FLOOD The Region’s Most Pervasive Hazard · groundwater table rises and shows itself as flooding...
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HAZARD DESCRIPTION Flooding is a natural cycle of streams and rivers. It occurs when the volume of precipitaon or melng snowpack water exceeds the capacity of river banks to keep flowing waters contained. Of all natural hazards that affect Thurston County, floods are the most common and, on an annual average basis, the most costly. Thurston County has been declared a major Federal Disaster Area 17 mes. Several factors determine flood severity: • Precipitaon intensity and duraon • Soil saturaon condions • Topography and ground cover • Amount of snow Four types of flooding occur in Thurston County: 1. River or stream floods occur with prolonged heavy rainfall, a rapidly melng snow pack or a combinaon of these. 2. Flash floods result from intense storms dropping large amounts of rain within a short period of me onto watersheds that cannot absorb or slow the flow. 3. Tidal floods result when extremely high des combine with low atmospheric pressure, excessive run-off, or strong northerly winds. The des can also enhance flooding in delta areas when rivers or creeks are at, or near, flood stage. 4. Groundwater flooding occurs when there is a high water table and persistent heavy rains. The situaon is caused in areas where an upper, thin layer of permeable soils overlies an impermeable layer of hard pan. As the ground absorbs more and more rain water, the groundwater table rises and shows itself as flooding in areas where the land surface is below the water table. The 1998-1999 flood is the groundwater flood of record. FLOOD The Region’s Most Pervasive Hazard The Devastang Effects and Costs of Floods Aside from inundaon of lands and property, floods can result in: • Death or injury to people, pets, and livestock • People leſt stranded or isolated for long periods of me • Physical destrucon of buildings and other structures • Contaminaon of water sources and treatment systems • Foul sepc systems • Destrucon of electrical and heang systems • Damage to, destrucon of, or closing of bridges, transportaon routes, railroads, and pipelines and ulity systems • Limited access for emergency responders and crical personnel The potenal consequences are enormous as the following summaries of damages from flooding in 1996, 1997 and 2007 demonstrate: The December 1996 and March 1997 winter storm and groundwater flooding: • Inundated approximately 200 homes countywide • Contaminated approximately 200 drinking water wells • Caused widespread failures of on-site sepc systems • Severely impacted a number of business operaons • Cost Thurston County government in excess of $340,000 • Cost other government enes & ulies in excess of $750,000 • Cost uninsured private property losses in excess of $1.75 million The December 2007 flood and severe storm:* • 25 rescue missions (17 by helicopter) to rescue 63 people • 400 properes affected, 96 households isolated by washout of Cedar Flats Road • 44 road and bridge closures with damages likely to exceed $2.7 million • LOTT Alliance Wastewater Treatment Facility overloaded by runoff and forced to discharge 1 million gallons of untreated stormwater per hour into Budd Inlet *Note: Data for the 2007 flood is sll being analyzed. December 2007 Chehalis River Basin Flooding. Photo courtesy Thurston County Cooper Point Rd/Black Lake Blvd Intersecon: West Olympia urban flooding. Photographer unkown. Flooding and debris over Independence Road, December 2007. Photo courtesy Thurston County FLOODING AREA CONSIDERATIONS IN CRITICAL AREAS ORDINANCE Delineaon of Floodplains The Thurston County Floodplain Map delineates the boundaries of 100-year floodplains and areas of high groundwater flooding. Flooding has and will connue to occur outside of these boundaries. The floodplain maps are based on flood of record and the current Flood Insurance Rate Maps prepared by the Federal Emergency Management Administraon. Important Things to Know: Many of the flood development rules originate from FEMA. • New homes/structures: • In order to reduce risks associated with flooding and to set aside areas where stream courses naturally fluctuate, new projects could not encroach on a floodplain or channel migraon area. Floodplains and channel migraon areas are also frequently associated with streams, their riparian habitats, and wetlands. Repairs, maintenance, alteraon of exisng homes/structures: • Buildings already located within exisng floodplains or channel migraon areas could be repaired, maintained or altered. If the building is completely within the floodplain or channel migraon area, the repairs, maintenance or alteraons would need to occur within the exisng footprint. If a poron of the building lies outside the floodplain or channel migraon area, the footprint could be expanded in that area, subject to county building codes (14.38 TCC). Restoraon or rebuilding due to a • natural disaster or accidental destrucon: Restoraon or rebuilding of legally established structures located wholly within a floodplain and damaged beyond repair or destroyed by accident, fire, explosion or flooding could be allowed, subject to county building codes (Title 14 TCC). Discreonary restoraon or rebuilding: • For proposed reconstrucon, replacement, or alteraon other than a natural disaster or accidental destrucon, structures with more than 50 percent damage or replacement would be treated as new structures and would have to be repaired or rebuilt in accordance with current state and local ordinances that regulate new construcon in a floodplain. (The 50 percent is based on the fair market value prior to the damage or reconstucon occurring.) Thurston County’s and FEMA’s goal is to avoid having homeowners rebuild in dangerous flood-prone areas and to avoid repeve losses. NEW Floodplain - areas that are susceptible to being inundated by floodwaters. Riparian Habitat Area (buffer) Riparian Habitat Area (buffer) EXISTING CAN STAY EXISTING PARTIALLY OUTSIDE FLOODPLAIN Remodeling, maintenance repair can expand footprint on this side Remodeling, maintenance repair must be done in existing footprint. EXISTING - NATURAL DISASTER EXISTING - DISCRETIONARY REPLACEMENT > 50% FAIR MARKET VALUE Can rebuild within existing footprint if natural disaster causes damage beyond repair (flooding, fire, accident) subject to conditions and in accordance with Thurston County building codes. For proposed reconstruction, replacement, or alteration other than a natural disaster; structures with more than 50 percent damage or replacement, based on the fair market value prior to the damage or reconstuction occurring, would be treated as new structures. Same rules apply for channel migration zones.
Transcript of FLOOD The Region’s Most Pervasive Hazard · groundwater table rises and shows itself as flooding...
HAZARD DESCRIPTIONFlooding is a natural cycle of streams and rivers. It occurs when the volume of precipitation or melting snowpack water exceeds the capacity of river banks to keep flowing waters contained.
Of all natural hazards that affect Thurston County, floods are the most common and, on an annual average basis, the most costly. Thurston County has been declared a major Federal Disaster Area 17 times.
Several factors determine flood severity: • Precipitation intensity and duration • Soil saturation conditions • Topography and ground cover • Amount of snow
Four types of flooding occur in Thurston County:
1. River or stream floods occur with prolonged heavy rainfall, a rapidly melting snow pack or a combination of these.
2. Flash floods result from intense storms dropping large amounts of rain within a short period of time onto watersheds that cannot absorb or slow the flow.
3. Tidal floods result when extremely high tides combine with low atmospheric pressure, excessive run-off, or strong northerly winds. The tides can also enhance flooding in delta areas when rivers or creeks are at, or near, flood stage.
4. Groundwater flooding occurs when there is a high water table and persistent heavy rains. The situation is caused in areas where an upper, thin layer of permeable soils overlies an impermeable layer of hard pan. As the ground absorbs more and more rain water, the groundwater table rises and shows itself as flooding in areas where the land surface is below the water table. The 1998-1999 flood is the groundwater flood of record.
FLOOD The Region’s Most Pervasive HazardThe Devastating Effects and Costs of Floods
Aside from inundation of lands and property, floods can result in:
• Death or injury to people, pets, and livestock • People left stranded or isolated for long
periods of time • Physical destruction of buildings and other
structures • Contamination of water sources and
treatment systems • Foul septic systems • Destruction of electrical and heating
systems • Damage to, destruction of, or closing of
bridges, transportation routes, railroads, and pipelines and utility systems
• Limited access for emergency responders and critical personnel
The potential consequences are enormous as the following summaries of damages from flooding in 1996, 1997 and 2007 demonstrate:
The December 1996 and March 1997 winter storm and groundwater flooding:
• Inundated approximately 200 homes countywide
• Contaminated approximately 200 drinking water wells
• Caused widespread failures of on-site septic systems
• Severely impacted a number of business operations
• Cost Thurston County government in excess of $340,000
• Cost other government entities & utilities in excess of $750,000
• Cost uninsured private property losses in excess of $1.75 million
The December 2007 flood and severe storm:*
• 25 rescue missions (17 by helicopter) to rescue 63 people
• 400 properties affected, 96 households isolated by washout of Cedar Flats Road
• 44 road and bridge closures with damages likely to exceed $2.7 million
• LOTT Alliance Wastewater Treatment Facility overloaded by runoff and forced to discharge 1 million gallons of untreated stormwater per hour into Budd Inlet
*Note: Data for the 2007 flood is still being analyzed.
December 2007 Chehalis River Basin Flooding. Photo courtesy Thurston County
Cooper Point Rd/Black Lake Blvd Intersection: West Olympia urban flooding. Photographer unkown.
Flooding and debris over Independence Road, December 2007. Photo courtesy Thurston County
FLOODING AREA CONSIDERATIONS IN CRITICAL AREAS ORDINANCEDelineation of Floodplains
The Thurston County Floodplain Map delineates the boundaries of 100-year floodplains and areas of high groundwater flooding. Flooding has and will continue to occur outside of these boundaries. The floodplain maps are based on flood of record and the current Flood Insurance Rate Maps prepared by the Federal Emergency Management Administration.
Important Things to Know:
Many of the flood development rules originate from FEMA.•
New homes/structures:• In order to reduce risks associated with flooding and to set aside areas where stream courses naturally fluctuate, new projects could not encroach on a floodplain or channel migration area. Floodplains and channel migration areas are also frequently associated with streams, their riparian habitats, and wetlands.
Repairs, maintenance, alteration of existing homes/structures: • Buildings already located within existing floodplains or channel migration areas could be repaired, maintained or altered. If the building is completely within the floodplain or channel migration area, the repairs, maintenance or alterations would need to occur within the existing footprint. If a portion of the building lies outside the floodplain or channel migration area, the footprint could be expanded in that area, subject to county building codes (14.38 TCC).
Restoration or rebuilding due to a • natural disaster or accidental destruction: Restoration or rebuilding of legally established structures located wholly within a floodplain and damaged beyond repair or destroyed by accident, fire, explosion or flooding could be allowed, subject to county building codes (Title 14 TCC).
Discretionary restoration or rebuilding: • For proposed reconstruction, replacement, or alteration other than a natural disaster or accidental destruction, structures with more than 50 percent damage or replacement would be treated as new structures and would have to be repaired or rebuilt in accordance with current state and local ordinances that regulate new construction in a floodplain. (The 50 percent is based on the fair market value prior to the damage or reconstuction occurring.) Thurston County’s and FEMA’s goal is to avoid having homeowners rebuild in dangerous flood-prone areas and to avoid repetitive losses.
NEW
Floodplain - areas that are susceptible to being inundated by �oodwaters.
Riparian Habitat
Area (bu�er)
Riparian Habitat
Area (bu�er)
EXISTING CAN STAY
EXISTING PARTIALLY OUTSIDE FLOODPLAIN
Remodeling, maintenance repair
can expand footprint on this side
Remodeling, maintenance repair must be done in
existing footprint.
EXISTING - NATURAL DISASTER
EXISTING - DISCRETIONARY REPLACEMENT > 50% FAIR MARKET VALUE
Can rebuild within existing footprint if natural disaster causes damage
beyond repair (�ooding, �re, accident) subject to conditions and in accordance with Thurston County
building codes.
For proposed reconstruction, replacement, or alteration other
than a natural disaster; structures with more than 50 percent damage
or replacement, based on the fair market value prior to the damage or reconstuction occurring, would be
treated as new structures.
Same rules apply for channel migration
zones.
