I have no idea where you obtained the technical specs for constructing this feature, but they are entirely incorrect. The feature built as you described is guaranteed to fail. Your dimensions are fine, only a reservoir to allow for free flowing water to supply the pump is a must. Your set-up as described will rapidly clog with wind blown debris resulting in pump failure and a very labor and time intensive cleaning task. Matrix blocks, available under different brand names, are the usual choice to create the open, free flowing void under a surface layer of gravel along with a pump vault that will allow for ease of access for pump maintenance and for flushing the reservoir when required.