1. Map-Based Hydrology and Hydraulics
ArcView
Input Data
DEM
ArcView
Flood
plain maps
CRWR-PrePro
Hec-GeoRAS
HEC-RAS
Water
surface
profiles
HEC-HMS
Flood
discharge

2. Austin Digital Elevation Model
Waller Creek

3. Austin Watersheds

4. CRWR-PrePro Digital Elevation Model Stream Map
ArcView-based preprocessor
for HEC-Hydrologic Modeling
System (HEC-HMS)
Control point locations
Soil and Land Use Maps
HMS Basin File

5. DEM Watersheds for Austin

6. Selected Watersheds and Streams
Mansfield Dam
Colorado
River

7. HMS Schematic Prepared with CRWR-PrePro
Mansfield Dam
Colorado
River

8. HMS Model of the Austin Region

9. HMS Results
Watershed 155
Junction 44

10. GIS-Based Flood Modeling
Terrain analysis using Digital Elevation Models
Flood hydrology model for the Austin region
Creating flood plain maps for Waller Creek
Real time flood emergency management

11. Map-Based Hydrology and Hydraulics
ArcView
Input Data
DEM
ArcView
Flood
plain maps
CRWR-PrePro
AvRAS
HEC-RAS
Water
surface
profiles
HEC-HMS
Flood
discharge

12. Colorado River Network
1:100,000 scale
Developed from
EPA River Reach File 3
(the predecessor of the NHD)

13. City of Austin Stream Network
Developed from 1”=100’
Capco Areal photogrammetry
1:1200 scale

14. Stream Definition: Waller Creek
Austin Watersheds with Streams derived from Aerial Photographs
Streamlines generated by the aerial photographs are not always continuous.

15. Information for Correcting Stream Network
DEM
Contours
Storm sewers
Orthophotos

16. Resulting Corrected Stream
Subsequent steps:
Verification of corrected streams by flood hydrologists.
Running “tracer” program to connect arcs.
Burning of streams into DEM.

17. Waller Creek HMS Model

18. Flood Plain Mapping

19. Connecting HMS and RAS

20. Map-Based Hydrology and Hydraulics
ArcView
Input Data
DEM
ArcView
Flood
plain maps
CRWR-PrePro
AvRAS
HEC-RAS
Water
surface
profiles
HEC-HMS
Flood
discharge

21. HEC-RAS: Background
River Analysis System model of the U.S. Army Corps of Engineers
Input = cross-section geometry and flow rates
Output = flood water elevations
Cross-Section Schematic
HEC-RAS is the Hydrologic Engineering Center River Analysis System. HEC is an office of the U.S. Army Corps of Engineers. HEC-RAS is a computer model designed to aid hydraulic engineers in stream channel analysis and floodplain determination. The model results are typically applied in floodplain management and flood insurance studies in order to evaluate floodway encroachments.
To analyze stream flow, HEC-RAS represents the stream as a set of cross-sections along the channel. The model input parameters primarily consist of channel geometry descriptions and water flow rates. At each cross-section, bank stations are identified. These points are used to divide the cross-section into segments of left floodway, main channel, and right floodway:

22. Waller Creek
Watersheds
Network
Channel

23. HEC-RAS: Cross-Section Description
Points describe channel and floodway geometry
Bank station locations
Water surface elevations and floodplain boundaries
At each cross-section, several geometry parameters are required to describe shape, elevation, and relative location along the stream:
River station (cross-section) number.
Lateral and elevation coordinates for each terrain point.
Left and right bank station locations.
Reach lengths between adjacent cross-sections
Manning's roughness coefficients.
Channel contraction and expansion coefficients.
Geometric description of any hydraulic structures (bridges, culverts, weirs, etc.).
After defining the stream geometry, flow values for each reach within the river system are entered and you can run the model.

24. Discharge at a Particular Cross-Section

25. HEC-RAS: Output Graphical Text File
For steady gradually varied flow, the primary procedure for computing water surface profiles between cross-sections is called the standard step method. The basic computational procedure is based on iterative solution of the energy equation. Given the flow and water surface elevation at one cross-section, the goal of the standard step method is to compute the water surface elevation at the adjacent upstream cross-section.
The output of the model comes in two primary forms: a graphical xyz perspective plot, and in ASCII text format. The picture is nice and the values useful for analyses, but there is no relationship to geographic reality.
At this point, the hydraulic engineer would typically return to the original contour maps showing the cross-sections, and plot the water elevations. In this manner, the floodplain extent is determined. This could quickly become tedious if the goal is to evaluate different flow scenarios. My work aims to automate the floodplain mapping process.

26. Floodplain Mapping: Plan View
So by following the procedure outlined in the previous slides, you can make a floodplain map such as this. I think the orthophoto is superior to topographic maps in that is allows you to see the landscape as it really appears. Using zoom tools in ArcView, the user can easily compare the location of the floodplain versus that of structures of interest, such as roads and buildings. By clicking on the nearest cross-section, you can determine flood elevation.
However, a two-dimensional map such as this shows only flood extent. Depth information would also be useful. A three-dimensional floodplain representation is required for this type of analysis. But before I discuss 3D floodplain mapping, I need to introduce some basic GIS concepts.

27. 3D Terrain Modeling: Ultimate Goal