ArcGIS REST Services Directory
Home > services > Emergency_Management (MapServer) > Dam_Breach_Willow_Creek_WR_6A Help | API Reference
JSON

Layer: Dam_Breach_Willow_Creek_WR_6A (ID: 11)

Parent Layer: Dam Breach Zones

Name: Dam_Breach_Willow_Creek_WR_6A

Display Field: Label

Type: Feature Layer

Geometry Type: esriGeometryPolygon

Description: For full documentation on the methods used please see the full Dam Assessment Report for South Zumbro WR-6A (September 2015) at: X:\2015_USDA_NRCS_Dam_Assessments\WR-6A\2015_Sept_South_Zumbro_WR-6_DamAssessment_Report.pdfHDR|Kleinfelder Joint Venture (JV) was retained by the Natural Resources ConservationService (NRCS) to complete a breach routing study and develop an inundation map showingthe area affected by a breach of the South Zumbro WR-6A Dam (dam). The dam is an earthenembankment structure on Willow Creek located approximately 7.1 miles, during high flows,upstream from the South Fork of the Zumbro River. The dam was designed and built by theNRCS in 1989 at a cost of $2,237,604, primarily for flood control purposes and stormwatermanagement (NRCS, 1994). The dam is currently owned, operated and maintained by the Cityof Rochester (“City”) (City of Rochester, 2005). After initial construction, the NRCS classifiedthe South Zumbro WR-6A Dam as a High Hazard Dam, where failure may cause loss of life,serious damage to homes, industrial and commercial buildings, important public utilities, mainhighways or railroads. The current NRCS dam hazard classification is High Hazard. The Stateof Minnesota dam classification is Hazard Class I, also indicating that a dam failure has thepotential for loss of life and serious impacts on utilities, industrial, or commercial properties.The purpose of this study is to perform and document a limited identification of immediatestructural concerns, develop inundation mapping, and assess the hazard classification of thedam. This assessment report describes the calculations and assumptions used to develop aninundation map for a sunny day dam breach scenario, showing the downstream area affectedby a breach. A summary of transportation structures, inhabited structures and facilities affectedby a dam breach is provided along with the estimated extent of flooding and travel time after thebreach formation. The NRCS, City of Rochester, and Minnesota Department of NaturalResources furnished information for this study, which included:As-built original drawings, dated 9/13/89Reservoir Design DataAnnual Operation and Maintenance Inspection Reports:National Engineering ManualTechnical Release No. 60 : Earth Dams and ReservoirsTechnical Release No. 66 : Simplified Dam-Breach Routing ProcedureThe WR-6A dam consists of the following components: earthen embankment, principalspillway/outlet structure, and auxiliary spillway. According to the as-built drawings, the earthenembankment is approximately 1,720 feet long, 54 feet high, and has a 14-foot wide crest. Thetop of the settled dam is at a surveyed elevation of 1145.0 feet (Unknown Datum Reference).The upstream face of the dam embankment has a slope of 2.5H:1V with a 20- foot wide bermlocated at an elevation of 1112.4 feet. The downstream face of the dam embankment has aslope of approximately 2H:1V with a 12-foot stability berm at an elevation of 1120.0 feet. Boththe upstream and downstream slopes of the dam embankment are vegetated.The principal spillway includes a reinforced concrete standard D x 3D covered top riser with a24-inch x 24-inch slide gate in the wet well to serve as a drawdown feature. A 42-inchreinforced concrete pipe outlet barrel was constructed using 20-foot pipe sections. Reinforcedconcrete anti-seep collars were used at 20-foot intervals along the center sections of the dam.According to the original design, the pipe discharged into a 23’-4” foot long and 17’-6” foot wideconcrete impact basin that includes a baffle wall to dissipate the energy. After the impact basinthe water flows into an 18 foot wide outlet channel with 4H:1V slopes. An 8-inch PVCfoundation drain was constructed at the toe of the downstream face of the dam. The drainsoutlet at the location where the principal spillway pipe discharges into the impact basin. A 170-foot wide broad-crested auxiliary spillway is located to the west of the dam. The spillwaychannel is approximately 12 feet deep and is bounded by earthen banks of 2H:1V slopes. Thebottom of the spillway is grass covered with some exposed bedrock. The control section isgrouted across the spillway and is keyed into the embankment by a minimum of 4.4 feet inheight.A bathymetric survey (McGhie and Betts, 2007) estimated available open storage below theauxiliary spillway at 1,543 acre-feet. The design data for the dam reported storage 2,418 acrefeetfor both flood control and the normal pool (NRCS, 1994). There is some apparentdiscrepancy between the bathymetric survey and reported dam design data.A site visit was conducted on November 4, 2014 to visually assess the condition and structuralintegrity of the dam embankment, outlet structure, auxiliary spillway, and outfall pipe. TheNational Operation & Maintenance Manual (NOMM) Inspection Checklist was completed duringthe inspection and is provided in Appendix B. Photographs to document the current condition ofthe dam were taken during the inspection and are included in Appendix C. Past inspectionreports from 2010 to 2014 indicated issues which include:Erosion and stabilization requirements on the embankmentRemoval of woody vegetation from the embankment areaRemoval of debris from the inlet worksRepair and maintenance of the embankment vehicle access roadA breach hydrograph was developed using simplified methods contained in NRCS TechnicalRelease TR-60 (TR-60) (USDA, 2005) and TR-66 (USDA, 1985). The evaluated dam breachfailure mechanism was a sunny day failure with pool elevation at the top of the dam. Themaximum breach outflow rate was determined using guidance contained in TR-60. Thecalculated peak discharge estimate used for this analysis was 82,200 cfs. Once this flow ratewas established, methods outlined in TR-66 were used to develop an outflow hydrograph foruse in a HEC-RAS hydraulic model. The HEC-RAS hydraulic model produced inundation extentboundaries, time of arrival estimates, and estimates of maximum depth, velocity and discharge.The model domain was laid out to encompass the region from the toe of the Dam extendingdownstream to the confluence of Bear Creek and on to the confluence with the South Fork ofthe Zumbro River.The maximum depth results are presented in Figure 3 – Dam Breach Maximum Depth Map.Figure 4 to Figure 7 provides detail views of the maximum inundation depths. Table 4 –Estimated Number of Buildings Within the Breach Flood Zone lists the number of buildings inthe breach flood zone. Of the 707 identified buildings, approximately 77% of these are in the 0to 3 feet maximum flood depth areas. A total of 162 buildings could be impacted with floodheights of more than 3 feet. There were two municipal buildings, four schools, and threehospitals in the breach flood zone.Several road crossings were selected as locations for reporting of results. Time of arrival afterbreach initiation, maximum water surface elevation, and approximate maximum discharge weretabulated for each of these sites. The results are given in Table 5 – South Zumbro WR-6A DamBreach Model Results. The dam breach flood is predicted to arrive at the South Broadway and40th Street Bridge in approximately one-half hour after the start of the breach formation. Thelow chords of the bridges upstream of this location are impacted and upstream roadwaysovertopped by between 2.1 and 2.4 feet. Storage in the South Broadway area is predicted toattenuate and delay the flood. The flood is predicted to arrive at the Highway 52 Bridgeapproximately one hour after the breach formation. No low chord or roadway overtopping ispredicted at this location. The flood flow is further attenuated at this location, with the floodpredicted to arrive at the 11th Avenue Southeast Bridge approximately two hours after thebreach formation. The low chord of the 11th Avenue Southeast Bridge is impacted androadway overtopped by two feet. The flood is predicted to arrive at the 12th Street SoutheastBridge approximately three hours after the breach formation, with impacts to low chord androadway overtopping of 1.3 feet. The dam breach flood is generally contained within the 100-year floodway at the confluence with the South Fork Zumbro River.The results show that the flood wave would travel down the drainage quite rapidly, arriving atHighway 52 in just over one hour, and would arrive at the confluence with the South Fork of theZumbro River in just over four hours. Flow depths and water surface elevations were tabulatedat several primary road crossings. Maximum depths varied between the road crossings, butresults indicate the maximum depths would exceed 10 feet at all road crossings examined. Adam breach would result in a potential loss of life by impacting 707 homes and businesses andfour road crossings. Within the buildings existing in the downstream dam break flood zone,there was identified two municipal buildings, four schools, and three hospitals. WR-6A iscurrently a High Hazard dam according to the NRCS and a Hazard Class I using the state ofMinnesota dam hazard classification criteria. This examination indicates the rating shouldremain as high hazard.

Copyright Text: 2015, HDR|Kleinfelder Joint Venture (JV) was retained by the Natural Resources Conservation Service (NRCS)

Default Visibility: false

MaxRecordCount: 1000

Supported Query Formats: JSON, geoJSON

Min Scale: 250000

Max Scale: 0

Supports Advanced Queries: true

Supports Statistics: true

Has Labels: false

Can Modify Layer: true

Can Scale Symbols: false

Use Standardized Queries: true

Supports Datum Transformation: true

Extent:
Drawing Info: Advanced Query Capabilities:
HasZ: false

HasM: false

Has Attachments: false

HTML Popup Type: esriServerHTMLPopupTypeAsHTMLText

Type ID Field: null

Fields:
Supported Operations:   Query   Generate Renderer   Return Updates

  Iteminfo   Thumbnail   Metadata