Hydrology of an urban environment
SSA2215 Summary (6) © Lim Fang Jeng Page 1
Hydro-Geomorphological Changes
Hydrological Changes Geomorphological Changes
Alteration of - flow duration - flood frequency
- modification of runoff from individual storms - Change in water quality & hydrological amenities
Change of
- Channel size, width, depth - Associated depositional form - Drainage density
Fluvial Processes
River system : made up of branched network of stream channels and adjacent slopes Drainage basin : Area of collection and distribution of water and sediment
Stream discharge
Q = Av
Q= Discharge; A: wetted cross-sectional area; v=mean velocity of water flow Hydrograph : Graph measuring the temporal changes in channel discharge due to precipitation Base flow : Perennial flow that is in most rivers, relative constant
discharge between flood events (from Ground water) Quick flow : Effect of runoff generated by the precipitation event Analysis of Hydrograph
- Rising Limb: distribution of rainfall intensity, hydraulic strength, ground slope, hydrological conditions of soils
o Steepness: increase in channel discharge o Sharp peak: high impervious cover
- Recession Limb: less steep, withdrawal of shallow subsurface water and basin storage
o More dependent on basin characteristics
Channel forms and hydraulic geometry Size of alluvial channels:
Width (w)=aQb Depth (d)=cQf Velocity (v)=kQm
River Systems in Singapore
- 40 small, yet separate drainage basins in Singapore - 75% of basins less than 13 km2
- 11 drainage administrative catchments
Impact of Urbanization on River Flow
- Basin storage reduced
- Interception loss (removal of vegetation)
Hydrology of an urban environment
SSA2215 Summary (6) © Lim Fang Jeng Page 2
- Building of sewage system and storm drains affects the timing of flood hydrographs
- Rising Limb : extremely steep - High peak discharge
- Lag time shorter
Flood
- River cannot accommodate water o Overbank flooding o Flash flood
- Bank-full discharge: Discharge when river is at full stage - Flooding is a problem in an urbanized environment - Causes economic loss, health hazard, loss of life - Drainage Masterplan
o Layout and land requirements
o Control levels for regulating the threshold platforms of future developments to achieve effectual drainage
o Special facilities to regulate hydraulic conditions - Reduce flood-prone area
Modification of river channels in Singapore
- Canalization – straightening of river channels, lining the sides with concrete
- Increase the discharge of water during storm by increasing the flow velocity (1-3 m/s) - 8 stormwater ponds, collect stormwater and direct to Bedok Reservoir for treatment
- Sungei Api Api: canal embrakments were made by replanting mangroves (increase aesthetic value) - Sungei Kallang: aesthetic waterway with liner park along the canalized riverbank
- Geylang River: Deepened river bed to create permanent waterbody during low tide - Sungei Tampines: Build underground diversion channel
Reservoirs and water resources in Singapore
- 14 dams in total
- Currently 1.4 million cubic meters/day - Rise threefold in 10 years’ time - NEWater and seawater desalination - Buy water from Malaysia and Indonesia
Impact of Canalization
- Ecological decline in natural rivers
- Extinction of water organisms (52 29 (1990))
- Change in water quality and sediment load (affect river ecosystem) - Affected habitats of freshwater organisms
- Anadromous : enter river to spawn (salmon, shad, striped bass,etc) - Catadromous: Live in fresh water but returning sea to spawn
Hydrology of an urban environment
SSA2215 Summary (6) © Lim Fang Jeng Page 3
Drainage System in Singapore
- Drains are designed to remove/dispose excess water - Type of drains
o Small tributary drain: Transfer water from impervious surfaces to monsoon drains
o Medium sized drain: Divert water from road surfaces to main drains or to transfer water from one main drain network to another
o Large open canal: Collect and channel excess storm water to sea - Might have different gradients
o Construction impracticalities o Initial topology influence
o Type of bedrock prior to construction
Sedimentation
- Causes pollution in the surroundings - Caused by residential activities - Accumulation of waste products, etc. - Bukit Batok case study
o Accumulation of industrial effluent, causes gravel particles held together by oil deposits.
Trap sediments, cannot be removed by running water o Massive excavation
Soil erosion
Debris washed into drains, sediment built-up o Drains with higher gradient will be less sediments
However, building drains with large gradient will increase water flow and it will endanger the surroundings
Not economical o Results in small-scale flooding
o pH value in drains with many sediments are lower (acidic)
o Sediments will have experienced various alterations chemically and physically - Countermeasure
o Public education
o Environmental awareness
o Leave leaves on the soil ground to provide nutrients for trees instead of dumping it into the drains to encourage sediments
o Build grass ground around the drains – purify water
