On Site Detention
Based on the Upper Parramatta River Catchment trust OSD Handbook

On site detention, discharge control pits

LEGEND:-
User Input Calculated
result

Background

This program is based on the methods recommended in the "On-site Stormwater Detention Handbook" developed by the Upper Parramatta River Catchment Trust (UPRCT). Available here.
This text is well respected in the industry. The general principles have been adopted by many Authorities, and it is cited as a reference in the overriding text the "Australian Rainfall and Runoff" (ARR).

So why do we need 'On Site Detention'?

Before a site is built on, it has a certain runoff.
The process of building something on this site, normally generates more runoff, because there's likely to be more impervious areas. As in more or bigger roofs, and more paved areas.

The powers that be, don't like more runoff, so they insist that there shall be no worsening effect with the new construction.
This is achieved by creating a storage area known as an on site detention basin or tank. (OSD)

The site runoff pours into this basin, but the outflow is restricted with an orifice, so water is coming in faster than it is going out. The remainder is held in storage. This will drain out later. This has the effect of reducing the peak outflow.
And everyone goes away with a warm inner glow.

Note

In the good old days we analysed the flow coming in with one storm, and the flow going out with the same storm, and twiddled all the numbers until we achieved this 'no worsening' effect.

But these days we have hundreds of storms to analyse. And the whole thing was getting out of hand.
No two people got the same answer, and nobody got the same answer twice. There were just too many variables.

Fortunately some Councils have done all the hard work analysing the storms, and now just give us the answers.
i.e. The permissible site discharge (PSD) in litres/sec/hectare, and the site storage requirement (SSR) in cubic metres per Hectare.

This programme assumes that you have this information.

If you don't, you may be able to get it from another Council in the same catchment area, or calculate it yourself the hard way using a computer programme. Something like" Drains" or "RORB".

The latest thinking is also to design for minor storms as well as major storms. Again if the Council hasn't given you this information it is advisable to allow for it anyway. This can be done by using information from the "Queensland Urban Drainage Manual" (QUDM) for the relevant Minor storm AEP, and from the UPRCT graph for the corresponding Storage requirement. (Refer below)

Using the program

Start from the top and work your way down pressing all the calculate buttons in order.
If you decide to change anything, repeat this process. ie pressing all the calculate buttons in order, even if there are already results in this section.
These results may need to be overridden with the new data.
Should you find that the Basin calculated with this program will not fit your desired site, there's no need to worry, as at the end of the program there are a lot of variables to play around with, allowing us to store various percentages of water in some sort of underground system.

If you are new to all this, it may help to start off with some sample data to play around with, so use the "Import example Data" button above.

from Queensland Urban Drainage manual.

Percentage of total storage for minor storms.

e.g. For a 1.5 year ARI the required storage for the minor storm = 66% of the total storage.

Site Data

Site Details(info?)

Site Area sq.m

Total Roof sq.m

Grassed Area sq.m

Site Runoff (grassed) Coefficient (C) (info?)

Storm Intensity (5 min 1% AEP) mm/hr

Number of Dwellings/rainwater tanks on site.

∴ Paved Area sq.m
(= siteA - roofA - grassed areas)

∴ Total site effective impervious area (sqm)
= RoofA + 0.9*PavedA + C*grassA

∴ Total site runoff(L/s)
= Effective Area*Inten/3600

∴ site Area / dwelling (sq.m)

∴ Roof Area / dwelling (sq.m)

Parameters from Council PSD (SRD) & SSR

(Not allowing for any bypass, refer next section.)

Note: Some Authorities use SRD "Site Required Discharge" instead of PSD "Permissible site Discharge"

OSD Tank nomenclature from UPRCT Fig 5.1

OSD Tank nomenclature in 2D from UPRCT Fig 5.1

Extended Detention SSRL Site Storage Requirements cu.m/ha

Total Detention
Total Storage (SSRT) cu.m/ha

PSDL (Primary Outlet) L/s/ha

PSDs (From whole site) L/s/ha

Minimum Primary orifice size mm

Minimum Secondary orifice size mm

∴ SSRL Storage Volume lower cu.m

∴SSRT Total Storage Volume cu.m

∴ Maximum Flow from Primary orifice (L/s)

∴ Max Flow from Secondary orifice (L/s)

ByPass calculations - Adjusted PSDs

Max equiv impervious area allowed to
bypass OSD %

Bypassed Area-Grassed (sqm)

Bypassed Area-Roofed (sqm)

Bypassed Area-Paved (sqm)

Total bypassed area
RoofA+grassedA+pavedA(sqm)

Total site area draining to OSD (At)
Site area - bypass area (sqm)

Max Equivalent impervious Area(sq.m)
allowed to bypass OSD

Bypassed Area (equiv. Impervious) (Ab)
(= RoofA+GrassedA*C+PavedA*0.9) (sqm)

from BCC Ch. 7 Storm Water Drainage
Mod. PSD = PSD x (At / [ At+Ab]) (L/s/Ha)

Adjusted primary orifice flow (L/s) info?

Adjusted secondary orifice flow (L/s)
(from BCC Ch. 7 Storm Water Drainage)

Rainwater Tank Calculations

The calculations assume that the same size rainwater tank and roof is installed on each dwelling.
If there are no dwellings as in a commercial or industrial bldg, and you still wish to use part of a tank for OSD, then enter '1' in the 'number of dwellings/Rainwater Tanks' box above, in the 'Site Data' section.

Water Demand on Rain Water Tank

Daily demands can be estimated using the following average daily demands for Western Sydney, as reported by Coombes and Kuczera (2003) (reter pp 240).

Estimated Daily water Demand on a rainwater Tank

Percentage of total water use demand on Rainwater tank

Number of Occupants/dwelling

Connected to RWT:- WC Ldry HC

Estimated Total indoor water usage. (L/d)
(includes Town water)

Estimated Demand on RWT(default) (L/d)
(RWT is not for potable use)

It is assumed WC and HC are connected to the RWT. These combinations may be changed.

Want to use a different demand(L/d)
on the RWT?, enter here

Allow min % of roof draining to tank. %
derived by rearanging the Dynamic air space formula below.

%age of roof water draining to each tank %

placeholder for notes and warnings

Total rainwater Tank volume (Litres)

Min Volume that triggers Top-up (Litres)

Max allow Dedicated Air Space(Litres)
per Dwelling Tank (refer notes below)

Dedicated Air Space used (Litres)

Place Holder for notes and warnings

Maximum tank PSD (PSD for minor storms)(L/s/ha)

Max Head to Centre of Tank Orifice (mm)

Maximum Tank Discharge (L/s)
= PSD * site Area/dwelling

Tank Orifice diameter (mm)

Residual Vol (L)
(Max Tank Vol - Min Top-up Vol)

Dedicated Air Space

Based on the analysis of the results reported by Cardno Willing, 2004 the following reductions in the SSR values may be allowed subject to Council approval.

• 50% of the dedicated airspace can be credited against the required extended detention volume (SSRL);
• 100% of the dedicated airspace can be credited against the required overall detention volume (SSRT);

subject to:
• a Maximum Dedicated Airspace credit no greater than the ratio of the area of roof discharging to the rainwater tank to the lot area times the overall site storage volume that is required;

Extended Detention
Dedicated Air Space Credit(Litres)

Total Detention
Dedicated Air Space Total Credit(Litres)

Dynamic Air Space

UPRCT 4.2.8 The rainwater tank dynamic airspace at the start of a storm is calculated using (refer Figure 5.2):

Dynamic Airspace available (kL) = 8.7 x Nett Tank Vol (kL)^1.05 x Roof Area (m2)^-0.5 x Demand (kL/d)^0.35

where:-
Nett Tank Volume = Max dynamic storage = Total Tank Volume - Dedicated Airspace - Top-Up Volume
Roof Area = (Roof area per dwelling) * %age draining to tank.
Demand = As calculated above.

Daily demands can be estimated using the above average daily demands for Western Sydney, as reported by Coombes and Kuczera (2003) (refer above).

The reduced SSR values due to dynamic rainwater tank airspace is calculated using:
SSRL= PSD_Lower - (1 ,950 x Dynamic Airspace (kL)^2.10 x Roof Area (m2)^-1.50
SSRT= PSD_Total - (1 ,650 x Dynamic Airspace (kL)^2.30 x Roof Area (m2)^-1.50 )

Maximum Dynamic Storage (Nett Vol) (L)
(from Nett Tank Vol formula above)

Daily demand on RWT (L)

Dynamic storage available at the start
of the storm (L)

Extended Detention
Dynamic Air Space Credit(Litres)

Total Detention
Dynamic Air Space Credit(Litres)

Tank Storage Credits

Combined Rainwater Tank Credits per tank
(from above) Extended detention cu.m

Combined Rainwater Tank Credits per tank
(from above)Total Detentioncu.m

Max allowable Rainwater Tank credit cu.m

Total credit (in SSRL) for all tanks cu.m

Total credit (in SSRT) for all tanks cu.m

End of Rainwater Tank section

OSD Calculations Storage Volumes & allowable discharges

Extended Detention Orig SSR Volume (extended detentn SSRL)cu.m

Total Detention
Original Total Storage (SSRT) cu.m

Total credit for all tanks (exten'd det'n) cu.m

Total credit for all tanks (Total det'n) cu.m

Extended detn storage less creditscu.m
(Primary storage)

Total detention storage less credits cu.m

Flood Detention (secondary) storagecu.m
(total - extended det'n)

OSD Discharge (Secondary outlet)L/s
from above

OSD Discharge (Primary outlet)L/s
from above

Overflow Weir & top water level Calculation

RL of Min Habitable Floor Level(FFL) (m)

RL of Minimum Garage floor Level (m)

Req'd freeboard to Habital Floor Level. (m)

Length of Overflow Weir (m)

Flow to OSD = Peak flow over weir(L/s)
= Equiv imperv flow to OSD*inten/3600

Depth of Flow over Weir mm

Maximum TWL(m)
working down from FFL

Maximum level of overflow weir(m)
= (Max TWL - Depth over weir)

Water Levels and Ponding Depths

Discharge control pits for On Site Detention

Ponding Depths

Maximum TWL (from above)(m)

Adopted TWL flood detention (m)
(must be lower than Max TWL)

Try ponding depth over Primary grate (mm)

Try ponding depth over secondary grate
for ARI 100 (1% AEP) mm

∴ RL of Primary grate (m)

∴ RL of secondary grate(m)
(TWL extended detn)

Orifice Calculations

Orifice Formula

Primary (Lower) Estimated Downstream Flood Level (FL)
1.44yr ARI 50% AEP (m)

Secondary (Upper) Estimated Downstream Flood Level (FL)
100yr ARI 1% AEP (m)

RL of Orifice Centre-line primary
(must be above d/s FL) (m)

RL of Orifice Centre-line secondary
(must be above d/s FL) (m)

Number of Orifices Primary

Number of Orifices Secondary

Orifice discharge coefficient. Primary DCP
(normally 0.61)

Orifice discharge coefficient. Secondary DCP
(normally 0.61)

TWL extended detention
(RL secondary Grate) (m)

TWL flood detention (From above) (m)

Design Head to Orifice Centre.
Primary DCP (m)

Design Head to Orifice Centre.
Secondary DCP (m)

Design flow through Primary Orifice/s (L/s)

Design flow through Secondary Orifice/s(L/s)

Primary Orifice dia. (mm)

Secondary Orifice dia. (mm)

Results Summary

Site Storage requirements less tank credits
if any. (SSR lower) Extended Det'n (cu.m)

Site Storage requirements less tank credits
if any. (SSR upper) Flood Detention (cu.m)

Site Storage requirements (SSR total) cu.m

PSD (Q) through Primary orifice
(from above) (L/s)

PSD (Q) through Secondary orifice
(from above) (L/s)

Orifice dia in Primary DCP (mm)

Orifice dia in Secondary DCP (mm)

Ponding Depth over Primary DCP (mm)

Ponding Depth over Secondary DCP (mm)

Check basin/tank areas

Lets see if the design will fit on the site.
If it doesn't, the program will also work for various other configurations. Scroll down for some possible examples.

The Method
The object is to adjust all, or any of the 6 available variables (in the white boxes) to ensure that the basin will fit on the site, and the ponding depths are acceptable.

Once this is achieved, if you have changed the ponding depths, you must then go back to the main program and adjust these ponding depths.
This is necessary, so that the orifice sizes, levels and depths can be recalculated.

Notes
Storing water underground, and/or increasing the basin area will decrease the ponding depth.
Conversely storing less water underground, and/or decreasing the basin area will increase the ponding depth.
Steepening the slopes will also have the effect of increasing the ponding depths.
Flattening the slopes have the effect of decreasing the ponding depths.

On Site Detention- Cross Section diagrammatic

Storage Configuration.

What is the Side slope required, (S2)
in the form of 1 in ? (0 = vertical)

What is the Base slope required,(S1) (0=flat)
in the form of 1 in ?

Length of longest base side Lb1 (m)

Length of base Lb2 (m)

% of Primary storage under ground (%)

% of Secondary storage under ground (%)

Vol of Primary storage under ground (cu.m)

Vol of Secondary storage under ground
(cum)

∴Vol of Primary storage Above ground
(Vol1) cu.m

∴Vol of Secondary storage Above ground
(Vol2) cu.m

Surface Area needed to get the required
Total above ground volume. (A3) (sq.m)

Ponding depth on the primary grate(D1)
(mm)

Top water Surface Side length (Ls1) (m)

Ponding depth on the secondary grate
(D2) (mm)

Top water Surface Side length (Ls2) (m)

Total ponding depth (D3) (mm)

Other configurations

OSD1

OSD2

OSD3

OSD4

If using OSD3:-
Length of weir2 (if required) (m)

Height 'H3' required for weir 2 (mm)
(Note 25mm freeboard has been added)

Head on Primary orifice H1
(Also depends on the d/s flood level) (mm)

Primary Orifice 1 diameter (mm)

If using OSD4:-
Adjust the RL's of the orifice, and the ponding depths in the main program.

However if there is no requirement for minor storms, the major orifice may be put at the base of the tank.

Note the use of a sump. This effectively makes total use of the tank depth.

On Site detention with added underground storage Plan View

A modular rainwater storage system from Spel stormwater Australia.

A modular rainwater storage system from ACO Australia.