PrePro2004: Comparison with Standard Hydrologic Modeling

Procedures

Rebecca Riggs

April 29, 2005

Hydrologic Modeling The use of physical or mathematical

techniques to simulate the hydrologic cycle and its effects on a watershed

Most commonly used to simulate stormwater runoff in a watershed

HEC-HMS

US Army Corps of Engineers Hydrologic Modeling System

Used to electronically simulate precipitation-runoff processes of dendritic watershed systems

Outputs include peak flows and runoff hydrographs

Outputs used directly or in conjunction with other programs

3 Components of HMS

1. Basin Model Watershed Parameters

2. Meteorologic ModelPrecipitation and Evapo-transpiration Parameters

3. Control Specifications Start and End Date/Time of Simulation

Basin Model

Watershed parameters: sub-basins, reachs, junctions, sources, sinks, and reservoirs

Commonly data is derived from contour maps (USGS or survey data), land use maps, and soil maps

Tedious and time consuming

Sub-basins

Reach Parameters

PrePro2004

A GIS pre-processor which extracts hydrologic information from spatial data for HMS modeling

Add-in tool in Arc-GIS interface Much faster than conventional

methods for calculating basin model parameters

Study Area

College Station, Texas Castlegate Subdivision Spring Creek, before confluence with

Lick Creek

Study Area

PrePro2004 Basic Steps

1. Gather data2. Fill sinks, create FDIR grid, create FACC grid3. Construct stream network4. Add inlet, outlet, and/or reservoir5. Delineate watershed6. Extract HMS elements7. Calculate parameters8. Calculate curve numbers9. Export data to IDM10.Export IDM to HMS11.Setup HMS project

1. Gather Data

DEM, stream vector data, mask grid, soils vector or grid data, land use vector or grid data

DEM source: USGS seamless Soils and land use (zoning) from

COCS Stream vector data from US EPA

Lower Brazos arc data

2. Fill sinks, create FDIR grid, create FACC grid

Fill: Cell elevation raised to lowest surrounding cell elevation

FDIR: Flow direction grid FACC: Flow accumulation grid

32 64 128

16 1

8 4 2

3. Construct Stream Network• Chose cell threshold = stream drainage area

4. Add Inlet/Outlet/Reservoir

5. Delineate Watershed

6. Extract HMS Elements

Under Vectorization tab in Watershed Delineation tool

Creates shapefiles to be directly imported to a target geodatabase

Elements include watershed, reach, junction, source, and reservoirs

Following element extraction can merge basins, but this was not selected in this project

7. Calculate Parameters

Generates longest flow path for each sub-basin, HMSCode, and extracts slope and elevation data from DEM

Data stored in ‘watershed’ and ‘reach’ layers

8. Calculate Curve No.

• Need soils data, land use data, watershed data (previously created), and CN lookup table

Curve No. Continued

• Vector data was used, but raster data could be used if available

• Soils data was given by individual HSG, not a % of each

• Zoning was used for land use, then equated to the NLCD land use codes manually

• After calculating CN and impervious cover are stored in the ‘watershed’ layer

9. Export Data to IDM

• Prior to this step gage weights can be calculated, a frequency storm is used in this project so gage weighting was skipped

• Exports data from previous steps to project and basin geodatabases (preferably empty)

• Following this step time of concentration for each subbasin is calculated (Calculate Parameters)

10. IDM to HMS

• Creates input files for HMS from data stored in basin geodatabase– Basin file– Meteorologic file (if gage data is used)

• Must propogate fields with chosen methods for loss rate, transform, and routing

11. Import to HMS

• Import basin and meteorologic files into HMS

• Import background map and grid data

Results

Basin Comparison

Comments

• The sub-basins would have been more similar had I added more user defined outlets

• Time savings is huge

• Good for large scale projects, but difficult to accurately define watersheds with tool on small scale (development scale)

Questions?