Floodplain Modeling &Mapping with GIS

Andrew Smith, Bennett Grider, and Angira Mondal

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Introduction | Background➢ In December of 2015, FEMA released new flood zone maps for

Anoka County.➢ Many 100-yr floodplain delineations resulted in new bank-initiated

assessments➢ Homeowners were notified of flood insurance premiums up to

$5,000/yr➢ FEMA began work in 2011, but did not incorporate new 2012

LiDAR data.➢ Approximate flooding zones are not calibrated to landform well➢ Forces property owners to undertake FEMA LOMA process

○ Requires elevation survey and certificate of property and structures

○ Extended time periods necessary for assessment by FEMA and potential LOMA-Out As Shown certification

➢ Experience as GIS Analyst with FEMA DFIRM floodplain mapping projects with H&H engineering firm - floodplain delineations

Traditional Solutions

➢ ArcGIS Hydrology toolset➢ Engineering solutions

○ XPSWMM○ Conceptual/physical models○ HEC-RAS, engineering parameters

■ Steady/unsteady flow sims,■ Hydraulics of culverts, bridges..■ Calculations for water storage, regression analysis, etc.

➢ Typically calibrated by field studies or correlation to observed flooding behavior

Our Methods

➢ 2D cut-fill analysis w/landform➢ May be accomplished with multiple tools and data formats

○ Terrain and TIN data analysis methods○ DEM and raster analysis methods○ Specialized software tools designed for hydrologic & hydraulic analysis - many non-GIS

➢ Choice of preparing and using DEMs format○ 2012 LiDAR derived-1m resolution DEM.○ Existing LiDAR elevation products - contour lines, building footprints

➢ Justification○ High-resolution data and strong influence of contour lines.○ Methodology used in H&H and GIS based flood-mapping industry for FEMA.

■ Approximate study or re-delineation areas only.

➢ Verification of model applicability using DEM data - shown later

DEM

Elev Raster

Raster Diff

Mapping Cross-Sections & FEMA Specifications➢ FEMA redelination guidelines used.➢ Cross-Sections should be placed close enough to

provide reasonable approximation of the flooding source’s profile based on terrain.

➢ Additional Cross-Sections should be placed at major changes in the width of the 1% annual-chance floodplain.

➢ Typically, cross-sections are placed perpendicular to the stream centerline.

➢ This approach generally works well and produces reasonable results, except where streams are highly sinuous or have multiple branches… “Guidance for Flood Risk Analysis

and Mapping”, May 2016, FEMA

Model

Model (cont.)

Base Layers

● User-specified cross-sections● 1-foot resolution LIDAR DEM● Visual examination: detailed

study matches with elevation

Raster Output from Model

● Direct result with no cleanup● Close fit with well-defined

boundaries● Importance of elevation

Vector Output from Model

● Gap filling, cleanup of small isolated areas

● Some small inaccuracies caused, but overall improvement

Model Testing | Output Validation● Detail Study area testing● Use of effective XS elevations● Comparison with effective delineation

Feature Compare ResultData Management Tools > Data Comparison > Feature Compare

➢ Structure removals… mostly➢ Additional flooding concerns

Modeling Results -Unnamed wetland, Ditch 66

Modeling Results -County Ditch 66

➢ Consistency of inundation➢ Hydraulics, XS shaping, structures➢ Potential flooding areas

➢ Removal and non-removal➢ Problematic area for effective

delineation➢ Potential flooding areas

Modeling Results -County Ditches 66 & 27

➢ Removal and improved accuracy➢ Hydraulics, XS shaping➢ Difficult areas to model...

Modeling Results -County Ditches 27

Results Review

➢ Combined analysis

➢ Aerial Imagery roles

➢ LiDAR Collection

➢ LiDAR products

➢ Raster resolution

➢ Structure awareness

➢ TIN generation

➢ Raster analysis

➢ Raster resolution matching

➢ Results clean-up

➢ Vector conversion

➢ Vector generalization

Effective FEMA Floodplain Delineations

Limitations

➢ Fundamental difficulties of modeling complex dynamic systems➢ Other significant variables like basin characteristics and soil characteristics

were not considered➢ Multiple Linear Regression may provide biased estimates

○ Measurement error in the dependent variable ○ Underfitting of model

Conclusions

➢ The city of Ramsey is and should be concerned about the accuracy of approximate 100-yr floodplain delineations that became effective in December 2015.

➢ Our project has used GIS to provide an update, as well as a different approach, to modeling these approximate flood zones.

➢ A new detailed Letter of Map Revision for problematic approximate floodplain delineation areas should be completed.

➢ Full engineering H&H methods should be applied for the new study.

Questions?