permeable pavement hydrologic modeling - etouches · permeable pavement hydrologic modeling robin...

4/20/2012

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Permeable Pavement

Hydrologic Modeling

Robin Kirschbaum, PE, LEED AP

Alice Lancaster, PE

April 25, 2012

Presentation Overview

Overview of Hydrologic Modeling

Performance Standards

Modeling Guidelines, Tools, Concepts

Permeable Pavement Types

Applications Flow Control

Water Quality Treatment

Slope Considerations

Peak Flow Reduction

Advanced Tools

Hydrologic Modeling

Q: What is hydrologic modeling?

A: Use of mathematical equations to estimate runoff based on: weather patterns

landuse

soil

topography

Source: http://www.und.nodak.edu/

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Hydrologic Modeling

Q: Why do we use hydrologic models?

A1: Characterize hydrologic conditions Predeveloped

Current

Post-project

A2: Design mitigation

A3: It’s fun!

Source: http://www.und.nodak.edu/

Hydrologic Modeling

Q: When does hydrologic

modeling enter into your

project?

A: Start to finish

preliminary design (sizing)

final design (optimization)

demonstrate requirements met

(permit submittals) Source: http://www.und.nodak.edu/

Performance Standards

Flow Control Non Exempt Receiving Water-

Ecology requirement to match the flow duration to predeveloped condition (typically forest)

Combined Sewer or Capacity Constrained Basins-

Local requirements are typically peak-control based

Water Quality Infiltrate 91 percent of runoff file through soils meeting

Ecology treatment requirements

Wetland Protection Ecology guideline to maintain wetland hydroperiod

Volume I App 1-D, Guide sheet 2

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Hydrologic Modeling Performance Standards

LID (Draft 2012 Ecology Manual)

Match 8% 2-yr to 50% 2-yr pre-developed durations

Source: Ecology Presentation on Municipal Stormwater General Permits West.

Washington LID Preliminary Draft Requirements, May 26, 2011.

http://www.ecy.wa.gov/programs/wq/stormwater/municipal/LID/LIDppMay2011.pdf

Flow Control MR #7

LID MR#5

Modeling Tools

Single-event models Appropriate for conveyance sizing

Continuous models Required for sizing flow control (MR7) and

treatment (MR6) BMPs

Simplified sizing tools Represent BMP footprint area as % Imp. Area (“sizing factor”)

Prescribed design criteria

Engineer not needed for small projects (e.g., <10,000sf imp.)

GSI-Calc available for western WA Lowlands

Jurisdiction-specific sizing tools also available (e.g., Seattle, Bellevue,

Edmonds, Kitsap County, Pierce County)

Modeling Tools Single-Event Methods

Input single storm event

Output peak flow rates

Typical methods

SCS

SBUH

StormShed

SWMM

HEC-HMS

SWMM

SUSTAIN

Ru

no

ff (

cfs

)

Rain

fall (in

ch

es)

Rainfall (in)

Unmit. Runoff (cfs)

Mit. Runoff (cfs)

Time (hrs)

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Modeling Tools Continuous Models

Input long-term rain and

evaporation

Output continuous runoff,

peak flow, & duration

Typical programs

HSPF

WWHM

MGS Flood

KCRTS

SWMM

SUSTAIN

Time (hrs)

Ru

no

ff (

cfs

)

Rain

fall (in

ch

es)

Rainfall (in)

Unmit. Runoff (cfs)

Mit. Runoff (cfs)

Modeling Tools Simplified Sizing Tools

Kitsap County: Pavement sized as function of

contributing impervious area and precipitation

BMP Design Infilt.

Rate (in/hr)

Forest Standard Sizing Equation

M B

Permeable Pavement Facility

6 inch

ponding

depth

0.25 0.1100 - 1.0536 Area (sf) = Impervious Area (sf)

x [M x Precip. (in) + B] 0.5 0.0187 + 0.4945

1.0 0.0048 + 0.3531

Permeable Pavement Surface

Slope <=

2%

0.13 – 0.249 0.005 0 Aggregate Depth (in) =

M x Precip. (in) ≥ 0.25 0.01 0

Kitsap County Pre-Sized Calculator GSI-Calc

Modeling Tools Simplified Sizing Tools

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Current Modeling Guidelines

Implicit Method (2005 LID Manual) Model as mix of landscape/impervious on underlying soil type

See Section 7.1 of 2005 LID Manual

Advantage: simple approach

Drawback: greatly underestimates flow control credit

Explicit Method Model impervious surface area draining to an infiltration trench

• Infiltration through surface

• Storage in aggregate discharge subbase

• Exfiltration to native soil

• Overflow

MGSFlood4 and WWHM4

Advantages: better represents design, higher flow control credits

Drawback: slightly more time consuming than implicit method

Modeling Tools HSPF Basics – Model Inputs

Meteorological Data Rainfall (5-min, 15-min, hourly)

Evaporation (daily)

Land Cover Types Impervious areas

• Slope

Pervious areas • Vegetation

• Soil type (A, B, C/D)

• Slope

Regional calibrated parameters (Dinicola 1990)

BMP Configurations

Permeable Pavement Types “Surface” Type: Designed to manage rain falling on the permeable pavement area

(no inflow from adjacent areas).

Rainfall

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Permeable Pavement Types “Facility” Type: May be designed to manage runoff from other areas

Deeper aggregate subbase

Measures to create subsurface ponding when subbase is sloped

Run-on from other areas may

be directed to facility

Thicker subbase to

provide storage

Rainfall

Subsurface ponding

depth controlled

Model Representation Gravel Trench Parameters

Subbase infiltration rate

Wearing Course Overflow

Subbase Aggregate

Key :

Aggregate layer

thickness and porosity

Subsurface ponding

depth in storage reservoir

controlled by overflow or

berms in subbase

Pavement area*

* May include additional contributing area

Model Representation Model Configuration


Subbase Aggregate

Key :

Pavement area (and area

draining to it, if any)

Precipitation

Evaporation Infiltration

Neglected

Gravel trench area

Aggregate courses above

overflow invert not modeled

Theoretical riser

Poin

t of

Com

plia

nce

Void

Storage

Overflow

Note: Only aggregate under overflow invert modeled

Runoff/Interflow

To GW

Where flow control standard

must be met

Infiltration

to native

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Model Representation Gravel Trench Routing

Stage

(ft) Area

(sf)

Storage

(cf)

Infilt.

(cfs)

Overflow

(cfs)

0.0 0 0 0 0

0.1 10,000 1,000 0.0579 0

0.2 10,000 2,000 0.0579 0

0.3 10,000 3,000 0.0579 0

0.4 10,000 4,000 0.0579 0

0.5 10,000 5,000 0.0579 0

0.6 10,000 6,000 0.0579 0

0.7 10,000 7,000 0.0579 31.8

0.8 10,000 8,000 0.0579 87.1

Ex. SSD Table

0’

Ex. Cross Sections

0.6’

Exfiltration

Storage


Subbase Aggregate

Key :

Overflow Elevation = top of pavement

or invert of overflow pipe

Model Representation Sloped Pavement

Exfiltration

Wearing Course

Subbase Aggregate

Key :

Subbase slopes 0 – 2%

can neglect lateral flow

subsurface storage depth modeled = aggregate thickness

Rainfall

Lateral flow along relatively

impermeable subbase

Model Representation Sloped Pavement (>2%)

Exfiltration

Wearing Course Check Dam

Subbase Aggregate

Key :

Check dam Max. WSE

d1 d2 Average d2, d2

Overflow

Subbase slopes 2 – 5%

cannot neglect lateral flow

subsurface storage depth modeled = average water depth before berm overtopping* or overflow

*function of slope, check dam height, and check dam spacing

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Model Representation Underlying Soil

Minimum Correction Factor

Permeable

Pavement Surface

Permeable

Pavement Facility

Not Receiving Run-on 2 2

Receiving Run-on from

Area < twice facility size

NA 2

Receiving Run-on from

Area > twice facility size

NA 4

Design Infiltration Rate Requires Correction

Treatment credits when 91% influent infiltrated

through soils meeting Ecology requirements

Ecology Approved Correction Factors for Kitsap County

Applications

Duration Control (Creek Protection)

Water Quality Treatment

Sloped Design Consideration

Peak Flow Reduction

Flow Control in Creek Basin WWHM4 Example – Explicit Method

Site in King County

Soil is till (0.25 inch/hour design infiltration rate)

Permeable pavement facility is 10,000 sf

Receiving run-on from 5,000 sf of additional area

Design goal = Ecology forest duration standard

Size aggregate depth (ave. subsurface ponding depth)

SIZING FOR FLOW CONTROL GOAL →

May NEED TO BE THICKER TO SATISFY OTHER

DESIGN GOALS (EX. LOADING)

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Precipitation/Evap. Data Select county and location on map

Computational Time Step 15 minutes

Option Menu

Sizing for Flow Control

Predeveloped Basin Select area, soil type, land cover and slope


15,000 sf

Predeveloped Basin Select area, soil type, land cover and slope


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Sizing for Flow Control Developed Mitigated Basin Area contributing runon to permeable pavement….

5,000 sf

Lateral

Impervious Basin

Sizing for Flow Control Developed Mitigated Basin Continued: Route to Porous Pavement Module

Impervious land surface over

gravel trench with infiltration

Sizing for Flow Control Developed Mitigated Basin Continued: Characterize Porous Pavement

Area

Pavement

Section

Infiltration to

Native Soil

SSD Table

Name

POC

Pavement depth +

freeboard above surface

Depression

storage before runoff (weir flow over edge)

(reduction factor =

inverse of correction factor)

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Stage Storage Discharge Table


Overf

low

Flow Duration Curve- Developed Unmitigated (Impervious)


50-yr

½ 2-yr

GOAL

Flow Duration Curve- Developed Mitigated (with Porous Pavement)


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Iteratively Sized Storage Aggregate Depth to Meet Duration Standard


Infiltrates

almost 100% runoff

Only 2”

required to meet goal

Flow Frequency Results


Use Gringorten or

Weibull Method for low annual flows

Performance & Infiltration Rate Example: Permeable Pavement in King County designed to achieve DOE forest

standard (Facility receiving runoff from 100% of permeable pavement area)

0

2

4

6

8

10

12

14

16

18

20

0.00 0.25 0.50 0.75 1.00

Design Infiltration Rate (inches/hour)

Ag

gre

gate

Dep

th (

inch

es)

Permeable Pavement Facility

Permeable Pavement Surface

Example Permeable Pavement Aggregate Depth by Infiltration Rate

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Water Quality Treatment Same WWHM4 Example

Sizing for Treatment Percent Infiltration- at least 91% of entire runoff file

Infiltration through soils meeting Ecology treatment soil requirements

Facility sized for flow control infiltrates much more than 91 percent

Further Analysis WWHM4 Example

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Hydrograph

Further Analysis

Report

Further Analysis

Time Series Export

Further Analysis

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Slope Considerations MGS Flood V4 Example

Site near WSU Puyallup Extension

(Extended Precip. Puget East 40 )

10,000 sf of till forest converted to 5,000 sf porous

pavement receiving runoff from 5,000 sf of impervious

area

1 in/hour saturated hydraulic conductivity

Evaluate berm spacing as a function of slope and depth

Evaluate peak flow reduction


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•Subtract

porous

pavement area

•Rainfall and

evap applied

separately


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• Trench bottom too steep

• Increase trench depth or add berms

• First, try adding 1 berm…


• Adjust # cells and trench

cell length

• 200 ft porous pavement

length / 2 cells 100 ft

trench cell length


• Beautiful!

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• Next, let’s try removing

the berm and increasing

the trench depth to 1.5 ft


• That works,

too


• for a 3% slope, 5 berms

spaced every 40 feet would

be needed

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Peak Flow Reduction

Draw Hydrographs

Output Report

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Resources LID Technical Guidance Manual http://www.pierce.wsu.edu/Water_Quality/LID/LID_manual2005.pdf

(Draft 2012 Manual does not yet have modeling section developed)

WWHM http://www.clearcreeksolutions.com/

MGSFlood http://www.mgsengr.com/MGSFlood.html

HSPF http://water.usgs.gov/software/HSPF/

WDMUtils http://www.epa.gov/waterscience/basins/b3webdwn.htm

Questions and Answers

???

Contact Information

Alice Lancaster, PE [email protected]

Robin Kirschbaum, PE, LEED [email protected]

http://www.pierce.wsu.edu/Water_Quality/LID/LID_manual2005.pdf

http://www.mgsengr.com/MGSFlood.htm






http://water.usgs.gov/software/HSPF/

http://www.epa.gov/waterscience/basins/b3webdwn.htm

mailto:[email protected]



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