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

Amount and Frequency of Recurring Costs N/A

Manatee County Government Administrative BuildingFirst Floor Chambers

February 8, 2018 9:00 am

Stormwater Design Criteria

Planning Commission February 8, 2018

Presented by: Thomas Gerstenberger, P.E.

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.

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?

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.

Stormwater Retention Ponds

Wet Retention Pond Dry Retention Pond

Most modern stormwater management systems utilize retention ponds for design and permitting.

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).

Wet Retention Pond

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.

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.

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.

Dry Retention Pond

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

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.

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.

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.

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.

Stormwater Design Criteria

Map exhibit of watersheds subject to a flow reduction and/or additional water quality treatment.

Floodplain Mitigation

This may look like a typical wet retention pond, however this is actually a floodplain compensation area.

Floodplain Mitigation

This is an aerial view the floodplain compensation area including the associated subdivision and a separate wet

retention pond.

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.

Floodplain Mitigation

The Floodplain Exhibit shown here provides a representative example of floodplain impact (fill) and

the compensation area (cut).

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.

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.

Rattlesnake Slough Watershed

Rattlesnake Slough Watershed

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.

Watershed Studies

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.

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.

FEMA 2014 FIRM

FEMA 2014 FIRM

Stormwater Design Verses Storm Event

How does modern stormwater compare to the August 26 & 27, 2017, event?

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.

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.

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.)

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.

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.

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.

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.

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 Number 3Slide Number 4Slide Number 5Slide Number 6Slide Number 7Slide Number 8Slide Number 9Slide Number 10Slide Number 11Slide Number 12Slide Number 13Slide Number 14Slide Number 15Slide Number 16Slide Number 17Slide Number 18Slide Number 19Slide Number 20Slide 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 Number 38Slide Number 39Slide Number 40Slide Number 41