Non-cavity drains usually have rectangular or trapezoidal profile in cross-section, hydraulic analysis of which is developed. Trapezoidal cross-sections in non-cavity drains facilitates their higher water absorption capacity at approximately the same level of water drainage capacity in comparison to pipe draining system. The water drainage capacity of a non-cavity drain can be improved by installing a pipe drain inside a non-cavity one. However, no hydraulic analysis of such combined drain has been developed. It is assumed that motion of water in both non-cavity part of a drain and pipe one changes gradually. In non-cavity part of the drain, transient flow takes place and it is turbulent in pipe one. Laminar flow is observed in non-cavity and pipe parts of the drain only near the source and has almost no influence on pressure losses lengthwise along the drain. There are obtained calculation dependencies to determine specific afflux to every part of the drain as well as the depth (head in the pipe part of the drain) at the source of drainage and at the section of maximum depth. This was achieved by solving a system of two equations describing the flow of water in non-cavity and pipe parts of the drain.
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