The first and most important part of the problem to be determined is the final disposition of the drainage water, or in other words, the outfalls. Having fixed these points, the lines of the canals collecting and delivering the water, can then be suitably arranged.
There are, in the main, three localities where it can be disposed of, namely: the Mississippi River, Lake Pontchartrain and Lake Borgne. It will not require more than the mere statement, that the river should not be the receptacle of the drainage water. The ground slopes naturally from the river levees back to the swamps and the lakes. The surface water therefore flows away from the river. To reverse its flow would require deep and large canals through the city to the river. As the latter is usually higher than the surface of the largest part of territory to be drained, all the water would have to be pumped, and during freshets the lift would be considerable.
Economical drainage outfalls are to be sought either in Lake Pontchartrain or in Lake Borgne; at present the former is used for this purpose.
It was above stated that the ordinary drainage flow and the water from slight rain storms, although considerably improved after most of the sewage has been separated therefrom, will still be somewhat polluted, and its outfall should therefore be selected so that no objectionable consequences can arise. It was also stated, that the water from heavy rainfalls will so dilute any objectionable matter that its disposal can be accomplished without reference to any but economical requirements.
The advantages of and objections to Lake Pontchartrain receiving the drainage are as follows:
It is nearer to the center of the city than Lake Borgne and the canals discharging the water will therefore be shorter and less expensive. It is at present used for an outfall, and several canals already lead to it from the city, making a continuance of this method of disposal less expensive than a different method. On the other hand, numerous pleasure resorts are located along the shore. It is therefore desirable to protect the lake water even from slight pollution. The lake is land-locked, which prevents a rapid tidal flow in and out, or any material circulation of its water. Under these circumstances, as is the case at present, a pollution becomes a serious matter. A discharge into the lake of the ordinary flow and of that from light storms should therefore be avoided. The water from heavy storms on the other hand can be discharged into it without detriment.
p24 The advantages of and objections to Lake Borgne as a receptacle for the drainage are as follows:
The borders are mostly uninhabited and a slight pollution of the water has no disadvantages. It is open to the Gulf, and the tides rise and fall more rapidly. The greater fluctuations cause a more complete dispersion and a more rapid removal of the drainage water. The mean level is several inches lower than Lake Pontchartrain. The distance from the city to the lake is somewhat greater, but Bayou Bienvenu, which by dredging can readily be made of sufficient size to convey the drainage water, runs from the lake nearly to the levees of the city and can be utilized, without detriment, to receive the drainage flow. It is, also, the natural outfall for the drainage of a large part of the city.
In our opinion, the proper place therefore to discharge the daily drainage from the City of New Orleans, and the more or less polluted water from light storms, is Lake Borgne. Although farther from the city, it possesses, as indicated, a number of material advantages over Lake Pontchartrain as a drainage receptacle, which make it decidedly preferable.
The amount of rain water which must be discharged after heavy storms is, however, so great, that it becomes impracticable to carry all of it to Lake Borgne. But as the water is then less objectionable in quality, it can be delivered with impunity at the shores of Lake Pontchartrain. Therefore, only the water from light rains and the daily flow should be taken in a canal to Lake Borgne, and relief canals at suitable points along the route should take the excessive quantities to nearer outfalls situated along the shores of Lake Pontchartrain. A discharge into the latter will then take place only occasionally, and after the first wash of heavy storms has been directed towards Lake Borgne. The character of the drainage water delivered into Bayou Bienvenu and the Lake, will by no means be sewage, nor resemble the water now observed in the drainage canals of the city, as the proposed sewerage system has a capacity sufficient to prevent the foul water of the built‑up parts of the city from reaching the drains, and for delivering all of it into the Mississippi River.
The various plans and suggestions for carrying the drainage to the outfall, which have been before the committee, were carefully compared as to efficiency and cost of construction, pumping, and general maintenance, with the result that the following general features of a plan are recommended as accomplishing the most efficient and comparatively least expensive drainage of the city, both ultimately and for the present.
In the statement of the general requirements for a drainage system, the territorial limits were designated for which it should provide improved drainage, that is adapted to present as well as future conditions, when the population has materially increased, buildings are more densely crowded and the paving of streets is more extended and impervious. The territory embraced within these limits has been divided into drainage sections as follows:
The First Section includes the area between the river and the lake, and between the upper limit of the city and a line extending along the New Canal and thence by Howard avenue, Camp street, Poeyfarre street, Magazine street and midway between Gaiennie and Calliope, between Fulton and S. Peters, and between Thalia and Melpomene streets, to the river. This section contains 8,887 acres.
The Second Section includes the territory between the First Section and the Bayou St. John, Carondelet Navigation Canal and Toulouse street. Its area is 5,384 acres, of which 1,293 are between Broad street and the river, and 4,091 between Broad street and the lake.
The Third Section lies between the lower limit of the Second Section and the line of People's avenue, Gentilly Ridge, London avenue, Broad street, St. Bernard avenue, Claiborne avenue and Elysian Fields. It includes 1,118 acres between Broad and Hope streets, Florida avenue and the river, and 3,564 acres between Broad and Hope streets, Florida avenue and the lake.
The Fourth Section comprises the territory between the lower limit of the Third Section and Gentilly Ridge, People's avenue, Florida avenue and Independence street. Its entire area, 2,153 åcres, is between Gentilly Ridge and the river.
The Fifth Section embraces the territory between the lower line of the Fourth Section and the lower line of the city, and contains 2,878 acres.
The territory on the right bank, designated as Algiers Section, comprises the territory between the upper parish line, lower limit of U. S. Reservation, Mississippi river and rear protection levee, containing 948 acres, and is treated as a separate section.
The boundaries and areas of these sections are shown on Plate IV.
p26 The reasons hereinbefore presented have led to the conclusion that it is desirable to deliver the greatest practicable volume of drainage water from the entire city through a canal connecting with the headwaters of Bayou Bienvenu and thence into Lake Borgne. Therefore, a main canal leading through the area to be drained, and to which the branch canals and drains of the several sections described are tributary, is made a fundamental feature of the plan.
The location of this "main canal" is shown on Plate IV. It extends from the intersection of Nashville avenue and Claiborne street in a direct line to the present head of Broad street at Napoleon avenue, and along Broad to Hope street at the intersection of Bruxelles street, and thence along Hope street and into and down Florida avenue. The connecting canal from Florida avenue to Bayou Bienvenu will be dredged, and certain reaches near the head of the Bayou enlarged by dredging, and cleared by the removal of sunken logs.
The contours and profile of Plate II show that the main canal is located in the lowest depression between the river and the Metairie and gentilly Ridges, and therefore, in the drainage of the city, it makes the greatest use of gravity that the topography allows.
The duty which this canal is to perform is the removal, not only of the daily flow during periods when there is little or no rainfall, but all run‑offs from storms in the several sections, which do not exceed, in any part of its length, its ultimate discharging capacity. It also serves as a basin for the collection of storm-water (to the extent of its cubic contents), during the time necessary to get the pumps started.
It is provisionally designed with a main (or high-water) channel, and a sub (or low-water) channel. The width of the high-water channel is about 70 feet, and the maximum depth at or about 5 Cairo datum, or 15 feet below the mean natural surface (20 C. D.) on the line of its location. These dimensions have been limited by considerations of convenience and safety in construction and with a view to a discharge of 3,000 cubic feet per second.
It is proposed to line the entire bed and sides with smooth and permanent material in order to insure cleanliness, and a good velocity and discharge. When crossing the navigation canals, special study will be required as to the best detailed method of so doing. But in no case will there be any practical difficulties. The water will probably be best carried across and under the same by large iron pipes, in such a manner as to avoid a depression or siphonic action, and any trouble in maintenance.
As this main canal, in its entire length and dimensions, will only be required when, to the run‑off from the city in its present condition, is added that which is estimated for the improvements and extensions of the next 50 years, and as it is not included among the earlier parts of the construction which are recommended further on in this report, for the purposes of affording immediate relief, it is wise to leave the final determination of its exact dimensions to the experience which will be acquired as the work progresses.
p27 Branch canals start from the main canal, and then diverge with constantly decreasing capacity, separating into main drains, branch drains, and surface gutters, which extend to the limits of each section, however remote, on either side of the main canal. The system is shown on Plate IV, and has, in addition to that furnished by the main canal, a large storage capacity.
The details of the plan include the re‑grading and lining of the street gutters. These will be of shorter length, of smaller size and discharge than at present, and constructed in such a manner as ultimate location and other conditions will determine. When their necessary capacity would become too large, branch drains are substituted, which have smooth and permanent sides and bottoms, and are generally to be covered. Their dimensions should be such as to admit of location under the sidewalks. Their direction and location is controlled by the contours of the ground, so as to give the greatest efficiency in collecting the drainage.
The volume of water accumulating in these branch drains is intercepted by main drains, whose general direction is at right angles to the former. This volume requires dimensions, which, in some cases, admit of a location under the sidewalk, but in others require it to be under the road‑way. In both cases, they are covered and provided with man‑holes for inspection, cleaning and repair. These main drains in turn discharge into branch canals, which lead directly into the main canal. Their construction is similar to that of the main and branch drains, but they have greater dimensions. These details of location, excepting for the surface gutters are plainly shown on Plate IV. A list of the branch canals in each section and a tabulation of their discharges is given below, while the same information concerning all the other lines of drainage is given in more detail in Table XI.
The drainage of the First Section is collected into eight Branch Canals, which are tributary to that part of the Main Canal located immediately above the New Orleans Navigation Canal and Pumping Station No. 1. Of these Branch Canals, six on the reply-side are located respectively on Claiborne, Nashville and Napoleon avenues and on Toledano, Melpomene and Venus streets; and two, on the lake-side, on Claiborne avenue and Venus street. The last mentioned is in its functions at different times either Branch or Relief canal, as explained below.
The drainage of the Second Section is collected into two Branch Canals, tributary to the main canal between the two Navigation Canals, which is served by Pumping Station No. 2, located immediately above the Carondelet Navigation Canal. Of these two branches, one is on St. Louis street on the river-side of Broad street, while the other, occupying the location of the present Toulouse Canal, drains the area on the lake-side of Broad street, and has the same alternate functions as the Venus street Canal above mentioned.
p28 The Branch Canal on St. Louis street for the drainage of this section between Broad street and the river, is well located and entirely practicable. Should the city have the opportunity to acquire, for drainage purposes, the use of that part of the Carondelet Navigation Canal, situated between its head at Basin street and the lake side of Broad street, at a reasonable cost and without prejudice to the navigation and commercial interests of the community, it could be used to advantage for these purposes, and in lieu of the St. Louis Canal. The construction would be cheaper on the line of the Navigation Canal than on St. Louis street, as a covering to restore the present St. Louis pavement would not be required. A crossing of the Navigation Canal by tunnel or siphon would also be obviated, although this feature does not present any special difficulty or great cost. On the other hand the re-establishment of commercial and navigation facilities by the construction of a basin and wharves at or above Hagan avenue and the building of streets leading thereto will be a necessary part of this alternative, and it is quite probable that the cost of these, together with that of acquiring the partial use of the Navigation Canal, and the opposition with which this, as a new proposition, might be met, would nullify any advantages the suggestion possesses.
The drainage of the Third Section is brought through four Branch Canals to the Main Canal. Those on the river-side are, respectively located on Orleans and St. Bernard avenue. Those on the lake-side enter the Main Canal from Orleans street and London avenue, and the latter also serves both as a Branch and Relief canal, as do those in the First and Second Sections already mentioned.
In the Fourth Section there are five Branch Canals tributary to the Main Canal. Three, from the river-side, enter at St. Anthony, Washington and Lafayette streets, while two, from the lake-side enter at Marigny avenue and Washington street.
In the Fifth Section are four Branch Canals. They are located on Louisa and France streets and Jourdan avenue, while one comes from the lower limits of the section.
In Algiers, there will be a pumping station on Lawrence street discharging the drainage of this section into an outfall canal, leading into Bayou Barataria.
The principal Canals leading thereto are on Lapeyrouse street, Vallette street and Canal avenue.
The proposed plan thus provides for the thorough drainage of every part of the city, within the boundaries described, which other plans do not.
It is less expensive than any of the others before us, if they were all made equally efficient and built in the same substantial manner as proposed further on. It is also less expensive in maintenance than any of the others including pumping and keeping the channels clean, if they are to accomplish the same results in drying the wet grounds and in removing the storm-water; and if the channels are equally well built.
Also, it is superior in its sanitary effects on account of the removal of the ordinary polluted drainage to Lake Borgne and the improvement of the condition of the shores of Lake Pontchartrain; also on account of the better velocities throughout the system by which any foulness of undesirable deposits are much reduced.
The run‑off to be collected into the minor canals and discharged through them into the main canal, is computed from the run‑off curves represented on Plate VIII described above in which due consideration is taken of heavy rain-falls recurring with sufficient frequency, to make their entire and immediate removal necessary, of the surface slope, of the character of the drainage areas, and of the present and prospective density of population.
These curves are based upon the results of the surveys, gaugings and observations made by the City Engineer's Department under the advice of the Advisory Board during the past two years and upon a comparison of these results with those of similar observations in other cities presenting like conditions.
p30 The amounts in the fourth and fifth columns of the foregoing table, represent the run‑off which may occur from any of the areas of the territory according to their differences in extent, slope and density of population. The discharge given for each branch canal may, at times, reach the main canal; but in any one section, the greatest run‑off from its several sub-divisions will not reach the main canal at the same time. The tributaries are of different lengths. They come from tracts with different surface slopes, extent, shape and character. One section is a narrow strip, from the river to the lake. At one end is the most densely built and completely paved part of the city, with a steep, short and direct run‑off to the main canal, while at the other is the low flat area, thence to the lake, with long leads and light grades.
Another section is roughly semi-circular, with the Branch Canals converging from several directions with approximate regularity, from comparatively uniform slopes, and areas more homogeneous in the character of their settlement than is the case in other sections.
Every section, particularly the second section, comprises extremely varied conditions of urban and suburban drainage.
These different conditions determine the relativeness of discharge from the branches into the Main Canal, and the effect thereof must be considered in estimating the volume to be dealt with at any one time, or at any one pumping station.
While every tributary to the Main Canal, or to any canal immediately above a pumping station may at sometime during a storm run completely full, it is very improbable that all of them will ever run full at the same time.
In each of the sections again, the run‑offs from the several Branch Canals reach the Main Canals at the pumping stations may, therefore, be greater in some instances than the differences of the time of their delivery into the Main Canal.
Finally, for the reasons just applied to the run‑offs within the different sections of the city, and for the entire drainage area of the city at the same time, and with equal intensity and duration, it is still more improbable that the run‑offs of the different sections will all combine so as to pass through any one of the sections of the Main Canal at the same time.
Therefore, it is certain that a reduction can be made from the sum of the volumes given in the preceding table. The conditions are so various, and admit of so many combinations, that this reduction is largely and properly a subject for the judgment and experience of the constructing engineer.
The plan now being described, contemplates the completion of every part and detail, Main and Branch Canals, Drains and Branch Drains, Pumps and Out-fall Canals, with a view of meeting the requirements of the city with the development and extensions that may p31 occur within not less than fifty years. Even were it financially practicable, it is evidently unnecessary and unwise to do a large part of the desired work for a long time to come.
As the canals and drains are intended to deliver the surface run‑off to the pumping stations, the completed pumping power will not be required until the entire completion of the drains and gutters. Under these conditions, during the time of construction and use of those parts of the plan which it may be decided to put in at present, and from time to time, there will be ample opportunity for the observation and experience required for a revision of the ultimate figures, as herein given, if this should be necessary. This course insures efficiency and economy.
The amounts given in the fourth, fifth and sixth columns of the foregoing Table are, therefore, excessive, for the purpose of estimating the required pumping power. The reductions proposed are stated in detail in a description given below of the different pumping stations.
But with any reduction, which, for these reasons, can be made from the volumes in the table, the remaining amounts are in excess of the discharge which can be obtained by the natural slope of a canal built under the existing topographical conditions.
The dimensions of a canal or drain are calculated not only from the quantity of water to be removed, but also from the velocity with which the water can flow, for, the smaller this is, the larger must be the cross-section. Velocities are determined by the slope and character of the perimeter of the channel. They are limited in this case by the available slopes and the nature of the channels.
When these are lined with masonry, the velocity can be as great as obtainable by any slope available in New Orleans, perhaps 8 feet per second. When the channels are not lined, they are subject to erosion and the velocity should not be greater than that which prevents a destruction of the bed. In earth channels, though protected somewhat by the growth of vegetation, and considering that the greatest flow and therefore velocity is not continuous, but occurs only after rainstorms, the computations have been based on a maximum velocity of 3½ feet per second. When the slope would cause a greater velocity the channel is assumed to be lined. But in unpopulated districts, where this would be unnecessarily expensive at first, it is desirable to reduce the slope temporarily by increasing the sectional area.
The least velocities of flow in the drains and canals should be greater than those usually assumed for sewers, because heavy materials, such as sand, etc., are carried along in them. The least velocity masonry lined conduits for the removal of storm-water has been assumed at three feet per second, and for the removal of the ordinary flow, to prevent deposits and gradual filling up, at six feet per second.
Pumping this ordinary flow and that of small storms in the Main Canal, will, of course, increase the velocity by increasing the slope, and therefore the discharge.
With the above assumption the slopes of the canals and drains have been obtained, and in view of the large quantities of storm-water to be dealt with, the slopes were made as great as possible.
In the larger canals it is advisable to have special sub-channels of sufficient capacity to convey the ordinary flow. This channel can then be made to deliver its charge with the best velocity and concentration of the volume, instead of spreading out over the wide section necessary for storm-water removal, thus facilitating deposit and causing an unsightly appearance.
For storm-water removal, the sections adapted for the numerous canals and drains are governed by the feasibility of making the excavation, by the necessary highest elevation of the water, and by the required capacity to discharge given amounts of water. As a rule they should be shallow and wide.
The plan proposed, and for which estimates are furnished, includes the lining of the sides and bottoms of most of the canals and drains and the covering of the smaller ones, between Broad Street and the p33 river and in the built‑up part of the city, in a durable manner and with a smooth interior surface. Eventually, nearly all of the drains and canals will require a similarly lining, but for a long time this necessity will not exist. But whenever they are permanently improved, their section should be made large enough for all time.
Wherever practicable and necessary, the drains and branch canals should be covered, so as to gain the use of the surface of the street, guard against the danger of persons and animals falling into them, and to protect them from an excessive amount of street rubbish being thrown into them.
It has been stated that covered drains and canals would become offensive. But this is not to be expected after the sewerage system has been completed and the greatest portion of foul matter removed from the drainage. In Paris the street water and house sewage, excepting most of the excrementitious matter, are carried off in covered channels with wide and untrapped openings at the gutters to receive the drainage, and without being offensive.
The special manner of covering, as well as the many difficult problems of detail that must be considered in the execution of the proposed drainage plan, in order to achieve success and at the same time be economical, should be left to the constructing engineers.
The surface of the ground along the line of the Main Canal is rally level, ranging from 19.5 to 21.5 C. D. The length of the canal is seven miles, the width is provisionally assumed at about 70 feet, and the depth is limited by the stability of the subsoil, and varies from 14.5 to 16.5 feet below the natural surface. Where it discharges into Lake Borgne, the mean tidal level is 21.26 C. D., but this may be increased three feet or more in great storms.
The Main Canal is to serve two distinct purposes. In the first place, it is intended to remove the ordinary drainage flow to the possessed outfall, in a proper manner. Owing to the great distance, the absence of fall, the fact that such ordinary flow should be given a suitable velocity, and in order to prevent sanitary objections in the future, it is absolutely necessary to lift this ordinary flow at a sufficient number of points so as to give it artificially the slope which cannot be had naturally by the topography. The plan, therefore, includes a series of main pumping stations, five in number along the Main Canal.
Secondly, the Main Canal is to carry away the water from rain-falls to such an extent, that the first wash from the surface of the streets, will as far as practicable be discharged into Bayou Bienvenu and Lake Borgne, rather than in Lake Pontchartrain. It has been considered that a rain-fall giving a run‑off of 3,000 cubic feet per second, at the lower end of the main canal, might be sufficient for the above purpose, and the canal has been provisionally designed for this quantity. For small rain-falls it will seldom be necessary to operate all the pumps, as any flow greater than the ordinary, will require a lighter slope for a given velocity, and the water can flow by gravity past several of the pumping stations without objection. In fact, there will be times after moderate rain-falls, when the water slope in the Main Canal can be made sufficient by the operation of merely the terminal pump at Jourdan Avenue, to finally deliver the run‑off beyond the drainage limits. Therefore, provision must be made at each of the pumping stations on the Main Canal, for allowing, at certain times, an unimpeded flow, and at other times, for holding the artificial head which the pumps will make.
When the capacity of the lower end of the main canal becomes insufficient to carry off the water from a storm, then it is necessary that the pumps deliver the excess into the branch relief canals, to flow to the auxiliary pumping stations, hereinafter described, and thence into Lake Pontchartrain. This surplus water will as a rule, be wholly unobjectionable in its character, and if it is discharged into Lake Pontchartrain will give no offense whatever.
Provision must therefore be made at the pumping stations for these several duties.
The pumps must handle, at certain intervals of distance, the daily flow during periods when there is little or no contribution from rain-fall, so as to give fall and velocity for its discharge into Lake Borgne. p35 The volume of this dry-weather flow from the several senates, is estimated to be as follows:
These amounts are progressively cumulative, and for their removal the daily operation of the pumps, on the line of the Main Canal, will be necessary. The lift at each pumping station will be from three to four feet, except at Pumping Station No. 5, where it is delivered into the main out‑fall canal. As before described, a sub-channel of the Main Canal, as shown in Plate IX is designed to carry this flow within its banks.
During ordinary storms, the lift for high water pumping will be about the same as that for pumping the low water flow, but the volume will of course be much greater. Further details concerning these lifts and volumes, are given below.
If the diversion of the excess of volume from the Main Canal to Lake Pontchartrain were accomplished solely by the pumping stations along the line of the Main Canal, the distance to the lake with the slope necessary to properly discharge the required volume, would demand a very great lift; and the branch relief canals being in such a case on a high level, would not admit of the easy drainage of the areas situated between the ridge and the lake. These difficulties might be removed by the establishment of additional pumping stations, either on or near the lake shore, which would divide the lift, bring the canal to a low level and admit of the drainage of the adjacent lands. But there are objections to this location of the pumps, as follows:
1st. The daily or dry-weather drainage, seeking this canal, would have to be removed from the lower end into lake Pontchartrain, and require the daily operation of these outlying pumps, as well as of those along the Main Canal, and causing the continuous pollution of the lake which it is desired to prevent in the future.
2d. The bottom of the relief canal, at its lower end, would be at a depth where the stability of the sub-soil is doubtful.
Auxiliary pumping stations centrally located between the ridge and lake could not be subject to such objections and would give more prompt and efficient drainage, and are therefore, recommended.
The plan provides for such locations at Bordeaux street, and Taylor and Monroe avenues, as shown in Plate IV. The lift is moderate and the intermediate reach between the two pumping stations, is lowered to a level which will serve for the rapid and complete drainage of the low tracts between the ridge and the lake.
In dry times, the drainage water of these lands will seek the low level reach of the branch canals, and flow by gravity through a by‑pass, provided at three of the main pumping stations, into the sub-channel of p36 the Main Canal, for a final disposal into Lake Borgne. During a higher flow in the Main Canal, these low level reaches of the branch canals serve for purposes of temporary storage, until a further rise of the water renders it necessary to start the auxiliary pumps at the other end of the reach. At no time should the water surface be allowed to rise sufficiently to flood the lands along the banks of the low level canal, or arrest its drainage.
Your committee is not prepared, at the present time, to give exact information regarding the capacity which should be given to all of the pumps to be erected at the different stations; because it depends to a large extent on the improvements that are intended to be made in the drainage system within the next few years. However, a general outline of the location and work required of each one, as forming part of the recommended plan, and approximately the volume and lift are given as follows:
The volumes assumed are smaller than those given in the table on page 29, for the reasons hereinbefore stated. Pumping stations numbers 1, 2, 3, 4 and 5, along the line of the Main Canal, are designated "Main Stations;" and numbers 6, 7 and 8, at the ends of the Branch Relief Canals, are designated "Auxiliary Stations."
Main Station No. 1 is located on the line of the Main Canal at Venus street, and disposes of the drainage of the First Section. The aggregate run‑off through the several branch canals of this section, as shown in table on page 29, during the greatest storms provided for, is 5,404 cubic feet per second. As the Branch Canals, tributary to this section of the Main Canal, converge from different directions, mainly from large areas of easy grades, and small density of population; and as they are of considerable length, and together with the Main Canal, afford a large storage capacity, for reasons assigned on page 30, a reduction of one-third may be made from the above amount. The run‑off to be pumped at this station, at any one time, is therefore assumed at 3603 cubic feet per second. But a further deduction of 865 cubic feet per second can be made, because this quantity enters the Branch Relief Canal between Pumping Stations numbers 1 and 6 from the lands of the section in of Metairie Ridge, and will be pumped at the Auxiliary Station No. 6.
The Main Station No. 1, therefore, deals with 2738 cubic feet per second, as follows: 1750 cubic feet are to be lifted from an elevation of 16 to 19 feet above C. D., and delivered into the Main Canal of the Second Section, and 988 cubic feet are to be lifted from an elevation of 16 to 20 feet above C. D., and delivered into the branch relief canal leading to the Auxiliary Station No. 6, at the intersection of Bordeaux and Upperline Canal. At the latter 865 cubic feet will be added, as mentioned above, giving a total volume of 1853 cubic feet per second to be lifted at this Station, in times of extreme rain-falls, from an elevation of 15 to 27 feet above C. D. This latter lift is not only the necessary head to effect a discharge at mean lake level, which is 21.26 feet above C. D., but also during exceptionally great storms, which may raise this level 3 or 4 feet.
p37 Following the same method of estimating, the present volumes to be provided for by pumps at the other Main Stations are as follows:
Main Station No. 2 on the Main Canal at the intersection of St. Louis street, according to the table on page 29, may receive a run‑of 2589 cubic feet per second from its two tributaries. The amount from the river-side, being 1458 cubic feet, is not materially reduced, but assumed at 1400 cubic feet, while that from the ridge is reduced from 1131 to 700 cubic feet, giving an aggregate of 2100 cubic feet per second. The run‑off from the Second Section to be provided for at Station No. 2 in great storms is therefore 1400 cubic feet per second. To this is added the volume pumped at Main Station No. 1 from the First Section, namely 1750 cubic feet.
Main Station No. 2 therefore deals with 3150 cubic feet per second, as follows: 2250 cubic feet are delivered into the Third Section of the Main Canal, with a lift from elevation 17 to 20 feet above C. D.; 900 cubic feet are delivered into the branch relief canal with a lift from elevation 17 to 19 feet above C. D., and conveyed to the Auxiliary Station No. 7, at the intersection of Taylor Avenue and Orleans Street. Here are added the 700 cubic feet assumed as being tributary to the Branch Relief Canal, giving a total volume of 1600 cubic feet to be lifted from an elevation of 14.08 to 27 feet above C. D., and flow to the lake by gravity.
Main Station No. 3, on the main canal at Help street, according to the table on page 29, receives an aggregate run-off of 1727 cubic feet per second, of which 1150 cubic feet are from the river-side. The latter is assumed at only 1000 cubic feet, to which is added 2250 cubic feet, delivered from the Second Section, making an aggregate to be lifted at Pumping Station No. 3, of 3250 cubic feet per second. Of this quantity 2000 cubic feet are delivered into the Fourth Section, with a lift from elevation 17.4 to 20 feet above C. D., and the balance, 1250 cubic feet, into the Branch Relief Canal, with the same lift. At the Auxiliary Station No. 8, located at the intersection of London and Monroe avenues, this amount together with 300 cubic feet representing the run‑off from the rear lands, or together 1550 cubic feet per second, is lifted from elevation 16.40 to 27 feet above C. D.
Main Station No. 4, at Lafayette avenue and the Main Canal, according to the foregoing table, receives a total run‑off of 1256 cubic feet per second, which could now be reasonably reduced to 1000 cubic feet. This amount is increased by 2000 cubic feet from the Section above, making 3000 cubic feet per second to be disposed of as follows: 2500 cubic feet per second are discharged into the Fifth and last section with a lift from elevation 16.5 to 20 feet above C. D., and 500 cubic feet per second into the Peoples'º Avenue Canal with a lift from elevation 16.5 to 27 feet above C. D., sufficient for the water to flow into the lake by gravity.
Main Station No. 5 at Jourdan avenue receives 2500 cubic feet per second from the Fourth Section, and about 500 cubic feet per second as the reduced run‑off of the Fifth Section. These quantities are delivered with a lift from elevation 16.68 to 28 feet above C. D., into the canal leading to Bayou Bienvenu and thence into Lake Borgne.
p38 The drainage of Algiers on the right bank of the Mississippi river, as shown on Plate IV, is accomplished by a similar system of Gutters, Branch and Main Drains, and Branch Canals leading into a Main or Intercepting Canal located on Canal avenue. From a main pumping station, located near the intersection of Lawrence street and Canal avenue, the total run‑off of the section, amounting to 671 cubic feet per second, may be reduced to 400 cubic feet per second, and will be lifted from an elevation of 18 to 25 feet above C. D., and delivered into the outfall.
For the purpose of obtaining the fullest advantage of the proposed system it must be kept pumped down, throughout all its branches and mains, to the lowest possible level. Constant vigilance and care must also be exercised to keep both the larger and smaller canals free from deposits of every kind. They will not all be entirely self-cleansing, and neither this nor any other system will be worth building, unless ample provision is made for its care and operation.
In the foregoing is approximately stated: the daily flow, which requires to be lifted, at the various Pumping Stations, in order to keep the territory dry; the run‑off or storm-water flow which will probably have to be lifted and discharged into Bayou Bienvenu, and, during excessive storms, also into Lake Pontchartrain; finally, the capacity of the pumps at the several stations.
This stated capacity will not be at present required, but, as the population increases, and the surface, upon which the rain falls, becomes more impervious, proportionately more water will run‑off into the canals and require pumping. The rate at which this quantity will increase, must be left to future observations, which, from time to time, will determine how soon and to what extent the pumping power of each station should be increased.
As already mentioned, there is a large storage capacity in the canals, which will be utilized until the storm pumps can be put into operation. The problem which will require attention in the future, is not only to find the capacity of the pumps required, to throw out the greatest quantity of water that will enter the main canal in each respective section, after it has filled up and the limit of its storage capacity is reached; but also to pump out the stored quantity itself in that space of time within which it is desired to have the canals resume their ordinary condition of flow. These questions can be readily determined after some portions of the drainage system have been put in operation. It is impracticable to answer them with sufficient accuracy at present, nor is it necessary. In the future, pumps should be added as they are needed, and sufficient land should now be acquired at each station, and the structures be designed to admit of extension.
The present draining machines, though simple in their action and requiring but ordinary intelligence to operate, are nevertheless by no means economical for the purpose of lifting the water. Since they were built, the progress in the design of pumps has been considerable; and advantage should be taken thereof in establishing the Pumping Stations for the proposed work.
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Sewerage & Water Board
of New Orleans
1895 Advisory Board Report
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