This not a direct answer to your question but if you've not come across it before this document entitled "Guidelines for spate irrigation" produced by the Food and Agriculture Organization in 2010 may help answer your question.  It is very comprehensive.  You can download it freely from here: http://www.fao.org/docrep/012/i1680e/i1680e.pdf

Thank you Dr Lumbroso. This is helpful. However, the challenge we have been facing is that, given the strong elevation differences between the source (the mountains) and the potential command area (dry lowlands), flood comes with high velocity in a very short periods of time creating gullies and washing away rangelands. Are there experiences elsewhere on how to regulate the speed, minimize downstream negative effects and sustainably spill the flood to farms and rangelands for production use?

Dear Dr Amede

With respect to spate irrigation managing sediment is important.  This references on the  subject may be useful:

http://www.sswm.info/sites/default/files/reference_attachments/EMBAYE%202009%20Analysis%20of%20Spate%20Irrigation%20Sedimentation.pdf

There is a design manual for spate irrigation diversions here:

http://spate-irrigation.org/wp-content/uploads/2014/04/Quick-Start-Design-Manual-for-Spate-Works.pdf

There are a variety of other guidelines here:

http://spate-irrigation.org/guidelines/

This report on smart water harvesting may also be of help although it is quite simple:

http://www.sswm.info/sites/default/files/reference_attachments/NWP%202007%20Smart%20Water%20Harvesting.pdf

I hope that these are of help.

Best regards

Darren

May you please read about  " The improved traditional spate irrigation systems" in FAO 2011 Guideline. This is the best reference material currently available. 

Experience shows that the most successful spate irrigation improvement projects do not significantly alter the way spate irrigation is practiced, but combine the advantages of traditional systems with those of more permanent and less labour intensive structures (van Steenbergen et al., 2010). In most cases, low-cost, simple and maintenance-friendly technology should be used to improve existing traditional intakes (van Steenbergen et al., 2010). 

Strong integration of technical (engineering), institutional and socio--economic aspects could lead to successful spate irr.development. The users or farmers have to participate in all phases of a  spate irrigation project development from inception to management /maintenance. The engineers' role should be minimized as much as possible because the flood is unpredictable and full of risk.  

In the long-term, watershed/catchment rehabilitation can reduce speed and volume of the flood by improving infiltration. In the short term, building multiple intakes may help. In addition to engineering solutions, integrating trees into the system can help mitigate the problem. There is a recommendation from van Steenbergen et al. 2011: "It is better to allow large floods with excessive sediment load (>100,000 ppm) to by-pass upstream intakes to be diverted lower down the system, and provide additional livelihood opportunities in downstream areas in the form of rangeland and agro-forestry development. This is particularly useful in the lowlands of Ethiopia with large pastoral community where spate irrigation is yet to make in-roads. " Article Status and Potential of Spate Irrigation in Ethiopia