In view of the fact that the present water consumption of the City of Rochester exceeds the available yield of the existing sources of supply as now developed, the question of dependable yield of the present sources, if fully developed, and present sources augmented in the manner proposed in this project, is of prime importance. Fortunately the determination of available yield can be made with more definiteness and certainty in this case than is usual in similar water supply projects, partly because of excellent and long-continued records of rainfall, runoff and evaporation, some of which were established by Emil Kuichling and Edwin A. Fisher in the early days of the Hemlock Lake system and which have been maintained by the City Engineer and Water Departments down to the present time. In particular, the weir record of outflow from Canadice Lake is continuous, accurate and homogeneous and covers a period of 40 years.
Gaging stations have been established to supplement existing data for determination of the yield of the Livonia and Calabogue drainage areas. Gaging stations have been maintained by the U. S. Geological Survey for several years on Cohocton River at Avoca and Campbell. These locations are farther downstream than the proposed diversion area on Cohocton River. They give the runoff from much larger areas and are not fully representative of conditions under which diversion would be made.
The Research Division of the Soil Conservation Service, U. S. Department of Agriculture, has cooperated with the City by turning over to the Consulting Engineer advanced copies of several years' records of rainfall and runoff from small drainage areas and test plats in the adjacent to the upper Cohocton drainage basin at Arnot, Hammondsport and Cohocton. These data not only save the City the expense of collecting similar data but also provide a basis for making a determination of the yield of the diversion areas on Cohocton River as soon as surveys are completed. Several years would be required to collect similar data if it had not already been done. Since the runoff from the Cohocton area may not be required for a few years it is desirable that arrangements be made to have at least the most important of these records continued and the data supplied to the City.
The extent to which natural runoff can be regulated as required for use, other things equal, depends on the volume of storage available per square mile of drainage area. For the Hemlock system it is as follows:
Drainage Storage Storage
area billion galls. mill. galls.
sq. mi. ------------------ per sq. mi.
Total : 90%
Present system
Hemlock Lake + Canadice Outlet ..... 53.8 10.1 - - 188
Canadice Lake ...................... 12.6 1.26 - - 100
Total ...................... 66.4 11.36 171
Completed project (not including storage on Calabogue and Cohocton areas)
New Hemlock Reservoir (el. 403.) .. 63.0 15.75 14.20 226
Canadice Reservoir ................ 12.6 3.83 3.45 304
Total ..................... 75.6 19.58 17.65 239
The total storage at present available in the Hemlock and Canadice Lake is 11.36 billion gallons. This assumes a draft of Canadice Lake storage of 6 feet.
The total available storage in conjunction with this project would be 19.58 billion gallons, an increase of 8.22 billion gallons, or 72%. This would be made up as follows:
Increased storage in Canadice Lake ...................... 2.57 bill. galls. Increased storage in new Hemlock Reservoir .............. 5.65 bill. galls.
The increase of storage in Hemlock Lake Reservoir would in turn be made up of two items:
Increased storage in Hemlock Lake by raising flow line to elevation 403.00 ................................ 4.12 bill. galls. Available storage, Lower Basin .......................... 1.53 bill. galls. Total ........................................... 5.65 bill. galls.
These figures do not include storage which would be provided on Calabogue Creek and Cohocton River, the amount of which has not yet been precisely determined. The storage provided would, however, serve chiefly as temporary or detention storage. Most of the runoff from these streams occurs in larger stream rises at times when there is likely to be a large natural inflow into Hemlock Lake. The detention reservoirs would permit the runoff of the tributary streams to be held back until the runoff to Hemlock Lake from the same storm has been drawn and used, thus providing impounding space in Hemlock Reservoir without waste of water.
In addition a reservoir can be constructed on lower Canadice Outlet, with a dam on excellent rock foundation, about one-fourth mile upstream from the present Canadice diversion weir. This dam would provide storage needed for the regulation of the runoff from the 5.8 square miles of area tributary to Canadice Outlet. Raising the flow line of Hemlock Reservoir to elevation 403.00 and increasing the available storage at Canadice Lake will, however, provide adequate storage for regulation of the runoff for these areas and of most of the runoff from the areas diverted into Hemlock Reservoir. Construction of the reservoir on lower Canadice Outlet will not be needed until the runoff from these additional areas is brought in and then only if required to release storage in Hemlock Reservoir and so permit increased diversion from the Calabogue and Cohocton areas.
The dependable supply of water from a given drainage area or source depends not only on the quantity of runoff or water yield of the drainage area but also on the extent to which the water can be impounded or stored for use at time when required. A dependable supply of about 600,000 gallons per day per square mile can be adequate storage from Hemlock and Canadice areas. This figure cannot be applied to the other drainage areas involved in this project, partly because of differences of rainfall, geology and soil, which affect the runoff conditions, particularly on the Cohocton area south of the divide between the Great Lakes and Susquehanna River drainage basins. Another reason why they cannot be applied to other areas is because all runoff from Hemlock and Canadice drainage basins enters the reservoirs directly, while in case of Calabogue and Cohocton areas the proportion of the natural runoff which can be diverted is largely determined by reservoir and diversion conduit capacity. Precise figures for the amount of diversion from these areas will not be available until field surveys are completed and the economic sizes of reservoirs and conduits are determined. Preliminary field examinations and subsequent studies show that at least the quantities of dependable supply given in the subjoined table can be obtained from these areas and still permit an ample supply of water to be passed downstream for domestic and agricultural uses. In dry periods no diversion would be made from these streams.