Engineering problems that have been caused by the rains during the 2016 to 2017 season in Rovambira Street in Zengeza 2

Engineering problems that have been caused by the rains during the 2016 to 2017 season in Rovambira Street in Zengeza 2, a residential area in Chitungwiza.
The purpose of this report is to help the Chitungwiza Town Council to rehabilitate the sewage system and clear the drains in Rovambira street which have been blocked for over a decade.
Executive summary
Zimbabwe continues to be plagued by the threat of heavy rain and flooding in the wake of the passage of Tropical Cyclone Dineo across southeastern Africa in the week of 16 to 20 February 2017.Rain has pounded Rovambira street incessantly, resulting in flash floods which is a source of danger.Residents fear possible electrocution or that their children may drown in the huge pools of water in their backyards.People in Zengeza 2 now safe drinking water resulting to typhoid outbreak. Floods have resulted in extensive damage to infrastructure, including Rovambira street roads.Water flows from Hwata, Mazvimbakupa and Chidembo roads into this street because the drains have been blocked for the past 10 years.With no visible drainage system, residents depend on the sand bags to prevent water from flowing into their homes. Their efforts are in vain as water still finds a way into their homes although their efforts help to minimise the damage.

Wetlands control and reduce flooding because of the sponge-like characteristics that they have which allow them to absorb excess water. Due to the normal and above rainfall currently being experienced in the country, people who have built on wetlands in areas such as Budiriro 5, Monavale, Chitungwiza, Marlborough among others, are battling with flash floods on a daily basis, which has been exacerbated by the construction on wetlands. High rainfall in a little space of time causes flash floods and at times the drainage systems cannot cope. Secondly we have a lot of building taking place in areas that used to be open spaces. People are building in the wrong places without being allocated by authorities, for example houses on the opposite side of Lorraine drive (Bluffhill, Harare) and these are some of the issues we end up seeing; water getting into houses.
Stormwater conveyance systems (curbs and gutters, catch basins, storm drain pipes, culverts, ditches, detention ponds, etc.) are typically designed to convey runoff from “every-day” or minor rain events and may overflow during larger storm events. It is reasonable to expect localized flooding during larger storm events including thunderstorms, micro-bursts, etc. Maintaining a network of open ditches, culvert pipes and storm drain systems for the conveyance of stormwater, however these storm water facilities for the most part are only capable of conveying runoff which occurs as a result of the “every day” storm event. Storm events that are capable of generating large quantities of storm water runoff will overwhelm the existing storm drainage systems, resulting in flooding and in some cases flash flooding.As a property owner and a resident of Chitungwiza, you can help ensure that drainage systems are clean and functional. If you observe a plugged drainage facility, notify the responsible entity in accordance with the guidance above. Plugged drainage facilities cause runoff to overflow and may result in rerouting of runoff and minor flooding.Things you can do include:

? Visually inspect drainage facilities in your area and follow the drainage maintenance steps listed above to initiate cleaning of plugged drainage facilities. It is advisable to do this in the spring, fall and after any significant storm event.

? Install properly designed improvements such as ditches, pipes, etc. to protect your property from localized flooding.
Proper maintenance of stormwater drainage facilities is essential to provide continued operation of the stormwater conveyance system. Weeds, sediment, garbage, grass clippings and other debris can negatively impact conveyance capacity often leading to localized flooding. Maintenance includes the routine inspection and cleaning of ditches, storm drain inlets and pipes, driveway culverts, stormwater detention/retention ponds, and other drainage features. The storm water runoff needs to be taken by an effective storm water management system in order to recharge the ground water and capture the runoff for store or reuse in an aesthetically designed underground water storage tank.

Efficient use of rainfall measurements and runoff computation will help us to calculate the permeability of the soil in Zengeza.The falling head permeability test is a common laboratory testing method used to determine the permeability of fine grained soils with intermediate and low permeability such as silts and clays. This testing method can be applied to an undisturbed sample.


The falling head permeability test involves flow of water through a relatively short soil sample connected to a standpipe which provides the water head and also allows measuring the volume of water passing through the sample. The diameter of the standpipe depends on the permeability of the tested soil. The test can be carried out in a Falling Head permeability cell or in an oedometer cell.

Before starting the flow measurements, the soil sample is saturated and the standpipes are filled with de-aired water to a given level. The test then starts by allowing water to flow through the sample until the water in the standpipe reaches a given lower limit. The time required for the water in the standpipe to to drop from the upper to the lower level is recorderd. Often, the standpipe is refilled and the test is repeated for couple of times. The recorded time should be the same for each test within an allowable variation of about 10% (Head 1982) otherwise the test is failed.

On the basis of the test results, the permeability of the sample can be calculated as
K=2.3 a.L / (A.?t).Log(h_U / h_L)

in which we have

L: the height of the soil sample column
A: the sample cross section
a: the cross section of the standpipe
?t: the recorded time for the water column to flow though the sample
h_U and h_L : the upper and lower water level in the standpipe measured using the same water head reference
Permeability is the property of rocks that is an indication of the ability for fluids (gas or liquid) to flow through rocks. High permeability will allow fluids to move rapidly through rocks. Permeability is affected by the pressure in a rock.Permeabilities are more commonly in the range of tens to hundreds of millidarcies. A rock with 25% porosity and a permeability of 1 md will not yield a significant flow of water. Such “tight” rocks are usually artificially stimulated (fractured or acidized) to create permeability and yield a flow.

It is important that our drains are kept clean. Open drains must always be kept clean to reduce flooding. At the same time the city council should be consistent and collect garbage on time. The clearance of drainages is an ongoing process. It should be a daily thing that should happen throughout the year in preparation for the next rains. This is work that requires around US$3-4 million for the equipment to realign drains and redo pipework.