volume and the storage volume. Ponded volume is based on the flow rate and 

 the required retention time. Storage volume is based on the solids concentra- 

 tion entering the containment area, the depth of settled solids, the treated 

 effluent volume, and the concentration of settled material. 



54. To have good settling, a ponded depth of at least 2 ft and a mean 

 retention time of at least 60 min are required. To account for hydraulic 

 inefficiencies, a theoretical retention of 150 min for ponded volume is recom- 

 mended. To reduce short-circuiting, a length-to-width ratio of at least 3:1 

 is required for the secondary containment area. 



55. The settling properties of flocculated dredged material resulting 

 from chemical clarifications are very fluid. The settled material is expected 

 to accumulate near the culvert inlet. However, studies have shown that the 

 kinetic energy of the inflow is capable of keeping the inlet clear of mate- 

 rial. Studies also have shown that the average concentrations of settled 

 material at the interface between the supernatant and settled layer are 50 g 

 per litre, and the concentrations increase with increasing depth at a rate of 

 25 g/£/ft. Therefore, deeper basins store more material in a given volume due 

 to compaction (USACE 1987). 



56. Using the following project information, an example secondary con- 

 tainment area size may be designed: 



a. Project information. 



Volume of sediment to be dredged 64,000 cu yd 



Primary effluent solids concentration 162 mg/£ 

 Secondary effluent solids concentration 81 mg/£ 



o 



Volume to be treated 1.47 * 10 I 



Depth available for storage 4 ft 



and ponding 



Average flow rate 3 cfs 



b. Volume of settled treated material (assuming a storage depth 

 of 1 ft). 



o 



Mass of settled material = (0.162 - 0.081) g/A x 1.47 * 10 I 



= 1.19 x 10 7 g 



Average concentration of settled material 

 = [(2 x 50 g/A) + (25 g/l/ft x 1 ft)]/2 

 = 62.5 g/£ 



37 



