Deforestation of Pugu Forest and 2011 floods in Dar es Salaam, are they related?

The devastating floods which took their toll in Dar es Salaam were according to some reports caused by the heaviests rains in 57 years. Inproper drainage systems and bridges were additional culprits according to the report. Exceptional high tide pushed up the water level in the Upanga area with the lower parts of the area being flooded, most dramatically in the floodplains of the Msimbazi (Jangwani).

Of interest is the role the recent deforestation of the Pugu Forest may have played in the exceptional flooding. Map 1. illustrates the location of the Msimbazi River which was responsible for the floods in Jangwani, together with the Sinza River which joins the Msimbazi a few kilometres before they flow into the Indian Ocean at the Selander bridge.

 

msimbazi

Map 1

The green boundary in the map (click to enlarge) indicates the 22km2 Pugu Forest Reserve. Inside the reserve only two third drains to the Msimbazi. Using Google Earth some 5 km2 of cleared and exposed slopes can be distinguished.  Compared to the at least 100 km2 of high density buildup settlements within the same catchment the contribution to the flooding by the 5 km2 of environmental destruction in the Pugu Forest will be minor.

As a result the impact of the deforestation has been limited compared to the impact of loss of infiltration in the high density and unplanned settlements, of the storm water (156.4 mm at the airport in 24 hours according to the TMA website which is more than 10% of the annual total!?). In addition to the limited runoff contribution of the section of the Pugu Forest to the total Msimbazi runoff - also the precipitation in Pugu never reached the claimed 156.4mm for the airport area. In other words with the limited rains at Pugu (say less than 50 mm in total during the days preceeding the floods) the major cause for the floods should be found in the unplanned settlements.

Although we would have appreciated a different outcome as rallying point for the conservation of Pugu Forest. This first assessment does not seem to suggest a major role of the deforestation in the case of Msimbazi, when compared to the impact of unplanned constructions in high density settlements which result in devastating runoff with almost all stormwater which may have exceeded the total annual runoff rushing to the sea.

Unfortunately without any measuring (gauging) network both the influence of the upstream Pugu Forest as the contributions of the various high density settlements will not be known. Avoiding similar loss of lives and damage needs to start with hydrological measurents especially during storms, to start gradually improving the catchment and absorbtion of rainwater for groundwater recharge and for planned management of the continuous growing metropolis.