[Hydrology and Hydraulic Analysis Utilizing Terrain Data] [Barrett Goodwin]

Goal: Examine Terrain Data sets used for analysis −Terrain data sets −What are they? −DEM −Types of DEM used in H&H Modeling Actual: Examined products available as Terrain datasets in DEM format for use in H&H Modeling DEM compared in a side by side comparison two totally different kinds: USGS 30M DEM vs. LiDAR DEM at two foot cell spacing Performance Against Goals

USGS 30M Grid cell vs. 2ft DEM

Performance Against Schedule Plan: Goal was to measure the differences in output at different points of H & H Modeling −USGS 30M DEM and LiDAR DEM were ran through preprocessing for Hydrology development developed Actual: The preprocessing was ran utilizing ArcHydro tools extension in ArcGIS. Once for the 30M DEM and once for the LiDAR DEM. Metrics compared were: processing time, regression equation input data, and regression equation output.

Performance Against Quality Quality goal: Goal was to process all data need for comparison and make a comparison. Items that would have contributed to measuring differences would have been anomalies in comparing the outputs. Actual: When running the preprocessing for Hydrology no significant difference in size or shape was detected. Actual values where imputed into the regression equation for evaluation. Flows were developed for each dataset studied. Once again no significant difference was detected.

Regression Equation These are the perimeters/inputs to calculating flows in CFS.

Performance Against Budget Budget: Time was considered and measured for differing task. Cost of data was not considered as the data was procured previously. Actual cost/expenditures: The actual time spent to process the USGS DEM data for the selected area was around 10 minutes from beginning to developing the final flow. LiDAR DEM for the same selected area on the other hand took approximately 1 hour and 10 minutes from beginning to final flow calculation.

Processing Times Pre and Post Processing in ArcHydro tools

Final thoughts Conclusion: In conclusion the final flows developed where about 10 cfs in difference. In engineering terms this difference would be considered insignificant. The time to process the LiDAR data was on the order of 6 times the amount of the USGS 30M DEM data. This was for a relative small watershed cost in data, time, and resources go up exponentially as area increases. Which dataset would you choose?

Two part discussion Explained Hydrology The following is Hydraulics

Goal: Map the same output using two different Terrain datasets USGS 30M DEM vs. LiDAR DEM at two foot cell spacing. Actual: The output from the hydraulic model for the same area was mapped using two very different Terrain datasets. In essence this was the floodplain being delineated for the same area. Performance Against Goals

Performance Against Schedule Plan: In an efficient manner map both floodplains for a quick comparison. Actual: The two floodplains where outputted and visually compared for differences.

Performance Against Quality Quality goal: Produce output and visually inspect differences in floodplain. Actual: Floodplain differences were observed. The main differences where not necessarily in actual shape overall but in the rasteriztion of the data being compared. These differences where noted as being concerns for floodplain mapping.

Performance Against Budget Budget: Cost of data was not considered as it was previously procured. Actual cost/expenditures: Had the cost been incurred the USGS data would have been free while the LiDAR data could have been some where on the order of $ a square mile this translates to the study area costing roughly $45,000.

Conclusion: Quality of floodplain delineation diminishes with DEM resolution. This can be bad if peoples lives and property are at risk, as needed help evacuating or insurance might be overlooked. But one needs to weight the use of the floodplain being developed. For example if it is used for display purposes at a small scale then highly accurate floodplain boundaries are not needed and quicker, cheaper boundaries can be developed. Same would apply for some environmental uses such as tracing contaminants through a riverine system this level of detail might not be need..

Key Lessons

Lessons The use of the lowest reasonable resolution data is OK for Hydrology flow development and watershed delineation Cost and Time associated with LiDAR data is unacceptable to Hydrology flow development Floodplain delineation benefits by higher resolution DEM data such as LiDAR but is not necessary in all cases

Recommendations It is recommended that further research would need to be made in order to conclusively support these findings Research would include studies in differing terrain type such as coastal vs. mountainous. Flow development would need to be performed in these areas and compared. Floodplain development at different flood stages/conditions would need to be performed in these areas.

Sources AeroMap U.S. Company online website LiDAR Basics. NOAA Coastal Services Center. Remote Sensing for Coastal Management LiDAR. United States Geologic Survey USGS. Online List of Products, National Elevation Dataset NED 1 arc second. Yures, Gabriel. Online GIS Tutorials: Chapter 3 Digital Terrain Model (DTM) -1.htm -1.htm

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