p17  Part III
Essential Factors for Solving the Problem

The requirements of a drainage system as above noted, necessitate a thorough examination into all subjects, which materially affect the nature of the design. Without a fair knowledge of the essential factors which govern the same, it is impracticable to foresee just what effects would be obtained by a certain design, and therefore, to know whether or not it will answer the purpose.

First of all, it was necessary to have a topographical survey of the territory, indicating the existing and projected streets in their true position, and the character of the surface as to permeability, requiring a record of the extent of area paved and covered with buildings, or of partially built‑up, and of rural and swamp area; and further, a hypsometric survey of the entire area, indicating the height of every part of the surface by horizontal contour lines drawn at every foot of elevation above Cairo datum.

The information available when the committee entered upon its duties, was to a large extent meagre, crude, and unreliable. It was deemed unsafe to base upon it the design of so important a project as the one under consideration. Therefore as a necessary preliminary step, a complete survey was recommended. The results are now available in the shape of numerous maps, indicating the surface characteristics, and of numerous profiles, indicating the sections of streets, canals, etc. Independently of the use to which these surveys are put in considering the drainage problem, they will be use also for the solution of other municipal problems, and are therefore of permanent value to the city. For a detailed description of the topographical and hypsometric work, and for a list of the plans, etc., see Appendix III.

It need here only be stated in general terms that the territory to be drained is highest along the banks of the Mississippi River, where the elevation of the ground is on the average about 33 C. D., the ground sloping from the river towards the lake for a distance of about 6,000 to 9,000 feet from the river bank, where the average elevation of the ground is 20 C. D. The surface of ground from the foot of the slope to Lake Pontchartrain is practically flat, with the exception of a slight rise called Metairie and Gentilly Ridges, running substantially parallel with the shore of the lake and about 2¼ miles distant therefrom. The average elevation between these ridges and the lake is 22 C. D. A large portion of the basin between the foot of the slope and Metairie and Gentilly Ridges is below mean gulf level. The elevations of the territory which it is proposed to drain are more fully shown on Plate II. Under existing conditions the major portion of the territory between the ridges and the lake, as also a large portion of the basin between the river and the ridges, is swampy and practically waste land, affording very unsanitary conditions, and providing in its present condition no inducement for general improvement.

The improved portion of the City of New Orleans is situated on the most elevated part of the territory.

 p18  The New Orleans Navigation Canal, and the Carondelet Navigation Canal and Bayou St. John, extend from the lake to Rampart street, at about 4,000 feet from the Mississippi River. These canals are used for navigation purposes, admitting of commerce reaching the city from Lake Pontchartrain, and are leveed on either side to prevent the water from Lake Pontchartrain inundating the low portions of the city.

The present system of drainage does not extend to the lake side of the Metairie and Gentilly Ridges, and there are large areas between the river and these ridges which are not now drained. The present works consist of deep gutters on both sides of every street leading from the river to the foot of the slope, where they are intercepted by open canals leading to the draining machines. These are four in number, and arranged to deliver from an average elevation of 15 C. D., and discharge into Lake Pontchartrain. The system is, from every point of view, wholly inadequate, and the present conditions are further objectionable for the reason that all the foul and other waste waters from the improved portion of the city naturally flow down the slope from the river bank, through the numerous street gutters to the low ground in the rear, and thence to the draining machines, through artificial canals which have not the slope necessary to convey these waters with sufficient velocity, to prevent fouling of the canals, and the formation of obnoxious gases. As the foul waters accumulate in the canals, the several draining machines discharge the same into Lake Pontchartrain, where objectionable conditions are created by the pollution of the water of the lake. For a more detailed description of the present drainage system see Appendix II, and for the area drained, as also the location of the machines, see Plate III.

Another essential factor required, is a knowledge of the quantity of rain water that falls upon the territory to be drained during the heavy storms occurring several times every year. Heretofore, the available records stated only the total amount of water that had fallen within twenty-four hours, or occasionally in shorter times, and only at one point within the city limits.

In order to have an intelligent opinion regarding the true nature and character of the storms, it was necessary for the present purpose to have a continuous record thereof, by which the rate of precipitation at every instant, and the duration of the heaviest downpours is recorded. It was further necessary to know approximately the extent of the area over which a storm of great intensity usually prevails. Six self-registering rain-gauges were therefore placed at different points, as indicated on Plate III. On April 1st, 1894, these gauges were in working order and have since then furnished very satisfactory results. A few of the records of the principal storms, and the deductions therefrom, as well as a description of the method of recording the rain-fall are given in Appendix IV. In the same appendix is also found a table giving a list of rain-falls which exceeded a rate of two inches per hour in the States of Louisiana and Mississippi since the year 1872. Further, a table is inserted giving the heaviest daily rain-falls recorded in New Orleans by the United States Department of Agriculture, Weather Bureau.

 p19  The record of the storm of August 13th, 1894, illustrates the advantage of having a number of gauging stations. The precipitation was more intense during this storm than in any other which occurred during the time these investigations were being made. The total precipitation at the Park station was 2.08 inches, at the Dublin station 2.88 inches, at the Hall station 2.96 inches, at the Jackson station 3.55 inches, at the London station 3.60 inches, and at the West station 2.76 inches. The storm struck the city from the east and traveled westerly. The Jackson and London stations are the most easterly and the Park and Dublin stations the most westerly, showing that the storm decreased in intensity as it traveled.

The records of July 4th, 1894, further illustrate the varying intensity of a storm. On this date there fell at the Park station 2.84 inches, at the Hall station 1.02 inches, at the Jackson station 1.19 inches, at the Dublin station 1.08 inches, at the West station 0.20 inches, and at the London station 0.46 inches. All the records of these instruments have been carefully plotted and the volume of water falling upon different areas calculated and tabulated in Appendix IV.

Although the rain-fall observations have so far given excellent results, and perhaps sufficient to form safe general deductions therefrom, it is yet urgently recommended that they be continued for several years. The results cannot fail to more than warrant their cost when designing the details of the drainage plan in the future.

A third essential factor required to form a judgment upon the efficiency of any design, is the amount of water and the greatest rate at which it flows off the surface and enters the drains and canals during heavy rain-falls; in other words, the actual greatest "run‑off." No system of canals and drains for this city can be proportioned to do satisfactory work, to prevent overflow, or to thoroughly drain the subsoil, without a much greater knowledge of the subject than has heretofore been applied to the purpose. This information was formerly sought to be obtained by an estimate of the water lifted by the draining machines; but the record does not furnish the element of time; it does not state whether or not the "run‑off" was uniform or whether it was great at one time and small at another, the capacity of the draining machines being insufficient to remove the water as fast as it comes and prevent the flooding of extensive areas.

Another way of estimating the "run‑off" was to use old formulae, mainly in the shape of a given fraction of the hourly or daily rain-fall. The assumptions were furthermore taken from European practice, where the rain-fall differs materially in intensity and magnitude from that of the lower Mississippi Valley.

The want of reliable information on this subject compelled your Committee to obtain it as accurately as was practicable in the short time at its disposal. Actual gaugings were made of the water discharged daily at the several draining machines. With a satisfactory record of a given rain-fall, as mentioned above, it then became practicable to throw some light on the proportion of water that runs off, the remainder being evaporated or held back on the surface or in the ground. It also became practicable to estimate from these gaugings the  p20 ordinary flow during dry weather; a necessary knowledge to proportion the cross sections of the drains and canals, as well as the pumps to do the most efficient work at ordinary times.

There is no opportunity in the city to gauge the immediate "run‑off" from large areas, because the rain water is now held back, to a large extent, in gutters and street depressions. This fact prevents it from flowing off as rapidly as will be the case after the city has been supplied with modern pavements and other improvements which are indispensable to a modern large city.

There was at least one opportunity, however, to get approximately correct information on this subject for certain conditions, and it was an important guide for your committee in their final conclusions. This was in the area drained by the Melpomene Canal. Careful gaugings of the flow in this canal have been obtained during almost every heavy rain-fall which occurred within the past year. These gaugings, together with a knowledge of the extent and intensity of the rain-fall, and of the character of the surface, its impermeability and slope, have given a valuable factor in estimating the run‑off, notwithstanding that much of the water is still held back in gutters and upon the streets before it enters the canal. Aided by information obtained elsewhere it was practicable for your committee to estimate with a reasonable degree of accuracy, the effect of this retention on the basis of the physical character of the surface in New Orleans.

It is evident that the amount of "run‑off" holds a fixed relation to various factors. It will be greater as the rain-fall intensity increases. It will be greater as the imperviousness of the surface increases, or as the territory becomes paved, and covered with buildings; in other words, we might say that it increases in a certain ratio with the density of population. The "run‑off" increases, finally, with the slope of the territory. The steeper the slope, the quicker the water will reach a given point. The water from a distant area on a steep slope is therefore likely to meet the "run‑off" from a nearer one on a flatter slope and causes the respective quantities to combine.

To get information covering the effect of these factors, all the available experience in different parts of the country was consulted and compiled. The actual gaugings made in the Melpomene Canal were given their true value, and from the total results, interpreted to the best of our ability at the present time, several diagrams were prepared. Plate VIII shows the greatest probable rate of "run‑off" in cubic feet per second from areas extending up to •1200 acres and with slopes of the territory ranging up to three feet per thousand, about the steepest existing slope in New Orleans. A set of these curves is given for four different characters of surface; the set marked A represents the "run‑off" from densely built up parts; the set marked B applies to  p21 the area having small yards, or a medium density of population; the set marked C applies to the sparsely built up parts, or those having large yards; and the set marked D applies to the rural areas. These curves, therefore, indicate the maximum rate of rain-fall which it is proposed to provide for, and which is assumed to reach the drains and canals from the respective areas.

They do not warrant the assumption, however, that the discharge will never exceed the quantities given for it; in fact, it is certain that they will be exceeded, but at such rare and indefinite intervals that their consideration is not justified. It should also be remarked that the curves are based upon the assumption, that the present practice of storing the water upon the street surface will be discontinued, and that the water enters the drains promptly, as is the case in most other cities.

In proportioning the size of the canals, it is necessary to provide for future conditions, and an estimate was therefore made as to the probable future growth of the city. Little or no change is expected in the densely populated areas, nor upon others which can only be sparsely settled for some years. On the average it was not found to be judicious to assume an increase in the quantity of water running off, of more than fifteen to twenty per cent. in excess of that which would run off at present were the street storage abolished.

It is required, further, to know the quantity of ground water which will drain from the territory at ordinary times. It consists of the natural seepage from the ground, or in other words, of the water percolating into the drains and canals, and also of those waste waters from buildings, not removed by sewers. Until these sewers are extended over all of the inhabited territory, sewage will still mingle with that ordinary drainage flow, but it will decrease in proportion to their extension.

The seepage itself, when coming from the built‑up territory, will, however, always remain slightly polluted. It will not be as offensive as it is now, but it is nevertheless wise to provide against its becoming objectionable. For this purpose it should be removed in properly lined canals, and not in earth ditches, where any suspended matter may be deposited and where stagnation may take place. By flowing over a smooth, hard surface, and also at a velocity of several feet per second, these objectionable results will be prevented. None are found to exist where similar canals with masonry channels carry away the ordinary drainage flow from a city. This subject will be further alluded to in describing the proposed sections of the canals.

In order to ascertain the quantity of water to be provided for at ordinary times, we should recognize first, that it will be reduced to below the present quantity by two causes. One, is the removal of sewage from the drains and its disposal into the Mississippi River, the other, is the gradual decrease of rain-fall penetration into the ground by a greater building up and paving of the city. On the other hand, the quantity will in the future tend to be increased, by the fact that the drains and canals will be deeper than at present, thus furnishing better conditions for infiltration from the river and from the lake. In  p22 view of these opposing tendencies it has been concluded that a provision for the ordinary flow in the future should be based upon the present quantities, as deduced from the records obtained at the several draining machines, and as given in Appendix V. They have been adjusted to the proposed new drainage sections of the city, and represent the amounts probably to be lifted at the proposed pumping stations. The resulting effect of the proposed manner of removing the ordinary flow of water, will be to materially lower the water level over all parts of the territory under consideration, including the swamp areas, and to keep the entire territory permanently dry and available for improvement.

In the opinion of your committee, the principal reason why so many projects for the solution of the drainage problem have been proposed, and again abandoned, is the fact, that insufficient information was at hand to definitely, and positively, determine its hydraulic features, and that there could be no assurance regarding the exact effects, or the results, that would ensue upon the carrying out of any of the numerous propositions.

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