Referring to Figure E 532, different corners show the different drainage patterns involved and described in the discussions that follow.
E 392.1 Corner A
This shows the drainage approaching the curb return from both directions. The grades should be arranged for drainage to go completely around the return toward either the BCR or the ECR, whichever appears to give smoother grades. In either case, the low point is located outside the curb return area. This method requires only one catch basin for drainage. If there is not enough fall around the return, the flow line is elevated at some point within the return area (usually near the MCR), creating a flow line high point within the return area. In other words, the top of curb grade is established, and the curb face is reduced by elevating the flow line. The absolute minimum curb face that should be produced in this way is 4 inches at the high point of the flow line. See Subsection E 633.4, Curb Face Heights. The drainage is then directed away from this high point toward the two sumps that have been created. The two sumps are located outside the return area, one near the BCR and the other near the ECR. The water is then handled by a storm drain installation. If no storm drain system is available within a reasonable distance from the standpoint of economy, either a cross-gutter will have to be provided or the intersection and approaches may have to be remodeled as outlined in Section E 560, Remodeling of Improved Streets.
E 392.2 Corner B
This shows the drainage approaching the BCR and being directed around the return. If the grade around the return is too flat to meet the minimum standards as previously outlined, it may be necessary to follow a design pattern similar to that shown in corner A; that is, provide a high point in the flow line in the return area and direct the water toward a sump created at or near the BCR, or remodel the intersection or the intersection approach flow line grades as outlined in Section E 560, Remodeling Improved Streets.
E 392.3 Corner C
This shows another drainage pattern with a high point usually within the curb return area. If the existing grades from the curb return area are too flat, drainage can be facilitated by elevating the flow line at some point within the curb return area.
E 392.4 Corner D
This shows drainage toward the return area from the direction of the BCR and the ECR. A cross-gutter may be provided if a local street is involved, and a storm drain system is not readily available at that corner. The water must be diverted from corner D to corner E and either picked up by some drainage structure or carried farther downstream to some other point of pickup. The flow line elevations of corner D are set so that the ECR, BCR, and MCR are higher than point Y, the flow line PI. Normally, a straight grade is provided between the BCR flow line and point Y. A straight grade is normally set between point V and point W, the flow line PI of corner E. The elevation at point X is normally set ¾ -inch higher than the elevation at point Z for a 4-foot cross-gutter. In following this procedure, it will be found that the elevation at point Y will be only about 0.01 or 0.02 of a foot below the elevation of point X. Since the gutter at this point will have practically no capacity, any appreciable amount of dry-weather flow will tend to leave the confines of the gutter and head downstream diagonally across the pavement and then back into the cross-gutter. If the grade of the street from corner D to comer E is particularly steep, the fall from point V to point Z will be very great, and when the elevation of point X is set only ¾-inch above point Z, the elevation of point Y will be higher than that of point X.
To provide more water-carrying capacity at point Y, the elevation of point Y should be lowered and/or the elevation of point X raised. This can be affected by designing a short, mild vertical curve or dip instead of the straight grade between the BCR of corner D and point V, the flow line PI and, if needed, raising the elevation of point X to a maximum of ¾-inch above point Z. Since normally a straight grade would be designed between points X and U raising the elevation of point X to ¾-inch (a nonstandard hike-up of the outer edge of cross-gutter above the cross-gutter flow line), the nonstandard hike-up would then transition to a normal hike-up at point U above point P. Since it is desirable to reach a normal hike-up as soon as possible, elevations providing normal hike-up should be designated at the centerline of the street between points S and T, or preferably at the quarterline between points Q and R.
E 392.5 Corner E
Based on the indicated drainage pattern, corner E should be designed so that the elevation at the BCR is higher than the flow line PI elevation W and the ECR flow line elevation. The MCR flow line elevation should be set at a lower elevation than the elevation at point W and higher than the ECR flow line, to aid in confining the water to a narrow stream and directing it toward the curb line.
E 392.6 Corners F and G
As previously discussed, storm drain considerations may modify the drainage pattern indicated on these comers. It may be decided that due to lack of water-carrying capacity of the street, the grades should be adjusted to carry a minimum of water by way of the cross-gutter, with most of the water being directed around the returns and down the intersecting street. On the other hand, the intersecting street may have a very small capacity, and little or no water may be permitted to drain around the return. The water diversion is accomplished by adjusting the ECR and flow line PI elevations at corners F and G so that they are higher or lower than the BCR elevations. This adjustment controls the flow around the return and from the flow line PI’s.
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