Low Impact Development Construction of Facilities Dan Cloak

Low Impact Development Construction of Facilities Dan Cloak Environmental Consulting Construction Workshop, May 2, 2012

Topics 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

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 Scale Higher 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 Objectives Watershed and Stream Scale Site scale Reduce 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 small �Promote infiltration �Detain �Treat

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

Self-retaining Areas 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

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

Pervious Pavements 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.

Installation on slopes

Bioretention

Bioretention Overflow Underdrain

Treatment-Only Criterion i = 0. 2 inches/hour BMP Area/Impervious Area = 0. 2/5 = 0. 04 Surface Loading Rate i = 5 inches/hour

Flow Duration Criteria

Standard Section A V 1 0. 04 Overflow 2" 4" Soil Mix 18" Gravel 12" V 2 Φ = 0. 4

Sizing Factors Facility Design Soil Group A Bioretentio B n Facility C D A Flow. B through C Planter D A B Dry Well C D A Cistern + B Bioretentio C n D A Bioretentio B n + Vault C D Area Volume A V 1 (ft 2/ft 2) (ft 3/ft 2) 0. 07 0. 11 0. 06 0. 058 0. 092 0. 050 0. 042 0. 06 0. 050 0. 042 0. 130 0. 204 0. 020 0. 009 0. 013 0. 017 0. 04 0. 193 0. 210 0. 105 0. 063 N/A N/A Volume Rainfall Adjustme 2 nt for (ft 3/ft 2) Surface Area No min. Eq. 4 -6 No min. Eq. 4 -7 0. 066 Eq. 4 -8 0. 055 Eq. 4 -9* Not permitted in “A” soils Not permitted in “B” soils 0. 066 Eq. 4 -8 0. 055 Eq. 4 -9* N/A Eq. 4 -6 N/A Eq. 4 -7 Not permitted in “C” soils Not permitted in “D” soils N/A Eq. 4 -13 N/A Eq. 4 -14 N/A Eq. 4 -15 N/A Eq. 4 -16 0. 096 N/A 0. 220 N/A 0. 152 N/A 0. 064 N/A V Rainfall Maximum Adjustmen Release t for Rate Storage Volume Eq. 4 -6 No orifice Eq. 4 -7 No orifice Eq. 4 -8 Eq. 4 -10 Eq. 4 -9 Eq. 4 -11 Eq. 4 -8 Eq. 4 -9 Eq. 4 -6 Eq. 4 -7 Eq. 4 -10 Eq. 4 -11 No release Eq. 4 -6 Eq. 4 -7 Eq. 4 -8 Eq. 4 -9 Eq. 4 -17 Eq. 4 -12 Eq. 4 -10 Eq. 4 -11 No release Eq. 4 -12 Eq. 4 -10 Eq. 4 -11

Tributary Area n 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

Observe elevations 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

Structural 15"

Structural

Inlets

Outlets Overflow elevation

Fixing a low outlet grate

Gravel and Underdrain 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

Soil Specification Now included in NPDES Permit 60 -70% Sand ASTM C 33 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

Plantings Maintain design top of soil elevation Trees Incorporate into bioretention facility Account for surface roots

Maintenance Inspections 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

Resources Stormwater C. 3 Guidebook, 6 th Edition www. cccleanwater. org/c 3. 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