vided underground at the entrance to the caverns, and the sediment pumped to the existing treatment works. Controlled outflow from the surface storage would also be directed to the existing major treatment works.

The total volumes of the proposed multi-purpose storage is 35,000 acre-feet below ground and 45,000 acre-feet above ground, or a total in the system of 80,000 acre-feet, of which 20,000 acre-feet was considered to be normally needed for power development, leaving 60,000 acre feet normally available for pollution and flood control.

The tunnel system to deliver the combined sewer spillage to the storage and power development site or sites would be generally of the same pattern as for the Underflow Plan, with an interconnecting tunnel through Chicago's southside connecting the Mainstream and DesPlaines Tunnel System to the Calument Tunnel System.

Two locations for storage and power development are presented: one near the Calumet Treatment Works, and one near the West-Southwest Treatment Works. Chicago drainage plan (Illinois Division of Waterways)

This plan presented in a preliminary report in November, 1968, combines navigation, flood control and pollution control in the areas tributary to the Illinois Waterway upstream from Brandon Dam.

For flood control in the Lockport-Joliet Area, as well as for improved navigation, it is proposed to remove the Brandon Road Dam and Locks, and the existing Lockport Dam, Lock, and Controlling Works; to build new twin locks, dam and controlling works about two miles upstream from the existing Lockport Dam; and to deepen and widen the channel from Brandon Road to the new Lockport Locks, so as to lower water levels in this reach, about 34 feet below present water levels.

Upstream from the new Lockport Dam, the Sanitary and Ship Canal would be widened to 325 feet, with 150 feet of this width to be deepened 10 feet. The Lockport Dam would be designed to maintain dry weather water levels above Lockport, 10 feet lower than at present.

The widening would extend to Willow Springs Road and the 10-foot lowering of the surface water levels would extend along the Calumet-Sag Channel and Little Calumet River to the O'Brien Lock and Dam and along the Sanitary and Ship Canal to Throop Street. A new dam and lock would be built at Throop Street with a 10-foot differential head, and the O'Brien Lock and Control Works rebuilt to accommodate the lowered water surface.

For pollution control, the Division of Waterways proposed the installation of storm water detention and sedimentation tanks at combined sewer outlets. These would be of the flow-through type and would discharge all flows in excess of tank volumes as partially settled combined sewage into the surface waterways. Solids which settled in the tanks, together with the liquid retained at the end of each stormwater runoff period would be drained or pumped into the intercepting sewers of the Metropolitan Sanitary District. Screening and chlorination at the tank locations as well as mobile aeration of the waterways might be added to improve the pollution control.

Proposed plans

Because of the urgency of meeting the water quality standards, SWB-15, (2) established by the Illinois State Sanitary Water Board and approved by the Federal Government, studies were continued on the various solutions to the flooding and pollution problems.

In February 1968, a Technical Advisory Committee of prominent engineers in the field of sanitation and drainage was established to review the several plans advanced for solving the pollution and flood control problems of the Chicago Metropolitan Area. The Technical Advisory Committee was charged with establishing criteria by which each plan would be evaluated, and finally to make recommendations to the Flood Control Coordinating Committee on the plan or composite plan which would be suited to economically solve these problems.

Such a plan would be primarily aimed at solving the pollution of the waterway system caused by the overflow from combined sewers during rainfall periods and eliminating flooding of the waterway in time of heavy storms. The plan must provide for meeting the criteria established by the State Sanitary Water Board for each water course not only as to the quality of water but as to time of implementation. The recommended plan must be adequate to handle a recurrence of the greatest storm of record without requiring the discharge of river or canal water to Lake Michigan.

The final plan selected, although meeting the criteria established for pollution abatement and flood control, should be as broad and as multiple-purpose as pos sible. Such other areas as recreation, esthetics, navigation and power generation should be considered if economically justified.

Storage entrapment studies

Since there was considerable difference of opinion as to the effect of storage on entrapment of pollutants, a subcommittee with members selected to represent the three principal local agencies of the Technical Advisory Committee made some detailed computer studies involving the relationship between the volumes of underground storage (acre-feet) and trap efficiency (percent of total B.O.D. spillage that would be trapped in underground storage).

From this study, it appeared that the percent of entrapment varies greatly with the amount of storage until reaching between 15,000 and 20,000 acre-feet when applied to the 300 square mile combined sewer area. Above 20,000 acre-feet, the increase in entrapment is at a far slower rate per increment of volume. If capital cost per acre-foot were constant for all volumes, then to achieve a trap efficiency greater than 98 percent might appear uneconomically unwarranted. However, many other compensating factors in the cost of providing such storage volume must be carefully evaluated.

Present status of plan for pollution and flood control

The authors have generally agreed that the First Phase Construction, to be outlined in the subsequent paragraphs of this report, is compatible with the Metropolitan Sanitary District's, the City of Chicago's and the State Division of Waterways' proposed plans. This is with the complete understanding that as detailed design progresses on this First Phase, conveyance tunnel configuration, size elevation, storage volume, treatment of the overflows and locations may require some modifications to provide the most economical system. The extension of the underflow tunnel to the Des Plaines River at Lockport, lowering of the waterways for navigation and flood control, or generating hydro-electric power to offset a part of the capital cost under the Second Phase work, require further evaluation.

The elected officials of the various agencies, however, have not adopted the First Phase Construction, nor any of the three plans as of this writing. Implementation of the projects will depend on policies established and commitments made by these agencies.

In the following paragraphs, the complete Underflow-Storage Plan is presented only to provide the reader with the relationship of the First Phase to the overall plan.

Combined underflow-storage plan

A modified form of the initial Underflow Plan proposed in the 1966 report has been developed and is referred to as the Combined Underflow-Storage Plan. Since the Illinois Division of Waterways has solidified their recommendations regarding waterways improvements and requested action by the U.S. Corps of Engineers to reconstruct that portion of the Illinois Waterway through Joliet. As already stated, their recommendations include the removal of the Brandon Road Dam and Lock and the existing Lockport Lock and Controlling Works, and the construction of a new dual lock and control gates in the waterway about 2 miles upstream from the existing Lockport Lock. This will extend the water levels of the Dresden Pool upstream to the new Lockport Lock, where the low-water differential water surface will become about 74 feet. The corresponding differential, during maximum flows, has not been accurately determined but will probably be about 70 feet.

The completion of the above described work will move the possible point of low-level discharge for an underflow tunnel, or tunnels, about 10 miles upstream

from that shown in the 1966 Chicago Underflow Report. The utilization of this low-level point of discharge for peak flows becomes more attractive.

A project, already approved by Congress and awaiting funding, provides for the widening of the channel to 225 feet.

The Chicago Drainage Plan now proposed by the Illinois Division of Waterways recommends the widening of this reach to 325 feet to accommodate barge tows which currently operate on the Illinois Waterway as far upstream as Brandon Road Pool and to increase flood conveyance capacity.

The Division of Waterways Plan also recommends a 10-foot deepening of the Canal for a width of 150 feet to further increase conveyance capacity.

The combined Underflow-Storage Plan herein recommended assumes a widening to 325 feet, without any deepening.

Combining storage with conveyance

The City of Chicago presented, in September, 1968, the Composite Drainage Plan, which considered the possibility of providing mined storage areas at four locations along the main tunnel, plus one in the Calumet Area and one along the DesPlaines River Tunnel. This was proposed in order to provide temporary detention storage closer to the various origins of spillage water and thereby reduce conveyance distances, and consequently the cost of conveyance tunnels. This concept of geographical spreading of the underground storage volume is further extended in the Underflow-Storage Plan.

It is now suggested that the main tunnels be re-sized so as to serve both as conveyance tunnels and as continuous storage reservoirs. The total length of underflow-storage tunnels is about 106 miles.

It is proposed that the principal tunnels be 26 feet wide and 50 feet high and have paved inverts plus sidewall lining (in their lower portions only). The principal or mainstream tunnels from Lockport to Lake Street would be Twin Tunnels.

It is also proposed that the inverts slope to low points opposite each of the three major existing treatment works. Pumping stations are proposed at these points having a combined pumping capacity of about 2,000 cfs, which is about equal to 4 of the total ultimate dry weather average flow through the three major treatment works.

The total storage volume of the tunnels shown for construction is equal to 18,000 acre-feet, or 1.12 inches of runoff from the entire 300 square miles of combined sewer area. It would accomplish, on a long term basis, an average of more than 98.5 percent entrapment of combined sewer B.O.D. spillages.

IV-CHICAGO UNDERFLOW PLAN

In 1966, the City of Chicago proposed a sewer project which would demonstrate the principles of the Underflow Plan but, of course, on a much smaller scale. The City of Chicago's Five Year Capital Improvement Program called for the construction of a new Auxiliary Outlet Sewer System to provide relief from basement and underpass flooding of an area bounded by the North Branch of the Chicago River, Irving Park Road, Oriole Avenue and Devon Avenue. Preliminary hydraulic studies indicated that a trunk sewer in the vicinity of Wilson Avenue from the North Branch of the Chicago River to Melvina Avenue with branches extending north and south to intercept and unload existing trunk sewers would provide the necessary flood relief for a direct drainage area of 3,620 acres.

The proposed sewer system in that program was designated the EastwoodWilson Avenue Sewer System and varied in size from a 2 barrel 13-foot by 13foot section at the lower end near the river to a 7.5-foot circular section at its upper end.