By following the plans described in the preceding pages absolute flood catchments may be provided above Little Falls on the Passaic Basin for 551.7 square miles, leaving only 221.2 square miles from which flood run-off would flow immediately. The accomplishment of this would involve the construction of Pompton reservoir, which would withhold all flood waters from the northern tributaries. It would leave unprovided for 20.2 square miles on the Rockaway, 71.7 square miles on the Whippany, 46.2 square miles on the upper Passaic, and 83.7 square miles tributary to the Central Basin and not included above.

Leaving Pompton reservoir out of consideration, and conserving flood run-off on the Ramapo, Wanaque, and Pequanac rivers, there would be absolute flood catchment up to a 12-inch run-off over 494.8 square miles above Little Falls. This would leave 278.1 square miles unprovided for, the run-off from which would not overburden the channel in the lower valley, provided, of course, that channel were improved to a maximum carrying capacity.

PREFERABLE RESERVOIR SITES.

The following table and discussion of preferable sites for flood prevention are taken from the report of the engineering committee of the northern New Jersey flood commission:

Table showing detailed facts regarding possible reservoir sites on Passaic drainage basin.

Reservoir.Area of watershed. Area of reservoir. Height of dam. Length of dam. Elevation of flow line. Storage, watershed. Storage capacity. Total cost.
Sq. mi. Sq. mi. Feet. Feet. Feet.Inches.Million c.f.
Ramapo 140 2.8 65 1,700 280 5.5 1,768 $900,000
Wanaque 83 2.1 60 1,200 275 7.7 1,491 1,000,000
Newfoundland 52 1.8 40 430 780 8 966 1,800,000
Rockaway 114 4.6 28 470 520 6 1,565 600,000
Millington 56 15.8 25 220 245 31 4,060 370,000
Great Piece 773 37 21 1,500 178.5 [C]9 8,950 2,625,000
Mountain View 380 13.4 42 2,150 202 8 7,200 3,340,000
Do..... 380 13.9 44 2,380 204 9 7,900 3,460,000
Do..... 380 14.3 46 2,470 206 10 8,700 3,590,000
Do..... 380 17.4 60 3,000 220 17 15,000 5,260,000

With the exception of the Millington reservoir site where the cost of the dam is a small factor, the elevation of flow line in the various reservoirs which determines the capacity was fixed so as to afford an approximate storage equal to a run-off of about 8 inches from the drainage area above each dam site. This amount is somewhat in excess of the run-off for the flood of October, 1903. It was found impracticable on the Rockaway reservoir site to provide for a storage greater than 6 inches. On the Wanaque the amount which can be stored falls slightly under 8 inches, while on the Ramapo it is possible to obtain only 5-1/2 inches, by reason of the fact that with a greater storage capacity the slack water would reach into New York State. The economical height for a dam at the lower end of the Great Piece Meadow, if such dam is provided with fixed discharge openings which will carry a maximum outflow of 12,000 cubic feet per second, will provide a reservoir which will dispose of a run-off of 9 inches on the drainage area above.

The following combinations of reservoir sites, with their respective drainage areas, proportional storage, and estimated costs, give the facts necessary for final deductions:

Site. Drainage area. Water collected. Equivalent area retarded. Cost.
Square miles.Inches. Square miles.
Ramapo 140 5.5 96.25 $900,000
Wanaque 83 7.7 80 1,000,000
Pequanac 52 8 52 1,800,000
Rockaway 114 6 85.5 600,000
Total 389 ..... 313.75 4,300,000
Ramapo 140 5.5 96.25 900,000
Wanaque 83 7.7 80 1,000,000
Rockaway 114 6 85.5 600,000
Millington 56 31 56 370,000
Total 393 ..... 317.75 3,870,000
Great Piece 773 4.5 435 2,625,000
Mountain View 380 8 380 3,340,000