FEMA Floodplain Revision Robbinsdale Park Area Reconstruction City Project No. 15-2253 / CIP No. 50421.3-1B FEMA Case No. 19-08-0857P
ROBBINSDALE PARK Background Who is FEMA?
Federal Emergency Management Agency (FEMA)
Establishes floodplain maps
Determines regulations and flood insurance policies What is a Floodplain?
The FEMA regulated floodplain is defined as the water surface boundary that is created during the 100-year storm event.
What is the 100-year storm event?
A storm intensity large enough to occur 1 time in 100 years on average.
This 100-year storm may also be called the 1% storm (1/100-years = 1%). This means in any given year we have a 1% chance of witnessing this intensity of storm.
This does not mean once this 1% storm occurs it will not be seen for another 100 years.
FEMA also defines a water surface boundary for the 500-year event too.
What is the 500-year storm event?
Just like the 100-year storm, it is a storm intensity large enough to occur 1 time in 500 years on average.
The 500-year storm event may also be called the 0.2% storm (1/500-years = 0.2%). The higher the probability of a storm occurring equals a lower amount of rainfall witnessed. 100-yr (1%) has less rainfall then the 500-yr (0.2%) What Type of Floodplains Exist?
FEMA has many types of floodplain designations but ultimately, they can be broken down into two (2) main types.
1.) Studied – survey and engineering data determines floodplain boundary
in-depth analysis completed
The amount of water flowing down the drainage as well as the anticipated height of the water is calculated.
2.) Unstudied – a “best guess” of floodplain boundary is determined
no in-depth analysis completed
No amount of flow or anticipated water heights have been calculated.
The intention of the Robbinsdale Park reconstruction project was to study the unstudied portion of floodplain running generally from the Fairmont Blvd. and E. Anaconda Rd. intersection to the intersection of Hawthorne Ave. and E. Meade St.
This portion of floodplain to be studied is part of a drainage area referred to as the Hawthorne Drainage Basin Pre-Project FEMA Floodplain
100-yr Studied
Meade. St. Hawthorne Ave. Hawthorne
100-yr Unstudied Grandview Dr. Grandview
500-yr Unstudied What is a Drainage Basin?
Stormwater follows gravity (from higher elevations to lower elevations)
A drainage basin is the total contributing area that flows to a single point (everything uphill)
Think of a drainage basin as a very large pool with a deep end and a shallow end.
If you were to fill the pool with water, the water would find its way downhill and ultimately end at the deep end. Thankfully, our drainage basin “pool” has outlets so water doesn’t build up and flood everyone in the basin.
Typically, the larger the drainage basin size, the larger the stormwater flows seen at the lowest point. 1.2 mi2 How Big is the Hawthorne Drainage Basin? 774 acres
Saint Patrick St.
Meade St. Monument
Health Hospital Hawthorne Ave. Highway 16
Skyline/Tower Road Intersection How Much Water is There?
About 1.0 mi2 drains into Robbinsdale Park from E. Fairlane Dr.
That equals About 777 cubic feet per second (cfs) of stormwater during the 100-year storm event
What does 777 cfs of water look like?
Fire hydrants flow at about 1,000 gallons per minute (gpm)
1,000 gpm equals 2.2 cfs
777 cfs equals 353 fire hoses
That’s a wall water 30’ wide and 26’ high every second
How do we reduce the amount of water and limit flooding?
Detention Cells Detention Cells
What is a Detention Cell?
It’s simply a pond that holds stormwater and slowly releases it downstream.
Think of it as a supersized bathtub with a slow drain.
Increasing the size of the pond allows for less water to be discharged downstream.
Benefits:
Slows down stormwater to a manageable rate
Minimizes flooding downstream of pond
Cleans stormwater (sediment and pollution) Many Detention Cells Are Throughout 8 in just the Hawthorne Rapid City Drainage Basin
Saint Patrick St.
Meade St.
Hawthorne Ave. Highway 16 Purpose of Robbinsdale Park Reconstruction Project Pre-Construction Conditions
Prior to reconstruction, the two detention cells in Robbinsdale Park were significantly undersized.
Most notably, the largest cell (southern cell) contained only 31% of the volume of the newly reconstructed cell.
Trees had grown in the existing cell and further limited the volume capacity as well as created maintenance issues for the City. Pre-Construction Conditions Cont.
Prior to reconstruction of Robbinsdale Park, downstream drainage improvements along Hawthorne Ave. were construction around 2002.
Improvements included the installation of an 8’x4’ box culvert to parallel the existing 48” diameter storm sewer.
Though additional stormwater can be taken underground with the new storm sewer, the detention cells in Robbinsdale Park were undersized and released too much water for this new stormwater infrastructure during large rain events.
The newly reconstructed detention cells at Robbinsdale Park are designed to hold back as much stormwater as possible so that the downstream storm sewer along Hawthorne Ave. is not overwhelmed.
This results in less stormwater needing to be placed above ground on the street and minimizes the chances of flooding occurring on properties downstream of Robbinsdale Park. Changes to the Southern Robbinsdale Detention Cell
Pre-Construction Mid-Construction
Near Final Post-Construction
Southern Cell – Pre-Construction: Approx. 11 acre-feet of storage
Southern Cell – Post-Construction: Approx. 35.5 acre-feet of storage Parts of the Southern Robbinsdale Detention Cell
Impact Structures
These structures lower the velocity of stormwater entering the detention cell.
This helps minimize erosion in the detention pond.
The structures works by creating a sudden change in direction of the stormwater. Parts of the Southern Robbinsdale Detention Cell
Outlet Structure
This structure controls how much water is released from this detention cell.
As water fills the detention cell, different sized openings regulate the total flow of water.
The bottom 3.5’ of the detention cell is designed for a very slow release of water.
This slow release allows sediment and other pollutants to settle out of the water and not be released downstream into Rapid Creek. Parts of the Northern Robbinsdale Detention Cell
The northern detention cell is significantly smaller than the southern cell.
The purpose of the detention cell is to regulate stormwater discharging from the subdivision directly west of Robbinsdale Park and to act as additional volume for any stormwater that the southern detention pond cannot handle.
The northern pond is equipped with two outlet structures to regulate stormwater flows.
The largest outlet structure is found near the eastern boundary of the detention cell and acts as a junction for underground stormwater infrastructure.
This junction connects the storm sewer of the southern and northern ponds to the City’s existing stormwater infrastructure. Changes to the Floodplain FEMA Process to Change Floodplain
The Robbinsdale Park Reconstruction project has gone through the FEMA LOMR process and has been approved.
LOMR stands for Letter of Map Revision
Approval of a LOMR by FEMA officially changes the floodplain outline as well as the flows and depths of a floodplain.
The LOMR process for this project included the following:
Revision of hydrologic model (how much stormwater)
Revision of hydraulic model (where and how the stormwater moves)
This new hydraulic model incorporates all the stormwater improvements within Robbinsdale Park as well as existing downstream and upstream infrastructure. Where to Find Floodplain Maps and Changes
FEMA has established a website to search and look at current floodplains for the USA.
www.fema.gov/national-flood-hazard-layer- nfhl
Click the “NFHL Viewer” link on the page
Please note that the changes to the floodplain around Robbinsdale Park will not be officially changed until October 5th, 2020. Until the October date, the current “unstudied” floodplain is still effective. Changes to Floodplain Boundaries
Full floodplain changes Pre-Construction Floodplain Post-Construction Floodplain have been included on this slide.
The next sixteen (16) slides are zoomed areas of interest to show the changes in more detail. Changes to Floodplain – Area 1 Changes to Floodplain – Area 2 Changes to Floodplain – Area 3 Changes to Floodplain – Area 4 Changes to Floodplain – Area 5 Changes to Floodplain – Area 6 Changes to Floodplain – Area 7 Changes to Floodplain – Area 8 Changes to Floodplain – Area 9 Changes to Floodplain – Area 10 Changes to Floodplain – Area 11 Changes to Floodplain – Area 12 Changes to Floodplain – Area 13 Changes to Floodplain – Area 14 Changes to Floodplain – Area 15 Changes to Floodplain – Area 16 May 18th, 2018 Storm Unexpected Testing of Detention Cells
In drainage engineering, many assumptions need to be made before finalizing a design. One of the largest contributors to the design is determining what storm intensity to plan for. For the Robbinsdale Park Reconstruction Project, the detention cells were designed for the 100-year storm event (1% chance). As discussed earlier, this means in a given year there is only a 1% chance of this magnitude storm occurring. The reconstruction within Robbinsdale Park was completed on Monday, May 14th, 2018. Four days later, a high magnitude storm occurred over the Robbinsdale area of Rapid City that put the detention cells’ design to the test. From the conditions witnessed during the May 18th storm, the engineering design assumptions for the Robbinsdale detention cells can be checked. The remaining portion of this presentation focuses on what occurred on May 18th as well as how the Robbinsdale Park Reconstruction Project significantly reduced the potential for flooding downstream of the Park. May 18th, 2018 Storm
At approximately 6:15 p.m. a rainstorm began in Rapid City
The storm quickly intensified, most notably over the Hawthorne Drainage Basin
2018 was the 3rd wettest year ever recorded for Rapid City Storm Intensity
Official National Weather Service (NWS) rainfall total was only 1.79 inches.
Unofficially, rain intensity appeared much higher in portions of the Hawthorne Drainage Basin
4.5 inches from residential rain gage
NWS did release interpolated rainfall depth based on radar
2.6 inches in Robbinsdale area
Because of this significant amount of rainfall in such a short timeframe, major flooding started to occur within the Robbinsdale Area
Image taken from social media Flooding Images E. Fairlane Drive (Directly Upstream of Robbinsdale Park)
Asphalt was torn out of the street section due to high velocities
Diameter of Tire is Approximately 26 inches
Image taken from social media E. Fairlane Drive The stormwater in the street was fast enough to rip up portions of asphalt patching Other Robbinsdale Flooding
Meade Channel Near Intersection of Maple Ave. and E. St. Anne St. Same Location as Depicted to the Left but with No Flooding
Image taken from social media E. Main St. in front of School of Mines Other Rapid City Flooding
Meade Channel Near Intersection of Meade St.
Image taken from social media
Same Location as Depicted Above but with No Flooding
Image taken from social media Robbinsdale Detention Ponds Southern Detention Cell – Taken the Morning of May 18th Southern Detention Cell – 7:30 PM (approx. 30 min after storm)
The detention cell has reached maximum volume capacity
35.5 acre-ft of water volume. The equivalent of a football field with 27 feet of water over it. Southern Detention Cell – 7:30 PM (approx. 30 min after storm)
Emergency spillway was activated
Excess flows from southern detention cell are transmitted to northern pond Northern Detention Cell – 7:30 PM (approx. 30 min after storm) A car was pushed from E. Fairlane Drive into the Southern Detention Cell Model Check Model Check
From witnessed observations seen during and after the storm as well as infield measurements, a back calculation can be performed to determine an estimate of how much water was within E. Fairlane Drive during the May 18th storm event.
With the aid of social media, many images were readily available showing E. Fairlane Drive during the storm event. These images can be used to estimate the height of the water.
After the stormwater had receded from the street section, measurements were taken showing location and elevation of debris lines along E. Fairlane Drive.
Once the imagery and infield measurements were compared and analyzed, a flow within the street was estimated.
This estimated flow can then be compared to the predicted results of the engineering models to determine if the floodplain mapping is correctly predicting where the water is going as well as determine if the detention cells operate as designed. Observations Images
Water in parking lot
Diameter of Tire is Approximately 26 inches
Image taken from social media Observations Debris Lines Infield Measurements Debris Lines
Revised 100-yr Floodplain
Debris Line Not Found Infield Measurements Debris Line Analysis Results
Average depth of infield measurements:
1.85 feet
Average depth per FEMA LOMR:
1.95 feet
Difference of 0.1 ft (5%) May 18th Conclusions
Due to the 5% difference in average stormwater depth along E. Fairlane Drive, along with photographic evidence, as well as rainfall data from the NWS, it was concluded that the May 18th, 2018 storm is approximately equivalent to the anticipated 100-yr storm event.
Stormwater Flows
E. Fairlane Drive Street Section:
577 cubic feet per second
48” Dia. Storm Sewer Within E. Fairlane Dr.:
200 cubic feet per second The detention cells performed as expected
Significant flooding was prevented downstream of Robbinsdale Park.
No flooding recorded along Hawthorne Ave. during the May 18th storm event How to Minimize Flooding What is the City Doing to Minimize Flooding?
Hire engineers to study the problem
Need to know what we are up against and determine what is the best way to move forward
How would you move a refrigerator across town?
By hand?
Vehicle?
Moped?
Truck?
Semi?
Engineers can utilize the newer modeling techniques and computer software to see what future improvements have the most benefit. Keep in mind flood mitigation within urbanized areas is very difficult.
Older subdivisions didn’t incorporate drainage very well (if at all).
Property owners have encroached and changed drainage patterns over time.
Limited room to install infrastructure.
Road right-of-way is only so big.
Do homeowners want big infrastructure sitting in their backyard? What is the City Doing to Minimize Flooding?
Engineering recommendations eventually turn into construction projects.
Construction plans don’t happen overnight.
They take time and money.
Reconstruction is not cheap (about $1,000 a foot to reconstruct a residential street)
Stormwater Drainage Utility Fee (Started in 2013)
Averages $36/year per residential property
Generates about $2.4 millions per year
$1.0 million allocated for maintenance of existing infrastructure
$1.4 million for new construction projects
Robbinsdale Park Drainage Improvements: $1.6 million What Can You Do to Prevent Flooding?
The City can help on major drainages like creeks, roads, drainage easements, but homeowners also have a responsibility for their property as well
Use your downspouts
Discharge water at least 10’ away from your house
Have a sump pump
Cover your window wells
Increase vegetation in your lawn and minimize gravel/concrete/asphalt
Grade your lawn to move water away from your house
Don’t place items in drainages (fences, landscaping, furniture) What Can You Do to Prevent Flooding?
Be a good neighbor
If you see something wrong, help out
Keep drainages clear
Your bridge across the stream looks nice but can it withstand a flood?
What will you do if that bridge clogs up a culvert?
Do you want to be the reason your neighbor got flooded?
If it looks like there is a major problem call someone
The sooner you report an issue the quicker a repair can happen Flood Insurance
National Flood Insurance Program (NFIP)
Required if you are in the FEMA floodplain and have a federally backed loan
Some private mortgages will require it as well
Coverage is meant for reimbursement of structures
Not items such as fences, cars, mold and mildew that is not flood related
Most policies can be purchased through your insurance agent
www.floodsmart.gov Flood Insurance
You don’t have to be in a FEMA floodplain to buy flood insurance
You can obtain flood insurance even if you live on top of a hill
Think of it like car insurance
1 in 103 odds of a being in a fatal motor vehicle related accident in your lifetime
1 in 100 odds of being affected by the 100-year storm event
Your Home - Protect your most expensive asset
www.floodsmart.gov How Do I Know If I’m At Risk?
Look for the common signs
You’re in a low lying area
You see a lot of water when it rains
There’s a lot of drainage infrastructure around (inlets, culverts, ditches, etc.)
Ask the professionals
Ask the City: 605-394-4154 (Public Works)
Ask an Engineer
Research
FEMA Maps
City of Rapid City Website Avg Total Total Yearly Precipitation - Rapid City, SD Precipitation: 16.7”30 2018 Final Thoughts 25
We will continue to deal with flooding 20 All we can do is minimize the chances of 15 effecting people and property.
(inches) 10 The City is trying to fix older infrastructure, but this requires time 5 1972 Flood
and significant funding. Precipitation Yearly Total 0 1940 1945 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 2020 Know the odds Year These large flooding events can occur at any time
The 1972 flood occurred on an average rainfall year while the May 18th flood occurred on the 3rd wettest year on record. Flooding can be isolated to an area
On May 18th, the Robbinsdale area was affected significantly while the surrounding areas saw very little rainfall