FLOODING Norfolk and the Region. Kristen M. Lentz, P.E., Acting Director of Public Works 8/25/2010

Transcription

1 FLOODING Norfolk and the Region Kristen M. Lentz, P.E., Acting Director of Public Works 8/25/2010 1

2 Purpose Provide a briefing on an important issue for Norfolk and the region Section I Scientific overview of flooding Future projections Potential consequences Section 2 Flood mitigation tools Section 3 Norfolk s Flood Mitigation Program Activities currently underway Recommended program expansion Policy considerations, next steps and conclusions 2

3 Acknowledgements Numerous individuals and organizations provided valuable information and input into this presentation including: Norfolk Residents and Businesses U.S. Army Corps of Engineers (ACOE) Virginia Institute of Marine Sciences (VIMS) Hampton Roads Planning District Commission (HRPDC) Fugro World Wide Multiple City Departments 3

4 Section 1 Scientific Overview of Flooding Future Projections Potential Consequences 4

5 Why Flooding Occurs In Norfolk, flooding is caused by rainfall, tides or a combination of both Rainfall Flooding Rain intensity exceeds capacity of the storm drain system Tidal Flooding Greater challenge and primary focus of this presentation Caused by tidal variations and storm surges Directly related to land elevation and proximity to coastline Can exacerbate rainfall flooding 5

6 Understanding Tailwater Where Precipitation Runoff Meets Tidal Waters Precipitation Drainage Pipe Opening Runoff Tailwater Rain Water Tidal Water Ground TYPICAL SUBMERGED OR PARTIALLY SUBMERGED STORM WATER OUTFALL Tailwater phenomena causes rainfall and tidal flooding to be inextricably linked. 6

7 Understanding Storm Intensity Terminology Scientists describe a rainfall event or a tidal storm by its recurrence interval, which is the probability it will occur at least once in a given span of time A 100-year storm (either tidal or rainfall): Has a probability of occurring once every 100 years 1% chance of happening in any given year Since these are probabilities, more than one 100-year storm can occur during any given period of time A lower intensity storm event recurs more often than a higher intensity event 7

8 Storm Intensity and Norfolk s Drainage System Largely over 60 years old and primarily designed for the 2-year rainfall storm Consists of: 18 million linear feet of pipes 28,100 structures 260,832 linear feet of ditches 13 City maintained storm water ponds 9 underpass pumping facilities the Downtown floodwall and its pumping station Since 2000, new and replacement facilities have been designed for 10-year storm 8

9 Understanding Flood Elevation Terminology Using data from previous storm events, scientists predict the elevation of flood waters for storms of various intensities Base Flood Elevation- is the predicted elevation of the tide, not including any wind-driven wave action or rainfall The Federal Emergency Management Agency (FEMA) produces maps that show the predicted 100-year base flood elevation for Norfolk 9

10 Flooding Datum Basics The 1988 North American Vertical Datum (NAVD 88) is the standard reference point used for measuring tide elevations Many other datums exist for various applications; however, NAVD 88 is the standard for FEMA s Flood Insurance Rate Maps (FIRM), National Oceanic and Atmospheric Administration (NOAA) and Norfolk tide gauges, elevation certificates and other property surveys At Sewells Point Normal high tide is about 1.1 NAVD 88 Normal low tide is about -1.5 NAVD 88 10

11 Hampton Roads Major Tidal Events This chart shows the 7 most significant tidal events that have occurred in the last 80 years. Note that 4 of the 7 occurred in the last 10 years. NAVD88 (FEET) ft ( year Flood Plain) Hurricane 1960 Ash Wed Storm 1998 Twin Nor'easter 2003 Hurricane Isabel Normal High Tide 2006 November Nor'easter 2008 September Storm 2009 November Nor'easter Storm Tide Elevations at Sewells Point 11

12 Sewells Point History Since 1930, relative mean sea level at Sewells Point has been documented to have risen 14.5 inches Rate of sea level rise has been increasing in recent years and is projected by some to continue increasing (NOAA, Tides and Currents, Sea Level Trends, 2010) 12

13 Tidal Flooding Projections Tidal flooding has occurred with greater frequency in recent years a trend that can be expected to continue due to several factors: Weather events of greater severity (related to increased ocean water temperatures) Sea level rise Land subsidence (land sinking) 13

14 Land subsidence is occurring faster in Hampton Roads than surrounding areas Filled areas sink faster than others Areas near the Chesapeake Bay impact crater are shifting Land Subsidence Average rate of subsidence in Hampton Roads is estimated by the Chesapeake Bay Program s Scientific and Technical Advisory Committee at 0.57 inches per decade 14

15 Relative Sea Level Rise Land Subsidence Relative Sea Level = Sea Level Rise + Land Subsidence Among Scientists there is a wide range of future sea level rise projections Sea Level Rise Mid-range future projections of Relative Sea Level Rise for Hampton Roads are 2-4 inches per decade 15

16 FEMA Flood Hazard Map year Flood Elevation 7.6 FT (NAVD88) In 2009 FEMA increased Norfolk s base flood elevation by 0.3 feet over their 1980 prediction 16

17 U. S. Relative Sea Level Change Hampton Roads rise has been the greatest of any location on the east coast Sewells Point 0.174in/yr EPA

18 Potential Long-Term Impacts of Relative Sea Level Rise More frequent and severe flooding Flood insurance required for more properties and at higher cost Public facilities and infrastructure more susceptible to flooding Transportation disruptions Reduction in suitable land for development/redevelopment Environmental effects including loss of wetlands and wildlife habitat Businesses and residents may relocate inland from coastal areas 18

19 Flooding- A Regional Challenge Flooding has been recognized as an important regional issue for Hampton Roads ODU s Dr. James Koch is calling upon local leaders to make flooding a priority issue. Flooding, then may be the issue of the 21 st century for Hampton Roads. Yes, our transportation challenges are immense, but even the best roads are of no use if they are under water. (Daily Press Editorial January 2010) Agencies such as the U.S. Navy and businesses such as Northrop - Grumman have already realized the importance of this issue and have hired consultants to help them understand how relative sea level rise could impact the future of their organizations. Such information will be used in crafting future business plans. 19

20 Section 2 Flood Mitigation Tools 20

21 Flood Mitigation Tools General Overview Improve conditions, but cannot guarantee against all future flooding Many types with widely ranging costs and results Different forms Physical Plans or regulations Informational Educational More than one tool can be utilized for a single area Norfolk has implemented many of the available tools in varying degrees and locations 21

22 Flood Mitigation Tools Precipitation Storm Water Ponds -Create additional rainfall drainage system capacity by providing storage that slows the flow of runoff Storm Pump stations add capacity by carrying rainfall out faster Larger pipes or ditches Carry rainfall out faster Allow tide in faster Flooding 22

23 Flood Mitigation Tools Tide Gates Tide gates at the end of storm drain pipes keep the tide out Provide flood protection if there is no precipitation, however. Rainfall runoff backs up and can cause flooding deeper than the tidal flooding Often malfunction and are maintenance intensive Clogging with debris Salt water corrodes metal and deteriorates rubber seals 23

24 Flood Mitigation Tools Major Structures Floodwalls and levees Can provide continuous tidal protection Require pump stations to pump rainfall out Significant visual and environmental impacts Costly Seawall: Galveston, Texas 24

25 Flood Mitigation Tools Flood Gates Permanent facility that provides temporary protection only as needed Located at entrances to waterways Allow boat traffic except when activated Require pump stations to address rainfall Storm Surge Gate: The Netherlands 25

26 Flood Mitigation Tools Design Considerations for Major Structures Including Flood Walls and Flood Gates Multiple design considerations must be investigated in order to determine whether a major flood control structure is feasible, and, if so, its estimated cost 1. Width of span 2. Depth of water and boat traffic 3. Storm design height 4. Sub-surface soil conditions for seepage and support 5. Existing underwater structures/remnants 6. How structure affects water flow process (scouring effect) 7. Protection of adjacent areas 8. Storage capacity for rainfall / pumping station 9. Environmental impacts and permitting 26

27 Flood Mitigation Tools Inflatable Dams A relatively new type of flood gate for use in a coastal environment with the potential for significantly lower costs Water or air filled bladder that is inflated by a pump station when tidal events are expected Requires a concrete support sill or foundation with structural considerations similar to traditional flood gates 27

28 Flood Mitigation Tools Shoreline Protection Sand dunes, beaches, wetlands or living shorelines can be constructed or expanded to protect coastal properties 28

29 Requirements that new facilities be built above the 100-year flood plain or another defined elevation Finished floors Parking areas Roads Programs to raise existing structures Purchase and demolition of flood prone structures Flood Mitigation Tools Adaptation of Structures 29

30 Flood Mitigation Tools Plans and Regulations Comprehensive land-use plans Optimize the use of flood-prone properties Parks and public amenities adjacent to waterways Enhance public access to rivers, bays and lakes Potential to raise property values for all rather than a few Opportunity to improve environment by creating water quality buffers and wildlife habitat Minimize the impact of flooding Leave flood-prone properties undeveloped 30

31 Flood Mitigation Tools Plans and Regulations Zoning can be used to reduce the susceptibility of future development to flooding through regulations that address Finished floor elevations Land use densities Lot sizes Land-to-building ratios 31

32 Flood Mitigation Tools Public Information and Notification Public information, warnings and notifications can be highly effective and relatively inexpensive mitigation tools Timely notice of expected higher tides Hazards of driving through flood waters Information regarding higher elevation roadways Shelter Locations Evacuation requests and routes Information and notification can utilize one or more Brochures Broadcast media Reverse 911, or similar cellular service calls Internet Physical signs and barriers 32

33 Flood Mitigation Tools Public Education Newer residents, visitors and tourists may not be aware of flooding risks. We can educate them on Tips for protecting property, such as identifying appropriate places for parking vehicles during flooding events The risks to garages, sheds and basements and other personal property located below the home s finished floor All residents need to be reminded periodically Value of flood insurance, even for those outside of the 100 year flood plain 33

34 Section 3 Norfolk s Flood Mitigation Program Current Program Overview Recommended Program Expansion Policy Considerations, Next Steps and Conclusions 34

35 Norfolk s Current Flood Mitigation Program Consists primarily of: Federal Assistance Programs and Projects Maintenance of Existing Storm Water System Flood mitigation projects that respond to problems and fit within the city s approach and cost-benefit funding strategies 35

36 Current Program Overview The National Flood Insurance Program (NFIP) Norfolk participates in the NFIP which is a federal program enabling property owners in member localities to purchase private flood insurance at subsidized rates based on flood insurance rate maps (FIRM) identifying 100-year flood plain boundary As a result of our voluntary participation in the Community Rating Program, the federal government provides a 5% reduction in flood insurance rates in Norfolk Norfolk requires the finished floors of all new structures to be at least 1 foot above the 100-year base flood elevation, but the finished floor elevations of many older structures are lower City sends letters annually recommending the purchase of flood insurance for all properties in the vicinity of previous flood insurance claims 36

37 Current Program Overview FEMA s Hazard Mitigation Program Provides limited funding to address repetitive loss properties which are defined as residential properties covered under a NFIP flood insurance policy that have had: At least four NFIP claim payments exceeding $5,000 each, or Two separate claim payments that, when totaled, exceed the market value of the building Funds can be applied to Raising structures Acquiring property Over the last 5 years, utilized by Norfolk to raise the finished floor of 24 structures (out of 148 applications) at a cost of $3.5 million (average of $146,000 per property) Residents are required to contribute 5% of the cost of raising the structure 37

38 Lockhaven/Meadowbrook Larchmont/Edgewater Willoughby East Ocean View Larrymore Lawns Repetitive Flooding Loss Sites Repetitive FEMA Claim Sites Lambert s Point Colonial Place/Riverview The Hague/Ghent Lafayette/Winona Spartan Village Ingleside Raised Homes through FEMA Grants Flood-prone Areas Willoughby East Ocean View Larrymore Lawns The Hague/Ghent Colonial Place/Riverview Ingleside Larchmont/Edgewater Lochaven/Meadowbrook Lambert s Point Lafayette/Winona Spartan Village Repetitive Loss is defined as a residential property covered under a NFIP flood insurance policy that has: At least four NFIP claim payments exceeding $5,000 each OR Two separate claims payments that, when totaled, exceeded the market value of the building. 38

39 Current Program Overview Downtown Floodwall and Pumping Station The City s Downtown floodwall, gates and pumping station were built in the late 1960 s by the U.S. Army Corps of Engineers to protect approximately 100 acres of the Downtown Business District from tidal flooding. They have performed well during the November 2009 Nor easter and other events The pump station is needed to pump out rain water that would otherwise be held in by the flood wall The floodwall is currently being evaluated for recertification by FEMA 39

40 City Hall Ave

41 Flood Reduction Projects CIP - 5 Year Plan FY11 FY12 FY13 FY14 FY15 Total Neighborhood Flood Reduction Program Storm Water Quality Improvement Program Storm Water Facility Improvement Program $1.45 $1.45 $1.45 $1.45 $1.45 $ Street Flooding Citywide Fairmount Park Street & Drainage Improvements TOTAL $6.35 $6.35 $6.35 $6.35 $6.35 $31.75 * in millions 41

42 Flood Mitigation Projects Currently Underway Cleaning and rehabilitation of large drainage lines to improve drainage Norfolk State University area pipes that drain Spartan Village Tidewater Drive area by Ruffner Middle School The Hague Haven Creek Raising the roads to reduce tidal flooding Bourbon Street Richmond and Surrey Crescents Extend outfall pipes and install pumping equipment at 15 th and Lea View Replacing the Masons Creek culvert Bulkhead replacement in Winona Living Shoreline Projects Knitting Mill Creek Haven Creek 42

43 Norfolk s Future Flood Mitigation Program An expansion of our existing flood mitigation program is needed to address our increasing coastal flooding challenges Proactive and strategic Long term project implementation plan (20 yrs) that includes project prioritization and funding mechanisms Based upon a city-wide analysis of needs and cost effectiveness of projects The City has hired Fugro, an internationally experienced coastal engineering firm headquartered in the Netherlands to perform the city-wide analysis and assist us in developing the plan 43

44 City-Wide Flooding Analysis Information on flood depths is needed to determine the best mitigation tools for an area Fugro is creating a model that forecasts flooding conditions throughout the City based on tide elevation data Installed 4 additional tide gauges throughout the City to provide better water surface elevation data Utilized Light Detection and Ranging (LIDAR) mapping to provide more frequent ground surface elevation data Model to be completed early

45 Tide Gauge Locations 45

46 LIDAR vs. Spot Elevation Mapping LIDAR : million elevation points city-wide Older spot elevation techniques: 3.6 million points city-wide LIDAR (Light Detection and Ranging) mapping provides 70 times as many elevation points as typical aerial mapping Norfolk obtained LIDAR mapping data in 2009 Provides basis for more accurately forecasting flood depths HRPDC and scientific and engineering communities are encouraging all Hampton Roads localities to obtain LIDAR data 46

47 Development of an Expanded Flood Mitigation Program Using the modeling information, an alternatives evaluation will be performed for various levels of flood protection Cost of implementing the flood mitigation tools Damage repair costs avoided General economic impacts Recommend prioritization of projects/activities a long-term implementation plan with associated funding mechanisms Consultant to complete study and proposed plan by end of

48 Focused Study Areas Due to unique geographical characteristics, three areas were identified for focused studies to be completed by the end of 2010 The Hague Pretty Lake Spartan Village/East Brambleton Area Study results to be evaluated for potential preliminary design inclusion in the FY12 CIP 48

49 Recommended City Council Action Extensive policy level discussions are needed prior to project driven decisions Policy considerations need to include: When and how to determine the acquisition of flood-prone properties Adoption of adjustments to zoning regulations City administration identifying for City Council approval alternative financing options such as special taxing districts Prioritization of criteria for major flood improvement projects City Council should also consider establishment or appointment of a Flood Management Advisory Committee; with the Mayor and another member of Council, city staff, and members of the public with interest and expertise to contribute to the effort of advising the City on best practices. The committee would not be involved in establishing priorities or specific projects. 49

50 Federal Funding Availability Flood Damage Reduction Nonstructural Corps will study under a 50/50 cost share flooding problems and potential nonstructural solutions Flood Damage Reduction - Structural, Section 205 $7 million maximum project size Current federal fiscal year funding nationwide is $50 million If project meets federal criteria, is approved through a feasibility study, and funding is available, the Corps may participate in completing the design and construction of the project at a 65/35 Corps/City cost share Larger flood control projects require U.S. Congressional appropriations (earmarks) 50

51 Funding Strategies An expanded flood mitigation program will require funding on a long-term basis The existing storm water fund, general fund CIP and currently available federal funds will need to be supplemented We are researching innovative funding mechanisms nationally Public or Public-Private partnerships Special improvements tax districts 51

52 Additional Next Steps Brief City Council on results of the three focus area studies and provide periodic briefings on status of city-wide coastal flooding study (January, 2011) Incorporate flooding vulnerability as part of the comprehensive plan update (ongoing, target date for completion is 2012) Work with HRPDC and Corps of Engineers to develop public information tools on coastal flooding for regional use (started discussions; will be ongoing) 52

53 Conclusion Norfolk recognizes that addressing flooding challenges is important for the continued quality of life and economic vitality of our city and the region We are working to build upon our existing storm water program to further address our current flooding challenges and prepare for those of the future Norfolk has stepped forward in the region to engage local, regional, national and international experts and partners to assist us in fully analyzing the flooding issue and all of the potential mitigation options. The comprehensive long-term flood mitigation program currently under development will assist us in making the best strategic and cost effective use of resources to further enhance the livability of our coastal city. 53