compound flood hazard modeling

benefits of small floods

disease regulation, water regulation, water quality, flood supply, recreation

benefits of extreme floods

primary production, soil formation, drinking water, disease regulation, tourism

FEMA floodplains definition

any land area susceptible to being inundated by floodwaters from any source

BFE

base flood elevation

BFE def

height of floodwaters (flood depth) resulting from a 100-year flood event FROM MSL

floodway def

a designated area reserved to discharge the base flood (avoid constructions)

flood fringe def

an area that can contain slow-moving or standing water

pools def

places where water is stored

fluxes def

ways in which water moves among pools

climate change and human activities affect the water cycle in terms of

quantity, quality, timing, and use

natural functions of floodplains

water resources, ecological and biological resources, societal resources

urban development alters ___ and the natural functioning of floodplains

river flow

examples of human impacts on natural floodplains

diversion or obstruction of river flow (dams)

increase of flow depth and inundation extent (regrading or filling)

Human impacts on catchments/watersheds

Decrease of infiltration (impervious areas)

Decrease of evapotranspiration (deforestation)

Increase of surface runoff (faster flow transport)

pluvial flooding

intense rainfall (flash flooding) or long-sustained moderate rainfall.

fluvial flooding

accumulation of large volumes of excessive rainfall and snowmelt

coastal flooding

one or more combinations of high tides, storm surge, and waves that are superimposed on relative mean sea level.

Compound flooding

multiple (non-) extreme flood drivers that occur in combination or closely succession and cause greater impacts than any single source of flooding in isolation

what type of flooding

fluvial

pluvial

coastal

compound

Flood hazards

refer to the duration, depth, and velocity of floodwaters

Duration

length of time in which floodwaters from river discharge or water level exceed, or remain above, a given threshold

flood duration formula

volume / peak

Depth

vertical distance between water surface elevation (WSE) and ground elevation (GE) with respect to a common vertical datum (e.g., NAVD88)

depth equation

WSE - GE

Flood Velocity

average rate of floodwaters moving through a floodplain, river/tidal channel, inundated area, or drainage network

flood impacts

refer to fatalities, injuries, and extensive infrastructural damage

hydrodynamic flood impacts

force due to moving water

hydrostatic flood impacts

vertical and lateral forces due to standing water

debris Flood impacts

floating and rolling debris from floodwater

soaking Flood impacts

damages to building materials (wood)

sediment Flood impacts

trapped in building material and structures

infrastructure Flood impacts

damage to critical infrastructure and utlities

safety and health Flood impacts

mental stress, fatigue, mold, mosquitos, etc.

V zones

coastal areas of high hazard. The “E” in VE indicates that a predicted elevation of water (e.g., the top of the waves in a 1%-annual-chance flood) has been determined and is designated on the Flood Insurance Rate Maps (FIRM).

A Zones

areas subject to inundation by a 1%-annual-chance flood (e.g., fluvial or coastal) that do not meet any of the criteria for being designated as a V Zone.

A Zones in coastal areas

subject to breaking waves (heights < 3 ft or 0.91 m), storm surge, and wave runup (depth < 3 ft) that may cause erosion and scour.

The Limit of Moderate Wave Action (LiMWA)

boundary between moderate (MoWA) and minimal wave action zones (MiWA)

X Zones

outside the areas inundated by the 1%-annual-chance flood. Zones beyond the SFHA limit.

Shaded X Zones

areas subject to inundation by the 0.2%- annual chance flood.

Unshaded X Zones

areas where the annual probability of flooding is less than 0.2%.

AO Zones

areas of 1% annual chance of shallow flooding, usually sheet flow on sloping terrain, where average depths are between 1 and 3 feet.

AH Zones

Areas with a 1% annual chance of shallow flooding, usually in the form of a pond, with an average depth ranging from 1 to 3 feet

AR Zone

Areas with a temporarily increased flood risk due to the building (or restoration) of a flood control system such as a levee or a dam

Exceedance Probability (EP)

chance that one or more floods occurring during any period (n) will exceed a given flood threshold

EP can be approximated with the

binomial distribution

according to FEMA, floodplain management is a

community-based effort to prevent or reduce the risk of flooding, resulting in a more resilient community

floodplain detrimental impacts

infrastructure damage, human displacement, financial loss, waterborne diseases

floodplain beneficial impacts

groundwater recharge, nutrient and sediment distribution, soil fertilization

floodplain inconsequential impacts

temporary disruption of services, dirty driveways, puddle formation, pedestrian detours

structural floodplain management measures are

designed to keep floodwaters away from building infrastructure

nonstructural flood management measures are designed to

keep building infrastructure away from floodwaters

FEMA management strategy 1

avoid hazardous , uneconomic, or unwise use of floodplains through floodplain regulating and redevelopment of policies

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.

(Re)Development of policies

considering the design and location of public services, utilities, and critical facilities

Land acquisition

preserving open space and permanently relocate buildings away from floodplains.

restore and preserve

natural resources and functions of floodplains

flood-proof building

elevating new buildings and retrofitting existing ones

fema flood management strategy 2

Develop projects/infrastructure that control floodwater.

• Dams and reservoirs along streams/rivers to store excess water upstream from developed areas.

• Dikes, levees, and floodwalls to keep water away from developed areas

Channel restoration or alteration to

avoid overbank flooding

(FEMA strategies) runoff control measures

for developed areas

(FEMA strategies) land treatment to

hold more water through infiltration

(FEMA strategies) detention ponds to

store on-site excess runoff

(FEMA strategies) flow diversion

around developed areas

(FEMA strategies) strategy 3

Assist individuals & 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

(FEMA strategies) strategy 4

Renew the vitality and purpose of natural floodplains through:

-Land use regulation: protect floodplains, wetlands, and coastal barriers.

-Policies: design and location of public services, utilities, and critical facilities.

(FEMA strategies) preservation

land acquisition, open space, permanent relocation of buildings, restoration of floodplains, wetlands, and habitats

(FEMA strategies) information and education

make people aware of natural floodplain resources and functions and how to protect them.

(FEMA strategies) tax adjustments

provide a financial initiative for preserving lands or restoring lands

(FEMA strategies) beach nourishment

protect inland development and maintain natural flood protection features

flood events are characterized using

spatial, temporal, or spatiotemporal data which include gridded and time-series data

spatial data examples

Satellite imagery

Radar data

LiDAR data

Drone imagery

temporal data examples

Tide-gauge data

River-gauge data

Rain-gauge data

High-water marks

Spatiotemporal data examples

Reanalysis products (CFSR – ERA-5)

Inundation maps (hydrodynamic models

Complete duration series:

all available data records

partial duration series

data records that exceed a given threshold value

Annual exceedance series

partial duration series in which the number of data values is equal to the number of years

extreme value series

partial duration series containing the largest or smallest values in equal intervals

baseflow

low flow rate added to a channel from the subsurface (e.g., groundwater)

bankfull flow

flow rate that fills a channel to its maximum capacity

flood flow

flow rate that overtops the natural/artificial banks

peak flow

maximum flow rate within a period of interest (e.g. annual peak flow)

The time between occurrences (X ≥ x T) is defined as

the recurrence interval (𝜏)

return period (T)

average of recurrence intervals E(𝜏) (T is the average time until the next occurrence of an extreme event.)

streamflow that has a return period of 100 years is called the

100-year flood

an area that gets inundated from a 100-year streamflow is considered the

100-year floodplain

the 100-year floodplain is used in practice for

flood hazard management

time series examples

river flow, rainfall, water level

in flood frequency analysis, we work with

continuous variables (so data points are independent)

histograms are

graphs that show the frequency distribution of a random variable. Histograms classify data into various “bins” or “range groups” and count how many data points belong to each of those bins

kernel density estimation is a

nonparametric method to estimate the probability density function of a random variable

univariate analysis in flood modeling

analysis of flood return periods for independent variables

block maxima (BM) consists in

dividing data records to nonoverlapping periods of equal size or duration (blocks)

selecting the maximum data record from each period is referred to as

annual maxima (AMAX)

BM ensures that extreme data are

independent

BM ignores

other extreme values within the same block that might be even higher than those from other blocks

peaks-over-threshold (POT) samples data records by

selecting extreme values that exceed a given threshold (e.g. percentiles -% of data below or at a given threshold)

POT method requires that extreme values are

independent and identically distributed (IID)