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Glen Canyon Dam Technical Work Group 

Agenda Item Information 

March 16-17, 2009

Agenda Item

River Stage and Archaeological Sites 

Action Requested

 Motion requested. (The following motion is recommended by CRAHG. However, no motion is 

presumed to be made unless and until a TWG member makes the motion in accordance with 

the TWG Operating Procedures.) 

The TWG recommends to the AMWG that the existing virtual shorelines generated by HECRAS model are good predictors of river stage and are reliable predictors of the inundation of 

archaeological site surfaces. However, river stage is not the only consideration employed for 

determining which archaeological sites need to be treated. Other proximate, secondary and 

tertiary causes must be considered in determining archaeological site condition and the need for 

treatment. Additional modeling is necessary to evaluate which combination of variables has the 

most explanatory value in assessing current site condition. The current monitoring and treatment 

of archaeological sites should continue while the utility of alternative models is investigated. It 

should be noted though that further exploration of these models will most likely have budget 

implications. 

Presenter

Mary Barger, Chair, CRAHG 

Previous Action Taken

 By AMWG: AMWG passed the following motion at its September 2008 meeting by a vote of 

12-6: 

To direct the Technical Work Group to review the flow levels (as indicated by the currently 

available shorelines of the HEC-RAS model) associated with each of the 158 archaeological sites 

that have been identified for monitoring and/or mitigation of impacts, and to report this 

information and any recommendations with regard to how these data would fit into the process 

of making choices of sites to be monitored and/or impacts mitigated to the AMWG at its next 

meeting, with the provision that any recommendation will not alter the choice of sites selected 

for impacts mitigation in FY09. 

 By TWG: TWG passed the following motion at its October 2008 meeting: 

The CRAHG will review the revised virtual shoreline analysis, in relation to archaeological sites, 

and bring recommendations to the TWG at its next meeting focusing its review on the first part 

of the AMWG motion assessing the utility of flow lines as simulated (with 

uncertainty) by the HECRAS model and other error sources. 
River Stage and Archaeological Sites, continued 

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Background Information

The CRAHG met on January 06, 2009, and were presented with a report on the virtual shorelines 

analysis (HEC-RAS) by Hoda Sondossi of the Grand Canyon Monitoring and Research Center and 

with an analysis by Mike Berry. As a result of those presentations and a subsequent discussion 

among CRAHG members, the assessment of the HEC-RAS model produced the following 

observations: 

The existing virtual shorelines model (as generated by HEC-RAS) is a good predictor of river stage. 

The current analysis does not provide accurate velocity predictions. The existing virtual shorelines 

model is a good predictor of the inundation of site surfaces. Previous analyses have considered river 

stage using other models, to determine mitigation for impacts to sites. HEC-RAS appears to be a 

more accurate tool for this type of analysis. There may be other ways to analyze the HEC-RAS 

model data to help prioritize sites for treatment, but these have not yet been explored. 

Mike Berry’s analysis which was intended to test the usefulness of the HEC-RAS model in 

predicting archaeological site condition. Based on Mike’s preliminary analysis, HEC-RAS, in 

combination with other variables, explains a negligible amount of variability in the data (site 

condition). Thus, based on this analysis, it may not be a good predictor of current archaeological site 

condition. Better predictors may be site slope, other geomorphological variables, weather, 

vegetation, etc. Additional modeling is necessary to evaluate which variables, and combination of 

variables, are good predictors of current site condition. 

Stage is not the only consideration for the need to mitigate impacts to sites. Other considerations 

must be considered (as described above). 

The CRAHG notes that this analysis does not resolve any issues regarding TCPs. The model could 

potentially be used to evaluate inundation of TCPs, but it is only one of many variables that still 

need to be considered. 

The CRAHG also notes that the HEC-RAS model and the virtual shorelines is not an analysis of 

possible dam effects. It is merely an illustration of sites which may be inundated. 

Recommendations: 

Additional modeling is necessary to evaluate which variables, and/or combination of variables, are 

good predictors of current site condition. 

There may be other ways to analyze the HEC-RAS model and virtual shorelines to help prioritize 

archaeological sites for treatment and monitoring, but these have not yet been explored. It is unclear 

how this new information would improve on the current prioritization process for treatment and 

monitoring. Further exploration of these models may have budget implications. 

The current monitoring and treatment of archaeological sites should continue while the utility of 

these alternative models is investigated. 
River Stage and Archaeological Sites, continued 

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The Cultural Resource Ad Hoc Group’s Report 

To the Technical Work Group Regarding the 

Hydrologic Engineering Centers River Analysis System (HEC-RAS) 

Model’s Applicability to Archaeological Sites 

March 2009 

The Cultural Resource Ad Hoc Group (CRAHG) prepared this report and 

recommendations in response to a charge from the Technical Work Group 

concerning the applicability of the Hydrologic Engineering Centers River Analysis 

System (HEC-RAS) model’s virtual shoreline analysis to archaeological site location 

along the Colorado River within the Grand Canyon. 

The specific charge to CRAHG from the Technical Work Group stated: 

The CRAHG will review the revised virtual shoreline analysis, in 

relation to archaeological sites, and bring recommendations to the TWG 

at its next meeting focusing its review on the first part of the AMWG 

motion assessing the utility of flow lines as simulated (with 

uncertainty) by the HECRAS model and other error sources. 

The TWG charged the CRAHG in response to a motion from the AMWG passed on 

10 August 2008. In this motion the AMWG directed the TWG: 

To review the flow levels (as indicated by the currently available 

shorelines of the HEC-RAS model) associated with each of the 158 

archaeological sites that have been identified for monitoring and/or 

mitigation of impacts, and to report this information and any 

recommendations with regard to how these data would fit into the process 

of making choices of sites to be monitored and/or impacts mitigated to 

the AMWG at its next meeting, with the provision that any recommendation 

will not alter the choice of sites selected for impacts mitigation in 

FY09.

The CRAHG met on 06 January 2009 and were presented with a report on the virtual 

shorelines analysis (HEC-RAS) by Hoda Sondossi of the Grand Canyon Monitoring 

and Research Center. As a result of that presentation and a subsequent discussion 

among CRAHG members, the assessment of the HEC-RAS model produced the 

following observations: 
River Stage and Archaeological Sites, continued 

 Page 4 

The existing virtual shorelines model (as generated by HEC-RAS) is a good predictor 

of river stage. The current analysis does not provide accurate velocity predictions. The 

existing virtual shorelines model is a good predictor of the inundation of site surfaces. 

Previous analyses have considered river stage using other models, to determine 

mitigation for impacts to sites. HEC-RAS appears to be a more accurate tool for this 

type of analysis. There may be other ways to analyze the HEC-RAS model data to 

help prioritize sites for treatment, but these have not yet been explored. 

Mike Berry’s analysis was intended to test the usefulness of the HEC-RAS model in 

predicting archaeological site condition. Based on Mike’s preliminary analysis, HECRAS, in combination with other variables, explains a negligible amount of variability 

in the data (site condition). Thus, based on this analysis, it may not be a good 

predictor of current archaeological site condition. Better predictors may be site slope, 

other geomorphological variables, weather, vegetation, etc. Additional modeling is 

necessary to evaluate which variables, and combination of variables, are good 

predictors of current site condition. 

Stage is not the only consideration for the need to mitigate impacts to sites. Other 

considerations must be considered (as described above). 

The CRAHG notes that this analysis does not resolve any issues regarding TCPs. The 

model could potentially be used to evaluate inundation of TCPs, but it is only one of 

many variables that still need to be considered. 

The CRAHG also notes that the HEC-RAS model and the virtual shorelines is not an 

analysis of possible dam effects. It is merely an illustration of sites which may be 

inundated. 

Recommendations: 

Additional modeling is necessary to evaluate which variables, and/or combination of 

variables, are good predictors of current site condition. 

There may be other ways to analyze the HEC-RAS model and virtual shorelines to 

help prioritize archaeological sites for treatment and monitoring, but these have not 

yet been explored. It is unclear how this new information would improve on the 

current prioritization process for treatment and monitoring. Further exploration of 

these models may have budget implications. 

The current monitoring and treatment of archaeological sites should continue while 

the utility of these alternative models is investigated. 
River Stage and Archaeological Sites, continued 

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Draft TWG Motion: The TWG recommends to the AMWG that the existing 

virtual shorelines generated by HEC-RAS model are good predictors of river stage 

and are reliable predictors of the inundation of archaeological site surfaces. 

However, river stage is not the only consideration employed for determining 

which archaeological sites need to be treated. Other proximate, secondary and 

tertiary causes must be considered in determining archaeological site condition and 

the need for treatment. Additional modeling is necessary to evaluate which 

combination of variables has the most explanatory value in assessing current site 

condition. The current monitoring and treatment of archaeological sites should 

continue while the utility of alternative models is investigated. It should be noted 

though that further exploration of these models will most likely have budget 

implications.