An equalizer is a culvert placed to keep water on both sides of a fill at equal levels. This balances the water head on either side of an embankment and reduces seepage flow. They can also be used to equalize water bodies on either side of a road to prevent overflow and allow traffic to pass over.
Trenchless construction methods such as pipe ramming and direct pipe can be used to install equalizer culverts.
Trenchlesspedia Explains Equalizer
A culvert transfers water efficiently and provides a balance by maintaining the levels of the two water bodies that it connects. The culvert length equals the length of the slopes on either side plus the width of the roadbed. For example, if the embankment slope is 10-feet on either side, and the width of the roadbed is 20-feet, the length of the culvert should be approximately 40-feet.
Other factors such as traffic flow, height of the road shoulder above river bed etc. also need to be considered to design a culvert. The culvert should extend well beyond the road shoulder to prevent erosion from fast flowing water.