Informational Meeting- March 31, 2016 Boxelder B-2/B-3 Watershed Planning Study
Agenda Introductions Work Completed to Date Phase 1 Evaluations Alternatives Evaluations Economic Evaluations Draft Watershed Plan Development Investigation and Analysis Report Environmental Evaluation (EE) Questions/Discussions
Introductions NPIC Golder NRCS Guests sign-in sheet April 5, 2016 3
Dam Locations and Watershed April 5, 2016 4
Watershed Dam B-2 April 5, 2016 5
Watershed Dam B-3 April 5, 2016 6
FEMA Regulatory Floodplains April 5, 2016 7
Impact of New Hazard Classification The new hazard classification for both B-2 and B-3 has been raised from significant to high based on downstream development Colorado Dam Safety Requirements: Inflow Design Flood (IDF) Significant Hazard: 50% PMP Event High Hazard: 90% PMP Event NRCS Dam Safety Requirements: Inflow Design Flood (IDF) Full PMF Event Current spillways do not meet capacity requirements April 5, 2016 8
Regulatory Floodplain Benefits Dam Drainage Area (sq mi) Storage at Crest (acft) Q 100 without Dam (cfs) Q 100 with Dam (cfs) Sunny Day Breach Discharge (cfs) Residences in Breach Zone B-2 109 12,000 17,500 (20,080) 295 83,900 >1,000 B-3 61 6,410 4,950 (5,850) 175 54,500 >1,000 FEMA FIS: Boxelder Creek Upstream of I-25 near Wellington, Q 100 = 1,170 cfs Coal Creek at Town of Wellington, Q 100 = 830 cfs April 5, 2016 9
Dam Breach Inundation Maps April 5, 2016 10
Purpose & Need Dams were originally constructed to protect agricultural land from flooding Dams now protect developed portions within Larimer County, Wellington, Fort Collins and Timnath Hazard classification has been raised by the Colorado DWR from Significant to High (spillway capacity requirements have changed) Desire to maintain flood control benefits Develop alternatives to meet all current Colorado DWR and NRCS dam safety requirements Minimize environmental, socio-economic and cultural resource impacts and damages April 5, 2016 11
Watershed Planning Study Components Phase 1: Collection and Analysis of Information (May-Aug, 2015) Phase 2: Formulation and Evaluation of Alternatives (Sept-Mar, 2015) Phase 3: Watershed Plan Supplement Preparation (Apr-Jul, 2016) April 5, 2016 12
Work Completed to Date Reviewed Previous Work Developed Purpose and Need Statement Developed Work Plan Developed Public Participation Plan Performed environmental and cultural resources field surveys Completed preliminary hydrologic and Incremental Damage Assessment (IDA) evaluations Completed embankment stability evaluations Completed sediment deposition evaluations Developed alternatives for modifications Commenced economic evaluations Progressed Environmental Evaluation (EE) spreadsheets April 5, 2016 13
Public Participation Informational Meetings Public Meetings (3 scheduled) Web Page: http://npicwater.com April 5, 2016 14
Field Assessments Summary Conducted environmental and cultural resources field surveys Environment/Wildlife: Raptor nests Cultural Resources: Archaelogic sites identified in study area Irrigation turnout may require special consideration Wetlands: Wetlands delineation prior to construction 404 Nationwide permit Compensatory wetlands for disturbed areas No significant findings that would limit construction of modifications April 5, 2016 15
Geotechnical Evaluations Summary Large amount of subsurface data exists for B-2 and B-3 collected as part of original design Embankments and structures appear to be in good condition Geotechnical stability evaluations indicate no issues with embankment stability in their current condition April 5, 2016 16
Condition Assessment Summary Existing facilities are in relatively good condition requiring no major repairs Sediment deposition within the reservoirs has been minimal April 5, 2016 17
Hydrologic Evaluations Summary Incorporated USBR methodology consistent with State of Colorado guidelines Infiltration/soil based on GIS datasets USBR Unit Hydrograph methodology Evaluated General and Local (Thunderstorm) events Areal reduction for Local Storm event PMF reduced by 25-40% (General Storm) and 70-80% (Local Storm) from previous NRCS evaluations General Storm produces highest peak discharges Dams would overtop under the greater than 25% (B-2) and 40% (B-3) PMF events April 5, 2016 18
Determination of Inflow Design Flood (IDF) Based on current hazard classification, dam needs to safely convey 90-100 percent of the Probable Maximum Flood (PMF) event Both NRCS and State of Colorado criteria allow for determining a different design flood based on evaluating incremental damages Differences in peak discharges between no dam and dam breach scenarios used to screen for appropriate design flood Evaluated increase in flood depths and velocities associated with a dam failure versus no dam scenario for various ratios of the PMF event: Incremental depth increases are less than 2 feet The product of the incremental depth and average cross section velocity is less than 7 April 5, 2016 19
Preliminary Incremental Damage Assessment (IDA) Hydrologic model used to estimate ratios of the PMF for cases 1) with dam, 2) without the dam in place, and 3) assuming an overtopping dam failure Boxelder B-2 Boxelder B-3 PMP 90% PMP 75% PMP 50% PMP 40% PMP 30% PMP With Dam (Peak Flow, cfs) (El. Ft AMSL) Overtop Depth (feet) Without Dam (Peak Flow, cfs) (El. Ft AMSL) Dam Breach (Peak Flow, cfs) (El. Ft AMSL) With Dam (Peak Flow, cfs) (El. Ft AMSL) Overtop Depth (feet) Without Dam (Peak Flow, cfs) (El. Ft AMSL) Dam Breach (Peak Flow, cfs) (El. Ft AMSL) 156,545 157,193 231,444 84,910 85,238 136,769 5,577.8 3.4 N/A 5,576.1 5,492.2 3.2 N/A 5,490.5 140,845 141,474 230,219 76,395 76,714 135,059 5,577.5 3.1 N/A 5,576.1 5,491.8 2.8 N/A 5,490.5 117,293 117,895 225,476 63,616 63,929 129,262 5,577.1 2.7 N/A 5,576.2 5,491.3 2.3 N/A 5,490.4 77,926 78,596 206,477 42,176 42,619 113,467 5,576.3 1.9 N/A 5,576.0 5,490.3 1.3 N/A 5,490.1 61,483 62,877 193,600 32,898 34,095 N/A 5,575.9 1.5 N/A 5,575.8 5,489.7 0.7 N/A 5,489.7 41,714 47,158 174,999 22,589 25,571 N/A 5,575.4 1.0 N/A 5,575.4 5,488.4 0.0 N/A 5,488.4 April 5, 2016 20
50% General Storm Hydrographs April 5, 2016 21
75% General Storm Hydrographs April 5, 2016 22
B2 Incremental Damage Assessment April 5, 2016 23
B-3 Incremental Damage Assessment April 5, 2016 24
Incremental Damage Assessment Summary Hydraulic model evaluated for both the no dam and dam breach flood scenarios for various ratios of the PMF event Comparison of increase in depth and increase in depth-velocity product for the no dam and dam breach scenarios Event B-2 B-3 100-year Dam does not overtop Dam does not overtop 30-percent PMF Dam overtops; failure unlikely Dam does not overtop 40-percent PMF Dam overtops; incremental depth greater than 2 ; depth-velocity product greater than 7 50-percent PMF Dam overtops; incremental depth greater than 2 ; depth-velocity product greater than 7 75-percent PMF Dam overtops; incremental depth greater than 2 ; depth-velocity product greater than 7 in some areas Dam overtops; failure unlikely Dam overtops; incremental depth greater than 2 ; depthvelocity product greater than 7 in some areas Dam overtops; incremental depth less than 2 ; depthvelocity product less than 7 90-percent PMF Full PMF Dam overtops; incremental depth less than 2 ; depthvelocity product less than 7 Dam overtops; incremental depth less than 2 ; depthvelocity product less than 7 Dam overtops; incremental depth less than 2 ; depthvelocity product less than 7 Dam overtops; incremental depth less than 2 ; depthvelocity product less than 7 April 5, 2016 25
Recommended Inflow Design Flood B2: Full PMF Event B3: 90-percent PMF Event April 5, 2016 26
Preliminary Alternatives Future Without Project (No Action Alternative) Decommissioning Auxiliary Spillway Modifications Modify crest (ogee, labyrinth) Lower crest; install gates Widening/lowering of existing spillway Embankment Crest Raise Overtopping Protection Roller Compacted Concrete (RCC) Articulated Concrete Blocks (ACB) Geosynthetics/Turf Reinforcement Mats (TRM) Secondary Emergency Spillway New Secondary Spillway Fuseplugs in Embankment April 5, 2016 27
Future Without Project (No Action) Assumes no Federal or outside funding for modifications Breaching of embankment would likely be required by Colorado DWR Embankment and hydraulic structures would remain in place April 5, 2016 28
Decommissioning Design breach to meet all NRCS and State requirements Breach would need to be 200 feet wide with 3H:1V side slopes and armoured Demolition of all hydraulic structures and reclamation Re-establishment of stream channel April 5, 2016 29
Auxilllary Spillway Modifications Widening/lowering of spillway Increased spillway hydraulic efficiency Ogee crest structure Labyrinth crest structure Lowering of spillway and installation of fusegates April 5, 2016 30
Embankment Crest Raise Raise embankment crest to increase freeboard and spillway capacity Abutments and upstream development limit height of embankment crest raise Dam B-2 April 5, 2016 31
Overtopping Protection Roller Compacted Concrete (RCC) overtopping protection Alternative overtopping materials Geotextiles Articulated Concrete Blocks (ACB) April 5, 2016 32
Secondary Emergency Spillway Installation of a new secondary spillway on the abutment Installation of a fuseplug within the embankment April 5, 2016 33
Alternatives Considered in Evaluations No Action (Future Without Project) Decommissioning RCC Overtopping Protection TRM Overtopping Protection ACB Overtopping Protection Existing Auxiliary Spillway Widening/Lowering Auxiliary Fuseplug Spillway in Embankment April 5, 2016 34
No Action (Future Without Project)-B2 April 5, 2016 35
No Action (Future Without Project)-B3 April 5, 2016 36
Breaching vs. Decommissioning Breach Alternative Decommissioning Alternative April 5, 2016 37
Overtopping Alternatives April 5, 2016 38
Existing Spillway Widening-B2 April 5, 2016 39
Existing Spillway Modifications-B3 April 5, 2016 40
Auxiliary Fuseplug Spillway-B2 April 5, 2016 41
Auxiliary Fuseplug Spillway-B3 April 5, 2016 42
Estimated Construction Costs 1,2 Alternative B-2 B-3 Future without Project 3 $3.6M $1.6M Decommissioning 3 $3.8M $1.7M RCC Overtopping Protection $27.6M $11.6M TRM Overtopping Protection $15.2M $7.2M ACB Overtopping Protection $28.1M $12.7M Existing Spillway Modifications $8.6M $2.4M Auxiliary Fuseplug Spillway in Embankment $6.1M $2.4M 1. Costs are for planning purposes only and do not include permitting, legal and land acquisition fees. 2. Costs include 25% contingency. 3. Costs do not include additional costs for floodproofing, buyouts, downstream flood protection. Actual costs for implementation would be much greater. April 5, 2016 43
Recommended Alternatives Dam B-2 Future Without Project (No Action) Auxiliary Fuseplug Spillway in Embankment Dam B-3 Future Without Project (No Action) Existing Emergency Spillway Modifications Auxiliary Fuseplug in Embankment April 5, 2016 44
Impacts of Decommissioning Approximately 2,300 structures in downstream floodplain impacted as a result of decommissioning B2 (majority within Town of Wellington) Approximately 1,800 structures in downstream floodplain impacted as a result of decommissioning B3 (majority within Town of Wellington) Downstream structures constructed by the Boxelder Regional Stormwater Authority would not function as designed Several road crossings (including I-25) impacted Currently performing economic evaluations to estimate benefit-cost of the various alternatives Benefit: Reduction in damages of keeping dams in service Large benefit for events up to 100-year frequency event Minimal benefit for very extreme flooding events April 5, 2016 45
Potential Funding Sources Project Sponsors Federal Agencies NRCS - anticipated to fund up to 65% of the eligible project cost not to exceed 100% of construction costs FEMA Grants Stakeholders April 5, 2016 46
Watershed Plan Schedule Preliminary Watershed Plan for Internal Review (end of April, 2016) Draft Watershed Plan for NRCS Review (mid-may, 2016) Public Meeting No. 3 (mid-june, 2016) Final Watershed Plan (late-july, 2016) April 5, 2016 47
Discussion/Questions/Comments April 5, 2016 48