Informational Meeting- July 21, Boxelder B-2/B-3 Watershed Planning Study

Transcription

1 Informational Meeting- July 21, 2015 Boxelder B-2/B-3 Watershed Planning Study

2 Agenda Introductions Background & History Purpose & Need NRCS Watershed Planning Program Work Completed to Date Longer term tasks Questions/Discussions

3 Introductions NPIC Golder NRCS Guests sign-in sheet August 5,

4 Watershed Dam B-2 August 5,

5 Watershed Dam B-3 August 5,

6 Watershed Description The Boxelder Creek Watershed covers approximately 260 square miles and extends into southeast Wyoming Portions of unicorporated Larimer County, Wellington, southeast Fort Collins and Timnath are located within the FEMA regulatory floodplain. The Lower Boxelder Valley is irrigated, and it contains some of the best agricultural land in the county The Lower Boxelder Basin has developed into an agribusiness and agriresearch corridor, which includes the Colorado State University Agricultural Research The lower, southern portion of the Basin is more urbanized and is impacted by many existing man-made structures, such as streets and buildings August 5,

7 FEMA Regulatory Floodplains August 5,

8 Watershed Map August 5,

9 History of Flooding Boxelder Creek and its tributaries have a long history of flooding. An SCS report documents that from 1904 to 1969, there was a damaging flood every one to three years, on average, somewhere in the watershed. There have been 24 major documented storm events and numerous localized storms occurring over the irrigated land causing flooding and sediment and erosion damages as well. In 1947, the Coloradoan newspaper headline read, Violent Rainstorm Floods Large Area; Crop Losses Heavy. In this storm, as much as five inches of rain fell northwest of Wellington, damaging nearly 1,000 acres of grain, alfalfa, and corn as well as washing out bridges. In 1967, heavy rains again flooded the area, causing Boxelder Creek to overtop roads, which resulted in the death of a mother and her three daughters on a county road southeast of Wellington. Seven times that summer, county bridges in the Watershed were destroyed by flooding. August 5,

10 Background & History Original NRCSD Watershed Plan completed (original project sponsors included NPIC and local soil conservation districts) 1979 to B-2 & B-3 constructed as part of a NRCS funded project that included five (5) dams constructed NPIC has retained maintenance of the facilities August, 2008 Boxelder Regional Stormwater Authority established 2009 NPIC received approval for partial NRCS funding to investigate rehabilitation of the two dams (with support from original project sponsors) 2009 to NRCS completed updated hydrology and dam breach evaluations August 5,

11 Background & History (cont.) March, 2011 Project Sponsors Meeting July, 2013 State of Colorado Department of Water Resources raised the hazard classification of both dams from Significant to High 2014 NRCS/NPIC agreed to funding of planning studies May, 2015 Watershed Planning Study Initiated August 5,

12 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 Need to meet all current Colorado DWR and NRCS dam safety requirements Desire to maintain flood control benefits Minimize environmental, socio-economic and cultural resource impacts and damages August 5,

13 Impact of New Hazard Classification Colorado Dam Safety Requirements: Inflow Design Flood (IDF) Significant Hazard: 50% PMP Event High Hazard: 90% PMP Event August 5,

14 Existing Spillway Capacity - B2 Reinforced Concrete Lined Chute; 250 wide, 10 deep Existing Spillway Capacity ~ 17,820 cfs 100-year frequency event ~ 295 cfs (w/ dam); 17,500 cfs (w/o dam) 90% PMF Event (NRCS, 2010) 24-hour Duration Event ~ 248,800 cfs 6-hour Duration PMF ~ 350,800 cfs Dam Overtops by approx. 5-7 feet Sunny Day Dam Failure (NRCS, dam ~ 83,900 Poudre River ~ 51,600 cfs August 5,

15 Existing Spillway Capacity - B3 Uncontrolled Excavated Earth Channel; 400 wide, 8 deep Existing Spillway Capacity ~ 17,000 cfs 100-year Event ~ 175 cfs (w/ dam); 4,950 cfs (w/o dam) 90% PMF Event (NRCS, 2010) 24-hour Duration Event ~ 102,000 cfs 6-hour Duration PMF ~ 126,500 cfs Dam Overtops by approx. 5-7 feet Sunny Day Dam Failure (NRCS, dam ~ 54,500 Poudre River ~ 23,500 cfs August 5,

16 Regulatory Floodplain Benefits Dam Drainage Area (sq mi) Storage at Crest (acft) Q 100 without Dam (cfs) Q 100 with Dam (cfs) Breach Discharge (cfs) Residences in Breach Zone B ,000 17, ,900 >1,000 B ,410 4, ,500 >1,000 August 5,

17 Dam Breach Inundation Maps August 5,

18 Watershed Planning Study Components Phase 1: Collection and Analysis of Information (May-July, 2015) Collect and Review Existing Information Develop Purpose and Need, Work Plan and Public Participation Plan Environmental and Cultural Resources Assessment Hydrology/Hydraulic Evaluations Surveying Geotechnical Investigations Sediment Deposition Assessment Phase 2: Formulation and Evaluation of Alternatives (Aug-Dec, 2015) Formulate Alternatives (Public Meeting No. 1) Engineering Evaluations Alternatives Assessment (Environmental, Social and Economic Impacts) Preliminary Design Identification of Funding Mechanisms August 5,

19 Watershed Planning Study Components (cont.) Phase 3: Watershed Plan Supplement Preparation (Jan-April, 2016) Preliminary Draft (NPIC/NRCS Colorado Review) Draft (NRCS/Public Review) (Public Meeting No. 2) Final August 5,

20 Work Completed to Date Reviewed Previous Work Completed Developed Purpose and Need Statement Developed Work Plan Developed Public Participation Plan Performed Environmental and Cultural Resources Field Surveys (Environmental Evaluation) Commenced Hydrologic and Incremental Damage Assessment (IDA) Evaluations Commenced Embankment Stability Evaluations Commenced Sediment Deposition Evaluations August 5,

21 Public Participation Plan Informational Meetings Public Meetings (2 scheduled; 1 end of August; 1 after draft plan) Web Page August 5,

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23 Environmental /Cultural Resources Assessments Summary Conducted environmental and cultural resources field surveys Initiated consultation and discussions with NRCS Environmental Staff Initiated Environmental Evaluation (EE) documentation August 5,

24 Hydrologic Evaluations Summary NRCS PMF Evaluation (2010) NRCS Dam Breach Studies ( ) Boxelder Stormwater Authority Hydrologic Assessments (updated 2014) Golder Incremental Damage Assessment Evaluations (on-going) August 5,

25 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 Preliminary geotechnical stability evaluations indicate no issues with embankment stability in their current condition August 5,

26 Work to Be Completed in Next 3-months Develop and Evaluate Alternatives Engineering Evaluations Socio-economic Assessments Prepare Environmental Evaluation (EE) Documents Public Meeting No. 1 Investigate Potential Funding Sources August 5,

27 Alternatives to Be Considered No Action Decommissioning Modifications Spillway Upgrades Overtopping Roller-compacted Concrete Other Revetment August 5,

28 Potential Cost Saving Solutions Incremental Damage Assessment: Decrease the Inflow Design Flood and associated magnitude of modifications required Alternative overtopping materials Geotextiles Articulated Concrete Blocks Increased spillway hydraulic efficiency Ogee/labrynith crest structures Fusegates Fuseplug August 5,

29 Potential Funding Sources Project Sponsors Federal Agencies (NRCS) Stakeholders August 5,

30 Discussion/Questions/Comments August 5,