Osd desisn, sanitation in low cost

2.  OSD is only one aspect of the management of
the water cycle on a site.
 On-site Stormwater Detention (OSD) involves
the temporary storage and controlled release
of stormwater generated within a site.
Without adversely affecting the property, it
relies on thoughtful design and passive
engineering during site development to
achieve significant reductions in downstream
flooding.

3.  A Water Sensitive Urban Design (WSUD)
approach aims to better mimic the natural water
cycle and may include the installation of
rainwater tanks to harvest and re-use rainwater,
the treating and re-using of grey water and
implementing a range of measures at lot,
neighbourhood and regional scales to reduce the
rate and volume of runoff and to improve runoff
quality.
 In particular, it aims to mitigate the impact of
urban runoff during more frequent storms on
natural creeks and watercourses

6.  This section outlines the calculations that are
undertaken to determine the OSD
parameters.
 Includes
 Site data
 Key Parameters for an On-Site Detention
System
 Key Parameters of a Rainwater Tank
 Basic OSD parameters
 OSD storage bypass
 OSD calculations

7.  calculates the extended detention storage
volume and the overall storage volume based on
the total site area;
 calculates the volume of dedicated and dynamic
airspace that can be credited against the required
extended detention storage volume and the
overall detention storage volume (based on a
rainwater tank being installed for each dwelling);
 calculates the nett extended detention storage
volume and the overall storage volume based on
the total site area; and
 calculates the primary and secondary outlet
discharges (based on the adjusted SRDs);

8.  The site data includes the :-
 total site area (in ha);
 total roof area (in ha);
 area of the site draining to the OSD storage
(in ha); and
 number of proposed dwellings on the site

9.  With Site Data we can calculate :-
 the residual site area – Total site area less total
roof area (in ha). It is assumed that all roof
runoff is directed to the OSD storage;
 the area of the site bypassing the storage; checks
that the area of the site bypassing the storage is
less than 30% of the residual site area (bypasses
greater than 30% are not allowed)1;
 calculates the equivalent site area per dwelling
(in ha); and
 calculates the average roof area per dwelling (in
ha).

11.  The basic OSD parameters are reported in the
last figure..
 SRDL = 40 L/s/ha (primary outlet)
 SSRL = 300 m3/ha (Extended Detention)
 SRDU = 150 L/s/ha (secondary outlet)
 SSRT = 455 m3/ha (Flood Detention)

13.  The OSD Calculation spreadsheet:
 calculates the adjusted SRDs based on the
calculated percentage of the residual lot area
draining to the OSD storage

14. Four units are to be constructed on a 0.24 hectare site. A grassed area at the
rear comprising 10% of the site (0.024 hectares) falls steeply to the rear and
cannot be drained to the OSD storage. Each unit will have a 5 kL rainwater
tank that will collect runoff from 80% of each 150 m2 roof with rainwater to be
used for outdoor, toilet flushing and laundry in each 4 person household. An
airspace “credit” is to be claimed against the OSD storage requirements.
There will be no dedicated airspace. The minimum volume in the rainwater
tank at which top-up commences is 0 kL ie. when it is empty.

20.  Low cost technologies are implemented
mostly in rural areas..
 In rural situations, family pit latrines can
provide satisfactory and acceptable
sanitation. They are all that most people in
the developing world can afford. Given
sensitive guidelines and a little technical help,
families can build latrines for themselves, at
very low cost.

21.  A family pit latrine should be about 1.2m
diameter, or square, (the smallest dimension
that can be dug conveniently), with the pit
wholly above the water table. It should be at
least three metres deep and, if necessary to
attain this depth, the floor level of the
building above it should be raised above
ground level.

22. Pit latrines should be sited at least
50m from the nearest well or borehole.
When water is used for anal cleansing
this rule can be difficult to enforce and
hydro geological advice should be
sought.
A latrine is a permanent installation. In
unstable ground the pit walls should
be supported with timber, bricks or
blocks. Struts spanning across the pit
should be avoided as they will become
fouled and will cause smells.