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There’s a Dam There? Using GIS to Re-Inventory Alabama Dams By Jeff Zanotti.

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Presentation on theme: "There’s a Dam There? Using GIS to Re-Inventory Alabama Dams By Jeff Zanotti."— Presentation transcript:

1 There’s a Dam There? Using GIS to Re-Inventory Alabama Dams By Jeff Zanotti

2 2 Normal Neighborhood in Cullman

3 3 Dam Directly Uphill

4 4 Decatur

5 5 Unmarked Dam

6 6 Rainsville

7 7 Dam Breach Potential Risks Dam Failure Risk: 1] Potential of failure 2] Potential damage Potential of failure  Age of dam  Construction deficiencies  Inadequate maintenance  Seismic and weather events Apart from seismic and weather events age is a leading indicator of dam failure.

8 8 Recent Regional Failures of Large Dams  Big Bay Dam  Earthern dam near Purvis, MS  March 12, 2004  Destruction:  55 homes and mobile homes were destroyed  104 structures were documented as destroyed or damaged  0 fatalities

9 9 Recent Regional Failures of Large Dams  Kelly Barnes Dam  Earthern embankment dam in Stephens County, GA  November 6, 1977  Destruction:  27 homes and mobile homes were destroyed  2 college buildings were destroyed  39 fatalities

10 10 Dam Safety Programs  Association of State Dam Safety Officials (ASDSO) started in 1984 to create a national organization to improve dam safety  Alabama is the only state that lacks a dam safety program

11 11 Safe Dams  ADECA’s Office of Water Resources began efforts in 2008 to develop an inventory of dams in Alabama  These dams would be classified by their hazard potential  Once established the program will help public safety and emergency response operations in the event of a disaster

12 12 How to Start the Re-Inventory Process  What is already inventoried?  National Inventory of Dams (NID) Database  What are the resources available?  GIS Layers  What attributes of the dams should be reflected in the inventory?  Goal is to classify by potential hazard

13 13 National Inventory of Dams  NID has many of the largest dams in Alabama but suffers from drawbacks of a national dataset  Many dam locations are off spatially by a significant distance or have points that do not correspond to any existing dam  Underestimates the true number of potentially hazardous dams in Alabama

14 14 Utilization of GIS Resources  Aerials → NAIP aerials available for free download and/or Base Map imagery layers in ArcGIS 10  Existing Shapefiles  USGS Quadrangles Layer → used as a way to divide up Alabama into study areas  National Hydrology Dataset → stream centerlines to verify stream flow  DFIRM Databases → includes flood zones to show extent of flooding effects in area  Seamless USGS 7.5 Minute Topographic Maps → available for free through services.arcgisonline.com  USGS 10 Meter Digital Elevation Models (DEMs) → assists in portraying drainage area as well as assisting in possible flooding extents for more difficult areas

15 15 Identifying Risk Attributes  What is the maximum storage in volume the dam can contain?  How tall is the dam?  Is there a risk downstream of the dam?

16 16 Locating Visually  Study Area Logistics  Study areas broken up by quads  48 subsections to each quad  Aerial Identification  3 acres with a hazard less than ½ mile downstream  Greater than 10 acres  Hazard Verification  Use NHD, DEM, and/or DFIRM flood zones to verify hazard potential

17 17 Example of Potential Infrastructure Hazard

18 18 Example of Potential Occupied Building Hazard

19 19 Calculating Height in GIS  H 1 = Height of water at normal pool (assumed to be 3 feet lower than top of dam)  H 2 = Difference in contours of those immediately downstream of dam and immediately upstream of visible water in pond  D 1 = Distance from dam to upstream end of stream line for visible water  D 2 = Distance between the 2 contours used to calculate H 2

20 20 ArcMap Height Calculation Example

21 21 D 1 and D 2 in ArcMap

22 22 Calculating Volume and Area in GIS  H 1 = previously calculated height 3 feet below top of dam  A W = area of visible surface water as measured in GIS from aerial  A TOD = area of surface water at full pool  V TOD = volume for the pond at full pool

23 23 Assumptions for Calculating Height/Area/Volume  Pools are at normal elevation  Stream gradient under the pond is the same as the stream gradient beyond the pool  3 feet in elevation between normal pool and top of dam  Depth vs. Area is a linear relationship  Volume = 0.4 x top of dam depth x top of dam area for full pool

24 24 DEM Usage  DEMs were required to add in supplemental information about the dams like calculating drainage area for watershed ponds  Helped designate dams as having a hazard or not downstream in areas without flood zones

25 25 QA/QC  Since GIS can use queries to field calculate most of the values only the D 1, D 2, and H 2, as well as the drainage area and dam length, had to be recorded in the initial review.  Spatial Database Engine (SDE) was used to actually draw the D 1 and D 2 as their own layers in one dataset so it would be available to the GIS specialist doing the initial inventory as well as the reviewer for edits at the same time  Reviewer focus:  Making sure all dams were accounted for per quad  Hazards are verified  Dam location orientation is correct (using DEM)  D 1, D 2, and H 2 all make sense to ensure accurate estimations

26 26 Trouble Areas  USGS topos and aerials do not match up making D1 and D2 seem shifted

27 27 Trouble Areas  Accounting for dry storage

28 28 Trouble Areas  Unrealistic height estimations from USGS topos

29 29 Results  NID points are outdated and did not represent true number of dams with potential hazards  Volume and Height estimations allow OWR to propose new inclusions into the NID  OWR can further classify these inventoried dams by risk and perform field verifications on the most essential dams as needed

30 30 Results

31 31 Qualifications for Inclusion into NID  Has an impounding capacity at maximum water storage elevation of at least 50 acre-feet  Exceeds 25 feet or more in height  Will create a probable loss of human life in the event of failure or improper operation, regardless of height or storage capacity. (High Hazard)  Will create a probable loss of critical infrastructure in the event of failure or improper operation, regardless of height or storage capacity. (Significant or Moderate Hazard)

32 32 Questions?


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