In securing these mats rubble-stone is to be used in small quantities,and in some instances the dressed slope between high and low river will have to be more or less paved with stone.
Any one who has been on the Rhine will have observed operations not unlike those to which we have just referred;and,indeed,most of the rivers of Europe flowing among their own alluvia have required similar treatment in the interest of navigation and agriculture.
The levee is the crowning work of bank revetment,although not necessarily in immediate connection.It may be set back a short distance from the revetted bank;but it is,in effect,the requisite parapet.
The flood river and the low river cannot be brought into register,and compelled to unite in the excavation of a single permanent channel,without a complete control of all the stages;and even the abnormal rise must be provided against,because this would endanger the levee,and once in force behind the works of revetment would tear them also away.
Under the general principle that the local slope of a river is the result and measure of the resistance of its bed,it is evident that a narrow and deep stream should have less slope,because it has less frictional surface in proportion to capacity;i.e.,less perimeter in proportion to area of cross section.
The ultimate effect of levees and revetments confining the floods and bringing all the stages of the river into register is to deepen the channel and let down the slope.
The first effect of the levees is to raise the surface;but this,by inducing greater velocity of flow,inevitably causes an enlargement of section,and if this enlargement is prevented from being made at the expense of the banks,the bottom must give way and the form of the waterway be so improved as to admit this flow with less rise.
The actual experience with levees upon the Mississippi River,with no attempt to hold the banks,has been favorable,and no one can doubt,upon the evidence furnished in the reports of the commission,that if the earliest levees had been accompanied by revetment of banks,and made complete,we should have to-day a river navigable at low water,and an adjacent country safe from inundation.
Of course it would be illogical to conclude that the constrained river can ever lower its flood slope so as to make levees unnecessary,but it is believed that,by this lateral constraint,the river as a conduit may be so improved in form that even those rare floods which result from the coincident rising of many tributaries will find vent without destroying levees of ordinary height.
That the actual capacity of a channel through alluvium depends upon its service during floods has been often shown,but this capacity does not include anomalous,but recurrent,floods.
It is hardly worth while to consider the projects for relieving the Mississippi River floods by creating new outlets,since these sensational propositions have commended themselves only to unthinking minds,and have no support among engineers.
Were the river bed cast-iron,a resort to openings for surplus waters might be a necessity;but as the bottom is yielding,and the best form of outlet is a single deep channel,as realizing the least ratio of perimeter to area of cross section,there could not well be a more unphilosophical method of treatment than the multiplication of avenues of escape.
In the foregoing statement the attempt has been made to condense in as limited a space as the importance of the subject would permit,the general elements of the problem,and the general features of the proposed method of improvement which has been adopted by the Mississippi River Commission.
The writer cannot help feeling that it is somewhat presumptuous on his part to attempt to present the facts relating to an enterprise which calls for the highest scientific skill;but it is a matter which interests every citizen of the United States,and is one of the methods of reconstruction which ought to be approved.
It is a war claim which implies no private gain,and no compensation except for one of the cases of destruction incident to war,which may well be repaired by the people of the whole country.
EDWARD ATKINSON.
Boston:April 14,1882.