Why is Kumalu Creek experiencing high mudflow and sediment deposition?

Introduction

Kumalu Creek is famously known in Morobe Province and for those of whom have traveled the Wau/Bulolo Highway to Wau, Bulolo and further to Menyama and Aseki from Lae may have experienced this section of the road at some occasions. It is famous because in 2002 after a heavy rainfall the small creek surprisingly exploded into a major stream transporting mountain of mud full of debris and sediment burying the entire Mumeng LLG Station.

Since this incident, the creek has been consistently supplying sediments from erosion and landslides upstream and covering the entire village and road section downstream of the creek towards Snake River confluence. The Papua New Guinea Government through Department of Works have been tirelessly budgeting and spending huge junk of funds trying its best to keep the Wau/Bulolo National Highway open by stationing road construction machines at the site to clear the debris for the public to cross the creek safely. However, this exercise has to come to no avail as the creek is naturally in the process of supplying more material downstream as long as weathering process continues upstream. In fact there is a lot of claims by contractors still awaiting payments which amounts to millions.

The Reason Why: Weathering Process

Sediment deposition is a process of weathering in which land erosion or slides happens upstream and is being transported by the energy of the flowing stream downstream. Deposition occurs when the slope of the riverbed upstream is lower than the downstream which influences the change in flow velocity of the stream causing it to pile up sediment as it flows downstream. As can be seen in the Google Earth image of the creek upstream, the creek is experiencing frequent major landslides that is able to constantly supply high volume of sediment downstream. Sediment Transport happens depending on the demand and supply process. As long as there is high demand downstream, the supply upstream will continue to flow until the creek bed reaches its equilibrium state at which the slope is 0%.

Recommendation

At present, the natural landscape or topography and geology encourages this natural process of weathering to continue the cycle given the river valley downstream with more fill space and weak soil stratum upstream that is easily erodible by pore water pressure and high rainfall. To cut the long engineering explanation short and in order to provide a long term solution we need to undertake Soil Erosion and Landslide prevention works at the peak and surrounding foothills of the creek. Undertake soil reinforcement works, plant vegetation with long root systems and conserve the area from any gardening or habitation and man-made grassland fires which are common in the area. Moreover, construct sediment prevention dams (Sabo Dams) upstream of the crossing and bank revetment works along the creek channel. Finally, we can construct a cross drainage structure that is suitable for the site, as it is now, any new structure constructed on site will be of waste of money and effort.