Some street sewers are so small that water overflows the manholes in the street during excessive rain storms. When a drainage system is connected to such a sewer it should be provided with a tide water trap, and if any fixtures in the building are located below the level of the street, a quick-closing lever handle gate valve (Fig. 13) should also be provided to cut off the water in case the tide water valve leaks. When there are no fixtures connected to the drainage system below the level of the street, the tide water trap should be located on the street side of the main drain trap. However, when fixtures are located at lower levels than the street surface, they all should discharge into one branch of the house drain, and the tide water trap and quick-closing valve should be placed on that branch. By this arrangement of the valve and tide water trap all fixtures above the street level can be used during overflow periods without overflowing the fixtures at lower levels, as would be the case if the gate valve and tide water trap were placed in the main drain near the main drain trap. Tide water traps should not be used in house drainage systems to the exclusion of main drain traps, but in combination with them. A combination house drain and tide water trap is shown in Fig. 14. (The side of the trap is broken away to show the interior.)

Fig. 12

Fig. 13