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MUD CREEK WATERSHED STUDY
PUBLIC MEETING NOVEMBER 7, 2013
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AGENDA Purpose of Study
Hydrology Hydraulics
Floodplain Mapping Floodplain Management Options
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PURPOSE OF STUDY Upstream Development
Near I-80 Bondurant
Flooding Issues Altoona Growth
Preemptive Management Strategies
Funding
NE
96th
Str
eet
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Mud Creek Watershed Study
HYDROLOGY
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HYDROLOGY
Definition “To study the occurrence, distribution, and
circulation of water on and below the Earth’s surface.”
In Other Words
How much water is in Mud Creek?
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HYDROLOGY
New Detailed Study From headwaters to confluence with Des Moines
River
Existing Conditions Future Conditions
Land use plan through 2030
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PRECIPITATION
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IOWA’S HISTORIC RAINFALL EVENTS
13.18 inch daily rainfall on 6/14/1998 (official record) – Atlantic, Iowa
12.52 inch daily rainfall on 7/2/1958 – Audubon, Iowa
12.02 inch daily rainfall on 7/17/1996 – Castana, Iowa
10.74 inch rainfall in 18 hours on 7/27/2011 – Dubuque, Iowa
21.70 inch daily rainfall on 9/18/1926 (unofficial record) – Boyden, Sioux Co. Iowa
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ANKENY YEARLY RAINFALL
0
10
20
30
40
50
60
70
Ankeny Airport Yearly Rainfall (inches)
Total
Average Annual Rainfall
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PROBABILITY OF ANOTHER FLOOD
What is the meaning of flood probability?
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WHEN TO EXPECT ANOTHER 100 YEAR FLOOD
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PEAK FLOOD FLOWS - CFS What is a CFS?
Cubic feet per second is a measurement of volume passing by a point every second.
7.48 gallons of water per second Mud Creek at NE 80th Street can convey
approximately 2,500 CFS with banks full.
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DESIGN FLOW DETERMINATION
Event Year Source Flow (cfs)
Location
Effective 1% Annual Exceedance Probability Flow
1982 Flood Insurance
Study
5,940 Highway 163
Updated 1% Annual Exceedance Probability Flow
2013 Snyder and Associates
8,600 Highway 163
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Mud Creek Watershed Study
HYDRAULICS
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HYDRAULICS New Detailed Study
From NE 27th Avenue To NE 62nd Avenue
NE 46th Avenue
NE 54th Avenue
NE 62nd Avenue
NE 27th Avenue
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HYDRAULICS Elevation & Survey Data Field survey of all structures LiDAR data
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HYDRAULICS How deep does the water get in Mud Creek?
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Mud Creek Watershed Study
FLOODPLAIN MAPPING
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FLOODPLAIN MAPPING
Floodplain Terminology Mud Creek Flood Mapping
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FLOODPLAIN
FLOODWAY
FLOODWAY FRINGE
FLOODWAY FRINGE STREAM CHANNEL
FLOODPLAIN TERMINOLOGY
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MUD CREEK FLOOD MAPPING
Mapping 100 Year and 500 Year Existing Conditions Future Conditions
No change in development practices up to 2030 Mapped using LiDAR data Compare to existing floodplain
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MUD CREEK FLOOD MAPPING Blue =Unofficial Existing
Green = FEMA
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MUD CREEK FLOOD MAPPING
Blue = “Unofficial” Existing Red = Future
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Mud Creek Watershed Study
FLOODPLAIN MANAGEMENT OPTIONS
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FLOODPLAIN MANAGEMENT OPTIONS
Minimum Management Criteria Above Minimum Criteria Regional/Site Detention
Water Quality Design
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MINIMUM MANAGEMENT CRITERIA
FLOODPLAIN
FLOODWAY
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FLOODPLAIN
FLOODWAY
MINIMUM MANAGEMENT CRITERIA
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FLOODWAY
MINIMUM MANAGEMENT CRITERIA
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MINIMUM MANAGEMENT CRITERIA
Minimum National Flood Insurance Program requirements Development allowed in Floodway Fringe 1’ above 100 year flood
Points of Consideration
Allows for them most developable area Highest risk for future flood impacts
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ABOVE MINIMUM MANAGEMENT
Polk County Criteria Fill criteria (No more than 5 feet of fill) Proximity to Floodway (Set back 25 feet)
Higher than 1’ above 100-year flood
No development in the floodplain
Points of Consideration Reduces developable area Lower risk for future flood impacts
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ABOVE MINIMUM MANAGEMENT Polk County Criteria
Setback
FLOODPLAIN
FLOODWAY
5 ft or less 25 ft 25 ft
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ABOVE MINIMUM MANAGEMENT
Higher than 1’ above 100-year flood
100 Year Floodplain
1’ Above
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ABOVE MINIMUM MANAGEMENT
No Development in the floodplain
FLOODPLAIN
FLOODWAY
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REGIONAL/SITE DETENTION
Effort to mimic pre-development hydrology
High cost to build and maintain Possible regional detention
Estimate of detention volume
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REGIONAL/SITE DETENTION
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WATER QUALITY DESIGN
Does not address increased volume Detain first 1.25’’ of precipitation
Treats up to 90% of all rain events Stabilizes stream flow
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MUD CREEK WATERSHED STUDY SEE MAPS AROUND ROOM WWW.ALTOONA-IOWA.COM – NEWS TO KNOW
PUBLIC MEETING NOVEMBER 7, 2013