PW/Engineering. June 20, 2013

Transcription

1 PW/Engineering June 20,

2 Three Questions Why is the Eastern Shore Drive Drainage Area Experiencing Recurrent Flooding? What can be done about it? What is being done? Comprehensively On an Interim basis 2

3 Why is the Eastern Shore Drive Drainage Area Experiencing Recurrent Flooding? 3

4 Recurrent Flooding & Sea Level Rise Storm water & flooding: A major citizen concern Flooding is increasing Sea level is rising 4

5 Sea Level Rise Information Sewell s Point: in/yr CBBT: in/yr Rise last 50 years: ~ 1 ft Projected rise next 50 years: 1 3 ft Result: Sea Level Rise Coastal + = Storms Increased and Recurrent Flooding in Many Areas 5

6 Ocean Park Lynnhaven Colony Cape Story Kings Grant Thalia Eastern Park Cape Henry Shores Oceanfront Windsor Woods Princess Anne Plaza Green Run Old Beach Ocean Lakes Glenwood Sandbridge Back Bay 6

7 30 th Street 7

8 Goodspeed Road 8

9 Dockside Vicinity 9

10 Bubba s 10

11 Eastern Shore Drive Neighborhoods 11

12 A Particularly Vulnerable Area Eastern Shore Drive Area -High Tides -Storm Surge -Rainfall 12

13 Flooding & Sea Level Rise Close-up of Eastern Shore Drive Area 13

14 What can be done about it? 14

15 Recurrent Flooding Adaptation Strategies Management/Retreat Prevent development in high risk areas Discourage rebuilding in high risk areas Accommodation Elevating buildings/flood proofing homes Evacuation routes/warning systems Protection Measures Focus of Today s Discussion 15

16 Recurrent Flooding & Sea Level Rise Adaptation Strategy: Protection Measures Engineering Flood Control Measures: Complex Costly O&M Intensive Limit to Effectiveness (LOS) LOS Exceedence = Major Flooding 16

17 What is being done? 17

18 Comprehensive Flood Control Measures 18

19 Comprehensive Flood Control Measures Major CIP: Eastern Shore Drive Drainage Improvement Programmed Funding: $53.13 M Final Cost: TBD Project Development Study Underway Identify Improvements: Pump Stations, Outfalls, Flood Control Gates, Flood Walls Lowest Lying Areas May Be Problematic Develop Implementation Plan (Phasing) Results By End of This Year 19

20 Interim Flood Control Measures Some have suggested installing backflow preventers to keep tides out 20

21 Interim Flood Control Measures Lynnhaven Colony Cape Henry Ditch 21

22 Interim Flood Control Measures Detailed analysis: Determine where backflow prevention devices will help reduce flooding from high tides Determine where backflow prevention devices will not increase flooding from rainfall Determine type of backflow prevention device appropriate for location (flexible check valve or automated sluice gate) 22

23 Interim Flood Control Measures For Each Location Evaluated: Determined tidal elevation at which flooding will occur, even with backflow preventer in place Determined elevations of streets and homes Calculated available storage for rainfall in each drainage basin Hydraulically modeled numerous rainfall / tidal elevation combinations to determine benefits and impacts of installing backflow prevention devices 23

24 Interim Flood Control Measures 39 Locations Considered 24

25 Interim Flood Control Measures Lynnhaven Colony - Tidally Adjacent Area 25

26 Interim Flood Control Measures Lynnhaven Colony - Tidally Adjacent Area Results Range of Effectiveness Up to overtopping elevations of 3.3 to 4.7 Feet (NAVD88) depending on location Up to 4 to 6 inches of rainfall depending on location Cost: $244,000 (5 Locations- 4, 5, 8, 10, 11) Houses Protected: 0 Garages Protected: 4 Linear Feet of Lynnhaven Drive Protected: 320 L.F. 26

27 Interim Flood Control Measures Lynnhaven Colony Cape Henry Ditch 27

28 Interim Flood Control Measures Lynnhaven Colony Cape Henry Ditch Results Range of effectiveness: Up to Overtopping Elevation: 5.4 Feet (NAVD88) Up to 1 inch of rainfall with tide gates closed Total Cost: $672,000 (2 Locations- 18 & 26) Houses protected: 4 Garages protected: 9 Length of Cape Henry Drive protected: 150 L.F. 28

29 Interim Flood Control Measures Cape Henry Area 29

30 Interim Flood Control Measures Range of Effectiveness Cape Henry Area Results Up to Overtopping Elevation: 5.2 Feet (NAVD88) Up to 1 inch of rainfall with tide gate closed Cost: $290,000 (Location 39) Houses Protected: 1 Garages Protected: 2 Linear Feet of Street Protected: 0 L.F. 30

31 Summary of Findings In eight locations backflow prevention devices can provide limited benefit by reducing the frequency of flooding from certain moderately high tides Backflow prevention devices cannot eliminate all flooding There has been at least three storm since 2009 for which the backflow preventers would have had absolutely no benefit: Nor Ida (November 2009) Elev. 6.9 Irene (August 2011) Elev. 5.5 Sandy (October 2013) Elev. 5.7 All elevations are in NAVD 88 datum. 31

32 Sewell s Point Tidal Surge Info Benchmarks and Events MLLW (FT) Category 4 (13 19 Feet Surge at High Tide) Category 3 (9 12 Feet Surge at High Tide) Category 2 (6 8 Feet Surge at High Tide) Hurricane 8.02' Above MLLW FT OCCURS ONCE EVERY 80 YEARS (EXTREME) 8.00 Hurricane Isabel 9/18/ ' Above MLLW 7.89 November Nor'easter 11/12/ ' Above MLLW 7.75 Category 1 (3 5 Feet Surge at High Tide) 7.56 Hurricane Irene 8/27/ Above MLLW 7.55 Ash Wednesday Storm 3/7/ ' Above MLLW FT OCCURS ONCE EVERY 20 YEARS (SEVERE) 7.00 Hurricane Sandy 10/29/ ' Above MLLW 6.81 Hurricane ' Above MLLW 6.72 Thanksgiving Nor'easter ' Above MLLW 6.63 Twin Nor'easter # ' Above MLLW 6.58 Columbus Day Nor'easter 6.52' Above MLLW 6.52 Nor'easter ' Above MLLW FT OCCURS ONCE EVERY 7 YEARS (MODERATE) 5.00 Twin Nor'easter ' Above MLLW 6.04 Hurricane Floyd ' Above MLLW 5.97 Hurricane Flossy ' Above MLLW 5.92 Hurricane Donna ' Above MLLS FT OCCURS 3 TIMES PER YEAR (MINOR) OCCURS 3 TIMES PER YEAR 2.56 MEAN HIGH HIGH WATER MHW (Average all High Tides) 2.56 MEAN SEA LEVEL 1.35 MEAN LOWER LOW WATER MLLW (Average all Low Tides) 0.00 Lowest Tide of Record 1/31/

33 Return On Investment Total Cost of Interim Improvements: $1.2 M Benefit is some reduction in flooding frequency for: 5 house 15 Garages 570 L.F. of Street Comprehensive improvements are budgeted and initial phases could begin in 2016 A decision on whether to fund the Interim Improvements is needed 33

34 If Authorized Two Step Process Procure / Install Check Valves at 5 locations in the Lynnhaven Colony Tidally Adjacent Area (Fall 2013, $244,000) Detailed Engineering / Permitting / Construction for 1 Check Valve and 2 Sluice Gates in Cape Henry Ditch (1 gate at West Great Neck and 1 gate at First Landing Lane, Check Valve at Sunstates Court; Construct Fall 2014, $962,000) 34

35 Decision Process Brief the City Management Leadership Team Brief the Bayfront Advisory Committee Brief the District Council Member Brief the City Council 35

36 QUESTIONS FOLLOW UP NEXT DOOR Report Located on VBGOV.COM under Public Works Stormwater 36