On-line storage vs. off-line storage

Design of storage pond is commonly divided into off -line storage and on-line storage. The on-line storage concept includes inclusion of storage facilities in series with the pipelines so that overflow at the storage facilities is allowed. One general application of on-line storage is to improve a large size of drainage pipes. Though for heavy rainfall situation, spare capacity of drainage pipes would be rapidly exhausted. Conversely off-line storage (for example underground storage tank) denotes to storage facilities in parallel with pipeline and return flow to the main pipeline is only allowed when outflow pipelines aren't surcharged.