Water Quality and Stormwater BMPs

Stormwater Best Management Practices (BMPs) are structural and non-structural controls designed to mitigate the water quality impacts of urban runoff. Urbanization increases impervious surfaces, leading to higher peak flows and increased pollutant wash-off (suspended solids, metals, nutrients, hydrocarbons).

Types of Stormwater BMPs

BMPs are selected based on site constraints, target pollutants, and regulatory requirements.

1. Retention Basins (Wet Ponds)

Wet ponds maintain a permanent pool of water.

  • Primary Mechanism: Sedimentation (settling of particles) and biological uptake of nutrients (nitrogen, phosphorus) by algae and wetland vegetation.
  • Key Advantage: High removal rates of dissolved pollutants and suspended solids.

2. Detention Basins (Dry Ponds)

Dry ponds temporarily store runoff and release it slowly through an outlet structure (typically over 24 to 72 hours). They are dry between storm events.

  • Primary Mechanism: Gravitational settling of coarse suspended solids.
  • Key Advantage: Excellent for peak flow control; less effective at removing fine particles or dissolved pollutants than wet ponds.

3. Vegetated Swales (Bioswales)

Broad, shallow channels with a dense stand of vegetation.

  • Primary Mechanism: Filtration by vegetation, sedimentation, and infiltration into the soil.
  • Key Advantage: Reduces runoff volume and peak velocities; ideal for highway medians and roadside drainage.

4. Sand Filters

Multi-chamber structures featuring a sedimentation chamber followed by a sand filtration bed.

  • Primary Mechanism: Physical filtration of fine suspended solids and absorption.
  • Key Advantage: Highly effective for small, highly impervious sites (like parking lots or gas stations).

5. Infiltration Basins and Trenches

Excavated basins backfilled with stone or sand to temporarily store water and allow it to infiltrate directly into the underlying soil.

  • Primary Mechanism: Infiltration and filtration.
  • Key Advantage: Excellent for volume reduction and groundwater recharge; requires permeable soils (HSG Group A or B) and must not be placed near groundwater tables.

Water Quality Volume ($WQV$)

The Water Quality Volume ($WQV$) is the volume of stormwater runoff generated by the “first flush” of a storm (typically the first 1.0 to 1.5 inches of rain). This volume contains the highest concentration of wash-off pollutants.

Governing Equation:

$$WQV = \frac{P \cdot R_v \cdot A}{12}\text{ (acre-feet)} = 3630 \cdot P \cdot R_v \cdot A\text{ (cubic feet)}$$

Where:

  • $WQV$ = Water Quality Volume (cubic feet, $\text{ft}^3$ or acre-feet)
  • $P$ = Design rainfall depth for water quality (inches; typically 1.0 inch or 1.5 inches)
  • $A$ = Contributing drainage area (acres)
  • $R_v$ = Runoff coefficient (dimensionless), calculated based on watershed imperviousness: $$R_v = 0.05 + 0.009 \cdot I$$ (where $I$ is the percent imperviousness of the drainage area, e.g., if the site is $65\%$ impervious, $I = 65$).

Worked Example: Water Quality Volume and BMP Sizing

A new commercial shopping plaza has an area of $8.5\text{ acres}$ and is determined to be $75\%$ impervious ($I = 75$). The local municipal code requires treating the runoff from a $1.2\text{-inch}$ design storm ($P = 1.2\text{ in}$) using a sand filter basin. The sand filter is designed to have a depth of $3.0\text{ feet}$ to store the required $WQV$.

1. Calculate the volumetric runoff coefficient ($R_v$) and the required Water Quality Volume ($WQV$) in cubic feet. 2. Calculate the minimum surface area ($A_f$) of the filter bed if the storage depth is $3.0\text{ feet}$ (assume a rectangular basin with vertical walls for simplicity).

Solution:

Step 1: Calculate $R_v$

$$R_v = 0.05 + 0.009 \cdot I$$

Using $I = 75$:

$$R_v = 0.05 + 0.009 \cdot 75 = 0.05 + 0.675 = 0.725$$

Step 2: Calculate the required Water Quality Volume ($WQV$)

$$WQV = 3630 \cdot P \cdot R_v \cdot A\text{ (cubic feet)}$$

$$WQV = 3630 \cdot 1.2\text{ inches} \cdot 0.725 \cdot 8.5\text{ acres}$$

$$WQV = 3630 \cdot 1.2 \cdot 0.725 \cdot 8.5 = 26,843.85\text{ ft}^3 \approx 26,844\text{ ft}^3$$

Step 3: Calculate the minimum filter bed surface area ($A_f$)

Assuming the sand filter pond stores the entire $WQV$ with a design storage depth of $h = 3.0\text{ ft}$:

$$A_f = \frac{WQV}{h}$$

$$A_f = \frac{26,844\text{ ft}^3}{3.0\text{ ft}} = 8,948\text{ ft}^2$$

Technical Pitfalls

  • Percent Imperviousness Unit: In the formula $R_v = 0.05 + 0.009 \cdot I$, $I$ must be entered as a percentage (e.g., $75$ for $75\%$), not a decimal ($0.75$). Entering $0.75$ results in an $R_v$ of $0.0567$ instead of $0.725$, which yields a massive error.
  • Unit Conversion: The factor $3630$ converts acre-inches to cubic feet ($43,560\text{ ft}^2/\text{acre} \div 12\text{ in/ft} = 3630$). If the question asks for $WQV$ in acre-feet, divide the result by $43,560$ or compute it directly as $\frac{P \cdot R_v \cdot A}{12}$.
  • Storage Volume vs. Filter Footprint: Make sure to check whether the question is asking for the total runoff volume ($WQV$) or the physical area/footprint of the basin.