Stormwater Management in Site Design: Detention, Retention, BMPs, and Low Impact Development

How architects apply stormwater management regulations and low impact development strategies during the site planning phase, including detention and retention systems, best management practices, and green infrastructure techniques that maintain or restore predevelopment hydrology.

ARE · Project Planning & Design

Overview

Every time you pave a parking lot or place a building footprint, you change how water moves across a site. Rain that once soaked into soil now sheets across impervious surfaces, picking up pollutants and overwhelming downstream infrastructure. That shift from natural infiltration to rapid runoff sits at the center of stormwater management on the ARE.

The Clean Water Act and local zoning ordinances require architects to address stormwater runoff during the site planning phase. You need to understand the difference between detention (temporarily holding water and releasing it slowly) and retention (capturing water and keeping it on-site through infiltration or reuse). Both strategies aim to replicate predevelopment hydrology, meaning the site should handle rainfall the way it did before construction.

Best management practices (BMPs) are the specific tools you use: bioretention cells, permeable pavements, grassed swales, rain gardens, green roofs, and cisterns. Low impact development (LID) takes these individual BMPs and weaves them into a site-wide design philosophy that manages stormwater at the source rather than piping it to a single collection point.

For the PPD exam, expect questions that hand you a site scenario and ask which BMP or LID strategy applies. You won't need to design a full drainage system, but you do need to know which tools match which site conditions and regulatory requirements.

Essential

Urban development changes how water moves. Natural ground cover absorbs rainfall, filters it through soil layers, and releases it slowly into groundwater and streams. Pave that same ground and you get fast, contaminated runoff that overwhelms pipes, erodes stream banks, and dumps pollutants into waterways. That transformation is the reason every jurisdiction requires stormwater management plans. ## Detention vs. Retention: Know the Difference These two terms trip up candidates constantly. Detention means temporarily storing stormwater and releasing it at a controlled rate. A detention basin fills up during a storm, holds the peak volume, and drains through a restricted outlet over hours or days. The water leaves the site eventually. Detention controls the rate of runoff. Retention means capturing stormwater and keeping it on-site permanently (or until it infiltrates into the ground or evaporates). A retention pond has no outlet; water leaves only through infiltration and evapotranspiration.

Professional

Consider a 12-acre mixed-use development on a previously forested site with moderate slopes draining toward a stream at the eastern boundary. The local ordinance requires stormwater management for any disturbance over 20,000 square feet, and the stream has a trout population sensitive to thermal pollution and sediment. The site designer faces a classic set of trade-offs. Conventional pipe-and-pond stormwater design would collect runoff from the entire site and route it to a large detention basin near the stream. This approach controls peak discharge rate but does nothing to address water quality or thermal impacts. Hot runoff from parking surfaces can raise stream temperatures 20 to 30 degrees F above rural baseline during summer storms. An LID approach distributes controls across the site. The parking areas get permeable pavement with stone reservoirs below, handling their own runoff through infiltration.

TLDR

Detention = temporary storage with slow release (controls runoff rate); Retention = permanent on-site capture through infiltration (controls runoff volume)
LID = distribute small-scale stormwater controls across the site instead of one big pipe-and-pond system
Key BMPs: bioretention/rain gardens, permeable pavement, grassed swales, green roofs, cisterns/rain barrels
The goal is predevelopment hydrology: make the site handle rain like it did before construction
Green infrastructure is defined in the Clean Water Act as measures using plant/soil systems, permeable surfaces, or stormwater harvest to reduce flows
Federal projects over 5,000 SF of site work must restore predevelopment hydrology; many local codes trigger at 20,000 SF disturbed area
Extensive green roofs: up to 6 inches growing medium, 15-30 psf saturated, sedums and hardy perennials
Intensive green roofs: 8+ inches growing medium, heavier, supports shrubs and trees
Reduce impervious surfaces by right-sizing roads, minimizing cul-de-sacs, using pervious materials
Protect existing natural features (forests, wetlands, stream buffers) before adding engineered BMPs

ELI5

Picture a field in a rainstorm. The rain hits the grass, soaks into the dirt, and slowly trickles down to a stream. Now picture that same field turned into a parking lot. The rain hits the pavement, can't soak in anywhere, and rushes across the surface in a big wave, picking up oil and dirt along the way. That wave slams into the stream all at once, flooding it and making the water dirty. Stormwater management is about making a developed site behave more like that original field. There are two main strategies. Detention is like putting a speed bump on the water. You build a basin that catches the rush of water and lets it drain out slowly through a small pipe. The water still leaves, but at a trickle instead of a flood. Retention is like putting a sponge on the ground.

AREProject Planning & Design

Stormwater Management in Site Design: Detention, Retention, BMPs, and Low Impact Development

2 min read218 words

Stormwater Management in Site Design

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