Discussion of Proposed MS4 Permit Design Standards

Language

Section E.12.b.3"The Permittee shall require all projects fitting the category

descriptions listed below to capture, infiltrate, and evapotranspire the runoff from the 85th percentile storm event to the maximum extent practicable. Runoff from the 85th percentile storm that cannot be captured, infiltrated, and evapotranspired must be

treated via a flow-through device designed to treat runoff at a flow rate produced by a rain event equal to at least two times the 85th

percentile hourly rainfall intensity for the applicable area, based on historical records of hourly rainfall depths."

Section E.12.b.3"The Permittee shall require all projects fitting the category

descriptions listed below to capture, infiltrate, and evapotranspire the runoff from the 85th percentile storm event

to the maximum extent practicable…”

• Type C and D soils convert between 50% and 90% of rainfall directly into runoff.

•Runoff volumes in these soils are extremely large

•It would not be possible to “capture” these volumes via Low Impact Development measures alone, and large retention would always be needed to hit this target. As a result, LID would be de-emphisized.

•Removing runoff for all events to the 85th percentile design event could have environmental consequences in some locations. Mostly to trees and vegetation.

Section E.12.b.3"The Permittee shall require all projects fitting the category

descriptions listed below to capture, infiltrate, and evapotranspire the runoff from the 85th percentile storm event

to the maximum extent practicable…”

• Should this language be limited to the difference in runoff volume between the pre-project and post-project condition?

• This would be how to measure of the impact of the project

• Would be consistent with other mitigation requirements under CEQA

• This standard may not produce improved treatment for some devices. For example, vortex style devices may not spin up during smaller more frequent storm events if they are oversized, reducing their benefits to trash screening only in those events.

• We understand that the new CA BMP Manuals may already prescribe that twice the 85th percentile precipitation rate be used on certain flow through BMP’s. Does this standard prescribe that 2x be applied twice resulting in 4x design?

Section E.12.b.3“…Runoff from the 85th percentile storm that cannot be

captured, infiltrated, and evapotranspired must be treated via a flow-through device designed to treat runoff at a flow rate produced by a rain event equal to at least two times the 85th

percentile hourly rainfall intensity for the applicable area, based on historical records of hourly rainfall depths."

SAMPLE PROJECTA REAL ROSEVILLE PROJECT &

THEIR FORWARD THINKING PLANTO MITIGATE STORMWATER QUALITY

(IN ANTICIPATION OF THE NEW MS4 PERMITS, and OBSERVING THE NEW CONSTRUCTION PERMIT STANDARDS)

SPECIFIC PLAN – PRE-DEVELOPMENT TERRAIN(NOTE LACK OF PRESENCE OF TREES AND VEGETATION)

CURRY CREEK FLOODPLAINS @ PROJECT

CURRY CREEK WATERSHED – SOILS

DETERMINING DEVELOPMENT IMPERVIOUS HYDROLOGIC IMPACT

- BASED ON ABCW REPORT ON DRY CREEK WATERSHED LAND USE IMPERVIOUS RATES

- CESI HAS ADAPTED REGIONAL FACTORS FOR DRY CREEK AND CROSS CANAL WATERSHEDS BY JURISDICTION, FACTORING

ZONING CODES (Nearly 500 types and factors)

BASE EXAMPLE FOR LID(FOR SINGLE FAMILY RESIDENTIAL)

EXAMPLE OF MAXIMIZED USE OF LID(FOR SINGLE FAMILY RESIDENTIAL)

DISCOUNT – NON-DIRECTLY CONNECTED IMPERVIOUS AREAS (PLACER SWMM – REMAINING HYDROLOGIC IMPERVIOUSNESS)

EXAMPLE OF MAXIMIZED USE OF LID(FOR SINGLE FAMILY RESIDENTIAL)

LID – TREE PLANTINGS – 3 PER LOT – HALF EA TYPE

EXAMPLE OF MAXIMIZED USE OF LID(FOR SINGLE FAMILY RESIDENTIAL)LID – DISCONNECTED ROOF DRAINS

EXAMPLE OF MAXIMIZED USE OF LID(FOR SINGLE FAMILY RESIDENTIAL)

LID – SEPARATED SIDEWALK (8% OF INTERIOR SIDEWALKS)

EXAMPLE OF MAXIMIZED USE OF LID(FOR SINGLE FAMILY RESIDENTIAL)

LID – SOIL AMENDMENTS AS NEEDED TO A 9% IMPERVIOUS

EXAMPLE OF MAXIMIZED USE OF LIDLID HYDROLOGIC IMPERVIOUS REDUCTION

BY LAND USE TYPE

EXAMPLE OF MAXIMIZED USE OF LIDLID VOLUME REDUCTION QUOTAS BY LANDUSE

EXAMPLE OF MAXIMIZED USE OF LIDLID REMAINING TREATMENT AT PIPE OUTFALLS AND OPEN

SPACE MITIGATION

EXAMPLE OF MAXIMIZED USE OF LIDHYDROGRAPH MODIFICATION BENEFITS OF COMBINED LID AND

OPEN SPACE PLAN FOR PROBABLISTIC EVENTS

Compare Pre to Post Mit LID

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200

400

600

800

1000

0.2 1 2 10 100 200

Recurrance

Flo

w PRE-PROJECT

POST-PROJECT

EXAMPLE OF MAXIMIZED USE OF LIDHYDROGRAPH MODIFICATION BENEFITS OF COMBINED LID AND

OPEN SPACE PLAN FOR 2 YEAR EVENT HYDROGRAPH

EXAMPLE OF MAXIMIZED USE OF LIDHYDROGRAPH MODIFICATION BENEFITS OF COMBINED LID AND

OPEN SPACE PLAN FOR PROBABLISTIC FLOW DURATION ANALYSIS