Bioretention sizing, media, and plants

Low Impact Development (LID) and Watershed Hydrology

Conventional civil engineering historically treated stormwater as a waste product to be collected in pipes and rapidly evacuated off site into municipal storm sewers and regional rivers. In urbanized landscapes, replacing natural forest cover with impervious rooftops, asphalt roads, and concrete flatwork causes severe watershed degradation:

• Runoff Volume Surge: Peak runoff and volume can increase substantially, causing extreme stream channel erosion, scouring out salmonid spawning gravels, and overwhelming combined municipal sewer networks.
• Loss of Baseflow: Preventing natural groundwater infiltration lowers regional water tables, causing Pacific Northwest streams to run dry or overheat during summer months.
• Nonpoint Source Pollution: Urban runoff carries concentrated environmental toxins—including heavy metals (copper from vehicle brake pads, zinc from tire wear), petroleum hydrocarbons (motor oils, fuels), excess nitrogen and phosphorus fertilizers, and suspended soil sediment.

Low Impact Development (LID) is an ecologically engineered approach that manages rainfall at the source. By incorporating decentralized, naturalized landscape features—such as bioretention cells, rain gardens, vegetated bioswales, and permeable interlocking concrete pavements—LID mimics pre-development hydrology through on-site infiltration, evapotranspiration, and biological pollutant filtration.

Rain Gardens and Bioretention Engineering

A rain garden (or bioretention cell) is an engineered, landscaped shallow depression designed to capture, detain, filter, and infiltrate stormwater runoff from designated impervious surfaces (roofs, driveways, parking lots).

Site investigation and sizing

Identify the contributing roof, paving, and landscape areas, local design storm, infiltration conditions, groundwater, utilities, and approved overflow route. Confirm infiltration feasibility with the applicable local test method. A small rain garden is not automatically sized at five to ten percent of roof area; required footprint depends on storage, soil conductivity, rainfall, and the design standard.

For an illustrative design prescribing a ten-percent surface-area ratio, a 2,400-square-foot roof implies 240 square feet. This arithmetic does not establish that ten percent is correct for another site. Calculate storage using effective ponding area and depth, and account for sloped sides rather than multiply the largest top footprint by full depth.

Setbacks from foundations, property lines, wells, septic systems, slopes, and groundwater come from applicable local criteria and site review. Do not present one statewide set of distances as universally mandatory. Infiltration on a slope or near a foundation can create geotechnical or moisture problems even when the planting looks attractive.

Media, drainage, and overflow

Use the jurisdiction's approved bioretention media and construction detail. Sand, organic matter, mineral soil, gradation, conductivity, and nutrient content are specified together. Arbitrarily combining three percentage ranges can produce an inconsistent mix. Ordinary rich planting compost is not automatically approved stormwater treatment media.

Protect the excavated basin from compaction and construction sediment. Provide the specified ponding depth, drawdown performance, underdrain where required by the design, and a safe overflow. An underdrain is not universally required merely because a soil is clay; some facilities infiltrate and others primarily filter and drain. Its invert controls storage and drawdown.

Use transition materials that prevent media migration into the drain layer. Keep cleanouts accessible. The overflow must discharge to an approved route without flooding a building or neighboring property. Local criteria establish the design and overflow storms, not a universal ten- or twenty-five-year number. Inspect the facility after storms and maintain inlets, sediment pretreatment, mulch, and plants.

Three-Tiered Bioretention Planting Zones

Match plants to the facility's actual inundation, drawdown, sun, and summer moisture. Wet-tolerant plants can occupy the bottom while better-drained species occupy higher positions, but a basin designed for temporary ponding is not automatically a permanently anaerobic wetland. Use the applicable local plant list and verify each species' tolerances.

Oregon iris (Iris tenax) should not be listed as a default plant for a deeply flooded basin bottom. Species selection also considers mature size and maintenance access; a large shrub must not block the overflow or inspection point.

Construction sequence and acceptance

Keep sediment-laden construction runoff out of the facility until the contributing site is stabilized. Otherwise the new soil surface may seal before the plants establish. Inspect subgrade conditions before placing media; a field discrepancy such as buried fill or unexpectedly high water needs design review. Confirm the finished media elevation and overflow level after settlement. An overflow set lower than intended reduces storage; one set too high can flood adjacent improvements.

For an illustrative basin with a constant two-hundred-square-foot horizontal storage area and six inches of ponding, simple rectangular storage is one hundred cubic feet. Real sloping sides change the area with depth, so use the actual geometry. Storage alone does not establish treatment performance: the rate of infiltration or underdrain discharge determines how soon capacity becomes available again. Record the acceptance checks, plant replacement responsibilities, and who will remove incoming sediment. Repeated ponding beyond the approved drawdown time is a diagnostic signal, not evidence that adding more wetland plants will correct the hydraulic problem.