ASFPM_2010_unique_approach_f

A Unique Approach to Integrating Local Municipal Needs

with Floodplain Mapping Updates

Robert Billings, PE, PH, CFMProject ManagerCharlotte-Mecklenburg Storm Water Services (County)

Danee McGee, PE, CFMProject ManagerCharlotte-Mecklenburg Storm Water Services (City)

City of Charlotte, Mecklenburg County, North Carolina

Charlotte-Mecklenburg“The Team”

Mecklenburg County Tim Trautman, Flood Mitigation Program Manager Bill Tingle, Floodplain Administrator Robert Billings, Project Manager- Remapping

City of Charlotte Tim Richards, Assistant City Engineer Jennifer G. Smith, Storm Water Division Manager Danee McGee, Project Manager- ETA Lead

Charlotte-MecklenburgBackground and History

Mecklenburg County includes:The City of CharlotteThe Towns of Cornelius, Davidson, Huntersville, Matthews, Mint Hill, and Pineville (8 NFIP Communities)

Land Area: 549 square miles

Uptown Charlotte


“Top of the hill” 5% of City floodplain Urban, national average is

7.5% floodplain


Local Flood Plain Mapping Current maps are result of County-

wide FIS completed in February 4, 2004 at a cost of $2,000,000

350 miles of detailed study streams 32 watersheds FEMA and Community flood fringe and

floodway Data reproduced in state-wide format

effective March 2, 2009 182 panels, paper and digital forms Remapping began in 2007 on two

basins , expanded in 2008 to include the majority of the remainder of the county


Flood Map Overview• Land Use and Topographic Data

1997• Future Build-out District Plans

1999• Adopted (Local – Regulatory)

5/22/00• Effective (FEMA - Flood Insurance)

2/4/04• Map Maintenance Strategy/Planning

2005-2006• Map Maintenance Implementation

2007 - present

FloodplainsCommunity (Future Conditions) Floodplain

FEMA (Existing Conditions) Floodplain

Community Encroachment Area

FEMA Floodway

Floodplain on PlansFebruary 2004 FIRM

Floodplain Regulations

and

The Flood Insurance Rate

Map (FIRM)

FEMA Floodplain Area

Community Floodplain Area

Charlotte-MecklenburgMidtown- A Case Study

Before

After

Charlotte-MecklenburgLocal Government

Mecklenburg County Budget - $1.1 Billion9 County Commissioners

SWAC – Storm Water Advisory Committee

City of CharlotteBudget - $1.39 Billion11 City Council Members

SWAC is a 9 member appointed board

SWAC is a “bridge” between City and County Stormwater Programs

SWAC’s charge is: To maintain consistent policy To identify possible duplication of

services

Charlotte-MecklenburgWorking Together

Mecklenburg County Floodplains and FloodwaysCounty Manager

Charlotte-Mecklenburg Storm Water ServicesCity of Charlotte

Capital ProjectsCity Manager

City Storm Water County Storm Water CDOT Planning Department (Commission) Consulting Engineers Citizen Task Force SWAC


Reasons for Needing a “Standards Document” Difficult to compare the results of the City and County/FEMA Analyses

Difficult to assess the downstream impacts from upstream improvements outside of FEMA system

City and County wish to improve the interoperability of two systems/approaches

Provide consistent base and clear direction on future modeling of other watersheds

Incorporate lessons learned from sensitivity analysis in the development of document


Reasons for Differences in City and County Analysis Techniques

City maintains “minor” system (<1 sq. mi.)

City drainage improvement projects often are upstream of, but end at the County/FEMA floodplain

In recent years, City has included consideration of impacts on FEMA streams from CIPs


Comparison of Differing Analysis Techniques

Parameter City County/FEMA

Project Size 200-500 acres (<1 sq. mi.) 1 - 40 sq. mi.

Project Focus

Planning, Design, and Constructions of

Drainage Improvements

Develop floodplain/floodway, and regulate constructions activities

within SFHA (Flood lands Permit).

ReferenceDocument

Charlotte-Mecklenburg Storm Water Design

Manual FEMA Guidelines and Specifications

SurveyDetailed survey of storm

water features

Countywide GIS with survey at structure crossings and sporadic

channel areas


Comparison of Differing Analysis TechniquesParameter City County/FEMA

Hydrologic ModelHEC-1/HMS/SWMM or

others HEC-1/HMS

Storm Pattern / Rainfall Depth

6-hour storm using Charlotte IDF depths SCS 24-hr Type II

Subbasins 15 - 30 acres Average 150 acresLoss

Methodology SCS Curve Number SCS Curve NumberHydrograph

Transform/Lag Time

Typically SCS Unit Hydrograph Using TR-55

Lag Time MethodSCS Unit Hydrograph Using TR-55

Lag Time Method

Routing Normal Depth, Level Pool Modified Puls

Calibration Typically none Limited Calibration

The Need for “Task Force Meetings” Process Assessment Identify Stakeholders and Define Objectives

“With recommendations from the City Council and Board of County Commissioners, County staff will select stakeholders who have property ownership or interests within the watershed. Each stakeholder group will reflect neighborhood representatives, engineers, real estate professionals, environmental advocates, developers and other interested parties.

Invitations for theTask Force



Goal of The Stakeholders Task Force:“To provide input on specific land related information that will be used in the remapping effort and to make recommendations regarding the specific land related information”

Their Task:Review adopted District Plans for their applicability to floodplain mapping.

Provide input on unique watershed features that may impact flood levels.


Estimates “existing” impervious area

Utilized: Rezonings, Planning Department ‘s Land use maps and existing aerials

No straight adoption of the Planning Department’s land use

Aerial photography to verify current development

Existing Conditions Land Use


Estimating full build out conditions

Planning Department’s Land Use plans as starting point

80 Future Land Use Categories

Translate to 12 categories impervious area categories for floodplain mapping

Comparison – Existing to Future

Future Conditions Land Use


Example Comment and ResponseComment: “greenway”Response: Recommend changing designation to Open Space.Information: Existing conditions land cover

0.25 to 0.50 Ac Residential0.50 to 2.0 Ac Residential

Future conditions land cover > 2.0 Ac Residential/Open Current percent impervious

Percent Impervious Assumption0.25 to 0.50 Ac Residential - 28%0.50 to 2.0 Ac Residential - 20%Open Space – 12%

This area is a greenway and should be Open Space inexisting and future land cover.


Plan on multiple meetings to reach consensus Meetings should have planned agendas, goals,

and documentation of decisions (i.e. Minutes) Be prepared to share about “Floodplains 101” Be prepared to answer, “Why do we need new

floodplain maps”.

Things to Keep in Mind….


Finish Briar Creek approvals

Concurrent Task Force Meetings for multiple watersheds

Start the process over again once the new maps are approved by FEMA in 2012

Next Steps

Charlotte-MecklenburgFuture of Map Maintenance

Identify Risk Hazard Mapping Map Maintenance

(keep risk current) Assess Risk

Flood Mitigation Planning

All-Hazards Planning Updated land data Elevation Certificates

Charlotte-Mecklenburg Comprehensive Approach

Charlotte-MecklenburgFuture of Map Maintenance

Communicate Risk Floodzone web application Merge map data with other data to

better communicate risk

Mitigate Risk Implement elements in Mitigation Plans Acquisitions Elevations

Charlotte-Mecklenburg Comprehensive Approach

Closing Thoughts…Take ownership of managing Flood Risks

Various levels of stewardship in mitigation Don’t need to do it all yourself (partnerships)

Think holistically Maps, Regulations, Mitigation Plans, Mitigation

Projects, Communication

Live in the present AND future Communicate Existing Risk Communicate and Regulate Future Risk Determine what type of “Future” mapping is

appropriate

QUESTIONS

Documents

ASFPM_2010_unique_approach_f