Osd desisn, sanitation in low cost

On Site Detention, Design Procedure For the Site Detention,

Low cost Appropriate Technologies for Sanitation

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.

INTRO

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

URBAN HYDRO CYCLE

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

THE DESIGN PROCESS of OSD

The OSD Calculation spreadsheet: 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);

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); andnumber of proposed dwellings on the site

Site data

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).

Key Parameters for an On-Site Detention System

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)

Basic OSD parameters

Key Parameters of a Rainwater Tank

The OSD Calculation spreadsheet:

calculates the adjusted SRDs based on the calculated percentage of the residual lot area draining to the OSD storage

OSD storage bypass

Worked exampleFour 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.

Outflow Rating Curve for OSD System with Primary (Lower) Orifice Outlet & Secondary HED Orifice Outlet

Low cost Appropriate Technologies for Sanitation 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.

Simple pit latrines 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.

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.