1. Map-Based Flood Hydrology and Hydraulics
David R. Maidment
Jan 10, 1998

2. Map-Based Hydrology and Hydraulics
Connecting ArcView with HMS for the Austin Region
Flood simulation using Nexrad data in Houston
HMS-RAS for Waller Creek in Austin
Creating flood plain maps in Waller Creek

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

4. Austin Digital Elevation Model
Waller Creek

5. Austin Watersheds

6. CRWR-PrePro Digital Elevation Model Stream Map
ArcView-based preprocessor
for HEC-Hydrologic Modeling
System (HEC-HMS)
Control point locations
HMS Basin File

7. Presentation Outline Using GIS to connect hydrology and meteorology
Representation of spatial objects in GIS
Terrain analysis using Digital Elevation Models
Geodesy and map projections

8. Study Region in West Austin
Hog Pen Ck
4 km
4 km

9. Watershed Delineation by Hand Digitizing
Watershed divide
Drainage direction
Outlet

10. 30 Meter Mesh Standard for 1:24,000 Scale Maps

11. DEM Elevations
720
720
Contours
740
720
700
680
740
720
700
680

12. DEM Elevations
Contours
700
680

13. Eight Direction Pour Point Model32 16 8 64 4
128 1 2

14. Direction of Steepest Descent1 1 67 56 49 53 44 37 58 55 22 67 56 49 53 44 37 58 55 22
Slope:

15. Flow Direction Grid 2 4 8 1 16
128

16. Austin West 30 Meter DEM Elevations in meters ftp://ftp. tnris. state
Austin West 30 Meter DEM Elevations in meters ftp://ftp.tnris.state.tx.us/tnris/demA.html

17. Flow Direction Grid 32 16 8 64 4
128 1 2

18. Grid Network

19. Flow Accumulation Grid3 2 3 2 2 2 11 1 1 11 1 15 1 15 2 5 2 5 24 1 1 24
Link to Grid calculator

20. Flow Accumulation > 5 Cell Threshold3 2 11 1 15 24 5

21. Stream Network for 5 cell Threshold Drainage Area3 2 2 1 11 1 15 2 5 1 24

22. Streams with 200 cell Threshold (>18 hectares or 13
Streams with 200 cell Threshold (>18 hectares or 13.5 acres drainage area)

23. Watershed Outlet

24. Watershed Draining to This Outlet

25. Watershed and Drainage Paths Delineated from 30m DEM
Automated method is more consistent than hand delineation

26. DEM Data Sources for Texas
Digital terrain models with 2’ or 4’ contours built from aerial photogrammetry
10m DE-DEMs, from 1:24,000 scale maps with drainage enforcement (experimental)
30m DEMs from 1:24,000 scale maps
1” seamless DEM of Texas (March, 1999)
3" (100m) DEMs from 1:250,000 scale maps (current state-wide coverage)

27. DEM Watersheds for Austin

28. Selected Watersheds and Streams
Mansfield Dam
Colorado
River

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

30. HMS Basin File Basin file is a text description of all
hydrologic elements
155 33 87
Subbasin 39
Reach
Junction

31. HMS Model of the Austin Region

33. Maps Served on the Web

34. Design Precipitation Input

35. HMS Control File

36. HMS Results
Watershed 155
Junction 44

37. Map-Based Hydrology and Hydraulics
Connecting ArcView with HMS for the Austin Region
Flood simulation using Nexrad data in Houston
HMS-RAS for Waller Creek in Austin
Creating flood plain maps in Waller Creek

38. Regional flood analysis in Houston
Study region

39. Study Region in West Houston Maps Developed using CRWR-Prepro by Seth Ahrens

40. Nexrad Rainfall for Storm of Oct 1994

41. Discharge in Buffalo Bayou at Katy October, 1994 storm

42. Calibrated Flow with HEC-HMS

43. Map-Based Hydrology and Hydraulics
Connecting ArcView with HMS for the Austin Region
Flood simulation using Nexrad data in Houston
HMS-RAS for Waller Creek in Austin
Creating flood plain maps in Waller Creek

44. Waller Creek DEM

45. Waller Creek Watershed Outlets

46. Waller Creek HMS Model

47. Flood Plain Mapping

48. Connecting HMS and RAS

49. Discharge at a Particular Cross-Section

50. Map-Based Hydrology and Hydraulics
Connecting ArcView with HMS for the Austin Region
Flood simulation using Nexrad data in Houston
HMS-RAS for Waller Creek in Austin
Creating flood plain maps in Waller Creek

51. HEC-RAS: Background
Hydraulic 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:

52. 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.

53. 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.

54. HEC-RAS: Data Translation
Data translation from HEC-RAS text file to dbase table
Bank and floodplain boundaries measured from stream centerline
In order to move into the GIS environment, the output data from HEC-RAS must be extracted. I wrote a computer program in ArcView’s scripting language, Avenue, to read the RAS output text file and write key stream geometry parameters to ArcView. Actually, most of the work presented in the remainder of this presentation has been done using Avenue programs. The parameters vary between cross-sections and include the following:
Station number
Location of the minimum channel elevation
Bank station locations
Reach lengths
Water surface elevation
At this point, all I’ve done is transfer the data from an ASCII text file to tabular format. It’s still not geographically referenced.

55. Digital Stream Mapping
Digital orthophotograph and road coverage used as a base map
User digitizes stream with mouse
Boundary points define the RAS stream
So at this point I have a representation of the stream in the HEC-RAS model. The next step is to create a representation of the stream in GIS, and then link the two. Using a digital orthophotograph as a base map, I digitized the stream using tools in ArcView. Digital orthophotos are aerial or satellite photographs in which scale distortions caused by camera tilt and relief have been removed. I also used a map theme of roads was used to assist in stream mapping.
The first step toward geographically referencing the cross-sections is to compare the definitions of the RAS stream and its digital counterpart. It's entirely possible for example, that the digital stream is defined to a point farther upstream than the RAS stream, or vice versa. Hence, it's necessary to delineate the upstream and downstream boundaries of the RAS stream on the digital stream in ArcView. To this end, an Avenue script was developed, with which the upstream and downstream boundaries can be established with a click of the mouse. Intermediate stream definition points corresponding to important RAS cross-sections such as bridges or culverts can also be defined.
When a point is clicked, the script determines the nearest point along the stream centerline and snaps the point to the digital stream.

56. 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.

57. 3D Terrain Modeling: Ultimate Goal

58. Web Links
For more information about GIS and Hydrology, see To obtain CRWR-PrePro to link ArcView with HEC-HMS, see To take an online spatial hydrology course, see