LID & Detention PondSizing Tool to Address Hydro-modification and Water Quality in Clackamas CountyLeah Johanson, Water Environment ServicesAndy Thayumanavan, Brown and CaldwellJanice Keeley, Brown and Caldwell

Agenda

» Background» Hydro-modification» Tool Criteria Development» Low Impact Development (LID) Sizing Tool» Detention Pond Sizing Tool» User Interface» Next Steps» Questions

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WES Surface Water Program Evolution

» Past emphasis on meeting regulatory requirements» Stormwater management at the “site level”» Movement toward goal of improving overall health of the

watershed» WES can best achieve this through effective and efficient

stormwater management» Partnering with others on non-stormwater-related

activities that will help WES meet this goal

Watershed Action Plans Project Purpose

» To develop basin specific plans that prioritize District activities and future investments for watershed management

Watershed Action Plans Study Area

Key Outcomes of WAP

» Better understanding of the conditions in the watersheds» Identification of high priority areas » Prioritized CIP/ programmatic changes» Early action items

– Update Stormwater Design Standards

Purpose of Sizing Tool

» Stormwater design standards updated to promote the use of Low Impact Development (LID) techniques, to promote infiltration and reduce hydro-modification impacts to streams

» Develop a simplified tool for development engineers to easily size LID and Detention BMPs to implement the new design standards

What is Hydro-modification?

The frequency and duration of geomorphically significant flows are the primary factors that control channel stability or instability

» Geomorphically significant flows range between:• Lower Threshold – bed material is mobilized• Upper Threshold – channel bank over-topping event

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Hydromodification Criteria Development

» Lower Flow Threshold – 0.42Q2 (ODOT)

» Upper Flow Threshold – Q10 (ODOT)

» HSPF Modeling Parameters (2006 SWMP MP, PWR)

» Rainfall Record

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Sizing Strategy – Flow Duration

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Upper Threshold Flow

Lower Threshold Flow

Sizing Strategy – Peak Flow

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Upper Threshold Flow

Lower Threshold Flow

Sizing ToolComputational Methods: Site Hydrology

» Developer Enters:– Site specific infiltration rate– Planned impervious area– Pre-developed land use– Drainage area hydrologic soil group – B, C, D

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LID Sizing Tool

» Facility Types (7 options)– Planter (Infil & Fil)– Rain Garden (Infil & Fil)– Vegetated Swale (Infil & Fil)– Infiltrator

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LID Sizing ToolComputational Methods: Facility Geometry

» Rain Garden Example

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Sizing Tool Results

» Finished Product – Web-based interface with underlying look-up table containing sizes for each possible iteration of drainage area land use, soil type, facility type and site specific infiltration rate

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Detention Pond Sizing Approach

» The detention pond sizing tool performs the following functions

– Configure pond geometry– Design outlet structure– Route post-project runoff– Compare pre-project and mitigated flow duration– Size the facility automatically to meet the WES HMP

requirements

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Computational Methods: Pond Geometry

» User inputs initial pond configuration including area, depth and side slopes

» Alternatively, the tool automatically provides initial pond geometry

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3:1

Overflow Weir

Bottom Orifice

Upper OrificeInfiltration

Depth

Length

Width

Detention Pond Interface

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Stage-Storage-Discharge Table

Example stage-storage-discharge table constructed by the PondConfig subroutine

Depth Index

i

Stage/Depth

(ft) D

Surface Area (ft2) SA

Volume Increment

(ft3) S

Cumulative Storage Volume

(ft3) S

Bottom Orifice

Discharge (cfs) Qbo

Middle Orifice

Discharge (cfs) Qmo

Weir Discharge

(cfs) Qw

Exfiltration Loss (cfs) Qinfilt

Total Discharge

(cfs) Qtotal

Storage-Indication

Term 2S/t+Q

1 0 6,457 0 0 0.00 0.00 0.00 0.000 0.00 0.0

2 0.14 6,591 900 900 0.09 0.00 0.00 0.031 0.06 0.6

3 0.28 6,726 918 1,818 0.13 0.00 0.00 0.031 0.10 1.1

4 0.41 6,862 937 2,755 0.16 0.00 0.00 0.032 0.13 1.7 . . .

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30 4.00 10,890 1,490 34,310 0.50 0.70 19.53 0.050 20.68 39.7

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Flow Duration Analysis

» Flow duration comparison is performed for pre- and mitigated time series hydrograph

» Complete time series is analyzed

» Range of interest is between lower and upper threshold flows

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Resizing the Pond

» If flow duration criteria is not met then the pond is resized iteratively

» The user can use the option to resize by either adjusting the area or depth

» Pond sizing is completed when hydro-modification performance requirements are met

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Next Steps

» Beta testing» Meetings with co-permittees» Testing and training

– ODEQ 319 Grant Funding» Web-based tool rolled out with new design standards» Continued monitoring and adaptive management

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Questions?

Sensitivity Analysis

» Slope» Thresholds» Time steps

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Computational Methods: Outlet Structure

» Use pre-defined outlet configuration that include two orifices and an overflow weir.

» The bottom orifice is sized to discharge lower threshold flow, QLT

» The orifice is sized to discharge the upper threshold flow, QUT.

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Compare Histograms

» Direct comparison between pre- and post-project durations for each flow bin is performed.

» A “Pass” or “Fail” result is generated for each flow bin within the lower and upper threshold flows.

» Results take into account the allowable 10% variance over the length of the curve (0.42Q2 to Q10).

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Flow Duration Comparison

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