Cascade and Multi-tiered Sortation

Complexity of Distribution Growing

A wide variety of forces are combining to reduce the average number of units shipped per order-line-item in distribution centers across all industries. The new order profiles are stressing the limits of traditional single pass sortation methods currently used in many distribution companies. Distribution bottlenecks are no longer cubic storage or picking rates, but are increasingly the number of discrete sort locations and the speed with which they can be "turned."

To meet the challenge of these increasingly complex production requirements requires a different approach to distribution center design - one that does not require the addition of new distribution centers which increase cost and exacerbate inventory management challenges.

Cascade (Multi-tiered) Sortation

Frequently, the optimum solution to this bottleneck is a cascade (also known as multi-tiered) sortation process. Cascade sortation brings product from a very large bulk pull to a "tier 1" sorter. The tier 1 sorter accumulates groupings of product that are released to "tier 2" sorter - hence the terms "cascade" or "multi-tiered" sortation.


In the above schematic example, the tier 1 sortation accumulates four hours worth of production and releases it to tier 2 sortation in thirty minute increments. This configuration assures that a pick-facing in reserve will be visited no more than twice per shift. The average replenishment frequency is 50 percent of the SKU count in any tier 1 wave. Since this increases the amount of materials consumed in each wave, the potential for full cases to be consumed in tier 1 induction is increased dramatically.

The tier 2 sorter can sort to a large number of cartons per wave. Since the sorter generates the carton contents, the adherence to the complex customer cartonization rules is systematic. A rapid "turn" rate at the tier 2 sorter is assured as all product is ready prior to initiating induction.

In short: cascade sortation breaks through the velocity barrier in the sortation process while improving productivity and accuracy. This is a perfect example of how intelligent distribution center design makes a distribution center more effective and a better solution than just building another distribution center.

Challenging Optimization

Cascade sortation based fulfillment process can deliver required throughput and accuracy. However, it requires careful analysis to assure that business processes and systems support its capability. Additionally, the optimization of a cascade sortation system poses many challenges.

The interconnections, interactions and interdependencies of each of the elements in a cascade sortation process are complex and not readily apparent. For example: if the decision was made to change the tier 1 wave from four hours to two hours it would greatly impact:

  1. The reserve picking process (productivity, % full case pulls, ...)
  2. The active replenishment process (frequency by SKU, time window available, ...)
  3. The active picking process (frequency of visiting a pick face, total travel distance, time window, ...)
  4. The tier 1 sorter size / cost
  5. The tier 1 sorter throughput
  6. The time spent ending / beginning tier 1 waves
  7. The number of accumulated bins moved to the tier 2 sorter
  8. The cost and complexity of conveyance to move from tier 1 to tier 2 sorter
  9. The number of outlets required in the tier 2 sorter
  10. The change-over time between tier 2 waves
  11. The impact on transit times between the tier 1 and tier 2 sorts
  12. Rules for jack-pot and handling known shortages on tier 2 sorter
  13. Overall systems responsiveness to new orders
  14. Systems responsiveness to change
  15. Optimum position in the process for additional services that may be required
  16. ... and other elements

The impact on some of these variables is clearly evident. However, when the impacts of several elements are combined the net effect is typically non-intuitive. When the interaction between system configuration and wave rules are evaluated for the anticipated variety of customer orders / wave profiles, a single iteration can take substantial effort to resolve. In addition to the effort, the chance for omission or error is substantial. Therefore to arrive at an optimized distribution center design with high confidence of achieving critical performance parameters is a lengthy and difficult task with a potential for serious error.

Design Tools

The forces that drive the design of cascade sortation systems are not unique to a company. However, the combined scale, scope and configuration of these forces are highly unique. You need to know rapidly ... what it will cost and what it will deliver.

Therefore, Consultleague has developed simulation tools that are tailored to answering your questions. This tool provides a means to rapidly define the optimized configuration and provide cost and performance data. Our experience addressing these powerful opportunities and challenges equip us to deliver a solution that is optimized and flexible.

The concept of cascading sortation and the interaction between the configuration of the sorters and the supporting equipment has been captured in a model. This simulation model will provide design guidance based upon your business parameters. These can be exercised against a variety of business scenarios to provide anticipated benefits and financial returns of the use of these techniques.

This tool builds on the invaluable experience gained in implementing cascade sortation processes. Projects can require investments in the tens of millions. The ability to optimize the investment and minimize risk makes the use of this tool a truly prudent business decision.

This tool can perform rapid iterations of multiple business scenarios against alternate design concepts. This tests the sensitivity of alternate design against various volume and mix variables.

The cascade sortation methodology is one component of intelligent and effective distribution center design, and is an extremely powerful way to meet the demands of today's marketplace. Getting there from here has just gotten easier.