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GREEN INFRASTRUCTURE RESILIENCY AND COMPUTER MODELING

JUNE 20, 2018

Association of State Floodplain Managers

Annual Conference

Session F8

Presentation by:

Rocky J. Keehn,

Common Sense Water Resources Engineering, LLC

ASFPM, GREEN INFRASTRUCTURE RESILIENCY AND COMPUTER MODELING June 20, 2018, R. Keehn

1

Why we need Green Infrastructure

ASFPM, GREEN INFRASTRUCTURE RESILIENCY AND COMPUTER MODELING June 20, 2018, R. Keehn 2

No People…No Problems –Rocky J. Keehn

The Challenge of GI Resiliency Design there are many perspectives of a “great” design


3

Omaha – Green Roof

Kearney, NE – Rain Garden From my Stormwater Tours

North Liberty, IA – Porous Pavers

ENGINEERS

LANDSCAPE ARCHITECTS

POLTICIANS

PUBLIC WORKS

ACADEMICS

SCIENTISTS

REGULATORY

Landscape Architect Perspective

184

Plants


4

“I hope most of these survive and if not will plant something else”

Engineer’s Perspective


5

OK…Green Engineer’s Perspective


“My name is going on the plans so how do I this not get sued”

What I learned in College at UNL, in the early 80’s, Definition of Hydrology

ESTIMATION OF SOME ASPECT of the

quantity of water available is of prime

importance in all water resources

engineering. The analysis is basic to the

planning, design, and operation of water

resource systems

My first Hydrology Book, circa 1980 UNL Engineering,

book publish date 1975


7

Hjelmfelt/Cassidy







resource systems




8

Hjelmfelt/Cassidy

Primary Focus = simple equations?







resource systems




Hjelmfelt/Cassidy

Common Sense

Proof it will work, modeling







resource systems




10

Hjelmfelt/Cassidy

Common Sense

Maintenance and Resiliency

Modeling for Proof and Function

My Design Event Philosophy,in Layman Terms

Think of design events as…• FREQUENT EVENT – Volume Control for most (90%) of all rainfall events (old way pick a

pretty arbitrary value)• STABILITY EVENT - Channel and flow system stability (i.e. 2-yr)

• NUISANCE EVENT – Limited complaints, maximum water people will tolerate (i.e 10-yr)

• FLOOD EVENT– Major Damage or if flooding occurs major ramifications (i.e. 100-yr)

• “ACT OF GOD” EVENT– The nearly unimaginable event or unpredicable.

Or better yet….RESILIENCY of your system…or can it (or you!) bounce back quickly from a wide variety of

rainfall impacts or is easy to “fix” after a storm event.

GREEN INFRASTRUCTURE: have to deal with all these events!


11

All GI’s can be modeled as “Detention” Ponds


12

Losses

UncontrolledOutflow

Losses

UncontrolledOutflowD

epth

In

crea

se

Dep

th

Incr

ease

Green Roof System Porous Pavement System

Case Study – Spring Lake Park, South Omaha Nebraska

13

ZOO

DOWNTOWN

AIRPORT

BOYS TOWN

SPRING LAKE PARK

Project is part of the City of Omaha’s Combined Sewer Overflow Program.

MISSOURI AVENUE SEWER SEPARATION/SPRING LAKE PARK

PROJECT


Grey to Green Project

36”

42”

48”

54”

416 acre watershed - 50%

residential/50% park.

Engineering goal, use green

infrastructure in the park

areas to reduce peak flows

so existing downstream

combined sewer would not

have to be upsized when

convert to a storm pipe.PIPES GI


Saved$5 Million

Green Infrastructure Summary – Lots of “GREEN” going on!

• 6 - Extended Dry Detention Basin• 5 – Dry Detention Facilities• 3 – Infiltration Basins• 2 – Retention Wet Ponds• 2 – Constructed channels (replace pipes)• 1 – Constructed Wetland (multiple cells)• 1 – Bioretention basin or “Rain Garden”

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Or Lots of Future Maintenance!


Impact of Green Infrastructure

100-year with no Green

Infrastructure 1065.0 feet


100-year with Green

Infrastructure 1063.1 feet

Entire watershed and all Green Infrastructure modeled in HydroCAD

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Phase 1 GI

Phase 2 GI

Grey System Downstream


Resiliency 101 - Preventing blowouts of GI during larger events!

Rain

Garden

36” RCP inlet into

Rain Garden

“Where

did that

BMP go?”


Blowouts

First Flush Diversions


Manholes Curb cuts

Blowouts

Overflow Structures as part of the GI


20

Blowouts

Planned overflows


21

First Flush Diversion, Modeling

1. Drainage Area

2. First Flush Diversion Manhole

3. Drop Manhole to Reduce Velocity

4. BMP or GI

5. Main Storm Sewer Line

6. Main Storm Sewer Line

1

2

3 4 6

5


22

Flow rate splits out of Green Infrastructure


Inflow into the first flush flow divert structure

All rainfall less then targeted water quality event, usually 90% of all rainfall events, flows into the GI/BMP

Volume in excess of the water quality event, bypass the BMP and flows to the main conveyance system.

Volume from the water quality event is detained, pollutants removed and then flows back into the main conveyance system

Volume in excess of the water quality event, depending on the amount, will flow either back into the main conveyance system or flow overland downstream.

HydroCAD Model – First Flush Structure


Top of Casting reached in Manhole, non-pressure assumption, flows go into BMP/GI

Invert is at highwater in

manhole for 1” rainfall in this

case

HydroCAD Model – GI


Not the control Structure

First Flush Model Results – 100 year

Watershed Runoff

Diversion Manhole Structure

BMP


Event Outflow

100-yr 67 cfs

Event Flow to BMP

Flow Bypassed

100-yr 33 cfs 34 cfs

Event Inflow Outflow Highwater in BMP

100-yr 33 cfs 33 cfs 1086.23 ft

Blowouts, Weirs and Velocity


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Storm Q to main conveyance (6” Orifice)

BMP Highwater

Flow over Weir

Depth over Weir

Velocity over Weir

1” Rain 1.4 cfs 1084.04 ft 0 cfs 0.00 ft 0.0 ft/sec

10-year 2.0 cfs 1086.14 ft 11 cfs 0.14 ft 1.0 ft/sec

100-year 2.0 cfs 1086.28 ft 31 cfs 0.28 ft 1.3 ft/sec

Natural Grass Overflow

Challenge was we had to construct 600-800 feet pipe over steep bank full of trees

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Is there a Green Infrastructure Solution?


Current outlet

from drainage

area

Yes, Infiltration Basin…but

29ASFPM, GREEN INFRASTRUCTURE RESILIENCY AND COMPUTER

MODELING June 20, 2018, R. Keehn

Needed a Pre-treatment BMP

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Pre-Treatment

Infiltration Basin


Pre-treatment Pond Outlets (No Diversion)

31ASFPM, GREEN INFRASTRUCTURE RESILIENCY AND COMPUTER MODELING

June 20, 2018, R. Keehn

All flow less than 1” goes through

standpipe

Emergency Overflow when rain

exceeds 1”

Model, No 1st Flush Pretreatment

Elevation of berm based on 10-year not overtopping


Modeling = It works!…I hope…

40- foot Overflow Weir


Event Qin Qinfiltration Qoverflow Velocity Overflow

1” 1.0 cfs 0.1 cfs 0.0 cfs 0.0 ft/sec

10-yr 16.6 cfs 0.1 cfs 16.5 cfs 1.4 ft/sec

100-yr 22.5 cfs 0.1 cfs 22.5 cfs 1.5 ft/sec

Downstream overflows – Maintenance headache?

34

Concrete Weir for

Flow Distribution


Event Velocity Overflow

1” 0.0 ft/sec

10-yr 1.4 ft/sec

100-yr 1.5 ft/sec

Downstream overflows – Maintenance headache?

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Looking Downstreamflows to a wide swale

Water just runs out of energyASFPM, GREEN INFRASTRUCTURE RESILIENCY AND COMPUTER MODELING

June 20, 2018, R. Keehn

Looking upstreamflows to a wide swale

Modeling helps check future issues

Water went over the maintenance road and caused bank erosion during rainfall

events less than 10-year event. The road was designed to become an

emergency overflow when it exceeded a 10-year event!

Checked the model and as-builts and found overflow structure was too high.

Lowered structure and have had no problems since. When designing for multiple events, project needs to be

constructed correctly to work.


Debris/trash issues and sediment buildup?



What is the value of a model for this situation?

Use model to check impact of debris build-up

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Assume less efficient if outlet partially blocked

Higher starting water surface. Also may be used to predict when

sediment needs to be removed


Green Infrastructure Channel

Pond/Channel replaced 600

feet storm sewer pipe and

from LID perspective

increases travel time of runoff

Needed to be designed for

base flows from seepage,

smaller BMP storms,

2-year, 10-year and 100-

year


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No Blowouts…upstream detention

Channel


Detention Pond

Resiliency and Vegetation

100-year define floodplain vegetation

HECRAS

Low flow over bounder drops

Low flow defines channel

HydroCADHydroCAD

Tailwater


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Modeling will estimate velocities in the channel to assure long-term stability

Results…looks pretty green!



Modeling showed a need for an Emergency Overflows – make it resilient!

Feb. 2017March 2016

FLEXAMAT

Detention Pond Overflow


Fully Vegetated and ready for the “Big Storm”

June 2018

Vegetation and Drawdown Time

Drawdown time about 16

hours for 2-year event


44

Complex of three wetlands in series for storage, habitat diversity and water quality improvement

Modeling what if’s…adds flexibility and checks changes

45

Two Stop Log

Structures to adjust

Wetland and Lake

levels

Two Gates…one to control

flow from detention pond to

channel and one to drain

the lake


Is riprap a good idea or does it increase maintenance costs?

46

FLEXAMAT

FLEXAMAT


Lake outlet very complex and fun to model!


Downstream protection during a “dam” situation


Worst Case,40 feet of head upstream. Think about the velocity at the outlet with this much head on a 48” culvert.

Modeling the entire Green Infrastructure Treatment Train..

49

SAFL Baffle in

residential

neighborhood

First Flush

Diversion

Manhole

Extended

Dry

Detention

Basin

Dry

Detention

Facility

Meandering

Channel with

Boulder Drops

Constructed

Wetland

Flat

Sloped

Channel

Spring

Lake

Combined

Pipe now

Storm Pipe

Missouri

River


Modeling the entire Conveyance Train

Dry Detention

Pond and gated

outlet pipe

Meandering

Channel with

Boulder Drops

Constructed

Wetland and

outlet pipe under

trail to Lake

Energy Dissipation

Structure to “Natural”

Channel

Spring Lake and

pipe under “F”

Street

Low flow culvert

and “Texas

Crossing” to

wetland

Roof/impervious

surface near

houses

Though the

yards

Down the

Street

Storm pipes

through the

park


50

GI level – What the Model tells us for this and other GI sites

Rate and volume of first flush flows and bypass flows – key

design component

Depth and time water is at various elevations during events –

helps with plant types

Velocity at outlets for a wide variety of flows – size erosion

control

Depth and occurrence of overflows – may be more than one

Erosion control of overflows


Conclusion

planning, design, and operation


52

Hjelmfelt/Cassidy

THINK:MAINTENANCE? RESILANCY?,

MODEL

RESILIENT PROJECT

Questions?

•THANK YOU


Rocky J. [email protected]

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