7 Dam Breach Potential Risks Dam Failure Risk:1] Potential of failure2] Potential damagePotential of failureAge of damConstruction deficienciesInadequate maintenanceSeismic and weather eventsApart from seismic and weather events age is a leading indicator of dam failure.
8 Recent Regional Failures of Large Dams Big Bay DamEarthern dam near Purvis, MSMarch 12, 2004Destruction:55 homes and mobile homes were destroyed104 structures were documented as destroyed or damaged0 fatalitiesFailed due to pipingInundated 14.3 miles
9 Recent Regional Failures of Large Dams Kelly Barnes DamEarthern embankment dam in Stephens County, GANovember 6, 1977Destruction:27 homes and mobile homes were destroyed2 college buildings were destroyed39 fatalitiesDam had been raised many times over the yearsWeather event exposed a structural weakness and insufficient designOccurred at 1:30 am helping to add to the high fatalities
10 Dam Safety ProgramsAssociation of State Dam Safety Officials (ASDSO) started in 1984 to create a national organization to improve dam safetyAlabama is the only state that lacks a dam safety programHaving a dam safety program allows a state to have legislative authority to administer dam safety regulatory programs
11 Safe DamsADECA’s Office of Water Resources began efforts in 2008 to develop an inventory of dams in AlabamaThese dams would be classified by their hazard potentialOnce established the program will help public safety and emergency response operations in the event of a disasterThrough 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) DatabaseWhat are the resources available?GIS LayersWhat attributes of the dams should be reflected in the inventory?Goal is to classify by potential hazardGIS 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 datasetMany dam locations are off spatially by a significant distance or have points that do not correspond to any existing damUnderestimates 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 10Existing ShapefilesUSGS Quadrangles Layer → used as a way to divide up Alabama into study areasNational Hydrology Dataset → stream centerlines to verify stream flowDFIRM Databases → includes flood zones to show extent of flooding effects in areaSeamless USGS 7.5 Minute Topographic Maps → available for free through services.arcgisonline.comUSGS 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 quads48 subsections to each quadAerial Identification3 acres with a hazard less than ½ mile downstreamGreater than 10 acresHazard VerificationUse NHD, DEM, and/or DFIRM flood zones to verify hazard potential
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 pondD1 = Distance from dam to upstream end of stream line for visible waterD2 = Distance between the 2 contours used to calculate H2Through making some general assumptions we can estimate the height and volume from the available layers
22 Calculating Volume and Area in GIS H1 = previously calculated height 3 feet below top of damAW = area of visible surface water as measured in GIS from aerialATOD = area of surface water at full poolVTOD = volume for the pond at full pool
23 Assumptions for Calculating Height/Area/Volume Pools are at normal elevationStream gradient under the pond is the same as the stream gradient beyond the pool3 feet in elevation between normal pool and top of damDepth vs. Area is a linear relationshipVolume = 0.4 x top of dam depth x top of dam area for full pool
24 DEM UsageDEMs were required to add in supplemental information about the dams like calculating drainage area for watershed pondsHelped designate dams as having a hazard or not downstream in areas without flood zones
25 QA/QCSince 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 timeReviewer focus:Making sure all dams were accounted for per quadHazards are verifiedDam location orientation is correct (using DEM)D1, D2, and H2 all make sense to ensure accurate estimations
26 Trouble AreasUSGS topos and aerials do not match up making D1 and D2 seem shifted
28 Trouble AreasUnrealistic height estimations from USGS topos
29 ResultsNID points are outdated and did not represent true number of dams with potential hazardsVolume and Height estimations allow OWR to propose new inclusions into the NIDOWR can further classify these inventoried dams by risk and perform field verifications on the most essential dams as needed
31 Qualifications for Inclusion into NID Has an impounding capacity at maximum water storage elevation of at least 50 acre-feetExceeds 25 feet or more in heightWill 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)
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