Atlantic
,,,, Beach), maximum structure footprint would be 1300 SF.
If the City adopts a Floor Area Ratio (FAR) of 0.45, as recommended by the
Winters & Co. report, this property could have an inhabited building area of 2250 SF.
"°" This could equate to a 1300 SF 1st floor and 950 SF 2nd story. For athree-story home
(maximum possible with 35 foot height limit), average floor area would be 750 SF
(25'x30').
With pool and pervious water surface area:
Assume an impervious pool deck of 350 SF.
SF.
This leaves 2150 SF for home and driveway coverage
Again assume a 400 SF driveway, maximum home footprint would be 1750
With the 0.45 FAR, this property would still have an inhabited building area of 2250 SF.
This could equate to a 1000 SF 1st floor and 950 SF 2nd story, with room fora 450 SF
garage.
Summary: adoption of a FAR restriction has potential for substantially greater impact on
home size than designating swimming pool water surface area as impervious.
With pool and 60% pervious water surface area:
Pool water surface counts for 180 SF impervious, with an impervious pool
deck of 350 SF.
This leaves 1970 SF for home and driveway coverage
SF.
Again assume a 400 SF driveway, maximum home footprint would be 1570
With the 0.45 FAR, this property would still have an inhabited building area of 2250 SF.
This could equate to a 1120 SF 1st and 2nd story, with room fora 450 SF garage.
Attachment 1
March 13, 2006 Regular Meeting
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