APPLICATION OF LIDAR IN FLOODPLAIN MAPPING

Imane MRINI

GIS in Water Resources

University of Texas at Austin

Source. Optech,Inc

Show how LIDAR technology can be used jointly with

GIS and hydraulic models to map the possible extent

of flooding, and to prove that it could be an invaluable

tool in flood mapping because of its high accuracy.

PURPOSE OF THIS PROJECT

WHAT IS LIDAR ?

Airborne laser system used to acquire x, y, and z coordinates of terrain, and consist of :

- Airborne Global Positioning System (GPS)

- Attendant GPS base station

- Inertial Measuring Unit (IMU)

- Light-emitting scanning laser.

Acronym for Light Detection And Ranging

HOW LIDAR WORKS

•Operating altitude: 400 - 2,000 meters.

•Laser pulse rate: up to 25,000 per sec.

•Swath width: up to 1,500 meters

at 2,000 meter altitude.

Source.BEG

PRECISION OF THE LIDAR DEM

LIDAR INSTRUMENT

All-Terrain Laser Mapper (ALTM)

LIDAR FACTS

-Elevation accuracy : 10 to 25 cm

- records first and last returns of individual pulses and intensity

Applications in :

Flood mapping

Forest management

Coastal management

Land cover classification

Atmospheric pollution monitoring

DATA PROCESSINGSource : BEG

(1) compute a differentially corrected aircraft trajectory using a GPS software,

(2) generate the raw x, y, and z data,

(3) grid the data to generate an “all-points DEM”,

(4) filter the raw data and re-grid to generate a “vegetation-removed DEM”

EFFECT OF VEGETATION REMOVAL

Before After

Study area : Waller creek watershed in Austin,TX

LIDAR data source : Bureau of Economic Geology- University of Texas at Austin

Type of Lidar data : 1m DEMs of the all-point and bare-earth data as interchange files ( e.00)

Spatial reference : UTM zone 14, datum NAD 1983

Softwares used : ArcGis ( Arctoolbox, ArcMap),Arcview , HEC-RAS+HecGeoRas extension

FLOODPLAIN MODELING

STUDY AREA

Waller Creek Watershed

ALL-POINT LIDAR DATA OF WALLER CREEK

BARE –EARTH LIDAR DATA FOR WALLER CREEK

FLOOD MODELING PROCEDURE

HEC-GeoRas

Preprocessing

LIDAR input data

1m grid DEM

ARCVIEW

Terrain model processing

Plan data

( Flow regime)

Geometric data

stream centerlines, riverbanks, floodplain boundaries, cross-

sections along the streams

HEC-GeoRas

Postprocessing

Flow data from HEC-HMS

HEC-RAS steady state simulation

Hydraulic modeling

Hydrologic modeling

Flood map visualization

Import file

Extracting geometric data

Use of Spatial Analyst ,3D Analyst and Hec-GeoRas extensions

Using HEC-GeoRas extension and the digitizing tool in Arcview, I generated :

- Streamcenterlines- Stream banks- Flow path centerlines- Cross section lines

STREAM CENTERLINES

STREAM BANKS

FLOW PATH LINES

CROSS-SECTIONS CUTLINES

Problem : Hec-GeoRas program only works with TINs

Solution : Convert grids to TINs in ArcMap using the best vertical accuracy to generate the maximum number of triangles in a reasonable amount of time

DIFFICULTY ENCOUNTERED

CONVERTING TINs TO GRID

EXTRACTED CROSS SECTIONS

PREPARING THE IMPORT FILE TO HEC-RAS

DIFFICULTY ENCOUNTERED AT THIS STAGE

An error message from the Hec-PreRAS program during the import file process.

• RAS stream geometry.

• Cross-section extracted from the TIN.

WORK TO BE DONE : MODELING WITH HEC-RAS

• Resulting water elevations.

PREVIOUS FLOOD MAPPING WITH TINs

(Source. Esteban Azagra)

Future work

Solving the problem of the HecRas import file, hopefully before December 6

Potential ideas to develop:

- Write a program that incorporate grid data in the preprocessing for HEC-RAS

- Obtain a more detailed TINs from the grid conversion

Dr. David Maidment, University of texas at Austin

Becky Smith, Bureau of Economic Geology, University of Texas at Austin

John Andrews, Bureau of Economic Geology, University of Texas at Austin

ACKNOWLEDGMENT