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

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

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

Austin Digital Elevation Model Waller Creek

Austin Watersheds

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

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

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

Watershed Delineation by Hand Digitizing Watershed divide Drainage direction Outlet

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

DEM Elevations 720 720 Contours 740 720 700 680 740 720 700 680

DEM Elevations Contours 700 680

Eight Direction Pour Point Model 32 16 8 64 4 128 1 2

Direction of Steepest Descent 1 1 67 56 49 53 44 37 58 55 22 67 56 49 53 44 37 58 55 22 Slope:

Flow Direction Grid 2 4 8 1 16 128

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

Flow Direction Grid 32 16 8 64 4 128 1 2

Grid Network

Flow Accumulation Grid 3 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

Flow Accumulation > 5 Cell Threshold 3 2 11 1 15 24 5

Stream Network for 5 cell Threshold Drainage Area 3 2 2 1 11 1 15 2 5 1 24

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

Watershed Outlet

Watershed Draining to This Outlet

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

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)

DEM Watersheds for Austin

Selected Watersheds and Streams Mansfield Dam Colorado River

HMS Schematic Prepared with CRWR-PrePro Mansfield Dam Colorado River

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

HMS Model of the Austin Region

Maps Served on the Web

Design Precipitation Input

HMS Control File

HMS Results Watershed 155 Junction 44

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

Regional flood analysis in Houston Study region

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

Nexrad Rainfall for Storm of Oct 1994

Discharge in Buffalo Bayou at Katy October, 1994 storm

Calibrated Flow with HEC-HMS

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

Waller Creek DEM

Waller Creek Watershed Outlets

Waller Creek HMS Model

Flood Plain Mapping

Connecting HMS and RAS

Discharge at a Particular Cross-Section

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

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:

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.

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.

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.

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.

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.

3D Terrain Modeling: Ultimate Goal

Web Links For more information about GIS and Hydrology, see http://www.ce.utexas.edu/prof/maidment To obtain CRWR-PrePro to link ArcView with HEC-HMS, see http://www.ce.utexas.edu/prof/olivera/prepro/prepro.htm To take an online spatial hydrology course, see http://campus.esri.com/campus/home/home.cfm