Flooding - Tidal and Rainfall. Presentation to City Council Informal Session May 24, John M. Keifer PE

Transcription

1 Flooding - Tidal and Rainfall Presentation to City Council Informal Session May 24, 2011 John M. Keifer PE

2 Purpose of Presentation 1. Review flooding causes and implications to the City 2. Discuss mitigation options 3. Present various strategies to handle flooding impacts 2

3 Overview Flooding is a global, national & regional issue City has undertaken extensive data gathering and engineering evaluations Key policy and funding strategies are needed at all government levels to meet the challenges 3

4 1. Flooding Causes and Implications 4

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

6 Rainfall and Tidal Flooding Tail Water Elevation 6

7 Precipitation Flooding Most all of the City s Planning Districts experience some level of flooding Some areas have no curb, gutter or collection systems Subsequent upstream development exceeds system capacity New and replacement storm water infrastructure are designed for 10-year storm Tidal flooding exacerbates precipitation flooding same pipes carrying out rain carry in tide 7

8 Areas with the most severe precipitation flooding (green) 8

9 Tidal Flooding 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 Sea level rise Land subsidence (land sinking) 9

10 Regional Geographic Settings Areas with the most severe coastal flooding include: CHESAPEAKE BAY Willoughby East Ocean View LAFAYETTE RIVER Colonial Place Larchmont / Edgewater Lafayette/Winona ELIZABETH RIVER Ghent Downtown Tidewater Drive Corridor BROAD CREEK Poplar Hall AREAS WITH REPETITIVE FLOODING LOSS CLAIMS 10

11 CURRENT CHALLENGES As sea level rises, increased depth, extent and duration of flooding events will occur Limitations of the current storm system: Become more significant Flooding will worsen unless actions are taken Funding for increased water quality regulations also needed 11

12 2. Mitigation Options - Coastal and Precipitation 12

13 City s Approach to Mitigate Flooding City has been proactive in recognizing and managing flooding challenges 1992 Created Environmental Storm Water Program 1994 & 2002 Developed Comprehensive Storm Water Master Plan for precipitation flooding. Total cost for projects identified over $100+Million Implemented numerous storm water system upgrades Hired Fugro, Netherlands-based firm to develop comprehensive plan for coastal flooding issues 13

14 Dedicated Capital Funding. Storm Water Facilities $950,000 Storm Water Waterfront Facilities $500,000 Neighborhood Flood Reduction $1,450,000 SW + $1,500,000 GF Water Quality Improvements $600,000 Total $5 million year 14

15 Storm Water Infrastructure 345 miles of Pipes 11.5 miles of Large Box Culverts 1,100 Outfalls 27,000 Storm Water Structures Major Maintenance and Improvements Average Age of Infrastructure: 60 years 50 miles of Non-Tidal Ditches & 33 miles of Tidal Ditches Anticipated Service Life of Infrastructure: 100 years Sliplining Existing Pipe & Repairs Mapping, Master Planning and Coastal Flooding Studies Major Storm Water System Upgrade and 15

16 Fairmount Park Rainfall Flooding Success Story In conjunction with water & sewer replacement full street improvement projects are completed Undersized pipes replaced to 10-year design Installed drainage inlets, curbs and gutters to improve water flow $18.2 million over 10 years 16

17 17

18 Other Notable Flood Improvement Projects Willoughby Pump Station Ocean View Outfall extensions Bulkhead Replacement Bourbon Avenue Downtown Pump Station and Flood Wall rehabilitation Pump Stations - underpass rehabilitation and emergency generators Ohio Creek (Norfolk State University) box culvert cleaning Coleman Place/Estabrook/Cromw ell Road Industrial/Overbrook Orange Avenue Curlew Drive Cape Henry Street Drainage Montclair Street Drainage Richmond Surrey Crescent Project 18

19 Coastal Flooding Comprehensive study Installed tide gauges throughout city Completed a City-wide assessment and began focused watershed evaluations to determine range of options that may be needed for effective flood relief Briefed City Council on coastal flooding (August 2010) 19

20 MITIGATION OPTIONS CONSIDERED IN FUGRO STUDY Public Information, Notification and Education Land use options & Government policy Zoning regulations Development requirements Minimum floor elevations for house Minimum floor elevation for garages and ancillary structures Flood proofing vulnerable elevations Foundation, structural, mechanical design requirements Property Purchase Infrastructure Approaches 20

21 INFRASTRUCTURE APPROACHES Infrastructure Structures and barriers Tide gates with pump stations Earth berms (and raised roads) Floodwalls & bulkheads Storm water system upgrades Retention ponds Increase drainage system capacity Raising of structures and roads Surge Side of Berm Surge Side of Wall Earthen Berm Flood Wall Land Side of Berm 21

22 TIDE GATE TYPE OPTIONS Evaluated multiple types of tide gates, including: Steel gates Obermeyer gates (hybrid of above steel gate with bladder) Inflatable dams Considerations Included: Navigation requirements (affects length and depth) Tidal flow Initial capital and future O&M costs Reliability Steel Gate Steel Gate Option Obermeyer Gate Option Inflatable Dam Option 22

23 COASTAL FLOODING - INITIAL FOCUSED STUDY AREAS Choices based on: Geographic setting Areas with natural constriction points Watersheds which lend themselves to specific, basin-wide mitigation options Areas of recurring damage 23

24 EVALUATION PROCESS USED FOR FOUR WATERSHEDS A GIS modeling tool, based on FEMA functions and processes, was completed. This tool provides consistency, comparisons and facilitates funding opportunities Components of the modeling tool: Modeling conditions utilizing tide gauge results Benefit/Cost (B/C) Analyses Compare cost of project with damages avoided Cost of capital project Cost of property purchase option Specifics on the time value of money 24

25 EVALUATION Process Modeling includes structures & contents utilizing the City parcel database and field verification was performed Not included in model are ancillary structures, vehicles, displacement, loss of use and city infrastructure Flood Level Crawl Space Flood Depth Ground Surface 25

26 WATERSHED CHARACTERISTICS Development Characteristic The Hague Pretty Lake Mason Creek Ohio Creek (includes Spartan Village) Total Structures 1,512 7,737 6, Single storm event potential damages - 10-yr Storm ($M) Single storm event potential damages 100-yr Storm ($M) Existing Tidal Protection No No Yes Yes Assessed value of the parcels in the four areas are similar Size and types of development within four areas vary 26

27 3. Coastal Flooding Study Results and Strategies 27

28 Project Scopes Three scopes of Projects funding and policy determine implementation 1. Watershed Area Flood Walls and pump stations Install pipes and new drainage systems 2. Neighborhood/small area Best Management Practices (BMP s) Raise streets Outfall extensions Purchase property 3. Local site specific (one or two houses; or one street or intersection) Install drainage inlets Install curb and gutters Raise a house or flood proof a structure 28

29 Areas Studied By Fugro - Most Effective Projects to Alleviate Coastal Flooding Watershed Project Elements Phased Approach Cost (in millions) * The Hague Floodwall, Gate, Pump Station & Additional Walls Yes $50 to $100 Pretty Lake Mason Creek Ohio Creek (includes Spartan Village) Floodwall, Gate, Pump Station, Additional Walls & Raising Roads Development Regulation changes, Pump Station, Forced Main & Additional Walls and Berms Property Acquisition Phase I, Pump Station & Culvert Phase II Yes Yes Yes *Further geotechnical/engineering studies needed to refine costs $40 to $80 $20 to $40 $20 to $40 29

30 THE HAGUE Fugro Recommended Project Project Elements Floodwall Tide Gate Pump Station Closure Walls Light Rail Bridge Brambleton Ave. Bridge Brambleton Ave. Bridge 30

31 THE HAGUE: Project Elements CAPITAL PROJECT to protect against coastal flooding (tidal surge): Tidal Barrier (floodwall) to protect against tidal surge Tide gate if navigation access required Pump station to remove rainfall runoff when gate closed Closure walls or berms across where land surface is low around basin/watershed perimeter 31

32 PRETTY LAKE Fugro Recommended Project Project Elements Floodwall Tide Gate Pump Station Road Raise Shore Drive Bridge Shore Drive Bridge 32

33 PRETTY LAKE: Project Elements CAPITAL PROJECT intended to protect against both coastal flooding (tidal surge) and rainfall runoff will require: Tidal Barrier (floodwall) structure to protect against inundation from tidal surge With tide gate if navigation access required Pump station to remove rainfall runoff when gate closed Raise roads 33 Raise structures

34 THE HAGUE & PRETTY LAKE Analyses Results The implications for The Hague and Pretty Lake are similar Each watershed flows to a body of water Neither area has any existing protection against tidal surge Flooding is wide-spread: Buy-out and/or raising of structures would be difficult to implement and will be less effective with future sea level rise For these 2 areas - significant capital investment is required to protect against coastal surge Mitigation requires tidal structures w/ pump stations plus other associated berms and walls Benefit/Cost ratios are positive Provides protection against coastal flooding 34

35 MASON CREEK: Project Elements FUGRO RECOMMENDED PROJECT Peripheral walls (berms) where land surface is low around creek Structure elevation Future Building requirements Property purchase ALTERNATIVE CAPITAL PROJECT Pump station to remove rainfall runoff when gate closed New storm culvert beneath Navy property 35

36 MASON CREEK: Summary of Fugro Options Mason Creek Watershed flows to an impoundment Existing flood gate under runway prevents tidal flooding Localized flooding due to rainfall when tide gate is closed Local property purchase, raising structures and/or walls are options Building code regulations to limit ancillary structure issues Significant investment is required to provide added flood protection Mitigation requires pump station & new storm drain under Navy property Benefit / Cost ratio is low 36

37 OHIO CREEK: Project Elements CAPITAL PROJECT Phased as Follows: Phase 1 Buyout of most vulnerable properties -50 year option Phase 2 Pump Station (reduces rainfall impact during tidal surge) year option Phase 3 Box Culvert (improves drainage system) Booker T. Washington H.S Spartan Village Norfolk State University 37

38 OHIO CREEK: Summary Ohio Creek (includes Spartan Village) watershed does not include a downstream impoundment (flows through culvert beneath I-264) Downstream tide gate protects against tidal surge Rainfall flooding: most severe in Spartan Village Most vulnerable property purchase Reconstruction above flood limits will restore value Eliminate most severe flooding area Pump Station Option Install new force main Cost of property purchase and pump station/ drainage improvements can be phased 38

39 Coastal Flooding Study Summary Projects to alleviate coastal flooding in the watershed areas studied are feasible but costly Pursue Federal funding Preliminary engineering/geotechnical studies for each watershed area needed to refine costs Explore Funding Mechanisms Further Consider Development Requirement Modifications 39