TO HELP CONTROL FLOODS

GEORGE R. PHILLIPS, BERNARD FRANK.

Many of our serious water problems have their roots in the misuse of land. The same human activities that aggravate water-shortage difficulties also contribute to uncontrolled water surpluses and all the misery and destruction they bring in their wake. Basically, the flood-control program of the Department of Agriculture aims at more than the repair of damaged watersheds. Even more important, it seeks to help those who now hold the land in trust to pass it on unimpaired so that the national health and strength will be maintained.

Large acreages of our finest bottom lands lie increasingly exposed to the threat of recurrent floods. Many farms are ruined beyond repair by the relentless cutting away of fertile fields that border on streams. The safety and productivity of the extensive industrial, community, water-supply, and other developments are seriously endangered by the murky flows that so often originate on the improperly handled crop, forest, or range lands.

Much more damage is caused annually on the average by the more frequent floods on the smaller tributaries than by the large, spectacular floods on the main streams. Flood and sedimentation damages alone now amount to well over 300 million dollars each year throughout the United States. More than 100 million dollars in losses occur on the Mississippi River system alone damages that are mostly above and beyond the growing losses in the storage capacities of reservoirs due to filling with material carried down from eroding watersheds.

Progress has been made in the building of works on our major waterways to reduce the flood losses. Comparable progress will have to be made in treating watersheds to reduce the greater aggregate damages we find on the smaller streams and to slow down the rate at which sediment is ruining reservoirs.

Engineering developments cannot by themselves overcome the problems of floods, because they operate only after the floodwaters have concentrated in the main channels.

We must begin where the floods begin. We must retard the runoff and reduce or prevent the loss of soil from the watershed lands themselves, before they have a chance to build up to destructive potentials in the channels.

Floods are caused by the excess rain water that is not absorbed and temporarily stored in the soil or contained within defined watercourses. The damage caused by these overflows and by the sediment they carry and deposit may occur in fields adjacent to upland watercourses, in bottom lands along small tributary streams, and in the flood plains of great rivers.

Programs to improve watersheds are designed to work with nature by retarding runoff and reducing soil losses. That is accomplished by managing the soil and protective plant cover and by stabilizing gullies, watercourses, and stream banks to help control the movement of water from the time it falls as rain or snow until it enters the rivers and waterways. Such an approach builds and conserves the soil so that it will function to best advantage as a natural reservoir and at the same time become more productive.

A highly important, lasting effect of stabilizing and improving the soil and vegetation on a watershed is the gradual restoration by natural processes of a better balance between stream-channel conditions and the runoff they carry. The amount, rates, and quality of stream flow reflect the characteristics of a watershed during any given period, so any betterment in these characteristics in time brings a corresponding favorable adjustment in the habits of the stream.

Certain intense or prolonged rains produce so much water that even watersheds in the best of condition can modify the occurrence of high flood flows only to a limited extent. Proper watershed measures carefully installed and maintained can lower the frequency of floods, especially the smaller ones, greatly lessen the chances of occurrence of swift flash floods in the smaller valleys, and materially reduce the silt load that adds to both the flood volume and the amount of damage.

The natural processes whereby the soil and plant cover of a watershed operate to reduce flood flows and sedimentation are described earlier in this chapter. Let us consider briefly here the role of the land in the operation of the phase of the hydrologic cycle that pertains to flood discharges.

Watershed lands influence flood flows and sedimentation by the manner in which they dispose of rain and snow melt. Flood runoff from the land occurs when rain falls or snow melts faster than the soil can absorb it. The ability of the soil to take in and hold back water is affected in turn by the kind and condition of the vegetative cover, as well as by the structure and depth of the soil. Surface runoff is the most destructive. It is a highly important factor in sheet and gully erosion and in the rapid formation of flood peaks.

Rapid subsurface discharge from pervious shallow soils or soils with an impervious layer near the surface can also contribute to flood flows. Because the storage capacity of such soils is limited, it is soon used up, and any additional water entering the soil quickly drains off into channels. Subsurface discharge, however, is less destructive than surface runoff, in that it seldom causes erosion.

The basic principle involved in watershed flood control is to increase the ability of the soil to absorb water and temporarily keep it from entering stream channels and to control the runoff movement of water so that it causes a minimum of damage.

That is accomplished (1) by restoring and retaining a good cover of vegetation and litter to protect the soil against compaction and erosion and to increase its intake and storage capacity and (2) by stabilizing gullies, waterways, and tributary stream channels. During the growing season, vegetation, especially heavy forest growth, can also increase the storage capacity of the soil by rapid and heavy transpiration.

THE FLOOD CONTROL ACT of 1936 provides the basis for current Federal flood-control activities. It is an outgrowth of many years of an increasing realization of the importance of watersheds in control of floods and sedimentation. It reflects a growing awareness of the important interrelations between the condition of the watershed soils and vegetation and the rates of runoff.

The Flood Control Act is a historic marker in the growth of Federal legislation. It recognizes that destructive floods constitute a menace to the national welfare. It declares that flood-control investigations and improvements of rivers and other waterways and their watersheds are of general concern. It fosters local responsibility by providing that the Federal Government participate with States and their political subdivisions in the improvement of the navigable waters and their watersheds.

The act provides that Federal investigations of watersheds and measures for runoff and water-flow retardation and soil-erosion prevention on watersheds shall be undertaken under the direction of the Department of Agriculture, and that Federal investigations and improvements of rivers and other waterways for flood control and allied purposes shall be under the direction of the Chief of Engineers, Department of the Army. The two Departments work very closely together to handle the Federal responsibilities for flood control. The act also provides for appropriate correlation of such activities with investigations and river improvements incidental to reclamation projects of the Bureau of Reclamation.

WATERSHED FLOOD-CONTROL activities are of two principal kinds : Investigations (preliminary examinations and surveys) of authorized watersheds, and installation of the watershed programs set forth in survey reports approved by Congress.

Preliminary examination reports contain such information as is necessary to determine whether watershed-treatment programs appear to be justified and whether surveys should be made. They are prepared largely from available data and generalized field examinations.

Survey reports describe the watersheds, their condition, flood history and flood damages, and outline remedial watershed programs and their estimated costs and benefits. They are prepared on the basis of field studies.

Both investigations and operations are conducted on a watershed or sub-watershed basis. This concept is consistently followed from the initiation of the preliminary examination to the completion of the work program. The surveys develop over-all estimates of the types and quantities of remedial measures and works, their anticipated costs, and their physical and economic benefits. They do not include plans for the location or designs for the construction of specific works at specific sites. Such detailed location plans and construction designs are prepared as a part of the flood-control work plans, the first step in carrying out authorized operations on the ground.

The proposed watershed-improvement programs are recommended to Congress for flood-control operations, where the estimated total benefits to all interests concerned exceed the estimated total costs.

In developing these programs, consideration is also given to the measures and practices that will help conserve water for beneficial use, reduce pollution, and benefit fish and wildlife.

The survey reports include recommendations for public lands as well as for privately owned lands. Authorized work is initiated on the basis of agreements between the Department of Agriculture and other public agencies concerned.

In general, measures and practices installed on the lands not owned by the Federal Government are maintained by local interests. However, the Department of Agriculture has a responsibility in the public interest to see that the installed improvements on such lands are properly maintained. In the case of any major works, special consideration is given to Federal responsibility for their maintenance or for Federal supervision of their maintenance by other than a Federal agency. Survey reports indicate the proposed maintenance responsibility for the recommended types of remedial measures, together with the cost-allocation estimates.

As a means of testing effectiveness of the watershed programs and improving knowledge of watershed functioning, survey reports often provide for measuring the effects of the work upon flood flows, sedimentation, soil deterioration, and other factors in sample subwatersheds.

WATERSHED PROGRAMS consist of such measures as the improvement of existing vegetative cover, the establishment of trees or other vegetation on denuded areas, and the protection of forests and grasslands from fire. They include the adoption of the best practices for the management of livestock and big game and of logging and forest-management practices that will maintain a good ground cover. They foster the proper use of close-growing and cover crops on tilled lands and such changes in land use and plant cover as will increase water absorption and retention in the soil and improve the soil storage capacity and crop production.