Map-Based Hydrology and HydraulicsArcViewInput DataDEMHEC-HMSFlooddischargeHEC-RASWatersurfaceprofilesArcViewFlood plain mapsCRWR-PreProHec-GeoRAS

Austin Digital Elevation ModelWaller Creek

Austin Watersheds

CRWR-PreProArcView-based preprocessorfor HEC-Hydrologic ModelingSystem (HEC-HMS)Digital Elevation ModelStream MapHMS Basin FileControl point locationsSoil and Land Use Maps

DEM Watersheds for Austin

Selected Watersheds and StreamsMansfield DamColoradoRiver

HMS Schematic Prepared with CRWR-PreProMansfield DamColoradoRiver

HMS Model of the Austin Region

HMS ResultsWatershed 155Junction 44

GIS-Based Flood ModelingTerrain analysis using Digital Elevation ModelsFlood hydrology model for the Austin regionCreating flood plain maps for Waller CreekReal time flood emergency management

Map-Based Hydrology and HydraulicsArcViewInput DataDEMHEC-HMSFlooddischargeHEC-RASWatersurfaceprofilesArcViewFlood plain mapsCRWR-PreProAvRAS

Colorado River Network1:100,000 scale Developed from EPA River Reach File 3 (the predecessor of the NHD)

City of Austin Stream NetworkDeveloped from 1=100Capco Areal photogrammetry1:1200 scale

Stream Definition: Waller CreekAustin Watersheds with Streamsderived from Aerial PhotographsStreamlines generated by the aerial photographs are not always continuous.

Information for Correcting Stream NetworkDEM

Contours

Storm sewers

Orthophotos

Resulting Corrected StreamSubsequent steps: Verification of corrected streams by flood hydrologists. Running tracer program to connect arcs. Burning of streams into DEM.

Waller Creek HMS Model

Flood Plain Mapping

Connecting HMS and RAS

Map-Based Hydrology and HydraulicsArcViewInput DataDEMHEC-HMSFlooddischargeHEC-RASWatersurfaceprofilesArcViewFlood plain mapsCRWR-PreProAvRAS

HEC-RAS: BackgroundRiver Analysis System model of the U.S. Army Corps of EngineersInput = cross-section geometry and flow ratesOutput = flood water elevations

Cross-Section Schematic

Waller CreekWatersheds

ChannelNetwork

HEC-RAS: Cross-Section DescriptionPoints describe channel and floodway geometry

Bank station locations

Water surface elevations and floodplain boundaries

Discharge at a Particular Cross-Section

HEC-RAS: OutputText FileGraphical

Floodplain Mapping: Plan View

3D Terrain Modeling: Ultimate Goal

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:

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.

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.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.