Pipe Design

'·reference : wsudcompliance.com.au / Austroads guide to road design part 5A
 
1. piped Network Element
ㅇ Dranage inlets
   - Capture surface runoff
   - control flodded limits(With, Depth, WxD)
   - redirect flows underground into pipe network
ㅇ Access chambers
   - Provide access for maintenance
   - Changes of direction, grade, level
   - At pipe junctions
ㅇ pipes
  - convey captured runoff from inlets
  - Multiple pipes interconnected within a system
  - it discharge into detention basin, Bioretention system, Rainwater haversting tank, Existing pit, Headwall outlet
 ㅇ outlets
  - allow piped network to discharge captured stormwater
  - headwall provide a stavle, controlled point of discharge
 
*runoff : 물의 흐름(명사)
 
2.Terminology

ㅇ pipe length measured to contre of pit for design purpoes
ㅇ pipe cover measured vertically from surface level to crown
ㅇ pipe slope(So), friction slope(Sf)
ㅇ Invert : Lowest point insde pipe
ㅇ Obvert : Highest point inside pipe
ㅇ Crown : Highest point outside pipe
ㅇ pit drop : Required to ensure free draining through pit
 
 
 
ㅇPressure Head
- the foce that water exerts due to the weight of water above it
- affects flow rate(how quickly water flows through the pipes.)
- The greater the pressire, the faster the water flow

ㅇ Hydralic Grade Line(HGL)
- HGL is preasure head in a pipe line(effective water level)
- preasure head at any point along HGL is vertical distance below that point
- Flow velocity is a fucntion of HGL(not pipe grade)
- Hf = SfL
  > Hf : head loss in pipe due to friction(m)
  > Sf : Friction slope (m/m)
  > L : Length of pipe reach(m)
- Hs = K * Vo^2 / 2g
  > Hs : head loss at obstrction, bend, junction(m)
  > K : Pressure change coefficient
  > Vo : velocity of flow in downstream pipe(m/s)
  > g : Gravitational acceleration
- Total Energy line(TEL)
  > velocity head = head pressure + Vo^2 / 2g
  > Total energy available to flow
  > HGL and TEL coincide where velocities are negligible

- Water Surface Elevation(WSE)
  > Hs = Kx * Vo^2 / 2g
  > Ku : junction pit pressure change coefficient
  > Kw : WSE change coefficient
  > 150mm freeboard(TWL) must be allowed above the WSE
 
 
 
- Open channel model
  > Assumes steady flow full but not under preasure
  > HGL matches or slightly lower with upstream of each pipe
  > Design flow determined by the rational method
  > V is calculated by manning's equation(V = 1/n*R^2/3*S^1/2),(Q = VA)
      (pipes treated as open channels with fully flow)

- Pressurised Grade Line
  >Assume steady flow full with preasure
  > HGL is above pipe obvert
  > Using Colebrook-White Equation

·

  - Unsteady flow model
   > Dependent on time, water level fluctuate.
   > Requires computer program for analysis

  - Tailwater Condition
   > Essential for backwater analysis
   > Determined by HGL(depend on Tailwater(TWL) in receiving water, Critical depth(dc) of flow in ourfall pipe, obvert level(OL) of pipe)
   > If TWL > OL, HGL = TWL
   > If TWL < 'Obvert level' and TWL > dc, HGL = 'obvert level'
   > If TWL < 'Invert level' or TWL < dc, HGL = dn

- Tidal Systems and  Flood gate
  > Flood gates prevent water backflow(opened when it's usual and closed when the tides come in)
  > Make higher hydraulic head loss associated with gate

- Connection into Existing Pipe network
  > Urbanisation increases discharges placing existing systems under stress
  > Potentially overwgelming dowstream pipes and channels
  > structure losses must be assed when connecting into existing system
  > HGL is a direct method but impractical so we need to estimate HGL(TWL)
  > befor adopting TWL, we need to know how existing system perform.