Design of a depressed sewer Design a depressed sewer, consisting of several pipes and an inlet and outlet chamber (see Fig. 5-13), to replace an existing single-pipe depressed sewer that has not performed properly because low velocities in the depressed sewer have increased sedimentation. Use ductile-iron pipes for the depressed sewers. The basic data and assumptions are as follows: 1. Length of depressed sewer = 135 m (440 ft) 2. Maximum sewer depression = 2.75 m (9.0 ft) 3. Diameter of reinforced-concrete gravity sewer to be connected by depressed sewer 750 mm (30 in) 4. Slope of existing 750-mm (30-in) sewer = 0.0022 m/m (ft/ft) 5. Minimum velocity in depressed sewer = 0.9 m/s (3.0 ft/s) 6. Design flows: (a) Minimum flow = 0.110 m/s (4 ft/s) (b) Maximum dry-weather flow = 0.255 m/s (9 ft/s) (c) Ultimate maximum flow equals the capacity of the gravity sewer 7. Available fall (invert to invert) = 1100 mm (3.6 ft) 8. Assumed head loss at inlet 120 mm (0.4 ft) 9. Available head loss for friction in depressed sewer=980 mm (3.2 ft) 10. Available hydraulic grade line (approximate) = 0.98 m/135 m = 0.0073 m/m 750mm 750 mm 375 mm Inlet chamber 500 mm 350 mm 400 mm Slide gate guide (typical) 500 mm -350 mm 500 mm 125 mm 350 mm Sectional plan 400 mm. 350 mm Section A-A 232 mm Invert elevation of 750 mm inlet 500 mm 232 mm 375 mm Section B-B Inlet chamber Figure 5-13 Definition sketch for depressed sewer. Note: mm x 0.03937 in. 750 mm Outlet chamber Section C-C Outlet chamber 750 mm A 400 mm 250 mm Invert elevation of 750 mm outlet