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February 8, 2018 Planning Commission Agenda Item #1
Subject Worksession regarding stormwater to be held after
completion of other agenda items. Briefings None Contact and/or
Presenter Information
Tom Gerstenberger, P.E., Stormwater Engineering Division
Manager
941-708-7450 ext. 7228
Action Requested
This is a worksession; no action required.
Enabling/Regulating Authority M/A Background Discussion
Materials to be provided at work session.
County Attorney Review Not Reviewed (No apparent legal issues)
Explanation of Other Reviewing Attorney N/A Instructions to Board
Records N/A Cost and Funds Source Account Number and Name N/A
Manatee County Government Administrative BuildingFirst Floor
Chambers
February 8, 2018 9:00 am
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Amount and Frequency of Recurring Costs N/A
Manatee County Government Administrative BuildingFirst Floor
Chambers
February 8, 2018 9:00 am
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Stormwater Design Criteria
Planning Commission February 8, 2018
Presented by: Thomas Gerstenberger, P.E.
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Can we make things better?
This is an aerial view of flooding along Pearce Drain during
August 26 & 27, 2017. The flood identified pros
and cons of the current state of modern stormwater design in
Manatee County.
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Overview • Stormwater Retention Ponds • Attenuation and Water
Quality Treatment Standards • Floodplain Mitigation • Watershed
Studies • Flood Map Modernization • Stormwater Design verses Storm
Event • What is next?
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Stormwater Retention Ponds
Pre-1985 Development Post-1985 Development
Modern stormwater design and permitting originated from state
regulations implemented in 1985. As evident
from this aerial comparison, the more recent development
includes retention ponds.
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Stormwater Retention Ponds
Wet Retention Pond Dry Retention Pond
Most modern stormwater management systems utilize retention
ponds for design and permitting.
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Wet Retention Pond • Most common form of stormwater retention. •
Artificially created pond or lake to provide
attenuation (storage) and treatment of runoff. Excavated
material typically used as fill on developments or roadways.
• Water quality treatment is typically a natural process of
absorption by aquatic plants in a littoral zone or residence time
associated with a permanent pool of water.
• Typically used for larger developments (greater than a ½
acre).
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Wet Retention Pond
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Wet Retention Pond
This is an example of a 6.4 acre wet retention pond for an
elementary school. The elementary school campus
(drainage area) is 30.4 acres including 5.85 acres of impervious
coverage, and the wet retention pond.
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Wet Retention Pond
How much water is stored in this wet retention pond? Pumped
empty, this wet retention stores over 2,000,000 cubic feet or
14,900,000 gallons (22 Olympic pools) from
normal water level to pond bottom.
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Dry Retention Pond • Typically used in small sites (less than a
½ acre) or
where space is limited. • Water quality treatment is typically
done by a
sidedrain or underdrain filtration system or percolation of
runoff into the ground.
• Seasonal High Water Table (SHWT) must be determined to prevent
muddy bottom and nuisance plant growth.
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Dry Retention Pond
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Attenuation and Water Quality Treatment Standards
This is a outfall structure typically found with any stormwater
retention pond. The design of the weir openings shown here are
normal water level to pond bottom dependent on the design
standards. This video reflects runoff
discharging through weir openings
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Water Quality Criteria • For a dry retention pond, treatment
volume is ½”
runoff from the site. This volume is designed to drawdown
(percolate) into the ground within 36 hours.
• For a wet retention pond, treatment volume is 1” runoff from
the site. Treatment volume is discharged in 120 hours and no more
than half in 60 hours.
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Water Quantity Criteria • Retention ponds attenuate (store) the
volume and
discharge runoff for a 25-year/24-hour storm frequency event
which equates to 8 inches of rainfall.
• As a standard practice, runoff rate of discharge in proposed
condition can not exceed runoff in a pre-development condition.
• Runoff rate of discharge is typically controlled by the weir
and orifice openings associated with an Outfall Control
Structure.
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Stormwater Design Criteria • For certain watersheds in the
County prone to
flooding, the County imposes a reduction in allowable
pre-development runoff rate of discharge.
• 50% reduction in allowable runoff has been required for
Bowlees Creek, Buffalo Canal, Rattlesnake Slough, and Pearce Drain
since 1989.
• 50% reduction in allowable runoff has been required for
Sugarhouse Creek, Slaughter Drain, and a portion of Mill Creek
since 2005.
• 25% reduction in allowable runoff has been required for Braden
River since 1999.
• Flow reduction is essentially a safety factor.
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Stormwater Design Criteria • For Lake Manatee, Evers Reservoir,
and
Outstanding Florida Waters (OFW), the county imposed 150% water
quality treatment. Sarasota Bay, Palma Sola Bay, Terra Ciea Bay
Aquatic Preserve are all Outstanding Florida Waters.
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Stormwater Design Criteria
Map exhibit of watersheds subject to a flow reduction and/or
additional water quality treatment.
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Floodplain Mitigation
This may look like a typical wet retention pond, however this is
actually a floodplain compensation area.
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Floodplain Mitigation
This is an aerial view the floodplain compensation area
including the associated subdivision and a separate wet
retention pond.
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Floodplain Mitigation
This is an aerial view of flooding along Pearce Drain during
August 26 & 27, 2017. Here is the same floodplain
compensation area occupied by flood waters.
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Floodplain Mitigation
The Floodplain Exhibit shown here provides a representative
example of floodplain impact (fill) and
the compensation area (cut).
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Floodplain Mitigation • Floodplain mitigation (also
referred to as “Cup-for-Cup”) is the balance (or surplus) of
compensation required for encroachment into the 100-year
Floodplain.
• “Cup for Cup” is literally replacing every cup of fill that is
placed in the floodplain with an equivalent empty cup.
• Compensation credited above Seasonal High Water Table.
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Watershed Studies • Modernized delineation of 100-year
floodplain
utilizing survey data, Record Drawing and permitting
information, and drainage modeling.
• Include GIS mapping and inventory database of drainage
infrastructure.
• Watershed delineation includes partitioning of drainage
sub-basins based upon topography and drainage structures.
• Drainage models are comprised of hundreds and thousands of
input files.
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Rattlesnake Slough Watershed
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Rattlesnake Slough Watershed
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Watershed Studies • The County has adopted watershed studies
for
Braden River (including Rattlesnake Slough), Buffalo Canal, and
Gamble Creek.
• The County has three watershed studies underway for Mill
Creek, Bowlees Creek and Pearce Drain.
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Watershed Studies
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Watershed Studies The County does not have modern watershed
studies available for: North County: McMullen Creek, Tampa Gap
Drain, Canal Road Drain, Government Hammock, Big Chimney Drain,
Carr Drain, and Slaughter Drain. South County: Sugarhouse
Creek/Glenn Creek, Williams Creek, Cypress Strand, Gates Creek,
West Branch Cedar Hammock, South Branch Cedar Hammock, and Palma
Sola Drain.
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Flood Map Modernization • The County recently began utilizing
the latest
version of the Flood Insurance Rate Maps (FIRM) effective March
17, 2014.
• These flood maps incorporate data from the Buffalo Canal
watershed study, digital topographic mapping (LIDAR), and GIS
mapping.
• The County is in the process of amending the FIRM panels
associated with Gamble Creek Watershed.
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FEMA 2014 FIRM
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FEMA 2014 FIRM
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Stormwater Design Verses Storm Event
How does modern stormwater compare to the August 26 & 27,
2017, event?
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Bowlees Creek Station
This is the rain and stream gauge Station on Bowlees Creek at
63rd Avenue East. This is one of 13 stations
throughout the county collecting valuable data.
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Pearce Drain Rainfall History
Pearce Drain station at 63rd Avenue East has been collecting
rainfall data since 2001. As reflected on the
chart above, highest daily rainfall totals occurred with T.S.
Gabrielle, T.S. Hermine, and most recently August 2017.
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Design Standard verses Observed August 2017
DESIGN STANDARD • Design Rainfall 25-year/24-hour= 8.0
inches
PEARCE DRAIN • Observed Rainfall Maximum 24-hour = 9.67 inches
(August 26th from 5 p.m. until August 27th 5 p.m.) • Observed
Rainfall Total Maximum 3-day = 13.25
inches (August 25th 8 p.m. until August 28th 8 p.m.)
BOWLEES CREEK • Observed Rainfall at Maximum 24-hour = 11.9
inches
(August 26th 7 p.m. until August 27th 7 p.m.) • Observed
Rainfall Total Maximum 3-day = 17.78
inches (August 25th 9 p.m. until August 28th 9 p.m.)
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Design Standard verses Observed August 2017 • The observed
rainfall event surpassed the design
standard! • The observed rainfall over 24-hour period at
Bowlees Creek and Pearce Drain were comparable to a
100-year/24-hour storm event.
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What is Next? • The most accurate flood data is available now
for
Buffalo Canal, Gamble Creek, and Braden River Watershed.
• Within 2 or 3 years watershed study floodplain delineation and
modeling will be available for Bowlees Creek, Pearce Drain, and
Mill Creek.
• Additional watershed studies are necessary for a number of
watersheds in Manatee County.
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What is Next? For development within adopted watershed studies:
All fill within the 100-year Floodplain shall be compensated by the
creation of an equal or greater storage volume above seasonal high
water table. Drainage modeling shall utilize the adopted watershed
study to demonstrate, in post-development condition, that no
adverse impacts are created to adjacent property based upon a
“no-rise” flood stage condition.
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What is Next? Flood stages are relative
elevations similar to a survey benchmark.
Floodplain mapping data, stages, and modeling are
only as reliable as the results provided and
accepted by the County. The goal is maintain or lower flood
stages, not increase flood stages
causing adverse impacts, to a “no-rise” condition.
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What is Next? • Future amendments and updates to the Land
Development Code and Stormwater Design Manual. • Drainage
modeling run for multiple storm event
scenarios, beyond the current 25-year storm event, such as
10-year, 50-year, and 100-year/24-hour storm events for attenuation
and discharge analysis. Possibly include a multi-day storm event
scenario such as a 100-year/3-day senario.
PC20180208DOCWork Session.pdfSlide Number 1Slide Number 2Slide
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Number 21Slide Number 22Slide Number 23Rattlesnake Slough
WatershedRattlesnake Slough WatershedSlide Number 26Watershed
StudiesSlide Number 28Slide Number 29FEMA 2014 FIRMFEMA 2014
FIRMSlide Number 32Slide Number 33Slide Number 34Design Standard
verses Observed August 2017Slide Number 36Slide Number 37Slide
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