3/11/2009

1

Potential of Glen Canyon Releases to Inundate 

Cultural Sites in Grand Canyon National Park: Cultural Sites in Grand Canyon National Park:

A GIS Analysis Using Modeled Virtual 

Shorelines, and Canyon-Wide Topographic 

Data

Hoda Sondossi Hoda Sondossi

Grand Canyon Monitoring and Research Center

928-556-7107

hsondossi@usgs.gov

Impetus for the Analysis

• Availability of 1) topographic, 2) cultural 

sites, and 3) virtual shoreline datasets

• Mandated by AMWG and TWG
3/11/2009

2

OUTLINE

• Bk d ac ground

• Methods

• Results

• Discussion

BACKGROUND:

Development of Water-Surface Elevations, and 

Virtual Shorelines

• 2 major components: 2 major components:

– 1) Water-Surface Elevations - modeling of cross-sections in 1 

dimension (HEC-RAS)

– 2) Virtual Shorelines Virtual Shorelines - modeling of the interaction of the 1D modeling of the interaction of the 1D

cross-sections with topography, in 3 dimensions 
3/11/2009

3

1) Water-Surface Elevation Modeling

• 1D model built in HEC-RAS (Hydrologic Engineering Centers River Analysis 

System), a “standard step” model;

• Uses 1D equations of energy and continuity to predict stage (water-surface 

elevation) for known discharges at specific cross-sections;

• 2,680 cross-sections generated between Lees Ferry & Diamond Creek, 

using high-resolution topography for stage above 227 m3/s (8,000 cfs), and 

synthetic bathymetry below;

• Model for prediction of stage ONLY—other parameters (e.g., bed 

roug , y) j hness, velocity) adjusted for each cross-section to predict accurate 

stage;

• Stage predicted to within:

– ± 0.4 m (1.31 ft) for discharge less than 1,300 m3/s (<46,000 cfs)

– ± 1.0 m (3.28 ft) for discharge ranging 1,300−2,500 m3/s (46,000-88,000 cfs)

– ± 1.5 m (4.92 ft) for discharge ranging 2,500−5,900 m3/s (88,000-210,000 cfs)
3/11/2009

4

Graphic of X-S

• Used topography generated from 2002 photogrammetry

2) Modeling Virtual Shorelines

• Assigned elevation values from 1D model at the 2,680 

cross-sections, interpolated between cross-sections to 

generated a 3D surface

• Generated “areas of inundation” by comparing the 

elevation of the 3D water surface layer with the topo 

layer
3/11/2009

5

In Cross-Section

modeled stage for each 

Discharge, at all 2,680 

cross-sections

Known topography
3/11/2009

6
3/11/2009

7
3/11/2009

8
3/11/2009

9

Virtual shoreline mapview

Why the new analysis?

What more was done?

modeled 

stage

error 

range
3/11/2009

10

Map view graphic

• Of different portions of a cultural site being 

i d t d b th f inundated by the range of error

RESULTS:

Basic Statistics of Cultural Sites Size Distributions
3/11/2009

11
3/11/2009

12
3/11/2009

13

Limitations of Analysis

• Only ground surface at archaeological sites is 

considered—how each flow level may affect a cultural how each flow level may affect a cultural

site has not been analyzed.

• Modeled water-surface elevations are based on current 

topography—changes in local topography (e.g., debris 

flow from side canyon) may change local stagedischarge relationships discharge relationships.

• Synthetic bathymetry suboptimal—Accurate bathymetric 

data may be used to update model in future.
3/11/2009

14

Mock-up of a cultural site with only surface artifacts and a gully. Modeled water stage indicates site 

being inundated, when the actual artifacts are not inundated. 

Cultural site

Virtual shoreline

Questions?
3/11/2009

15

CITATIONS

• Christopher S. Magirl, Michael J. Breedlove, Robert H. Webb, and Peter G. Grifiths. 2008. 

Modeling Water-Surface Elevations and Virtual Shorelines for the Colorado River in Grand 

Canyon Arizona Canyon, Arizona. USGS Scientific Investigation Report 2008 USGS Scientific Investigation Report 2008-5075

• G.W. Brunner. 2002. HEC-RAS River Analysis System: Hydraulic Reference Manual, 

Version 3.1. Davis, California, U.S. Army Corps of Engineers, Institute for Water Resources, 

Hydrologic Engineering Center.