On-Site Detention Plan-Checking Training Session

Internal training session focused on checking an On-Site Detention (OSD) design that has already been lodged.
Only the OSD component of a broader civil plan set is reviewed; other civil aspects (earthworks, roads, etc.) are assumed complete.
Trainees (Patrick and colleagues) learn to:
- Read plans, pit schedules and cross-sections.
- Extract governing parameters supplied by council via OSD 4 software.
- Populate Council’s in-house Excel “Detention Volume Calculation” template.
- Confirm that the design meets both Permissible Site Discharge (PSD) and Required Storage Volume criteria.
- Recognise how pits, pipes and tanks contribute to total storage, and why ≥ 60 % must be underground.

OSD (On-Site Detention) – temporary storage system that throttles storm-water runoff so post-development peak discharge ≤ pre-development target.
PSD (Permissible Site Discharge) – maximum flow rate council allows to leave the site.
Storage Volume – minimum volume that must be provided to attenuate the design storm.
Flow Control Pit (FCP) – chambered structure with inlet & outlet compartments; outlet has an orifice sized to limit flow.
Invert Level (IL) – internal base level of a pit/pipe.
Cover Level (CL) – finished surface level above a pit.
Top Water Level (TWL) – maximum water surface in detention system during the critical storm.
Critical Duration – storm duration that produces worst-case storage or discharge.
MC2 / Multicell – proprietary flow-control device; performance read from manufacturer’s chart.

Civil drawings (layout, long-sections, pit schedule, cross-sections).
OSD 4 report issued by design engineer (contains PSD, storage, critical duration, etc.).
Council Excel templates located via:
Excel → Templates → “Detention Volume Calculation”.
• Sheet 1 = calc engine (contains live formulas with cell tool-tips).
• Sheet 2 = explanation of equations (e.g.
Rational Method, continuity, orifice flow, Darcy-Weisbach, etc.).

The very bottom line of the OSD 4 print-out summaries the design targets. Example site values:
- $PSD_{req} = 8.79\;L\,s^{-1}$
- $V_{req} = 4.34\;m^{3}$
- Critical duration, total hard surface area, etc. captured for reference.

For every pit (starting at the outlet and working upstream):

Read Cover Level & Depth from the pit schedule.
Compute Invert Level via $IL = CL - Depth$ .
Record TWL (usually the top of the baffle/batter wall separating inlet & outlet chambers).

Typical rectangular pit volume:
$V_{pit} = Width \times Length \times (TWL - IL)$
Example – Pit 7 (FCP):
- Width = $0.6\,m$
- Length = $1.8\,m$
- TWL = 109.75\,m,
  IL = 109.01\,m\;\Rightarrow\;(TWL-IL)=0.74\,m
- $V_{pit7}=0.6\times1.8\times0.74=0.80\,m^{3}\;(rounded\;0.74\,m^{3}\text{ in session})$

Identify pipes whose upstream end is connected to an inlet chamber (storage-side). Pipes bypassing directly to the outlet do not count.
Use the higher IL of the two connecting pits for conservatism.
If multiple parallel barrels exist, multiply the length by the number of barrels.
Volume per pipe:
$V_{pipe}=\tfrac{\pi}{4} D^{2} L$
(ensure $D$ in metres, $L$ total length in metres).

Rainwater or reuse tanks sitting above natural ground = above-ground storage and may be excluded if ≥ 60 % underground rule already satisfied.
Underground tanks can be included provided they drain only through the throttled outlet.

Designer supplies outlet-orifice diameter (e.g. $77\,mm$ ).
Enter diameter in the template; embedded orifice equation: $Q = C_d A \sqrt{2g h}$
- $C_d$ = discharge coefficient
- $A$ = orifice area
- $h$ = head on orifice
Spreadsheet turns the PSD cell green when $Q{calc}\le PSD{req}$ .

When an MC2 cartridge replaces a plain orifice, discharge is obtained from manufacturer’s MC2 performance chart.
Process:
1. Determine inlet head $h$ at critical TWL.
2. Read corresponding discharge $Q_{chart}$ .
3. Insert $Q_{chart}$ into PSD field.

Cells auto-colour:
- Green – requirement met.
- Red – requirement not satisfied.
Two mandatory conditions:
1. $Q{calc} \le PSD{req}$
2. $V{total} \ge V{req}$

If TWL < IL for any element, resulting $\Delta h$ becomes negative ⇒ storage contribution set to $0$ (delete that row).

Plans must be plotted to true A4 scale (e.g. 1 : 100) so lengths can be measured.
Subdivision / title plans must match the current registered title; otherwise Titles Office rejects.

Parameter	Required	Provided (after data entry)	Status
Storage Volume	$4.34\,m^{3}$	$5.76\,m^{3}$	Green (OK)
PSD	$8.79\,L\,s^{-1}$	$8.70\,L\,s^{-1}$ (with 77 mm orifice)	Green (OK)

Always convert mm → m before area/volume calcs.
Start storage tally downstream and move upstream; quit once $V{total} \ge V{req}$ to save time.
For pipes with no dimension shown, use CAD or scale-rule to measure length directly on the plan.
Triple-check that pits feeding the outlet chamber are excluded from storage tally.
Watch for designs where tanks are above ground; do not accidentally credit them as underground.
If PSD or volume barely fails, suggest designer either enlarge pit, extend pipe length, or shrink outlet orifice.

Excel Template: Excel → Templates → “Detention Volume Calculation”.
Sheet 2 of template documents every formula; hover over any cell for built-in comments.
OSD 4 Software: Generates PSD & storage targets per council methodology.
Core Equations (embedded):
1. Orifice Flow $Q=C_d A\sqrt{2gh}$
2. Pipe Storage $V=\tfrac{\pi}{4}D^2L$
3. Rectangular Pit Volume $V= B\times L\times h$

Ensuring compliance avoids downstream flooding and liability.
Incorrect titles or scales delay registration; accuracy protects council reputation and project timelines.
≥ 60 % underground rule limits visual impact and maximises effective attenuation.
MC2 sizing must respect manufacturer data; misuse voids warranties and may breach approval conditions.

Marianne’s earlier session on OSD parameters explained how PSD & storage are derived in OSD 4; today’s lesson shows how those numbers are applied during plan checking.
Flow-measurement theory (head–discharge relationships) underpins orifice and MC2 checks, reinforcing fluid-mechanics topics covered in foundational hydraulics modules.