Verifying (testing) a stormwater biofiltration hydrologic model

Anna Lintern, E. Daly, A. Deletic

Monash Water for Liveability,

CRC for Water Sensitive Cities

Monash University

Background: • Management of stormwater is a problem

• One solution = biofiltration systems

Background Hydrologic

model Methods Results Conclusion

Background:

• Restores hydrology of

pre-urbanised catchment

• Models are invaluable

Piped outflow Exfiltration

Background Hydrologic

model Methods Results Conclusion

• Incorporated in software

Objective of project

To test a stormwater biofilter hydrologic model using an operating field system.

Background Hydrologic

model Methods Results Conclusion

Infiltration

Overflow Rainfall

Stormwater

inflow

ET Infiltration

Capillary Rise

Exfiltration

How the model works

Background Hydrologic

model Methods Results Conclusion

Ponding

Zone

Filter

media

Submerged zone

Pipe

Outflow Drainage

layer

Untreated

overflow

Rain Stormwater

inflow

Ponding Zone

Infiltration

We

ir h

eig

ht

Hydraulic conductivity of

the filter media, k

Ponding Zone

Filter media

Zone

Evapotranspiration

- Hydraulic conductivity of the filter media, k

- Soil moisture

Infiltration

Hydraulic conductivity of

the filter media, k

Ponding Zone

Filter media

Zone

Evapotranspiration

Drainage layer

Zone

Infiltration Capillary rise

Exfiltration to surrounding soil

Infiltration

Piped outflow

Methodology

Monash University Carpark biofilter

Background Hydrologic

model Methods Results Conclusion

Cell 2 Cell 3 Cell 1

Sandy Loam with Compost

and Mulch Filter media

Methodology

• Available data:

• Continuous inflow and outflows at 1 minute timesteps

• 8 months

• Model calibration

• Calibration parameter = hydraulic conductivity

• Calibrated for flow rates

• Objective functions:

• Nash and Sutcliffe coefficient, E:

• Unbiased function, φ:

• Model performance assessed in terms of:

• Outflow flow rates

• Outflow volumes

Background Hydrologic

model Methods Results Conclusion

Results:

Background Hydrologic

model Methods Results Conclusion

• All results use corrected input data

Results: Hydraulic Conductivity

Background Hydrologic

model Methods Results Conclusion

Measured k Coefficient of efficiency Unbiased function

k E k φ

Cell 1 123 mm/hr 190 mm/hr 0.59 120 mm/hr 4.08

Cell 2 144 mm/hr 180 mm/hr 0.91 170 mm/hr 0.24

Cell 3 77 mm/hr 50 mm/hr 0.68 60 mm/hr 0.4

Results: Flow Rates

Background Hydrologic

model Methods Results Conclusion

3. Event timing/duration predicted successfully

1. Flows are sometimes over or underestimated

2. Model performance affected by objective function

Results: Flow Rates

Background Hydrologic

model Methods Results Conclusion

Results: Volumes

Background Hydrologic

model Methods Results Conclusion

1. Total volumes are generally well modelled

Cu

mu

lati

ve

Ou

tflo

w V

olu

me (

m3)

(φ)

Results: Volumes

Background Hydrologic

model Methods Results Conclusion

Cu

mu

lati

ve

Ou

tflo

w V

olu

me (

m3) 18%

(φ)

Results: Volumes

Background Hydrologic

model Methods Results Conclusion

Cu

mu

lati

ve

Ou

tflo

w V

olu

me (

m3)

9% 12%

(φ)

Although the model works quite well, there are some limitations:

1. Model limitations: a. Outflows are sometimes over- or under- estimated 2. Assumptions:

a. Biofilter represented as a 1D system b. Homogeneity of hydraulic conductivity

Background Hydrologic

model Methods Results Conclusion

Model limitations and assumptions

Background Hydrologic

model Methods Results Conclusion

Hydraulic conductivity affected by: 1)Macropores 2) Soil porosity

Particle size

Particle shape

Roots

Compaction

Conclusions

1. A hydrologic model of a stormwater biofilter was tested 2. Main conclusions about model performance: a. Model is working well i. Appropriate for conceptual design stage ii. Appropriate for modelling pollutant loads b. Model performance dependent on objective function choice c. Model is not perfect 3. Further work: a. Calibrated model should be verified on an independent data set b. Uncertainty should be assessed

Background Hydrologic

model Methods Results Conclusion

Thank you