1-20-24 Handout 2Issues Related to Chapter 24 Stormwater Regulations
Gregory Powell, PhD, PE
1) Section 24-17 (Page 15) Definition of Impervious Surface
a) First sentence is correct, the rest of the definition needs to be modified to be correct.
b) While it is true that many surfaces made of concrete, asphalt, brick, or plastic are
impervious, the definition of an Impervious surface SHOULD NOT be based solely on the
materials of construction. It should be determined primarily on the infiltration rate.
c) There are many products made from concrete, asphalt, brick, and plastic that may be
partially or effectively PERVIOUIS. Examples include:
i) Permeable Pavers,
ii) Porous Pavers or "Turf blocks",
iii) Permeable Concrete or Asphalt,
iv) Pervious Pavers, and
v) Artificial Turf
d) To exclude these produces based solely on the material of construction is a significant
stormwater management mistake.
e) In Northeast Florida, rainfall events with an intensity of 4.0 inches/hour have a return
period of 100 -years (US Department of Commerce, Weather Bureau, Technical Paper No.
25). Also, most PERVIOUS surfaces in Atlantic Beach have an infiltration rate less than
4.0 inches/hour.
f) Consequently, if test data or a manufacturer's specification for a product made of
concrete, asphalt, brick, or plastic confirms a long-term infiltration rate greater than or
equal to 4.0 inches/hour, it should be classified as PERVIOUS.
g) SUGESTION TO CORRECTING THE DEFINITION OF IMPERVIOUS SURFACE:
i) In the 2nd sentence, insert the work "may" before the word "include".
ii) Insert the following sentence between the 2nd and 3rd sentence. "Construction
products or the surface materials that may be impervious, as discussed above, can be
classified as PERVIOUS, if test results or manufacturer specifications confirm that the
long-term infiltration rate is greater than or equal to 4.0 inches/hour, and the base or
lining is not impervious."
iii) Delete the last sentence. This statement about pools is incorrect (see #3 below).
2) Calculation of a credit for materials with infiltration rates less than 4.0 inches/hour.
a) IMPERVIOUS (%) = [1 -(tested infiltration rate (inches/hour)/4.0 (inches/hour))]*100
b) PERVIOUS (%) =100- IMPERVIOUS (%)
January 20, 2024 Page 1 of 3
3) Classifying all swimming pools as 50% impervious is incorrect.
a) Except for infinity pools, which should be regulated separately as 100% IMPERVIOUS, the
vast majority of standard swimming pools should be classified as 100% PERVIOUS.
b) I have provided City Staff with a spreadsheet, based on the Curve Number method, that
demonstrates this fact. And would be happy to provide it to anyone else that is
interested.
c) The vast majority of pools use a surface skimmer to remove floating debris, and the
skimmer is usually installed 4.5 to 5.5 inches below the top of the copping, which
defines the pool freeboard.
d) If the pool freeboard is 4.3 inches or more, runoff from the pool during the COAB design
storm will be equal to or less than the runoff from a typical PERVIOUS surface in Atlantic
Beach (i.e., the pool will not cause any EXCESS runoff).
e) If for some reason, the pool does not comply with these standard design specifications,
the spreadsheet I provided can be used to easily adjust the pervious/impervious credit
that should be allowed.
4) Section 24-89(c)(9) (Page 74-75) Volume Calculations for Onsite Storage
a) The onsite storage calculation uses a "Modified Rational" method. While this method
can be used to simplify the runoff volume calculation, the runoff coefficient (the C -value)
must be appropriate for the intended purpose. The runoff coefficient used in this
regulation is NOT correct.
b) The coefficients referenced in this regulation for IMPERVIOUS and undeveloped (i.e.,
PERVIOUS) conditions may be appropriate for a runoff rate calculation, depending on
the specified storm intensity, duration, and frequency. However, they are NOT correct
for a volume calculation using the design storm specified in Chapter 24 (i.e., 25 -year, 24-
hour, 9.3 inch event).
c) Using the standard Curve Number method, which is appropriate for calculating the
runoff volumes for the design storm, the runoff coefficient for the volume calculation
should be 0.44, NOT 0.92 as specified in this section of Chapter 24. Again, I have
provided City Staff a spreadsheet showing how this calculation should be done.
d) IN CONCLUSION: The current onsite storage formula is incorrect. It requires the
property owner to install more than twice the onsite storage as is necessary to meet the
stated objective of controlling excess runoff from impervious surfaces. This error can be
corrected simply by specifying that the runoff coefficient to be used in the formula is
0.44.
January 20, 2024 Page 2 of 3
5) Section 24-90(a)(4) and (b) (Pages 76-77) Artificial Turf
a) The fourth location condition, that Artificial Turf is prohibited within the dripline of a
regulated tree, is unjustified. The most obvious, but unstated, justification is that
Artificial Turf is proposed to be regulated as an IMPERVIOUS surface and that an
impervious surface may impact the tree.
b) If this is the justification, then why is this location criteria not applied to sidewalks,
roadways, driveways, and other surfaces that are also impervious? Regulations and
justifications should be consistent.
c) There are hundreds of examples within Atlantic Beach of impervious surfaces within the
dripline of a regulated tree, with no apparent harm to the tree.
d) Again, as with other construction material, Artificial Turf should be regulated
as PERVIOUS, if test data or manufacturer's specifications show the
infiltration rate is 4.0 inches/hour or greater.
6) Professional Engineering Certification
a) As a Professional Engineer I would suggest the following be included in
Chapter 24:
If a Professional Engineer (PE), licensed in the State of Florida, provides the
COAB with a signed and sealed engineering report for an onsite stormwater
management system (including necessary plans, specifications, and
calculations), and certifies that the system meets the objectives and
requirements of Chapter 24, only the City Engineer, or a licensed engineer
under the City Engineer's responsible charge, shall review the plans and
approve/disapprove the permit. Furthermore, If the COAB chooses not to
review the plans within a reasonable time (TBD), the permit should be
automatically approved. However, if the City Engineer requests further
information from the PE, the review clock stops until the PE responds.
b) This request is not intended to preclude a property owner from providing the
COAB with the necessary information for a permit. It is intended to ensure
that the work of a licensed Professional Engineer is reviewed by a similarly
qualified engineer.
January 20, 2024 Page 3 of 3