Engineers widely use plastic chambers for retention and/or infiltration of stormwater.  These systems are comprised of rows of open bottom arch-shaped chambers in a stone backfill.  Stormwater is stored in the chamber volume as well as the void space in the surrounding stone.  Manifolds for chamber systems typically consist of 12" to 24" HDPE corrugated pipe elbows and tees.  The chamber has an open bottom so water can pass freely from one row of chambers to the next.  One might ask, "Why do I need an inlet or outlet manifold if the water flows passively through the stone?".  

The inlet manifold serves two primary functions.  The first is to control the flow of stormwater into the system so that it flows quickly into multiple rows of chambers with minimal resistance.  This could be imperative based on the flow rate of the incoming stormwater to prevent any type of backwater condition.  The inlet manifold also minimizes the stormwater's velocity as it enters each chamber row, thus minimizing the chance that the stone bedding under the chamber is scoured, creating ruts or channels.  The second function of the inlet manifold is to provide easy access for inspection of the chamber rows.  Typically, 4" inspection ports in the top of the chamber provide a degree of inspection access, but it is limited.  If the chamber row is long, say 100 feet, then using these 4" inspection ports can be challenging to get a good look.  Using an inlet manifold allows access to larger inspection equipment, such as remote cameras, to help in visual inspection of the condition of each chamber row.  If you do not have a manifold and the inspection ports are insufficient, you may have to dig up the chambers for visual inspection if a structural problem is suspected.

The outlet manifold can serve the same maintenance access and flow control purposes on the downstream side of the system.  

One last thing to consider is the added storage capacity of a manifold.  Inlet/Outlet manifolds might provide that extra bit of storage you need on a project where a small footprint is crucial.

So remember, in chamber systems, even though the stormwater flows through the stone between chamber rows, there are still is a function for inlet and outlet manifolds.