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

2. Normal Neighborhood in Cullman

3. Dam Directly Uphill
Constructed in 1948; Sportsman Lake Dam

4. Decatur

5. Unmarked Dam

6. Rainsville

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. 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
Failed due to piping
Inundated 14.3 miles

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
Dam had been raised many times over the years
Weather event exposed a structural weakness and insufficient design
Occurred at 1:30 am helping to add to the high fatalities

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
Having a dam safety program allows a state to have legislative authority to administer dam safety regulatory programs

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
Through federal grants (Dept of Homeland Security: FEMA) were able to bring in AMEC as a contractor to assist

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
GIS is the best way to perform this re-inventory due to cost effectiveness and its ability to utilize available free resources

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. 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. 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. Locating Visually Study Area Logistics Aerial Identification
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. Example of Potential Infrastructure Hazard

18. Example of Potential Occupied Building Hazard

19. Calculating Height in GIS
H1 = Height of water at normal pool (assumed to be 3 feet lower than top of dam)
H2 = Difference in contours of those immediately downstream of dam and immediately upstream of visible water in pond
D1 = Distance from dam to upstream end of stream line for visible water
D2 = Distance between the 2 contours used to calculate H2
Through making some general assumptions we can estimate the height and volume from the available layers

20. ArcMap Height Calculation Example

21. D1 and D2 in ArcMap

22. Calculating Volume and Area in GIS
H1 = previously calculated height 3 feet below top of dam
AW = area of visible surface water as measured in GIS from aerial
ATOD = area of surface water at full pool
VTOD = volume for the pond at full pool

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. 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. QA/QC
Since GIS can use queries to field calculate most of the values only the D1, D2, and H2, 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 D1 and D2 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)
D1, D2, and H2 all make sense to ensure accurate estimations

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

27. Trouble Areas
Accounting for dry storage

28. Trouble Areas
Unrealistic height estimations from USGS topos

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. Results

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. Questions?