Plaza Grading: Accessibility, Minimum Slope, and Drainage
A 90-by-140-ft plaza lies between a new building entrance and a public sidewalk. The building finished floor is elevation 100.0 and the sidewalk is at 97.0. An accessible route about 100 ft long must run from the sidewalk to the entrance. Local code requires paving to drain at a minimum of 1.5 percent and caps drainage slope at 5 percent; accessibility standards limit cross slope on an accessible route to 2 percent. The client wants a large, level area near the entrance for cafe tables roughly 30 by 40 ft. An existing 8-in storm main lies under the sidewalk with an invert at elevation 93.5, and two 12-in area drains are proposed at the low corners of the plaza.
1. Determine whether the entrance can be reached by an accessible route without a ramp, and identify the governing constraint.
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Set the geometry. Rise = 100.0 - 97.0 = 3.0 ft over about 100 ft of route, which is 3.0 percent running slope. An accessible route may run at up to 5 percent, or 1:20, without being a ramp; handrails, intermediate landings, and the other ramp requirements begin above 5 percent. No ramp is required here.
The governing constraint is cross slope, which must not exceed 2 percent, or 1:48, at any point along the route. That, not the long slope, usually controls the plaza layout and the drain locations. Details that matter: a landing at the door of at least 60 in clear with slopes no greater than 2 percent in all directions and no ponding, a route at least 44 in wide, and a door threshold no more than 1/2 in and beveled. The common error is to draw a 5 percent ramp with handrails and landings because the arithmetic feels steep; at 3 percent that is costly, unnecessary, and a maintenance liability.
2. The client wants a 30-by-40-ft level terrace for cafe tables. Resolve the conflict between 'level' and the minimum drainage slope.
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A plane the client calls level still has to drain. Minimum paving slope is usually 1.5 percent to a positive outlet; across 30 ft that is 5.4 in of fall, which no one perceives as a slope but which drains reliably and covers construction tolerance. At 1.5 percent the terrace also stays under the 2 percent cross-slope limit, so the accessible route can cross it in any direction.
Design the terrace as a single plane falling to a trench drain along the building edge or to the nearest area drain. Avoid any step: a 6-in step is not permitted on an accessible route, and a beveled edge only works up to 1/2 in. If the client insists on truly flat, the alternative is a structural slab with a perimeter slot drain and a sealed membrane, which is expensive and still needs about 1 percent of fall.
Recommend the 1.5 percent plane and explain that tables on 1.5 percent feel level. Do not use 1.0 percent; it ponds within normal construction tolerance.
3. Check the drainage concept: are two 12-in area drains adequate, and what would you change?
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Tributary area = 90 x 140 = 12,600 sf = 0.29 ac. With a paved C of 0.9 and a 3.0 in/hr intensity, Q = 0.9 x 3.0 x 0.29 = 0.78 cfs, about 350 gpm. That is modest for two 12-in area drains on 12-in leaders, so capacity is not the problem; layout is.
Sheet flow on pavement should not travel more than roughly 100 ft before it reaches an inlet. With drains only at the two low corners, the path from the high corner is over 140 ft diagonally. Add a trench drain or a third inlet along the building edge, or reshape the plane so the high line divides the plaza into two shorter flow paths.
Verify that the leaders reach the 8-in main at invert 93.5 on positive slope; the plaza at 97.0 to 100.0 gives 3.5 to 6.5 ft of cover at the connection, enough for a 12-in line and a saddle. Finally, provide an overflow path, a scupper or channel, so a blocked drain cannot flood the entrance doors, and hold the doors at least one riser above the plaza.