the east africa symposium on best practices in pastoralist...

The East Africa Symposium on Best Practices in Pastoralist Areas 4th- 6th March 2014, Garden hotel-Machakos

Thursday, March 06, 2014 @ 2013 SASOL Foundation 1

J. Mutinda Munguti, CEO-SASOL, +254713402186, [email protected], www.sasolfoundation.co.ke

Why Sand dams?

The global food security is at stake. The world is grappling with soaring food prices. The effect is worse in Africa especially Sub-Saharan Africa where dwindling supplies, expanding demand, and changing weather patterns have adverse implications……

Unreliability of the seasonal rainfalls in the ASALs greatly contributes to poor crop and pasture production hence contributing to cyclic food insecurity.

To buffer the unreliability of rainfall, food production hinges on water retention (recharge & reuse) and moisture conservation. This is what this entire presentation is all about- SAND DAMS; a SMART technology that enhance water retention for improved food and pasture production to end extreme hunger and poverty.


What is a sand dam? A sand storage dam (or sand dam) is a weir build on and into

the riverbed of a seasonal sand river. The functioning of a sand dam is based on sedimentation of

coarse sand upstream of the structure, by which the natural storage capacity of the riverbed aquifer is enlarged. The aquifer fills with water during the wet season, resulting from surface runoff and groundwater recharge within the catchment.

When the riverbed aquifer is full, usually within one or two large rainfall events, the river starts to flow as it does in the absence of the dam. However, the groundwater flow through the riverbed is now obstructed by the sand dam, creating additional groundwater storage for the community.


Pictorial presentation


A newly constructed sand dam filled with flash water

How to Construct Sand dams: A PRA Approach

@ 2013 SASOL Foundation Thursday, March 06, 2014 5

Needs assessment Seeks to identify and

prioritize water needs ; Assesses the possible and

sustainable water source options;

Explores the maintenance cost and community capacity;

Agree on the technology.


Site selection and Siting Site Selection

Topography-Gradient of the stream at the point of interest; River morphology-checks the alignment(>50m) and nature of bank material ; Accessibility during development & utilization time; Participatory confirmation of the site. Carrying out site measurement.

Sitting; Check: Height of the banks(Hb)-Adequate enough to accommodate river flow; Firm soil formation-To support the structure and withstand abrasion. The slope of the stream should be ≤ 2m. to provide for adequate throw-back. Existence of natural water indicators or possibility of an underground aquifer. Presence of sand deposits; Coarse sand preferred. Previous channel shift from its original position. Presence of firm basement through probing.


Construction This entails:-

Marking out of the proposed point. Excavation up to firm basement. Laying out of reinforcement. Filling of the trench with mortar/hardcore/concrete. Construction of superstructure.


Hydraulic consideration This provides for the safety and usefulness of the sand dam; Parameters of interest:

C/d cross sectional dimensions,. Q-Flow rate, Qd-Design discharge, S-Gradient ,

The dam and spillway can be designed with the above parameters at hand. Stilling basin component is designed against the falling water hammer

subject to: Qd-design discharge Gf-Gross freeboard.


Design calculations Sand dams are designed against:-

Hydraulic & Structural failure.

The hydraulic & structural design component looks into: The spillway dimensions-height and length. The height of the useful part of the dam-up to spillway The height of the gross freeboard for dam safety. The length of the dam. The sufficiency of the stilling basin/rip-rap. Forces acting on the dam.

The design discharge is computed by use of the manning formula(Q(m3/s)=AR2/3 S1/n ) from the slope, cross-sectional dimensions.

The spillway dimensioning is developed from the discharge by the discharge equation (Qmann=CLH3/2)

Structural design focuses on vertical and horizontal forces which checks against sliding and overturning.


Sand Dams as a 3R Best Practice 3R = Retention+ Recharge+ (Re)use 3R is a practical IWRM concept which looks

into :- injection of more water into the buffer Increased retention time for outflow and raised

water table and.. Recirculation of water into the system.

Sand Dams Greatly promotes: MUS (Multiple Use Systems) Natural re-generation; Ground water replenishment. Groundwater salinity amelioration.

Thursday, March 06, 2014 11 @ 2013 SASOL Foundation


MUS

Parting shot: Choice to fail or succeed!

“The significant problems we face today cannot be solved at the same level of thinking we were at when we created them”

Albert Einstein


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