Mecklenburg County Floodplain Mapping
Summary Report
Background
Numerous communities across the United States
have to deal with out-dated Federal Emergency
Management Agency (FEMA) floodplain maps.
Floodplain maps are more than 10 years old for over
50% of the communities that participate in the
National Flood Insurance Program (NFIP).
These maps are used to:
• educate the public on potential flood risks
• set minimum flood elevations and “no- fill” areas
for land development activities
• set flood insurance rates
Floodplain Management Guidance Document:
Mecklenburg County is experiencing
significant development and the accuracy of the FEMA floodplain maps have been a
concern for some time now. In fact, the County’s Floodplain Management Guidance
Document, adopted in December 1997, recommended that the County evaluate the
FEMA maps and considering taking over the floodplain mapping responsibility from
FEMA.
Mecklenburg floodplain statistics include:
• 320 miles of FEMA streams
• Mapped by FEMA in the 70’s
• Mapping based on predicted 1995 landuse
• 2000+ structures in the floodplain
• Approximately 1530 insurance policies
• 754 claims, to date
• $13,000,000 in insured losses, to date
• Floodplain information can be viewed at:
http://www.co.mecklenburg.nc.us/coeng/Storm/
Mecklenburg County, NC
Floodplain Mapping
Executive Summary
Page 1
October 25, 1999
The Floodplain Guidance Document, as mentioned above, is a strategic plan with the
following objectives:
• To prevent or reduce the loss of life, disruption of services, and damages caused by
floods
• To preserve and restore the natural and beneficial functions of the floodplain.
With the continued increase in building activity and flooding in certain areas not depicted
on the FEMA maps, the County has recognized the critical need for accurate floodplain
maps. In addition, there has been a realization that new development in and around
floodplains must be regulated to protect from “future” flooding that could be expected
from increased flow caused by continued development in upstream watersheds. On June
1, 1999 the Mecklenburg Board of County Commissioners authorized funds for the
County to retain a consulting firm (Hayes, Seay, Mattern and Mattern) and work
cooperatively with the U.S. Army Corps of Engineers and FEMA to map the County’s
floodplains. This mapping project will cost approximately $1,400,000 where the County
will fund 60% and State and Federal agencies will fund 40% of the total project costs.
Flood Hazard Mitigation Program: To compliment Mecklenburg County’s Floodplain
Mapping initiative, the Flood Hazard Mitigation Program focuses on the reduction or
elimination of flood loss potential for the 2000+ structures built in the floodplain. Flood
Hazard Mitigation Plans have been completed for Upper Little Sugar, Briar, McMullen
and
Irwin
Creek
watersheds
and
can
be
viewed
at
http://engbs.co.mecklenburg.nc.us/storm/html/mitplans/plans.htm. Through the use of
these plans, a $12,200,000 grant has been secured to remove 116 structures from the
floodplain (see diagram below). The acquisition and removal of these structures will not
only eliminate the possibility for flood losses to occur on these properties again, but is
also gives Mecklenburg County an opportunity to increase open space, develop additional
greenways and/or construct wetland areas to improve water quality.
Mecklenburg County Storm Water Services has a long-term, balanced approach of
decreasing the cost of natural disasters due to flooding. This is demonstrated by the
commitment to map floodplain areas at the local level (to help in preventing losses for
undeveloped properties) and develop mitigation plans and seek funding for
implementation of mitigation projects (to prevent flood losses for already developed
floodprone properties).
Note: The Floodplain Management Guidance Document and the Flood Hazard
Mitigation Program uses the 100-year flood as the “standard” for mapping and
determining appropriate mitigation alternatives. It is important to note that the 100- year
flood is actually a flood that has a one (1) percent chance of occurring in any given year.
It is often incorrectly thought that once a flood of this magnitude has occurred, it won’t
happen again for another 100 years. This has been proven to be incorrect here in
Mecklenburg County based on the storm events that occurred in 1995 and 1997. Another
way to look at a storm event of this magnitude is that a 100- year storm event has a 33%
chance of occurring over the life of a 30-year mortgage.
Mecklenburg County, NC
Floodplain Mapping
Executive Summary
Page 2
October 25, 1999
Report Layout:
To try to answer some basic questions related to the new
floodplain maps, the following report is organized in a question and answer format. In
the remainder of this Summary Report, the following questions are adressed (based on
the data we have to date):
•
•
•
•
•
How Are the New Floodplain Maps Being Developed?
How long before the new maps will be ready?
What has been done to reduce the potential for new construction to flood in the
future?
Why is development still going to be allowed in the floodplain?
What are the impacts to property based on the new floodplain maps?
Mecklenburg County, NC
Floodplain Mapping
Executive Summary
Page 3
October 25, 1999
How Are the New Floodplain Maps Being Developed?
General Process:
Floodplain maps are created based on a hydrologic (amount of
water generated from a storm) and hydraulic (how high the water rises in the floodplain)
analysis. The information used in a hydrologic/hydraulic analysis includes, but is not
limited to, the following:
Landuse (past, present, and future)
Slope of the land
Creek characteristics (size, shape, and roughness)
Soil Types
Rainfall Amounts
Culverts, bridges, etc
There have been numerous improvements in the field of floodplain mapping since the
FEMA floodplain maps were developed in the 1970’s for Mecklenburg County. These
improvements allow for the development of more accurate flood boundaries and can also
estimate flood conditions that may occur in the future, such as ultimate development in
the watershed. In addition to the increased computing power, there has been a significant
increase in the collection of rainfall and flood data that can be used in verifying the
results of the hydrologic/hydraulic analysis.
The flood elevations and floodplain boundaries are developed using FEMA-accepted
computer models to simulate the effect of rainfall and subsequent runoff in a watershed.
The models are built based on the unique, physical characteristics (landuse, soils, slope,
etc) of the different watersheds in the County. The models are then calibrated or verified
by using actual storms that have occurred in the past (i.e. 1995 and 1997 storms) to
ensure that the models can adequately reproduce the observed events. This calibration
process includes evaluating measured rainfall and streamflow data from the existing gage
network maintained by the U.S. Geological Survey, under contact with Mecklenburg
County. To verify previous flood levels, individual property owners along stream
reaches are interviewed to determine the heights they witnessed during in specific storm
events. These testimonials from residents provide great benefit to ensuring the accuracy
of the models that are developed, as well as soliciting community involvement.
Mecklenburg County, NC
Floodplain Mapping
Executive Summary
Page 4
October 25, 1999
Once the computer models have been calibrated using actual storm events, various
landuse conditions (present, future and ultimate buildout) and floodplain development
assumptions can be loaded into the computer. Depending on the assumptions, a
floodplain map can be developed and used for flood insurance rating purposes (FEMA
Maps) and for the local land development regulatory program (FLUM Maps).
FEMA Mapping Process:
As part of developing the floodplain maps to be sent to
FEMA, the current (1999) landuse is loaded in to the computer programs and the analysis
is performed for various flood frequencies (10, 25, 50, 100 and 500-year floods) to
establish regulatory flood elevations and areas. The storm event of most importance for
regulatory purposes is the 100-year flood (1% chance annual flood).
Once the 100-year floodplain has been established based on current landuse, an
encroachment analysis is performed. The encroachment analysis removes storage areas
from each side of the floodplain until the original water surface elevation increases by a
certain value (surcharge). Mecklenburg County’s floodplain maps developed in the 70’s
were drawn based on a 1.0’ surcharge. This is the maximum surcharge allowed by
FEMA and essentially allocates the greatest amount of floodplain for development (flood
fringe) and the minimum amount for floodwaters (floodway).
Each community can decide the surcharge value for developing their floodplain FEMA
maps. For FEMA purposes, Mecklenburg County’s floodplain maps will be drawn with
a 0.5’ surcharge value. Therefore, the storage areas on each side will be taken away unit
the water surface elevation increases by 0.5’. When the 0.5’ surcharge is met, the
boundaries of the floodway (termed: FEMA Floodway encroachment lines) will be
established. Any fill in the FEMA Floodway (i.e. between the FEMA Floodway
encroachment lines) will require a variance by the local jurisdiction and approval by
FEMA.
Mecklenburg County, NC
Floodplain Mapping
Executive Summary
Page 5
October 25, 1999
The data from this analysis, based on current (1999) landaus and a 0.5’ surcharge value),
will be provided to FEMA for the development of their new Flood Insurance Rate Map’s
(FIRM’s). These maps will be used by real estate professionals, attorneys, insurance
agents, etc in determining flood insurance rates and the potential for a property loss due
to flooding.
Floodplain Landuse Mapping (FLUM) Process:
One of the shortcomings of the
FEMA maps is that FEMA will not draw floodplain maps based on future landuse
conditions. In developing areas of the United States, this is a significant concern because
as little as 6-months after FEMA floodplain maps are made available, the maps are
outdated due to development upstream. When FEMA re-maps a community every 10-25
years, the usefulness of the FEMA maps to guide new development is severely limited.
Therefore, Mecklenburg County will develop local Floodplain Landuse Maps (FLUM)
that will contain technical information that is more restrictive than FEMA and will be
used for new development.
Mecklenburg County, NC
Floodplain Mapping
Executive Summary
Page 6
October 25, 1999
Using the existing landuse and a 0.1’ surcharge value, the flood fringes and the floodway
will be drawn based on a 0.1’ surcharge. Therefore, the storage areas on each side will be
taken away until the water surface elevation increases by 0.1’ surcharge. When the 0.1’
surcharge is met, the boundaries of the FLUM floodway (termed: FLUM Floodway
encroachment lines) will be established.
This will set aside additional area as floodway (increase the carrying capacity of the
floodplain) and will reduce the amount floodplain that can be filled and built upon. This
information will not be sent to FEMA for flood insurance rating purposes. Instead, this
information will be used in the land development process to limit the amount of fill
placed in the floodplain of undeveloped properties or the re-development/expansion of
developed properties. Any fill in the FLUM Floodway (i.e. area between the FLUM
Floodway encroachment lines) will require a variance by the local jurisdiction only;
FEMA approval is not necessary.
In addition to increasing the size of the floodway, minimum finished floor elevations
(FFE) of new structures will be based on future (ultimate) development in the watershed
to help protect new development from flooding. The future landuse (defined as ultimate
build-out as shown in locally adopted District Plans) is loaded into the
hydrologic/hydraulic computer models. New flood elevations and floodplain areas are
recomputed and this data will be used for all new building permits issued in the County.
Mecklenburg County, NC
Floodplain Mapping
Executive Summary
Page 7
October 25, 1999
Once the future floodplain elevations are calculated, new development will have to be
constructed a minimum of 1 foot above the FFE.
Below is an example of a FLUM that contains the FEMA Floodway, FEMA Floodplain,
FLUM Floodway and FLUM Floodplain. The FLUM, and the information contained
therein, can be used to: rate an insurance policy; determine developable land areas;
understand the changes in the floodplain as development occurs in the watershed; etc.
What do we know about the watersheds the County has mapped?
Revised studies have been completed for the major portion of the Mallard and McAlpine
Creek watersheds. Both of these studies clearly indicate that the current FEMA flood
elevations are too low. In addition, future development will continue to raise flood
elevations and could worsen flooding conditions. The table below provides a summary
of the average increases in flood elevations, compared to the 1970’s FEMA maps, for the
existing and future conditions.
Mecklenburg County, NC
Floodplain Mapping
Executive Summary
Page 8
October 25, 1999
Average Change in Flood Elevation Compared to Old FEMA Maps
Stream
Existing
Conditions
(ft)
High
(ft)
Low
(ft)
Future
Conditions
(ft)
High
(ft)
Low
(ft)
Mallard Creek
1.9
7.9
-5.3
4.3
11.0
-4.1
Doby Creek
-1.2
3.9
-6.1
0.7
10.7
-4.6
-1.9
-0.1
-3.6
-0.2
1.6
-2.8
1.0
4.4
-4.6
2.9
6.0
-2.7
Campbell Creek
0.9
4.6
-2.7
1.6
5.2
-1.5
Irvins Creek
0.6
8.7
-4.1
3.0
10.9
-2.1
Drainage
Area (mi2 )
Doby Creek
Tributary
McAlpine Creek
An evaluation was also performed for Mallard Creek watershed to determine the
cumulative impact of fill in the floodplain fringe. The results of this study indicate that
based on the regulations adopted along with the 1970’s FEMA maps, continued filling in
the floodplain fringe could result in increases in flood elevations of almost 2.5 feet. The
old regulations allowed for the placement of fill in approximately 50% of the floodplain,
with the remaining 50% reserved for the passage of floodwaters. Additional regulatory
requirements being considered under the Surface Water Improvement and Management
(SWIM) initiative would further restrict development in some of these areas, resulting in
increases on flood heights of closer to 1.5 feet. A summary of these impacts is included
in the table below.
Impacts of Floodplain Fill on Flood Elevations (average changes)
Stream
0.1’ + SWIM Buffer
Elevation
Discharge
(ft)
0.5’ + SWIM Buffer
Elevation
Discharge
(ft)
1.0’ (old ordinance)
Elevation
Discharge
(ft)
Mallard Creek
3.0%
0.2
11.6%
0.6
19.3%
2.3
Doby Creek
1.0%
0.02
2.2%
0.1
13.9%
1.4
Doby Creek
Tributary
0.0%
0
0.0%
0
13.2%
2.2
Mecklenburg County, NC
Floodplain Mapping
Executive Summary
Page 9
October 25, 1999
How long before the new maps will be ready?
Updated mapping has been completed and adopted for Mallard, Doby and Doby Creek
Tributary. These maps are currently being used to require new development be built
above the future 100-year flood elevations. The hydrologic/hydraulic analysis for the
remainder of the County continues and as new maps become available for specific
watersheds, they will be brought forward to County staff for review. After the review is
complete, public hearing(s) will be set so the affected governing bodies can consider
adoption of the FLUM’s. The anticipated delivery schedule of the floodplain maps is
shown below. If someone wants to locate their property more precisely on the delivery
schedule map, they can access the Storm Water site at
http://www.co.mecklenburg.nc.us/coeng/Storm/
Mecklenburg County, NC
Floodplain Mapping
Executive Summary
Page 10
October 25, 1999
What has been done to reduce the potential for new construction to flood in the
future?
An Interim Policy has been developed by staff and adopted by the Charlotte City Council
and the Mecklenburg Board of County Commissioners. The Towns will consider the
Interim Policy in October and November.
Mecklenburg County, NC
Floodplain Mapping
Executive Summary
Page 11
October 25, 1999
The Interim Policy requires that any new development in or adjacent to the floodplains be
a minimum of 5.7 feet above the current 100-year flood levels (as defined on the FEMA
maps drawn in the 70’s). This policy implements a temporary standard so development
that is permitted before new maps can be adopted will be less likely to be built below the
new flood elevations based on future development. The Interim Policy is not applicable
for properties along the Catawba River. When the County’s FLUM’s are adopted, the
Interim Policy will no longer be applicable. The Interim Policy can be viewed at
http://www.co.mecklenburg. nc.us/coeng/Storm/.
How much improvement in flooding or water quality is there if development is not
allowed in any part of the floodplain?
Based on the Mallard Creek study, the average width of the FLUM Floodplain (total
floodplain based on ultimate buildout) is 611 feet. The average width of the floodway
assuming a 1.0' surcharge (the same surcharge value used in the 1970's floodplain
mapping) is 290 feet. The average width of the FLUM Floodway, which assumes a 0.1'
surcharge, is 454 feet (approximately 75% of the FLUM Floodplain). By changing the
surcharge value from 1.0' to 0.1 feet (using 1999 landuse), the amount of floodway has
increased by over 50%.
The computer models indicate that when that additional 50% floodway is set aside for
floodwaters instead of development, the increase in water surface elevations due to filling
in the fringe is negligible (see 0.1' + SWIM Buffer in the table above). The greatest
increase in flood heights is not filling in the floodplain, it is development in the ent ire
watershed upstream.
Based on the information collected as part of the SWIM Buffer Plan discussions, there is
not a measurable improvement in water quality if a buffer is increased from 227 feet (half
of 454 feet) to 305 feet (half of 611 feet). The current SWIM Buffer Plan calls for a
minimum 100' undisturbed water quality buffers, plus 50% of the FLUM flood fringe.
With this amount of undisturbed buffer being preserved right next to the creeks, as well
as the buffers on the creeks and streams upstream of the floodplains, water quality is
expected to improve significantly.
How does the SWIM water quality buffers relate to the new floodplain maps?
As it relates to where development is and is not allowed, please see the following
examples:
SWIM Buffer controls:
The current SWIM Buffer Plan calls for a minimum 100'
undisturbed water quality buffers, plus 50% of the FLUM flood fringe. If the 100’
undisturbed water quality buffer is greater than the FLUM Floodplain, there is no filling
allowed in the FLUM fringe areas (see below).
Mecklenburg County, NC
Floodplain Mapping
Executive Summary
Page 12
October 25, 1999
Floodplain regulations controls: If the 100’ undisturbed water quality buffer is less
than the FLUM Floodway, the 100’ buffer and the equivalent of 50% of the FLUM flood
fringe must remain undisturbed (see below).
There are other factors and issues one needs to consider related to the SWIM Buffer Plan.
The SWIM Buffer Plan can be viewed at:
http://www.co.mecklenburg.nc.us/coenv/inside.htm
Mecklenburg County, NC
Floodplain Mapping
Executive Summary
Page 13
October 25, 1999