Construction of Facilities

Low Impact Development

Dan Cloak Environmental ConsultingConstruction Workshop, May 2, 2012

Low Impact Development: What it is, what it does Runoff dispersal (“self-retaining areas”) Pervious pavements Bioretention

How facilities work and how they are sized What to watch for during construction

Topics

Conventional Urban Drainage Features

Impervious surfaces: roofs and pavement

Catch basins and piped drainage

“Collect and convey” design objective

Drainage Impacts

Site Scale Watershed and Stream ScaleHigher peak flows Flooding and scouring of stream beds

Lower time of concentration Flash flows

Runoff from small storms Discharge when runoff did not previously occur

Increased runoff durations Stream erosion at moderate stream flow rates

Greater runoff volumes Higher pollutant loading

Greater runoff energy Conveys trash and gross pollutants to streams

Decreased infiltration Lower and less frequent stream base flow

Dry weather discharges High pollutant concentrations during low flows

LID Design ObjectivesWatershed and Stream Scale Site scaleReduce peak flows Detain runoff on site

Increase time of concentration Slow runoff from leaving site

No runoff from small storms Infiltrate, evapotranspirate and reuse

Reduce duration of moderate flows Let runoff seep away very slowly

Reduce runoff volume Infiltrate and reuse where possible

Reduce runoff energy Detain and slow flows

Increase groundwater storage and stream base flows

Facilitate infiltration

Reduce pollutants in runoff Detain and filter runoff

Protect against spills and dumping Disconnect drainage and filter runoff

LID Drainage Principles Instead of “collect and convey,”

“slow it, spread it, sink it.”Avoid concentrating flows

Keep drainage areas smallPromote infiltrationDetainTreat

LID Design Steps Optimize the site

layout

LID Design Steps Optimize the site

layout Use pervious

surfaces and green roofs where possible

LID Design Steps Optimize the site

layout Use pervious

surfaces and green roofs where possible

Disperse runoff to landscaping

LID Design Steps Optimize the site

layout Use pervious

surfaces and green roofs where possible

Disperse runoff to landscaping

Direct drainage from impervious surfaces to bioretention facilities

Rationale Produce no runoff from

“small” storms Also called “zero

discharge” areas Criteria

Bermed or graded concave Slopes do not exceed 4% Amended soils and

vegetation to maintain soil stability and permeability

Inlets of area drains, if any, 3" minimum above surrounding grade

Self-retaining Areas

Draining to Self-Retaining Rationale

Self-retaining areas are capable of ponding and infiltrating some additional runoff

Criteria Ratio is not greater than

2:1 if treatment-only requirements apply

Ratio is not greater than 1:1 if flow-control requirements apply

Provide sufficient ponding/storage depth

Facilitate infiltration

Variations Pervious concrete Porous asphalt Porous pavers Gravel Open pavers Solid unit pavers set on sand/gravel Artificial turf

Common Characteristics Require a base course Base course depth varies with soil type More costly than conventional concrete or asphalt Installation on slopes requires careful design

Pervious Pavements

Minimal compaction of underlying soil Check base course depth

4 to 12 inches or more, depending on competence of underlying soils

Base course underdrains Consider omitting the underdrain and relying on

infiltration and lateral flow Perforated SDR 35 or equivalent Outlet elevation minimum 3" above bottom of base

course Underdrain may be routed to storm drain

Pervious Pavements

Pavers Rigid edge required Solid pavers require 3/8" minimum gap Joints filled with open-graded aggregate free of fines

Permeable concrete and pervious asphalt Installed by industry-certified professionals according to

vendor’s recommendations.

Pervious Pavements

Installation on slopes

Bioretention

Bioretention

Overflow

Underdrain

Treatment-Only Criterion

Surface Loading Rate

i = 0.2 inches/hour

i = 5 inches/hour

BMP Area/Impervious Area =0.2/5 = 0.04

Flow Duration Criteria

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

% Time Exceeded

Flow

(cfs

)

ImperviousMitigated Post-Project SitePre-Project SiteQ100.1Q2

IMP Reduces Impervious Runoff to Less Than Pre-Project Levels

Standard Section

Soil Mix

Gravel 12"

18"

4"2"

Overflow

A

V1

V2

Φ = 0.4

0.04

Sizing FactorsFacility Design

Soil Group

Area A

(ft2/ft2)

Volume V1

(ft3/ft2)

VolumeV2

(ft3/ft2)

Rainfall Adjustment for Surface Area

Rainfall Adjustment for Storage Volume

Maximum Release Rate

Bioretention Facility

A 0.07 0.058 No min. Eq. 4-6 Eq. 4-6 No orifice B 0.11 0.092 No min. Eq. 4-7 Eq. 4-7 No orifice C 0.06 0.050 0.066 Eq. 4-8 Eq. 4-8 Eq. 4-10D 0.05 0.042 0.055 Eq. 4-9* Eq. 4-9 Eq. 4-11

Flow-through Planter

A Not permitted in “A” soilsB Not permitted in “B” soilsC 0.06 0.050 0.066 Eq. 4-8 Eq. 4-8 Eq. 4-10D 0.05 0.042 0.055 Eq. 4-9* Eq. 4-9 Eq. 4-11

Dry Well

A 0.05 0.130 N/A Eq. 4-6 Eq. 4-6 No release

B 0.06 0.204 N/A Eq. 4-7 Eq. 4-7 No release

C Not permitted in “C” soilsD Not permitted in “D” soils

Cistern + Bioretention

A 0.020 0.193 N/A Eq. 4-13 Eq. 4-6 Eq. 4-17B 0.009 0.210 N/A Eq. 4-14 Eq. 4-7 Eq. 4-12C 0.013 0.105 N/A Eq. 4-15 Eq. 4-8 Eq. 4-10D 0.017 0.063 N/A Eq. 4-16 Eq. 4-9 Eq. 4-11

Bioretention + Vault

A 0.04 N/A 0.096 N/A Eq. 4-6 No release

B 0.04 N/A 0.220 N/A Eq. 4-7 Eq. 4-12C 0.04 N/A 0.152 N/A Eq. 4-8 Eq. 4-10D 0.04 N/A 0.064 N/A Eq. 4-9 Eq. 4-11

Tributary Area Drainage

area includes portions of roof and of parking lot

Drainage Management Areas

Drainage Management Areas

Grading and Paving

Grading

Roof leaders Runoff may

enter by sheet flow or be piped.

Roof leaders can be piped directly or spill across pavement

Ensure is consistent with DMA delineation

Protect during grading

Outlet structure Top of overflow grate Underdrain connection

Inlet Flow line at inlet Top of curb Top of adjacent paving

Soil layers Top of soil layer Bottom of gravel layer Bottom of soil layer

Observe elevations

Structural

15"

Structural

Inlets

Outlets

Overflow elevation

Fixing a low outlet grate

Class 2 permeable Caltrans spec 68-1.025 Typical to be slightly off

gradation spec on delivery No filter fabric Underdrain

Near top of gravel layer PVC SDR 35 or equivalent; holes facing down Solid pipe for 2' closest to outlet structure Cleanout

Gravel and Underdrain

Soil Specification Now included in NPDES Permit 60-70% Sand

ASTM C33 for fine aggregate 30-40% Compost

Certified through US Composting Council Seal of Testing Assurance Program

Submittal per Guidebook Option to accept test results for

a “brand-name” mix if volume is less than 100 cubic yards

Install in 8"-12" lifts Do not compact Do not overfill Leave room for mulch

Maintain design top of soil elevation

Trees Incorporate into

bioretention facility

Account for surface roots

Plantings

Inspect each new facility within 45 days of installation

Inspect 20% of all facilities each year Inspect every facility at least once every

five years

Maintenance Inspections

Stormwater C.3 Guidebook, 6th Edition www.cccleanwater.org/c3.html Or search “Contra Costa Stormwater”

See in particular: Chapter 5, Construction Checklist Design sheets at end of Chapter 4 Appendix B, Soils/Plantings/Irrigation

for Bioretention Facilities

Resources