**3.3.4 Discussion**

Arid areas are susceptible to flash floods mainly due to the lack of vegetation. Urbanization in such areas has further aggravated the problem, especially where human interference has obstructed the natural drainage pattern. Furthermore in arid regions it is difficult to predict the rainfall and surface run off characteristics and may cause negative complications during urban planning, (Hussein et al, 2008). This is true for most of the cities in arid regions such as Riyadh.

Fig. 29. Areas prone to hydrological hazards in Hanifa basin

As shown from the results above the Hanifa Basin is more important from the point of hydrological hazards mainly because of the large basin area as compared to Sulay basin. This implies that during the same intensity of rainfall in the region, water in the Hanifa Basin will be collected from a larger catchment as compared to Sulay Basin, thus bringing a greater volume of water to the main drainage. Secondly the Hanifa Basin has a more rugged topography as compared to Sulay basin, (see Figure 27 and Table 8) which means the velocity of the water will be much higher. Third and most importantly the bifurcation ratio of Hanifa Basin is much higher as compared to Sulay Basin, (Table 8). This is true specially for the 5th order streams which clearly means that more number of 4th order streams meet at different points to form a 5th order stream in Hanifa Basin, (Figure 28). From the perspective of hydrological hazards a high bifurcation ratio increases the chances of flooding as water from a given stream order, rather than spreading out when they meet the stream of the next higher order, tend to accumulate in one place.

Based on this observation, all the 4th order sub-basins of the Hanifa basin have been identified and demarcated (Figure 29). The mouth of these basins are the potential sites for hydrological hazards in case of heavy rainfall and should be carefully monitored during the rainy seasons in the region to prevent damage to life and property.

## **4. References**

156 Urban Development

It is the ratio of the basin area to the area of the circle having the same perimeter as the basin. The circulatory ratio for Hanifa basin is 0.31 and the circulatory ratio for Sulay Basin

The Hanifa basin shows a high relief ratio (3.30 m/km) and has an average slope percentage of 6.6 as compared to Sulay Basin which has a relief ratio of 2.29 m/km and an average slope percentage of 2.3. The results clearly suggest that Hanifa basin has a more rugged

Arid areas are susceptible to flash floods mainly due to the lack of vegetation. Urbanization in such areas has further aggravated the problem, especially where human interference has obstructed the natural drainage pattern. Furthermore in arid regions it is difficult to predict the rainfall and surface run off characteristics and may cause negative complications during urban planning, (Hussein et al, 2008). This is true for most of the cities in arid regions such

is 0.23. This factor is influenced by the lithological characteristics of the basin

Fig. 29. Areas prone to hydrological hazards in Hanifa basin

As shown from the results above the Hanifa Basin is more important from the point of hydrological hazards mainly because of the large basin area as compared to Sulay basin.

Circulatory ratio

**3.3.4 Discussion** 

as Riyadh.

terrain as compared to Sulay basin.

Slope


http://srtm.usgs.gov/data/obtaining.html


**1. Introduction** 

installed and the type of water use.

rainfall in those cities as compared to Melbourne.

**8** 

*Australia* 

**Rainwater Harvesting in Large** 

**Residential Buildings in Australia** 

Australia is one of the driest inhabited continents, with highly variable rainfall. A growing urban population and frequent droughts in recent years have made water supply a major issue in Australia. A number of alternative water sources have received attention in Australia including rainwater harvesting, grey water reuse and wastewater recycling. Among these, rainwater harvesting has received the greatest attention as rainwater is fresh in nature and can be easily collected and used for non-potable purposes. However, many Australians still show a reluctance to adopt rainwater harvesting systems. Statistics from the Australian Bureau of Statistics (ABS) show that about 47% of respondents say that the main reason for not installing a rainwater tank is the perceived 'higher cost' (ABS, 2011). Government authorities in Australia provide financial incentives to encourage home owners to install rainwater tanks. For example, Sydney Water Corporation in Australia offers a rainwater tank rebate of up to \$1,500 (here \$ is in Aus\$) depending on the size of the tank

Many home owners do not readily see the benefit of a rainwater harvesting system over the long term, which may be attributed to the limited understanding of the life cycle costs of the system. Domenech and Sauri (2010) investigated the financial viability of rainwater harvesting systems in single and multi-family buildings in the metropolitan area of Barcelona (Spain). In single-family households, an expected payback period was found to be between 33 to 43 years depending on the tank size, while in a multi-family building a payback period was 61 years for a 20 m3 tank. Imteaz et al. (2011) found that for commercial tanks connected to large roofs in Melbourne, total construction costs can be recovered within 15 to 21 years depending on the tank size, climatic conditions and future water price increase rate. Tam et al. (2009) investigated the cost effectiveness of rainwater harvesting systems in residential areas around Australia and found that these systems can offer notable financial benefits for Brisbane, Sydney and the Gold Coast due to the relatively higher

Notable research has been conducted on the relationship between rainwater tank sizing and water savings. Khastagir and Jayasuriya (2009) used water demand and roof area to develop a set of dimensionless number curves to obtain the optimum rainwater tank size for a group

Ataur Rahman, Joseph Dbais, Sk Mazharul Islam,

Erhan Eroksuz and Khaled Haddad

*University of Western Sydney,* 

