Low Impact Development
description
Transcript of Low Impact Development
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