Environmental Engineering Reference
In-Depth Information
Water Quality Mitigation
The water quality benefits reported for rain gardens
can be expected to remove a high amount of total suspended solids (typically,
70 to 90%), a medium amount of total phosphorus (approximately 60%), and a
medium amount of total nitrogen (often 40 to 50%). In areas with high sediment
loading, pretreatment of runoff can significantly reduce the amount of rain garden
maintenance required. See Chapter 6 for water quality compliance procedures.
Construction of a Rain Garden
The following is a typical construction sequence. However, alterations will be
necessary depending on design variations.
1. Install temporary sediment control BMPs as required by permitting author-
ity.
2. Complete the site grading, minimizing compaction as much as possible. If
applicable, construct curb cuts or other inflow entrance but provide protection
so that drainage is prohibited from entering the rain garden construction area.
Construct pretreatment devices (e.g., filter strips, swales) if applicable.
3. Stabilize grading except within the rain garden area. Rain garden bed areas
may be used as temporary sediment traps provided that the proposed finish eleva-
tion of the bed is at least 12 in. lower than the bottom elevation of the sediment
trap.
4. Excavate the rain garden to proposed invert depth, and scarify the existing
soil surfaces. Do not compact soils.
5. If applicable, install slotted underdrain and/or gravel drainage or storage
layer.
6. Backfill the rain garden with amended soil as shown on plans and specifi-
cations. Overfilling is recommended to account for settling. Light hand tamping
is acceptable if necessary.
7. Install an automatic irrigation system if applicable.
8. Prewet the planting soil at least 24 hours before planting to aid in settlement.
9. Complete final grading to achieve proposed design elevations, leaving space
for the upper layer of compost or mulch as specified on plans.
10. Plant vegetation according to the planting plan.
11. Apply the mulch layer.
12. Install erosion protection at surface flow entrances where necessary.
Maintenance of Rain Gardens
Rain gardens designed and installed properly require some regular maintenance,
most frequently during the first year or two of establishment.
1. Rain gardens will require supplemental irrigation during the first 2 to 3
years after planting. Drought-tolerant species may need little additional water
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