Week 7: Urban Water Systems

How do urbanisation and catchment clearing affect hydrology and waterways?

Vegetation loss → reduced interception and erosion resistance.
Impervious surfaces and connected drains → faster runoff and reduced infiltration.
Hydrograph: shorter lag time, steeper rising limb, higher peak and shorter flow duration.
Concentrated flows → channel erosion and increased sediment transport.
Channel straightening, riparian clearing and sand/gravel removal further degrade waterways.

What are the economic, social and water-security impacts of catchment degradation?

Flood damage to infrastructure.
Loss of productive farmland.
Increased water-treatment and dredging costs.
Loss of water-storage capacity.
Water security requires managing drought, flooding and water quality together.

Five water-security dimensions:
Household, economic, urban, environmental and resilience to water-related disasters.

What are the lecture’s three main approaches to improving urban water systems?

“Slow it down and clean it up” → manage runoff and remove pollutants.
“Take it out and use it” → harvest stormwater for suitable uses.
“Understand and fix our catchments” → target erosion and sediment sources.

How should stormwater-treatment devices be selected?

Match the water source, treatment objective, site conditions and available space.

Roof runoff: tanks, green roofs, biofilters and infiltration trenches.
Urban stormwater: wetlands, biofilters, swales, basins, porous paving and filter strips.
Light greywater: appropriately designed biofilters or green walls.
Greywater/blackwater or pre-treated wastewater: appropriately designed treatment wetlands.

Objectives include water quality, reuse, flood mitigation, flow attenuation, greening and cooling.

Describe gross pollutant traps, swales and porous pavements.

Gross pollutant traps:
Capture litter, organic debris and coarse sediment.
Require regular cleaning.

Swales:
Vegetated open channels, often beside roads.
Vegetation and infiltration slow/reduce flow.
Provide conveyance, some pollutant removal, flood mitigation and greenery.

Porous pavements:
Allow water through pavement layers.
Reduce runoff and remove some sediment/pollutants.
May recharge groundwater or drain downstream.
Performance depends on underlying soils; clogging requires maintenance.

Describe rainwater tanks and stormwater retention/retarding basins.

Rainwater tanks:
Store roof runoff for reuse, water security and flood mitigation.
Water may contain particles, metals, acidity or pathogens.
Match water quality and treatment to the intended use.

Retention/retarding basins:
Temporarily store stormwater and reduce flood peaks.
May include a pond that expands during storms.
Can provide community open space between storms.

How do constructed wetlands work, and what makes them effective?

Slow water → sediment settles.
Plants take up nitrogen and phosphorus.
Microbes in substrates/biofilms process pollutants.
Provide wildlife habitat and social amenity.

Effective design:
High, consistent plant cover.
Shallow, slow and evenly distributed flows.
Avoid excessive open water.

Types:
Subsurface horizontal/vertical flow, free-water-surface and floating treatment wetlands.

How do biofilters work, and how effective are they?

Vegetated systems with layered filter media.
Stormwater temporarily ponds, then infiltrates downward.
Pollutants removed through filtration, adsorption and biological uptake.
Design varies with site conditions and treatment objectives.

Lecture concentration reductions:

95% suspended solids.
50% nitrogen.
65% phosphorus.
90% heavy metals.
95% hydrocarbons.
1-log reduction in key pathogen indicators.

How does stormwater harvesting work? Compare open and closed storage.

Open storage:
Runoff → filtration/WSUD device → open water storage → pumping → treatment as needed → use.
Example: White’s Hill Reserve, Camp Hill.

Closed/above-ground storage:
Runoff → gross pollutant and sediment removal → collection sump → pumping/treatment → storage tank → use.
Example: Albert Bishop Park, Nundah.

Possible uses:
Irrigation, car washing, fountains, toilets and cooling.
Water quality/treatment must match the use.

What are the benefits of stormwater harvesting? Give the Brisbane case study.

Benefits:
Reduced drinking-water demand through fit-for-purpose use.
Improved water security and stormwater quality.
Reduced runoff volume and peak.
Healthier waterways, habitat and usable green space.
Community engagement and ownership.

Brisbane lecture case study:
26 schemes developed during 2007–2016.
Cost: $14.7 million.
Harvest: approximately 460 ML/year.
45 community-group partnerships.
Reported savings: $30 million.

How could you estimate Brisbane’s harvestable stormwater, and what are the challenges?

Suggested approach:
Map catchments → estimate rainfall and runoff → assess collection, treatment and storage → estimate usable yield → compare with demand.

Challenges:
Variable rainfall.
Different catchment surfaces and runoff rates.
Water-quality requirements.
Storage and infrastructure limits.
Losses and environmental needs.
Supply may not occur when demand is highest.

This is a suggested answer to the lecture’s discussion question, rather than an answer supplied on the slide.

How can catchment restoration target sediment pollution? Give a case study.

Sediment sources are unevenly distributed.
Use spatial analysis to identify high-contributing areas.
Target restoration and sediment-control measures there.
Build and maintain sediment bunds to keep sediment on land.

Lecture example:
About 10% of catchment area supplied about 60% of sediment.

Lockyer case study:
2,764 km of cleared channels.
Fixing 238 km could halve sediment loads.

Ephemeral watercourses:
Flow temporarily after rainfall and can transport sediment.