Green-Ampt Parameterization for Rainfall Excess

Determination

Harry Downing

SWFWMD

April 18, 2008

Green-Ampt (GA) Parameter Outline

• The District has changed to GA?• GA Process (General)• Data Sources for Developing GA Parameters• Look-Up Table Development (Soils and LU)• ArcHydro Tools for GA• Parameter Generation

• Questions

Rainfall Excess Process

Soil Parameter Sources

• NRCS - ‘Soil Survey’ and Soil Survey Geographical (SSURGO) Database - Depth to Water Table and Ksat

• IFAS - ‘Site Specific Soil Characterization’ – Soil Moisture Retention Information

• Table Information from Other Studies (Text Books)

• Site Specific Characteristics from ERP Applications

Soil Survey Information

Soil Survey Information Continued

Soil Moisture for 200 cm Depth to WTH

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Soil Moisture Content %

Soil Moisture

Saturation / Soil Porosity

Soil Look-Up Table

Land Use Look-up Table

Creative Land Use Look-Up Table

ESRI ArcHydro Tools

ESRI Sub-Catchment Assignment of GA Parameters

Questions?

Infiltration and Percolation Parameterization

Take You Through Process that the District has Used

Parameters Generation

1. Area Of Unsaturated and Saturated Infiltration

2. Perimeter Development

3. Hydraulic Properties of the Soil

4. Expected Depth to the Water Table (SHGWT or Higher)

5. Development of Parameters for Input Into ICPR

Area of Infiltration

Area of Unsaturated Infiltration

• First Make a Run with the GA Excess Option Only

• Obtain the Maximum Inundation Area and Elevation from the Model Results

• Develop a GIS feature class of the Inundation areas (area & perimeter) - units

• Use this Information for the Infiltration and Percolation Parameters for ICPR regarding initial area

Hydraulic Properties of Soils

• SSURGO and IFAS Data• ERP Information• Geotechnical Information Reports• Soil Testing

Location of Soil Bores Within Hernando County

Vertical Hydraulic Conductivity Results

Results from ERP Tests

SSURGO Results for Hydrologic ‘A’ Soils

• Minimum Value 0.028 inches/hr

• Mean 21.25

• Max 35 inches/hr

Assumptions

• Horizontal Hydraulic Conductivity Greater Than Vertical• Use of Vertical for Horizontal Should be Conservative

Depth to SHGWT

• Use SSURGO

• Potentiometric Surface Map

• Land Use Map

• Imagery

ICPR Parameterization

ICPR Parameterization Continued

Questions?

Floodplain ResultComparisons

Stage Plot Comparison at Node

Stage Plot Comparison at Node

Stage Plot Comparison at Node

Reasonableness of Floodplain

ITEMS TO CHECK !!

• Double Accounting - Between GA, and Infiltration and Percolation

• Area of Influence Around Infiltration Areas

Questions?