Flood Hazard Module

1. Flood Hazard Module Unit 1 – Introduction to Flooding Version: Nov 14, 2018

2. Floods • What do you know about flood or flooding? • When is the last time you heard about it? • Have you (or anyone you know) ever experienced a flooding situation? Are floods always bad or damaging? • Can you think of any reason(s) why floods might be beneficial

3. Pre-requisite Reading/Knowledge • Hydrologic Cycle and Processes • How does water gets into streams? • Why do we get ponding in some urban or rural areas?

4. Flooding - definition • Occurrence of water that is beyond the handling capacity of any natural or engineering system

5. What kind of flood do you see? Source: Jocelyn Augustino, FEMA. https://www.fema.gov/media-library/assets/images/54208#details Source: Jocelyn Augustino, FEMA. https://www.fema.gov/media-library/assets/images/50150 Riverine Flooding Coastal Flooding Shallow Flooding

6. What kind of flood do you see? Source: https://commons.wikimedia.org/wiki/File:Van_Buren_Maine_aerial_during_2008_flood.jpg Source: https://commons.wikimedia.org/wiki/File:Fort_Kent_Maine_Bridge_during_flood_2008.jpg Riverine Flooding Coastal Flooding Shallow Flooding

7. What kind of flood do you see? Source: https://www.usgs.gov/media/images/souris-river-flooding-1 Source: https://www.usgs.gov/media/images/boulder-creek-colo-flood-stage Riverine Flooding Coastal Flooding Shallow Flooding

8. What kind of flood do you see? Source: https://coastal.er.usgs.gov/hurricanes/sandy/photo-comparisons/newjersey.php Riverine Flooding Coastal Flooding Shallow Flooding

9. What kind of flood do you see? Source: https://coastal.er.usgs.gov/hurricanes/sandy/photo-comparisons/northcarolina.php Riverine Flooding Coastal Flooding Shallow Flooding

10. Floodplain Topographically: relatively flat land adjacent to a stream Geomorphologically: landform composed primarily unconsolidated sedimentary deposits (alluvium) from the adjacent stream Hydrologically: area subjected to periodic flooding from the adjacent stream Redrawn after: http://www.yourdictionary.com/floodplain A flat area of land adjacent to a stream, composed of alluvium, and is subjected to flooding or inundation during periods of high water in the stream.

11. Natural functions of floodplains • Water resources • Flood and erosion control • Groundwater recharge • Ecological/biological resources • Natural filters of sediments and nutrients • Enhances biodiversity • Habitat for fish and wildlife • Societal Resources • Recreational activities • Fertile agricultural land • Educational and research opportunities

12. Human impacts (riverine development) • Alteration of flood conveyance • Obstruction/diversion of flow

13. Human impacts (watershed development) • Decrease in infiltration • Increase in run-off and flooding Redrawn after: http://auburnhills.org/departments/community_development/low_impact_development/index.php

14. Human impacts (Coastal development) • Coastal erosion resulting from anthropogenic actions, including building structures and sand mining

15. How floods impact humans and property • Hydrodynamic force • Force due to moving water • Hydrostatic force • Vertical and lateral forces due to standing water • Debris impact • Floating and rolling debris from flood water • Soaking • Damage to wood, drywall and other building materials • Sediments and contaminants • Trapped in building material and structures after drying

16. Safety and health hazards • Damage to utilities – electrocution, drinking water contamination, sewage, gas leaks, fire hazard • Breeding ground for mosquitos, mold, mildew • Mental stress and fatigue

17. Floodplain management • Floodplain Management: “a decision-making process that aims to achieve the wise use of the nation’s floodplains.” “Wise use” means both reduced flood losses and protection of the natural resources and functions of floodplains.

18. Floodplain Management • Structural Measures: building of dams, levees and other structure to manage floods and floodplains • Non-structural measures – retain natural characteristics of floods and floodplains; regulations; policies

19. Floodplain management strategies and tools • Strategy 1: Modify human susceptibility to flood damage • Reduce disruption by avoiding hazardous, uneconomic or unwise use of floodplains by: • Floodplain Regulation: zoning codes to steer development away from hazardous areas or natural areas deserving preservation, establishing rules for developing subdivisions, and rigorously following building, health and sanitary codes. • Development and re-development policies: on the design and location of public services, utilities and critical facilities.

20. Strategy 1 continued • Land acquisition: to preserve open space and permanently relocate buildings from floodplains. • Floodproof building: by elevating new buildings and retrofitting existing ones. • Restore and preserve the natural resources and functions of floodplains

21. Strategy 2: Modify the impact of flooding • Assist individuals and communities to prepare for, respond to and recover from a flood by • Providing information and education to assist self-help and protection measures. • Following flood emergency measures to protect people and property. • Providing financial support through disaster assistance, flood insurance and tax adjustments. • Preparing post-flood recovery plans and programs to help people rebuild and implement mitigation measures to protect against future floods.

22. Strategy 3: Modify flooding itself • Develop projects that control floodwater. • Dams and reservoirs along streams/rivers to store excess water upstream from developed areas. • Dike, levees and floodwalls to keep water away from developed areas. • Channel restoration or alteration to make them more efficient, so overbank flooding will be less frequent. • Flow diversion around developed areas. • Land treatment to hold more water through infiltration • Detention ponds to store on-site excess runoff • Runoff control measures from areas under development outside the floodplain.

23. Strategy 4: Preserve and Restore Natural Resources • Renew the vitality and purpose of floodplains by re-establishing and maintaining floodplain environments in their natural state through: • Floodplain, wetlands and coastal barrier resources or land use regulation to steer development away from sensitive or natural areas. • Development and redevelopment policies on the design and location of public services, utilities and critical facilities. • Land acquisition; open space preservation; permanent relocation of buildings; restoration of floodplains and wetlands, and preservation of natural functions and habitats.

24. Strategy 4 Continued • Information and education to make people aware of natural floodplain resources and functions and how to protect them. • Tax adjustments to provide a financial initiative for preserving lands or restoring lands to their natural state. • Beach nourishment and dune building to protect inland development by maintaining the natural flood protection features.

25. Hydrology of Flooding • How to quantify and characterize a flood event? • This is typically done using the concept of return period or frequency of occurrence • Requires knowledge of probability and statistics related to streamflow time series

26. Hydrologic data series Complete duration series (All available data)

27. Hydrologic Data Series Continued Partial duration series: Magnitude greater than some base value. Annual exceedance series: is a partial duration series with # of values = # years

28. Hydrologic Data Series Continued • Extreme value series • Includes largest or smallest values in equal intervals • Annual series: interval = 1 year • Annual maximum series: largest values • Annual minimum series : smallest values

29. Return Period • Random variable: • Threshold level: • Extreme event occurs if: • Recurrence interval: • Return Period: Average recurrence interval between events equaling or exceeding a threshold

30. Annual Maximum Series for Wabash River at Lafayette, IN What is the return period of 75,000 cfs at this site?

31. Return period for 75,000 cfs • Number of times flow exceeded 75, 000 cfs in 114 years = 13 • Average recurrence interval = return period = 114/(13-1) = 9.5 years (approx. 10 years) • Probability of occurrence of 75,000 cfs every year = (1/10) x100 = 10 %

32. 100-Year Flood • A flow that has a return period of 100-years is called the 100-year flood • For most practical purposes (e.g., building house, buying insurances, etc.), area that gets inundated from 100-year flow is considered as the “floodplain” • Approaches to finding the 100-year flow and how to map the corresponding floodplain will be covered in Units 2-4 of this module

33. Summary • Floodplains provide many natural functions and benefits to the society • Some human actions alter the natural functioning of floodplains, thus increasing the flow discharge • A flood with very high discharge becomes a hazard • There are several ways to manage floodplains and mitigate the flood hazard • The 100-year flow is used in practice for flood hazard management