Transcript Document

Processing Geospatial Data
with HEC-GeoRAS 3.1
Texas A&M University, Department of Civil Engineering
CVEN 689 Applications of GIS in Civil Engineering
Professor: Dr. Francisco Olivera
Student: Brad W. Endres
28 April 2003
Aerial photo of Bolinas Lagoon, CA. USACE.
Demonstration of Capabilities
Abstract: The use of geospatial information is increasingly useful in hydrologic and hydraulic modeling through
computer simulation. Geographic Information Systems (GIS) software such as ArcView 3.2 and the Hydrologic
Engineering Center’s River Analysis System (HEC-RAS 3.1) are software programs that utilize digital spatial and
temporal information to model the flow of water on the earth’s surface. HEC-GeoRAS 3.1 extends the capabilities of
ArcView by extracting and formatting hydraulic parameters that are required to model streams and rivers in HEC-RAS.
As an interface between two distinct software programs, GeoRAS makes it possible to utilize digital data files where
calculations and drawings were conventionally done by hand. By employing these digital tools, engineers not only
increase the calculation accuracy, but they increase the number of scenarios that can be quickly modeled through
computer processing. Initially, an investment of time is required to master these software programs, but the savings in
time and money associated with detailed hydraulic analysis is worth the effort.
This depiction of the ArcView GIS 3.2 menu shows the data processing (drop down) menus of the HEC-GeoRAS extension.
PreRAS
Land Use
Theme
Stream
Centerline
After loading the 3D Analyst, Spatial
Analyst, and the GeoRAS extensions,
the user can add TIN data to begin the
project. The features of the Pre-RAS
menu allow the user to generate the
geometric data and the RAS GIS
import file that will transport the data in
a readable format for HEC-RAS. The
items in the drop down menu should
be followed in order to ensure
accurate completion of the geometric
data. Using the drawing tools of
ArcView 3.2, the user makes line
themes in the project. Engineering
judgment is applied when drawing the
center of the stream, its banks, and
the over bank flow paths.
Right Bank
Flow Path
Centerlines
Cross Section
Cut Lines
3-D scene of the Wailupe River TIN. Triangular
Irregular Networks provide precise elevation
data needed for channel bottoms.
3-D scene of the contours of the Wailupe
River TIN. Using the contour theme allows
for faster display of the background layer.
Bolinas1
HEC-RAS
This image represents the line themes that the user creates under the Pre-RAS menu.
Using basic ArcView drawing tools, the user creates the Stream Centerline, Main Channel
Banks, Flow Path Centerlines, Cross Section Cut Lines, and adds polygon themes as well.
Plan: Bolinas1968 Steady flow plan 4/27/2003
.004
Elevation (ft)
Also free at the HEC website, HECRAS performs one-dimensional steady
and unsteady flow calculations for a
natural or constructed channel. HECGeoRAS allows the transfer of
geometric data from ArcView to be
used in the Geometric Data Editor of
HEC-RAS. The user completes any
additional geometry, provides flow
data, provides boundary conditions,
and performs a steady flow analysis
(in this case). The resulting
calculations from solving the energy
equation indicate the water surface
profile and other data in tabular or
graphical form.
Extension, user’s manual, and example
data are available at
www.hec.usace.army.mil
10
Legend
5
EG 50 yr
WS 50 yr
0
Ground
Bank Sta
-5
-10
-15
0
200
400
600
800
1000
Station (ft)
This image from the Geometric Data Editor of
HEC-RAS indicates the successful transfer of
the RAS GIS import file of Wailupe River.
After entering the stream’s geometric data and
flow data, HEC-RAS computes the water
surface profile at each cross section.
HEC-RAS computes water surface profiles throughout the reach for steady gradually
varied flow (in this example). The X-Y-Z plot under the View menu illustrates the extent
to which the flood event exceeds the river banks. This data is exported to ArcView.
PostRAS
PostRAS
Following satisfactory results from
HEC-RAS, the RAS GIS export file is
loaded into the ArcView project using
the commands under the PostRAS
drop down menu. Floodplain
delineation, water depth, and water
velocity themes may be generated
using the PostRAS features. The base
themes that are created include the
stream network, cross sectional cut
lines, cross section surface lines, bank
station lines, water surface profile
bounding polygons, and velocity mass
points. A water surface TIN is created
independent of the terrain TIN. Next,
the water surface TIN and the terrain
TIN are rasterized to create the
floodplain – areas where the water
surface elevations are higher than the
terrain elevations.
3-D scene of the Wailupe contours with the
delineated floodplain.
ArcView makes additional grid and TIN
data from the previously rasterized water
surface TIN and terrain TIN. When
subtracted, a depth TIN results. The user
can select the identify tool and click on
the depth grid to obtain a discrete value.
If the velocity data was included in the
RAS GIS export file, a velocity TIN can
be generated using the velocity mass
point shapefile. The values are
interpolated between the points, the user
must consider this when viewing results.
Next, a velocity grid can be rasterized
from the velocity TIN. Again, discrete
values are interpolated between the
cross sections.
The depth grid is created. Dark blue indicates
deeper water.
The velocity grid is created. Dark blue
indicates faster flowing water.
Application to Study Area
Bolinas Lagoon, CA
Analysis with GeoRAS
PreRAS
Results
PostRAS
21.0 ft
Bolinas Lagoon, California is an environmentally sensitive estuarine habitat
located 15 miles north of San Francisco, CA. Development within the
watershed of the lagoon has caused accelerated sedimentation which is
slowly destroying aquatic life and disrupting the habitat of seals and birds.
The lagoon’s surface area is 1100 acres and consists of tidal channels,
mudflats, and marshes. The watershed is 16.7 square miles of steep ridges
and small streams with high velocities. USACE has studied several
restoration plans.
Using a TIN of the 1988 conditions of lagoon and known water surface elevations, the line themes and land
use themes were created for import to HEC-RAS for analysis of the main channel within the lagoon. HECRAS computes the water surface profile at each cross section, and flow and velocity distribution data if
selected. HEC-GeoRAS allows this information to be read into ArcView for the generation of the water
surface TIN and floodplain themes. A depth grid can be created to show the discrete value of water surface
elevation above the channel bottom. These GIS tools are effective in delineating a floodplain for a given flow
condition, as well as providing depth and velocity grids for further engineering analysis.
In comparison with a simulation using
1968 data, the main channel of the
lagoon appears to be getting deeper
while the overall elevation of the
lagoon is rising. This result is
consistent with the notion that
watershed runoff is entering the lagoon
at a higher velocity and with a higher
sediment load. The GIS analysis
suggests that the main channel within
the lagoon is experiencing scour due to
the higher runoff velocity. The deeper
channel may contribute to the
measured decrease in the tidal prism
for the lagoon. Although this
application should be validated with
further study, it demonstrates the
engineering utility and capability of GIS
tools.