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City of AtlantaMayor Kasim Reed

Department of Watershed Management

Raymond J. Wilke, P.E., CFMWatershed Director

Brian J. Rainwater, P.E.Civil Engineering Manager

March 27, 2014

ManagingUncharted

Waters

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Managing the Floodplainin “Historically Modified”

Drainage Basinsby Implementing a

Unique Approach inFloodplain Regulation

Uncharted Waters

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The intent of this presentation is to convey issues encountered by DWM during the first 9 months of implementation of the city’s revised Flood Area Regulations (Ordinance) and since the effective date of the latest FIRM panels for Fulton County.

AGENDA Background

The Challenge

Historically Modified

Flood Classification – Options and Implementation

Examples – Ordinance Issues and Opportunities

Mitigation

Take Aways (for “Smaller” and “Developing” Communities)

Purpose

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AP Photo/The Atlanta Journal & Constitution, Curtis Compton)

Background – Something To Think About

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The multi-year project by the State to re-examine the Upper Chattahoochee River portion of metro Atlanta’s flood risks is complete and maps have become effective

Much of what changed on the maps was based upon improvements in the available data and technology

More precise topography based upon LIDAR mapping has greatly improved the accuracy of flood mapping

Better modeling, better maps, better customer service – win, win, win

But could there be more that we could do to assist and protect our customers and to improve the quality our watersheds?

Current riverine models of the older, developed urban core are simply not sufficient in all circumstances

Background – Can We Do Better?

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Changes to the extent and elevation of the base flood in the “Traditional” Floodplain are not the only issues which should be considered for updated mapping

Older sections of Atlanta include combined sewer systems (CSSs) that are designed to collect rainwater runoff and domestic sewage in the same pipe

Photo by Alan Cressler, USGS.

Background ‐ The Work Is Not Finished

Sometimes, these pipes simply cannot handle the flows (wet weather events)

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Background - History

Former Streams and Branches are no longer surface features, but pipes buried in the ground.

Five Points, and the surrounding area, has undergone major modifications since this photo was taken of the Western & Atlantic Roundhouse (near Phillips Arena) in September of 1864

Present day location of Phillips ArenaPresent day CNN Center

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Background - History

Phillips Arena

Former Location of

The Roundhouse

DWM Offices

1864 picture taken approx. here looking north

CNN Center

MARTAFive Points

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As part of recent floodplain re-mapping for the Upper Chattahoochee Basin, DWM moved into “uncharted waters” regarding floodplain management

FM typically involves analyzing, summarizing, mapping and managing the “Traditional Floodplain”

DWM embarked upon a difficult yet prudent attempt to protect its customers by also including FM in the “Historically Modified Floodplain” (HMFP)

In order to implement regulatory changes in the HMFP, DWM also needed to update and modify the city’s Flood Area Regulations prior to the September 13, 2013 new Effective Date for Fulton County Flood Insurance Rate Maps (FIRMs)

Updated Flood Area Regulations became effective in June of 2013

The document included a section dedicated to the HMFP

The Challenge

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According to the MNGWPD Floodplain Model Ordinance:

• "Floodplain" means any land area susceptible to flooding.

• “Flood” or "flooding" means a general and temporary condition of partial or complete inundation of normally dry land areas from:

(a.) the overflow of inland or tidal waters; or (b.) the unusual and rapid accumulation, or runoff of surface waters from any source.

Any source?

What about localized ponding of stormwater?

Ponding from Combined Sewer Systems?

The Challenge

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According to the latest City of Atlanta Flood Area Regulations:

• Traditional floodplain means any area of land presently having natural or man-made contours of elevation at or below the base flood elevation shown on the flood maps AND those areas of land which are not so shown on the flood maps where inundation is likely to occur during a base flood as determined from evidence of prior flooding OR as determined by hydrologic studies to establish the probable depth of such floodwaters.

• Historically modified floodplain means an area, in a combined sewershed, in which significant portions of the historic "natural" stream have been enclosed in a pipe (excluding bridges, roadway culverts and short pipe segments) AND is subject to periods of severe inundation.

Why the distinction?

Traditional flooding and localized ponding are not the same

And they cannot be handled the same way

Historically Modified - Definitions

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Development limited the ability of the “piped streams” to carry resultant flows.

Pipe inundation and “localized ponding” were the inevitable result.

Historically Modified - “Localized Ponding”

Is this

“Localized Ponding”

or is it

Flooding

(September, 2009)

This area was not shown on previous

flood maps

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Historically Modified - Localized Ponding! Conventional thinking does not consider these “ponded” areas to be part of the Special

Flood Hazard Area

The city considered their inclusion on the updated Flood Insurance Rate Maps (F.I.R.M. panels) a critical factor in properly managing inundated areas.

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How to regulate stormwater ponding if it is not on the F.I.R.M.?

By adding the HMFP areas to the flood panels and updated regulations, DWM anticipates future investments related to site development and redevelopment within the HMFP to comply with the Stormwater Ordinance (water quantity and quality)

This will ensure that improvements to the condition of properties proximate to the HMFP will be properly managed and protected.

“Historically Modified” – Now Regulated

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DWM worked with and provided hydraulic modeling results to FEMA contractors as necessary to include the ponded areas on the newly effective September 18, 2013 F.I.R.M. maps.

In the Atlanta downtown and midtown areas, Fulton County F.I.R.M. panels now depict the ponded areas as Shaded Zone X.

Historically Modified - But Not Overly Regulated

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Modeling of the city’s urban core, in the combined systems area

Ponding was primarily due to inadequate conveyance capacity, not poor inlet conditions

Historically Modified – Areal Extent

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How should CSO Zones be classified?

If classified Zone A, residents affected would be required to purchase flood insurance

Since CSO flooding is more often a hydraulic problem, and not necessarily a hydrologic problem, would this be fair?

A CSO study is not necessarily connected and equivalent to riverine flood plain models

Thus, for the City of Atlanta, the CSO areas were shown on the new flood maps as shaded Zone X, as they are not identified by conventional FEMA practices or riverine modeling

These “Historically Modified Floodplain” (HMFP) areas required a DUAL SET OF criteria within the City’s Floodplain Ordinance for development and redevelopment

By investing as necessary to improve the conditions in the HMFP “bowl” areas, the adjoining properties will be properly protected

Flood Classification ‐ Options

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Flood Classification ‐ ImplementationWhat other steps is the City of Atlanta taking to relieve

CSO problems?

Flood Plain Ordinance revision – WHY?

• Study indicates flooded areas at some CS inlets (CSO condition)

• Ma and Pa Kettle want to build a deck/addition

Why must we buy insurance?

Never been flooded – lived here for 50 years

• Is study wrong? Are engineers wrong?

• Hydraulic Capacity versus Inlet Capacity

• Solution: Shaded Zone X

• other issues related to remapping, including interpretations of floodplain fill issues within CSO area in the urban core

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Flood Classification ‐ Implementation

Post Development Stormwater Management Ordinance Revision

New houses and large additions - manage first 1.0 inch of runoff

Promoting the use of Green Infrastructure

BMPs – Pervious Pavement

BMPs – Green Roofs

Water Quality?

Retrofiting?

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Ma and Pa Kettle Ma and Pa have lived at the same location for 35 years and have no/minimal loan Their property has not been, and is not currently, located in the “Traditional Floodplain” An existing storm inlet is located 75 feet from their home The front corner of their home is located in the HMFP created by the inlet What effects? Based upon the newly effective FIRM panels, the front corner of their house lies within

the limits of a “Shaded Zone X”

Examples

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• Pro – DWM was able to send a letter to Ma and Pa prior to the new maps becoming effective that their parcel had a “moderate risk” of flooding, and that they may want to “consider” purchasing a flood insurance policy

• Pro – Ma and Pa purchased a relatively inexpensive policy “in the event of” localized ponding

• Pro – Closer scrutiny of re-development at the property to prevent redirecting ponded water toward or onto neighboring properties

• Pro – FEMA does not require flood insurance in a “Shaded Zone X”• Con – Mortgage company now thinks that flood insurance is required• Con – Selling property could be more difficult if potential purchaser needs a

mortgage• Con – Closer scrutiny during Building Permit process if Ma and Pa want to add a

porch, deck, room

Examples

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Ms. Smith wants to add a garage on to her house Ms. Smith has lived at the same location for 18 years Her property has not been, and is not currently, located in the “Traditional Floodplain” Existing house is now located in a HMFP, and thus is located in a “Shaded Zone X” What effects? In order to mitigate the effects of flooding, Ms. Smith could grade her site to remove the

area planned for the addition out of the flooded area. HOWEVER: • Based upon the “former” flood area regulations, fill in the FP would not have been allowed.

Fill in the HMFP is allowed, as long as “compensation” for fill is provided

Examples

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Other things to consider:• Pro – DWM was able to send a letter to Ms. Smith prior to the new maps becoming

effective that their parcel had a “moderate risk” of flooding, and that she may want to “consider” purchasing a flood insurance policy

• Pro – FEMA does not require flood insurance in a “Shaded Zone X”• Pro – Closer scrutiny of re-development at the property to prevent redirecting

ponded water toward or onto neighboring properties• Con – Mortgage company now thinks that flood insurance is required• Con – Selling property could be more difficult if potential purchaser needs a

mortgage• Con – Closer scrutiny during Building Permit process if Ma and Pa want to add a

porch, deck, room

Examples

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Mr. Austin would like to build a carriage house for his aging mother Mr. Austin recently moved into his house 2 years ago His property has not been, and is not currently, located in the “Traditional Floodplain” An existing storm inlet is located 75 feet from his home The house is not located in the HMFP. HOWEVER… The location for the Carriage House is near the center of the HMFP Three sanitary easements also encumbered his property This particular HMFP is frequently inundated. What effects? NO NEW CONSTRUCTION POSSIBLE without mitigation It would be possible to mitigate the overall effect of the HMFP, but it would be financially

difficult for a homeowner to mitigate for just a single structure installation

Examples

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Elizabeth wants to develop a commercial piece of property Elizabeth inherited a commercial property and would like to develop it into a mixed use

development A large portion of the property is located in the HMFP Because of the extent of the HMFP, compensated fill is not possible Elizabeth’s engineer designs a vault, under a parking deck, to contain the HMFP volume

AND to meet the stormwater ordinance for water quantity and quality This “improvement” reduces the HMFP risk to adjacent property owners

Examples

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Mitigation

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Redevelopment can be part of the solution

Developers must comply with Stormwater Management and Floodplain Management Ordinances

• Includes provisions for new & re‐developed in HMFP’s

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Mitigation

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In order to dampen the effects of “ponding” in the yellow area shown in the graphic below, this city is also planning to install two underground storage vaults for flood control

Vaults are being considered in other areas as well.

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Take Aways – for “Smaller” and “Developing” Communities

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A lack of knowledge in the late 19th and the first half of the 20th centuriesregarding how “combined systems” would affect the environment led to regrettable decisions for many municipalities across this country

On behalf of the City of Atlanta, DWM has been tasked to utilize redevelopment to improve upon the past

Some of the combined areas cannot feasibly be amended

Bad history should not repeat itself

WE HAVE A CHOICE:

"We learn from history that we learn nothing from history“George Bernard Shaw

"Those who cannot remember the past are condemned to repeat it"George Santayana

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Do you have ditches, swales and areas that were riverine that are now piped?

Every community has areas where localized ponding occurs, yet not everyone in the community is aware of the condition

Consider using Shaded X in your community for localized ponding areas

Consider the results of development and redevelopment

Are areas experiencing new construction making things better or worse? (They should be better)

“Smaller” and “Developing” Communities have a chance to “Stay ahead of the curve”

Can you think of a recent stormwater pipe installation that may actually contribute to these sorts of problems (i.e. – did you design the pipe for Q100?

Are we saying design all stormpipes for Q100?

Take Aways – for “Smaller” and “Developing” Communities

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