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Raingarden Cost Estimate Excel Sheet Supplemental
A Comment on Costs
Costs in construction materials have been skyrocketing lately and tend to be pretty volatile (because it
takes oil to extract, make and transport materials). The only way to get a handle on unit costs is to call
around.
Specifications
Check with the city agency’s land development folks to see if they have standards. If not, here’s some
additional information on materials and installation:
Clean, washed aggregate means that the rock should be washed at the quarry AND washed carefully
on-site. Pile the rock up and hose it off. Scoop from the top and then wash it off again. Make sure that
wherever the runoff is going it’s either settling in a yard area somewhere or roads and other areas that
might carry that sediment downstream are protected with erosion control measures.
Uniformly graded aggregate means it’s all approximately the same size. In other words 1 ½” to 1”
rock, 5/8” to ¾” rock, AASHTO No. 57, or AASHTO No. 3 are all appropriate materials for the rock in
the rock trench. When you order it, just let them know that you want a rock with 30 – 40% void ratio and
you’ll be sure to have enough voids in the rock to store water.
For small facilities, the catch basin can be a 12”x12” catch basin or a beehive type made of metal,
concrete, or plastic. Gibson Steel in Eugene makes catch basins (and trench drains). The last time I
checked, they were a couple hundred bucks. When installing the catch basin, the native subgrade soils
will have to be compacted. Minimize this area through design and careful installation to protect the rest
of the bottom of the raingarden from compaction.
Outfall Rock at the inlet should be crushed aggregate (i.e. angular, not smooth like pea gravel)
because angular rock is more likely to stay in place. Rock size depends on the pipe diameter that’s
daylighting (showing the stormwater the light of day after being in a pipe) into the outfall. See below for
recommendations from the City of Portland:
Sustainable design isn’t about doing something neat, it’s about doing something right.
Spillway Rock for the outlet should be crushed aggregate (i.e. angular, not smooth like pea gravel)
because angular rock is more likely to stay in place.
Perforated pipe should be at least a 6 diameter corrugated HDPE. If it’s placed in soil, it should be
wrapped with a non-woven geotextile. No compaction of the subgrade will be needed for this pipe. Lay
this pipe flat, 0% slope.
Non-perforated overflow pipe should be a smooth pipe. The size of a pipe varies with the slope that
it’s installed with (not the slope of the ground, which could be very different). I really would recommend
getting an engineer to design this pipe for you including the diameter, the slope, the alignment, and the
connection point (usually a sewer pipe in the right-of-way or an appropriate outfall.) For planning
purposes, you need to know that this pipe will need at least 12” of cover (i.e. dirt between the ground
surface and the top of the pipe.) Drain rock will be installed under this pipe and the subgrade will be
compacted.
Non-woven geotextile is the same as the fabric used for french drains. Ask the guy at Home Depot
which geotextile you should buy if you have doubts. Overlap the geotextile a minimum of 12” at the
seams.
The catch basin can be a 12”x12” catch basin or a beehive type made of metal, concrete, or plastic.
Gibson Steel in Eugene makes catch basins (and trench drains). The last time I checked, they were a
couple hundred bucks.
The liner can be a 30 mil (minimum) plastic pond liner or bentonite mats (made of all natural clay). I’d
avoid PVC here again and find a polyethylene (PE) liner.
Sustainable design isn’t about doing something neat, it’s about doing something right.