Flash Flood Vulnerability of Rain Garden Homeowners in Rochester, MN for Week 6 of St. Mary U’s Advanced Modeling __________________________________________

Kevin Byrne October 13, 2009

 

Goal Analyze and map the flash flood vulnerability of homeowners on Rochester (MN) parcels who have – or are now installing – rain gardens on their property.

 

Context Rochester Public Works launched a program titled Realize Raingardens Rochester recently that funded partial subsidy for installation by residents. Four were approved this year. As a future rain garden owner I felt it would be useful to ponder how flash floods might impact such a garden, say 5 or 10 inch rainfalls.

 

Method

1. Ponder all the variables that might pertain to raingarden flash flood issues. 2. Contact Rochester Public Works for what they might have on the topic. 3. Gather, inspect, and organize all the assets. 4. Construct a basic layered ArcMap file as a base map. 5. Get well acquainted with Spatial Analyst, ArcToolBox’s Spatial Analyst

Tools’ Hydrology, and Xtools for data analysis. Then do it all! 6. Export lots of micro-maps to PDF since no one big map will tell the story. 7. Create a well-structured and clean custom .xls table that holds keys data. 8. Aggregate the three most significant criteria into a “summary” attribute

column to portray flash flood vulnerability for four lots with rain gardens. 9. Prepare a report and slide presentation.

How Can I Create a Rain Garden?

L A K E H O M E & C A B I N K I T • S E C O N D E D I T I O N

9

shoreland & w

ater quality

What Is a Rain Garden?A rain garden is a shallow depression filled withflood-tolerant shrubs, flowers and grasses. Itsmain functions are to collect and filter storm-water runoff, but rain gardens also add beautyto the landscape and may attract butterflies andbirds.

What Are the Benefits of Rain Gardens?Whether you live in the city or along a lake orriver, managing stormwater runoff is important.Rooftops, roads, driveways, and sidewalks arehard surfaces that prevent rainwater and meltingsnow from reaching the soil and soaking into theground. These hard surfaces also tend to collectnutrient-rich yard and pet debris, oil and radiatorfluid from autos, and other debris and pollutants.During a rain event or snow melt, fast-movingrunoff washes the nutrient-rich debris and otherharmful pollutants away, often directly into lakes,rivers, and wetlands. The greater amount andincreased speed of water flowing off hard surfacescan erode soil and carry it into our surface waters.In the summer, runoff is often warmed as it flowsover hard surfaces. If the warm runoff enterslakes and rivers directly, it can affect aquatic life.

A rain garden is one way to address all of theseproblems. Rain gardens are designed to collectstormwater runoff, preventing the runoff fromflowing directly into lakes, rivers, and wetlands.They allow runoff to soak into the soil, filteringout pollutants before entering the groundwater.Rain gardens also allow sediments to settle andplants to absorb nutrients.

Design and Placement of Rain GardensIt is best to sketch a design before you startdigging. Rain garden designs can be simple orelaborate, depending on your gardening interestand experience. When designing a rain gardenconsider garden placement, the size you need,the shape you want, the soil type, and the plantsyou’d like to include. You may need to designmore than one rain garden into your landscape to accommodate the runoff.

First, determine areas of your property that aresuitable for a rain garden. These will generally below areas that are the recommended distanceaway from other features. Rain gardens shouldbe placed 10 feet or more away from buildingsto prevent foundations and basements frombeing damaged by water. The rain garden shouldalso be placed 35 feet or more from septic systemdrain fields, 50 feet or more from drinking waterwells and well away from utility lines. Call theDigger’s Hotline (800) 242-8511 to locateelectrical, gas or telephone lines.

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Next, test the soil in areas that are both suitableand near the sources of runoff. The kind of soil inwhich you construct the rain garden is of utmostimportance. Remember, the purpose of raingardens is to absorb and filter runoff water, so thesoils need to be porous enough to soak up waterwithin 48 hours. Forty-eight hours is the standardbecause it’s likely to be the shortest periodbetween two rainstorms; and will prevent gardenplants from drowning and rain gardens frombecoming mosquito breeding grounds. A simpletest of a soil’s ability to absorb water is to dig awide hole 10 inches deep and fill it with water;if the water disappears within 48 hours, the siteis suitable for a rain garden. If your first site failsthe 48-hour test, test the soil at other potentialrain garden sites on your property.

Rain Garden Shape and SizeRain gardens can be designed in any shape—crescent or kidney shapes are attractive—but a long and narrow rain garden may be bettersuited if you are placing it between structures,such as a house and sidewalk.

The size of your rain garden will depend uponthe size of the roof, driveway, or other hardsurface being drained. Typical rain gardensrange from 100 to 300 square feet in size and, as a rule of thumb, will handle the runoff from a hardsurface that is about three times their size. Forlarger surfaces, more than one rain garden maybe needed to handle the runoff. For example,large roof tops may need a rain garden neareach down spout.

Selecting PlantsChoose plants appropriate for the soil type inyour rain garden and that will also toleratestanding water for up to 48 hours. Many nativeplant species are well suited for rain gardens.If you are constructing a rain garden near alakeshore or riverbank, you may be required to use native plants, depending upon localordinances—so check with your local Soil andWater Conservation District. For recommenda-tions on rain garden plants, see the websitesbelow.

Constructing and Planting a Rain GardenOnce the size, shape, and location of the raingarden has been decided and plants have beenselected, construction can begin. Lay out a ropeor garden hose in the desired shape to use as aguide for digging. The depth of the depressionmay vary from 4 inches to 10 inches. For bestinfiltration the bottom of the rain garden shouldbe level. If your garden is placed on a slope, usethe soil from digging to create a berm on thedownhill side of the rain garden. Excess soilshould be removed from the site.

L A K E H O M E & C A B I N K I T • S E C O N D E D I T I O N

Want to know more?INFORMATION ON THE WEB

For more guidance on rain garden design and construction visit:http://clean-water.uwex.edu/pubs/raingarden/rgmanual.pdf. http://www.pca.state.mn.us/publications/wq-strm8-14af.pdf

Copyright © 2006, Regents of the University of Minnesota. All rights reserved. For ordering information call (800) 876-8636, item #08241. For Americans with Disabilities Act accommodations, please call (800) 876-8636.

The University of Minnesota Extension Service is an equal opportunity educator and employer. Printed on recycled paper.

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How Can I Create a Rain Garden? continued

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Variables • Groundwater height • Bedrock height • Sun aspect • Slope • Soil type and properties • Surface runoff • Saturated hydraulic conductivity • Urbanity, i.e. porosity and proximity of neighborhood

surfaces

1005

2915

425

1865

US Hwy 52

US Hwy 14

35th

Ave N

W

1st St SW

7th St NW

West Cirle Dr

2nd St NW

W Center St1st St NW

24th St NW

17th

Ave N

W

15th

Ave S

W

17th Ave SW

11th Ave NW

6th St SW

Valleyhigh Rd NWCivic Center Dr NW

19th St NW

County Club Rd W

Folwell La SW

4th Av

e NW

8th Av

e SW

32nd Ave NW9th

Ave S

W

E US Hwy 52 Frontage Rd

Fox Valley Dr

10th St SW

5th Ave

NW

15th

Ave N

W

18th St NW

3rd St SW

11th St SW

16 1/2 St NW

2nd St SW

Lake S

t NW28th St NW

18 1/2 Ave NW

15th St NW

13th Ave NW

Elton

Hills

Dr NW

6th Av

e SW

23rd St NW

31st

Ave N

W

16th

Ave S

W

6th St NW

7th Av

e NW

12th Ave NW

10th

Ave S

W

29th St NW

17 1/2 St NW

25th St NW 21st Ave NW

Eagle La SW

5th St SW

27th St NW

36th Ave NW

18th

Ave N

W14th St NW

Aspen Pl

16th

Ave N

W

W US

Hwy

52 Fr

ontag

e Rd

7th St SW

23rd

Ave S

W

20th St NW

26th St NW

17th St NW

14th

Ave N

W

Memorial Pkwy

Unknown or No Streetname

16th St NW

Walden La SW

4th St NW

30th Ave NW

20th

Ave S

W

7th Av

e SW

19th

Ave S

W

18th

Ave S

W

30th St NW

22nd St NW

4th St SW

Lake

ridge

Dr N

W

19th

Ave N

W

9th St NW

26th

Ave N

W

18 1/

2 Ave

SW

Foxcroft Cir SW

8th St SW

15 1/

2 Ave

NW

16 1/

2 Ave

NW

10th St NW

Valkyrie La NW

12th St NW

9th Av

e NW

21st St NW

Fox Valley Dr SW

Whitney

La

25th Ave NW

Frontage Rd6th

Ave N

W

Audaz La SW

Industrial Dr NW

Opportunity Rd NW

Hill Ave S

W

Skyli

ne La

SW

31st St NW

3rd St NW10

th Av

e NW

5th Av

e SW

20th

Ave N

W

Clover La SW

15 1/2 St NW

25th St NW

11th

Ave N

W

4th St SW

20th St NW

13th

Ave N

W

17th

Ave S

W

16th

Ave N

W

15th

Ave N

W

18 1/

2 Ave

NW

6th Av

e NW

24th St NW

West Cirle Dr

17th

Ave N

W

1st St SW

7th Av

e SW

4th St SW

5th Av

e NW

35th

Ave N

W

26th St NW

US Hwy 52

US Hwy 14

US Hwy 14

5th Av

e SW

26th St NW

1st St NW

8th St SW

31st St NW

19th

Ave S

W

1st St NW

8th St SW

28th St NW

15th St NW

29th St NW

9th St NW

3rd St SW

17th

Ave N

W

Legendparcelssoil microregionstreamsother waterstreets

elevationHigh : 1260

Low : 958

Realize Raingardens Rochester2009 Sites Basemap

0.5

mile

425

425

1005

1005

425 LegendparcelsPCFFVULN

91%64%55%45%streamsother water

350

Feet

1005 LegendparcelsPCFFVULN

91%64%55%45%streamsother water

490

Feet

1865LegendparcelsPCFFVULN

91%64%55%45%streamsother water

400

Feet

2915

LegendparcelsPCFFVULN

91%64%55%45%streamsother water

370

Feet

 

What I learned • Soil classifications and theory • Definition – and kinds of – floods and rainfalls • Criteria for above: surface runoff, hydraulic conductivity, etc. • Criteria for rain garden planning • Slope derivation from a DEM • Directional flow derived from above • “Kindness of strangers” • Aggregating multiple criteria into a “summary” attribute

column to portray flash flood vulnerability

 

What’s still left to acknowledge and pursue • It’s complicated • It’s like art: there is lots of subjectivity • Your custom Excel table needs to be assembled perfectly or

table joins just don’t work • Directional Accum. is a good next step after Directional Flow • Buffering is trickier than I thought, need to work on that • I need to get better skilled at grid analysis and reclassification • I think GIS analysis of the “landscape architecture of rain

gardens” just might be a profitable endeavor or specialty

 

Software and References • ESRI ArcGIS (ArcMap) • ESRI and second party extensions

o Spatial Analyst o ArcToolBox: Spatial Analyst Tools: Hydrology o Xtools

• Documents from Allison Plume, Rochester Public Works • Files from Prof. John Ebert • Minnesota DEM • Literature: J. Dobur, 2000, “Analysis of Geographical

Distribution of Flash Flood Events,” NOAA/NWS, and others!