Flood Mapping: Case of Narok County, Kenya

Floods, described as the overflow of water on a particular region that is normally dry due to heavy  rainfall or other weather related events by Doswell III. 94% of all hazard related economic  damages are caused by floods, thus providing an incentive to look into it.

Narok county, specifically Narok town is a town that is located within an escarpment with a  permanent river (Enkare Narok River) that runs through it, which has two temporary  distributaries that have their mouth at the Enkare Narok River. The town’s vulnerability is such  that in a period of 5 years (2010-2015) floods have led to the death of 88 people, displaced 62  families, caused 29 serious injuries and has led to 3 missing people. The town has also suffered  economic losses as the floods render the highway impassable, which has led to the decline in  international tourists visiting the Mara and Hell’s gate.

This study’s objective was:

  1. To determine the major cause of flooding in Narok town,
  2. To map out the major flood hot spots in Narok town, and;
  3. To estimate the effects of floods in Narok

The method used to carry out the proposed objectives was through the use of hydraulic  simulation that would lead to the creation of a 1-Dimensional model of the Enkare Narok river.  This hydraulic simulation would require three main data inputs; the region’s Digital Elevation  Model, the river discharge rate and the river’s cross-section. These inputs lead to the creation of  a 100-year inundation model, where a 100-year model indicates that the region receives  moderate amount of floods as they are often prevalent in the March April May season and the October November December seasons.

Upon using the hydraulic simulation, the cause of floods is determined to be the Enkare Narok  River. This is shown as during the rainy season the two seasonal distributaries of the Enkare Narko  fill up and consequentially lead to the Enkare filling up and due to the lap-sided nature of the  river observed from the river cross-section, the river bursts out of the bank and thus leads to the  flooding of Narok town. Mapping of the hotspots was done through the created 100-year  inundation model that created a flood map over the digital elevation model through the use of  the river cross section and the river discharge rate. The map displayed that the main hotspots  are the main town area and the highway, this consequentially showed the flood effects.

Hydraulic simulation using HEC-RAS leads to production of potential flood zones which normal  flood prediction models using rainfall will be unable to do. The simulations provide the depth of  the water to be expected in a given flood zone, which can act as an early warning system for the area.

By Alvin Igobwa