IMPROVEMENTS IN CURB-OPENING AND GRATE INLET EFFICIENCY

Draining storm water quickly and efficiently from highways is an essential part of any highway program. Laboratory experiments were conducted to develop curb-opening and grate inlet efficiency curves for the Nebraska standard inlet (single and in series), the city of Lincoln canted inlet, a new grate inlet (single and in series), and an inlet affected by resurfacing. Experiments were performed for the on-grade inlets on a full-scale roadway surface that was treated with sand-imbedded paint to produce an average Manning's n-value of 0.016. The constant longitudinal and cross slopes were 3% and 2%, respectively. Supercritical flow prevailed over the flow range of 0.5 to 5 cu ft/sec. Results show that the Nebraska standard inlet provides about 20% greater efficiency than the equivalent AASHTO-type inlet. Canted inlet performance was only marginally better than that of the Nebraska standard inlet. The new grate inlet performance was very similar to that of curb-opening inlets. Inlets in series increased efficiencies by almost 20% over the efficiencies of single inlets. Finally, roadway resurfacing that covers inlet transitions reduces efficiency by about one-half.