CIVIL ENGINEERING

Question

A stormwater bioinfiltration system receives water from a residential area (area 10,000 m2 ; runoff coefficient C = 0.8). Your consulting firm is asked to design the biofilter to treat a 85 percentile rainfall, which is 1 in/h.

a) What is the amount of water (m3 ) the bioinfiltration system would receive for a rainfall lasting 1 h?

b) What should be the area (m2 ) of bioretention system (depth 0.5 m) that would infiltrate the received runoff volume to an underdrain without ponding? Assume the hydraulic conductivity of the filter layer is 200 cm/h and the hydraulic head gradient is 1.

Answer 1

Let's go through the problem step-by-step to determine the answers for both parts (a) and (b).

Part (a): Amount of Water Received by the Bio infiltration System

First, calculate the volume of water received by the bio infiltration system from the residential area during the rainfall event.

**Given:**

- Area of the residential area A = 10,000 m²

- Runoff coefficient C = 0.8

- Rainfall intensity I = 1 inch/hour (convert this to meters: 1 inch = 0.0254 meters)

The formula to calculate the runoff volume V

Substitute the given values into the formula:

V = 0.8 X 0.0254 X 10,000 m2

Now calculate the volume:

V = 203.2 m3

So, the bio infiltration system would receive 203.2 m³ of water for a rainfall lasting 1 hour.

Part (b): Area of the Bioretention System

To determine the required area of the bioretention system, we need to ensure it can infiltrate the received runoff volume without ponding.

**Given: **

- Received runoff volume V= 203.2 m³

- Depth of the bioretention system d = 0.5 meters

- Hydraulic conductivity K = 200 cm/hour (convert this to meters: 200 cm/hour = 2 meters/hour)

- Hydraulic head gradient i= 1

The infiltration rate (\(Q\)) is given by:

Q=K X i X A

Where:

-Q = infiltration rate (m³/hour)

- K= hydraulic conductivity (m/hour)

- A= area of the bioretention system (m²)

- i = hydraulic head gradient (dimensionless, given as 1)

To prevent ponding, the infiltration rate must equal the runoff volume over the duration of the rainfall event. The required area (\(A\)) can be found by rearranging the formula:

A=Q/(K X i)

The runoff volume (203.2 m³) is the same as the infiltration rate over the duration of 1 hour (since the rainfall lasts 1 hour):

A=101.6m2

Thus, the area of the bioretention system should be 101.6 m² to infiltrate the received runoff volume to an underdrain without ponding.