SUMMARY OF TECHNIQUES USED: Residential site (735 Homer Avenue, Palo Alto)
Technique
Dimensions
Total cost (a)
Unit cost
Average annual runoff reduction (b)
(1) Reused concrete
470 sq ft
$3,200
$6.80/sq ft
2930 gallons
(2) Unit pavers
845 sq ft
$19,600
$23.20/sq ft
5270 gallons
(3) Gravel trench
180 ft long
$7,000
$38.90/lin ft
3620 gallons (from 580 sq ft of roof runoff)
(4) Concrete pad (not permeable)
736 sq ft
$13,200
$17.90/sq ft
4590 gallons (runoff drains to Hollywood driveway)
(5) Hollywood driveway
720 sq ft
$12,600
$17.50/sq ft
4490 gallons
(6) Rain garden
78 sq ft
$2,100
$26.90/sq ft
3730 gallons (from 520 sq ft of roof runoff + 78 sq ft asphalt removed)
TOTAL
$57,700
24,630 gallons
Notes:
(a) Includes labor, materials, equipment rental, and dump fees – see DETAILED INFORMATION below for material costs.
(b) Based on estimated infiltration of at least 10 inches per year (average annual rainfall is 16 inches). Future monitoring data will refine this estimate.
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Figure 1. Site diagram (post-project)
DETAILED INFORMATION BY TECHNIQUE
(1) Patio made of reused concrete
MATERIALS: Existing concrete patio broken into large pieces (approximately 2 – 2.5’ in diameter) and relaid with rough surface facing up (for aesthetics) and gaps between pieces. Baby’s tears planted in gaps. BENEFITS: 470 square feet concrete removed, all rainfall infiltrates into ground (based on observations from 10/06 to 3/07)
(2) Unit pavers
Material
Dimensions, description
Unit cost
Unit pavers
845 sq. ft., paver style: San Francisco Rima made by Basalite*
$3.35/sq ft
Bedding material
1/4” crushed (angular) rock, 1” deep (under pavers)
$55/ton
Permeable base
3/4" crushed (angular) rock, 6” deep (under bedding material)
$52/ton
BENEFITS: 845 square feet asphalt removed, all rainfall infiltrates into ground (based on observations from 10/06 to 3/07) ADDITIONAL NOTES: Pavers placed on gradual slope away from buildings; California fescue seeded in gaps. LESSONS LEARNED:
Problem: Originally used Grasspave2 (plastic matrix with grass growing through it) for half of area between concrete pad and Building 4, and unit pavers for other half where cars park overnight. However, the grass did not survive the four car trips per day (in and out for two cars). Solution: Replaced Grasspave2 areas with unit pavers.
(3) Rooftop runoff to gravel trench
Material
Dimensions, description
Unit cost
Trench
180’ long, 16” wide, 18” deep
Excavation costs
Perforated drain pipe
180‘ long, 4” diameter, 1/2“ perforations
$8.99/10’ section
Filter fabric
180‘ long, 6‘ wide, wrapped around crushed rock to prevent surrounding soil from clogging void spaces
$1.00/ft (roll 6’ wide)
Crushed rock
3/4" crushed (angular) rock, 12” deep
$52/ton
BENEFITS: absorbs runoff from 580 square feet of roof, all rainfall infiltrates into ground (no overflow from 10/06 to 3/07) ADDITIONAL NOTES: Standard PVC pipe conveys water from downspouts to perforated pipe in trench. Pop-up emitter at end of trench serves as overflow outlet for when trench is fully saturated.
(4) Concrete pad: Detailed information not given for concrete pad because it is not a runoff reduction technique.
(5) Hollywood driveway (wheel tracks)
Material
Dimensions, description
Unit cost
Unit pavers
72’ long, each track 34” wide (408 sq. ft.), 27“ between tracks,
paver style: San Francisco Rima made by Basalite*
$3.35/sq ft
Bedding material
1/4” crushed (angular) rock, 1” deep (under pavers)
$55/ton
Permeable base
3/4" crushed (angular) rock, 6” deep (under bedding material)
$52/ton
Grasspave2
72’ long, 27” wide (seeded with an extra hearty fescue variety)
$2.85/sq ft
BENEFITS: 700 square feet asphalt removed, all rainfall infiltrates into ground (based on observations from 10/06 to 3/07) ADDITIONAL NOTES: California fescue seeded in gaps. LESSONS LEARNED: Problem:
Original 20”-wide tracks too narrow for long driveway where backing out required. Rutting occurred in grass along edges of tracks. The worst rutting occurred at beginning of driveway due to turning in from street. Also, irrigation runoff flowed down concrete tracks to street. Solution: Replaced with wider tracks made of unit pavers. Also, used pavers across full width for first 3’ of driveway.
BENEFITS:
78 square feet of asphalt removed, absorbs runoff from 520 square feet of roof, all rainfall infiltrates into ground (based on observations from 10/06 to 3/07)
*
We gratefully acknowledge Basalite’s donation of 11 pallets of San Francisco Rima pavers.
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Problem: Originally used Grasspave2 (plastic matrix with grass growing through it) for half of area between concrete pad and Building 4, and unit pavers for other half where cars park overnight. However, the grass did not survive the four car trips per day (in and out for two cars). Solution: Replaced Grasspave2 areas with unit pavers.
