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Floods can occur in any area, although to varying degrees.
If the property is on a hill or in an area that has not been flooded, the risk may be significantly reduced, but it is not eliminated.
A structure located within the floodplain is over 6 times as likely to incur damage from a flood as it is from a fire during the term of a 30-year mortgage.
Knowledge of flood risk is an important personal decision-making tool.
According to FEMA, a floodplain is an area that will be inundated by a flood from a river, creek, ditch, lake, or other source of flooding.
The floodplain is the area that would be under water for a 1-percent annual chance flood. Also called the 100-year flood.
On statistical average, it would occur once every 100 years, but could occur twice in same year or in back-to-back years ( SMC Floodplain Study, Tony Wolff ).
Floodplains (Cont.) Source: SMC Floodplain Studies for Bull Creek, Tony Wolff
Why Study Floodplains
To establish 100-year floodplain limits within or near development. This helps to protect natural resources, property, and persons.
Land use changes were increasing flood levels and, consequently, flood risk.
Flood study needed to take corrective and preventive measures for reducing flood damage (FEMA).
Identify risk-free zones to increase economic incentives for development.
Flood Damage Source: Tool Hire
Approximate Study – Defined Special Flood Hazard Area (SFHA), but Base Flood Elevations (BFEs) are not published.
Limited Detailed Study – Defined SFHA and the BFEs may or may not be published on the FIRM and in the Flood Insurance Report (FIS) report.
Detailed Study – The SFHA and the 0.2-percent annual chance floodplain are defined and the BFEs are published.
Moderate to Low Risk Areas
High Risk Areas
Zone AH (shallow flooding – ponding area)
Zone AO (shallow flooding – sheet flow)
Zone AR (building or flood control system)
Flood Zones (Cont.)
High Risk – Coastal Areas
Undetermined Risk Areas
Approximate Floodplain Study Techniques
Discharge – Drainage Area Relationship
Regional Regression Equation
Weir Depth Flow
Approximate Floodplain Study Techniques (Cont.)
Surveyed High Water Marks
Starting Water Surface Elevation
GIS based Techniques (GeoRAS, WISE, etc.)
Discharge – Drainage Area Relationship: Information obtained from an FIS report, other state or local floodplain studies, subdivision reports, and highways/roadway reports.
Regional Regression Equations: Regression equations based on gauged data from actual watersheds within the region of application and equations developed by USGS.
Watershed Modeling: All FEMA approved hydrology models such as TR-55, HEC-1, HEC-HMS, SWMM.
Rational Formula: Small watershed less than 1 square mile.
Normal Depth: Uniform, steady, and one-dimensional flow. Appropriate for development of equal or less than 50 lots or 5 acres.
Weir Depth Flow: This method is appropriate for the streams where flow is affected by downstream obstructions (bridges, culverts, dams, etc). or flow changes.
Surveyed High Water Marks: This includes water marks obtained from previous flood events for the stream of interest.
Starting Water Surface Elevation: One-dimensional and steady flow, step-backwater model.
Floodplain Mapping – GIS Based Techniques
Floodplain Delineation Tool
A Case Study on Ash Creek, Lackawanna County, PA
Hydrology: Regional regression equation was used to obtained base flood.
Hydraulic: HEC-RAS hydraulic model was used to obtained 100-year water surface elevation.
Floodplain mapping: GeoRAS was used to delineate the approximate floodplain.
Watershed and sub watershed were delineated using DEM. Arc Hydro extension was used to delineate the watershed boundaries.
Outlets points were generated upstream, at most junctions, and downstream of the streams.
Additional outlets points were added at the desired location to delineate the sub-watershed.
Regression equation was used to determine the peak flow for the studied stream.
Study area was covered in peak-flow region 1 in Pennsylvania.
Regression model used to compute the peak flood discharge:
Q = 10 A (DA) b (El) c (1+.01C) d (1+.01U) e (1+.1Sto) f
Q, peak flow, in cubic feet per second;
A is the intercept;
DA is drainage area, in square miles;
El is mean elevation, in feet;
C is percentage of basin underlain by carbonate bedrock, in percent;
U is percentage of urban area, in percent;
Sto is percentage of storage, in percent; and
b, c, d, e, and f, are basin characteristic coefficients of regression.
Peak-flow regions in Pennsylvania ( USGS Water-Resources Investigations Report 00-4189 )
Hydrology (Cont.) Regression model used to compute the peak flood discharge for region 1: Q = 10 A (DA) b (1+0.1Sto) f Where, Q, peak flow, in cubic feet per second; A is the intercept; DA is drainage area, in square miles; Sto is percentage of storage, in percent; and b and f, are basin characteristic coefficients of regression.
HEC-GeoRAS PreRAS Process Assign Stream name and Reach ID Assign Flowpath directions
Geometric data were imported into HEC-RAS.
Data include stream centerline and cross-section geometry.
Flows determined in hydrology used for HEC-RAS simulation.
Boundary condition was used as a known Water Surface Elevation (WSEL) or Normal Depth.
Hydraulic HEC-RAS Process (Cont.)
Run the steady flow model
Save the data and export the GIS data
HEC-RAS Process (Cont.)
Import RAS SDF file
Setup the PostRAS Layer
Floodplain Mapping PostRAS Process (Cont.)
Floodplain Mapping Smoothing
Helpful and expeditious method.
Powerful tool for visualizing and processing spatial data.
Tool is ideally suited for development and analysis of hydraulic model and approximate floodplain delineation.
Preliminary estimates indicate that approximate floodplain area can be delineated using the GeoRAS techniques in less than 2 months at an average of 1.5 – 2.0 miles/hr.