Effects Of Catchment Urbanisation Flashcards
2 results of Catchment urbanization
Catchment imperviousness
Connection/drainage (dominated by extensive stormwater pipe net work)
Three features of peak discharge- average recurrence interval diagram
Imperviousness 20, 40, 60%
- Urbanization effect most significant for high-frequency events(ecology).
- 20% imperviousness sufficient to cause significant increase in frequency of bank full discharge.
- From Q5 to Q0.5 slope of flood frequency curve is flatter.
Drainage connection effects on catchment hydrology?
Increased hydraulic efficiency of drainage system accounts for between 80% and 90% of the increase in peak discharge.
Implications for receiving water?
Increase flow velocity and volume( habitat scour and erosion, reduced habitat diversity)
Sediment mobilization and deposition(habitat smothering, reduced habitat diversity)
Very efficient pollutant mobilization and transport
What effects urbanization has on direct habitat modification?
Streams channelized to increase hydraulic efficiency, thus increasing pollutant transport, scour and reducing diversity of habitat.
Loss of riparian zone values(shade, OM).
Isolation from floodplain.
Effects of 城市化 on stormwater pollution
Increased pollutant generation
Increased delivery efficiency
Types of pollutants
Sediments, organics( oxygen demand substances), nutrients, other contaminants, oil and grease, metals(copper, lead, zinc, etc), pathogens.
Urbanization impacts are exerted on?
Catchment hydrology
Water quality
Direct habitat modification( instream and riparian)
Gross pollutants and litter
Urban debris flushed from catchment
Visual impact, threat to aquatic ecosystem(physical impacts and contamination of water quality)
Public perceives it as major water quality issue. Reflect 公众对暴雨管理策略的感知
Gross pollutant impacts are?
Aesthetic impacts, Aquatic habitat losses, Marine debris impacts, Clean-up costs, Litter is one of many management issues.
Monitoring results of gross litter
Most litter from commercial areas
Highest concentrations early in the event
Highest loads during high discharge
Subsequent storms, in the same day, produce gross pollutant trends
What are the types of gross pollutant?
Vegetation-dominant; 75% of total gross pollutant load
The rest 25% is litter
Carries lower 两个magnitude nutrient load than found in urban stormwater, BOD 高
Causes of gross pollutant
Litter - dropping of rubbish, bin overflows (recycling), tip-escapes
Vegetation - deciduous trees, lawn clippings
What comprise sediment and Ss
Inorganic/organic materials
Erosion catchment or instream, related to I and v of flow Construction activities Catchment wash off of organic materials Sewer overflow Airborne particulate matter
Impacts of sediment and Ss
Saltation of waterways
Smothering of aquatic habitats, reduction in discharge capacity, turbidity( reduced light penetration and plant growth), transport of particulate-associated pollutants(phosphorus, metals)
Smaller particles-harder to remove
Concentration of sediment
Measure range for SS 0-30,000 mg/l
Event Mean Concentration 55-1000
Recommended EMC 140 (40-500)
Nutrients sources
Atmospheric deposition Erosion Septic tanks Sewer overflow Fertilizers Kitchen wastes, detergents, Nitrous oxide(vehicle emission, ash)
Types of nutrients
Nitrogen ( nitrite, nitrate, ammonium, organic N)
Phosphorus -particulate and dissolved
Impacts of nutrients
Wujibifan
Excess leads to eutrophication-excessive plant growth
Algae blooms- release of toxins, impacts on agriculture, reaction, fisheries.
Micro-organisms involved in water quality influenced by stormwater?
Sources? Impacts?
Bacteria, viruses, fungi and Protozoa
Stormwater contains salmonella,
E. coli
Sources
Animal faeces, sewer overflows, ineffective septic systems, rural runoff.
Impacts - health, pathogens can cause hepatitis, gastroenteritis; closure of beaches, recreational waters.
- major public perception risk esp. For tourist areas
Dissolved oxygen 2 features
Utilized by all organic material inn the process of biodegradation and chemical oxidation.
Important for aquatic organisms, fish etc
BOD biochemical oxygen demand sources
Micro-organisms activities
Organic gross pollutants , leaf matter
Reduction of metals,
Microbial conversion of NH4 to NOx
Impacts of BOD
Reduced DO levels- stress to aquatic community(cause death); release nutrients from sediment(desorption of phosphorus and metals from anaerobic sediment) ; contribution to eutrophication, algae bloom; offensive odors.
Toxic organics and metals and their impacts
Excess of threshold will be toxic
Cd, Cr, Cu, Ni, Pb, Zn plus Hg, As
Impacts
Varied and complex-
interactions with water biophysical parameters( pH, temperature, redox potential)
Many metals found in particulate form ( transported with particles, not readily available for biological uptake )
Effects may be acute or chronic( bio accumulation , shellfish)
Cause of mutation which affect life cycle
Can influence other treatment process eg denitrification
Oils and surfactants
Increase oxygen demand
Often toxic and cancerogenic
Affect respiration
Persistent and difficult to remove
Sources
Vehicle operation, cleaning, maintenance
Oil handling and disposal, road runoff, detergents
What is the urban stream syndrome?
Changed flow Poor water quality Simplified habitat Reduced in-stream biodiversity More algae growth Less retention, more transport of pollutants
Are impervious areas equal to each other? If not which is important.
No. What is important is the amount of impervious area directly connected to a stream.
Effective imperviousness equation
EI = Total impervious area * connection %
Proportion of catchment which is impervious and directly connected to receiving water.
How to break connection?
By retaining the first x mm of runoff.
- permeable pavements.
- rainwater tanks.
- Swales.
- ponds and storage tanks.
Setting target for urban drainage
Environmental target, flow target, water quality target, concentration and loads
Integrated targets for drainage
Maintain pre-development ... Runoff volume, Runoff frequency, Ratio of base flow Pollutant concentration TSS, TP, TN, metals,
impacts of urbanization
Hydrology ( bigger floods, flashier) Water quality ( many types of pollutants, highly variable behavior) Ecosystem health ( directly related to connected impervious area)
Target flow, water quality, ecosystem health
