Report on water spreading in Clark County, Nevada as a means of controlling floods, May 9, 1949

United States Department of Agriculture Soil Conservation Service, Research Division of Irrigation and Water Conservation REPORT ON SPREADING OF WATER IN CLARK COUNTY, NEVADA AS A MEANS OF CONTROLLING FLOODS By Dean C. Muckel, Engineer Under the immediate Supervision of A. T. Mitchelson, State Project Supervisor Prepared under the direction of George D. Clyde, Chief Division of Irrigation and Water Conservation May 9, 1949 United States Department of Agriculture Soil Conservation Service, Research Division of Irrigation and Water Conservation REPORT ON SPREADING OF WATER IN CLARK COUNTY, NEVADA AS A MEANS OF CONTROLLING FLOODS 1/ By Dean C. Muckel, Engineer At the request of J. H. Wittwer, County Agent, Clark County Nevada, an examination was made during the week of April 25, 1949, of the possibilities of controlling floods by water spreading. The place of principal concern was Meadow Valley, Wash, north of Glendale, Nevada. Other places examined were portions of California Wash, northeast of Las Vegas, and Cottonwood Wash, west of Las Vegas. Floods occurring in Meadow Valley and California washes endanger life and property in the lower Moapa Valley. Cottonwood Wish was given a more or less cursory examination in the upper reaches and where springs occurred. The conclusions reached as a result of these examinations are reported in the following paragraphs. 1/ Prepared under the supervision of A. T. Mitchelson, State Project Supervisor and under the direction of George D. Clyde, Chief of the Division of Irrigation and Water Conservation. -2- Meadow Valley Wash There is now in Meadow Valley Wash a spreading system, embracing approximately 800 acres, all on the east bank of the main stream channel. This system consists of a concrete diversion structure across the main stream channel, eight earthen dikes at right angles to the stream channel, located so as to form basins to retard the surface water movement and permit the water to soak or percolate into the soils. Excess surface water not so absorbed is directed back into the main stream channel by an earthen dike and concrete weir at the lower end of the spreading grounds. The entire system covers an area about two and one-half miles long and one-half mile wide. The point of diversion is five miles north of Moapa, Nevada. Layout of spreading system. The layout of the spreading system appears to be satisfactory and fits the topography. Water is diverted into the first basin and when it fills, water in excess of the capacity of that basin passes to the next lower basin over a low weir located at the end of the dike farthest from the stream channel. The land rises gradually away from the stream channel, and consequently the height of the dikes decreases as they extend outward. The relatively even land surface and gradual slopes permit most of the soil surface within the confines of the dikes to be flooded by relatively low dikes. -3- The arrangement at the lower end of the spreading system by which excess surface water can be turned back into the main stream is a good feature, providing a measure of protection to property downstream. Soils and percolation rate. The soils of the spreading area are valley-fill material consisting of interbedded silt, clay, sand, gravel, cobbles and occasional boulders. Without actual measurement it is practically impossible to predict a rate at which any soil will absorb water. However, considering their source and general appearance, it is believed they are fairly absorptive. In studies made by this Division on the spreading areas of southern California and in San Joaquin Valley, it has been found consistently that the percolation rate on a given soil or area is not constant. It varies continually during a spreading run and from one spreading run to the next. Some soils, of course, have higher average rates than others. The only work done on desert washes with which the writer is acquainted is that reported in Transactions of the American Geophysical Union, Part I, 1942, pages 49 to 56, inclusive, by H. M. Babcock and E. M. Cushing. This paper, entitled "Recharge to Ground-Water from Floods in a Typical Desert Wash, Pinal County, Arizona", shows that the rate of infiltration over the wetted area for different floods varied from 0.14 to 2.09 feet per day, and averaged 1.08 feet per day. It was also found that the rate of infiltration for the summer-type flash- floods was less, and showed a wider range than those for the winter- type floods. The authors attributed this to the fact that floods of the summer type are of shorter duration and, on the average, carry more silt than those of the winter type. .4- Vegetation on a spreading area tends to benefit percolation. This does not imply that it should be left in those areas where free surface movement of water is desired, such as in channels or water ways near the diversions or weirs. Any vegetation located will tend to catch debris and silt, thus materially retard the movement of water over the land surface. Underground storage capacity. For successful spreading of water, there must be ample storage capacity underground to receive any water infiltrating into the soil. At the Meadow Valley Wash spreading area, there appears to be adequate capacity for any amounts of water likely to be spread. Examination of a dug well on the eastern edge of the spreading system showed the present ground water level to be more than 50 feet below the land surface. Fifty feet was the limit of visibility and no free water could be seen within that depth. Furthermore, the predominating vegetation over the spreading area was a desert-type growth, and was additional evidence that no water table existed near the ground surface. Silt. It is known that silt is detrimental to spreading areas, As there was no water flowing at the time of his inspection trip the writer had no opportunity to observe a silty condition. Observations with several local people did, however, disclose that silt may be a very important item in attempts to spread the waters of Meadow Valley Wash. A variety of statements were received regarding the silt content of the flood waters. These estimates ranged from 60 percent silt downward. It was not clear whether the term "silt" in all cases included -5- rolling bed load or merely suspended material. Rolling bed load can be controlled by properly designed diversion structures, but suspended material, which is by far the more detrimental to a spreading area, cannot be entirely controlled. Reports were received of silt being carried at times through the irrigation ditches in lower Moapa Valley, then into each irrigation furrow, practically sealing them temporarily from the entrance of water. It was said this silt originated in Meadow Valley Wash. Some evidence of silt deposits were noted in low spots of the spreading basins of the Meadow Valley Wash system, but they did not appear to be excessive. It must be mentioned, however, that these spreading areas have not been utilized to any great extent and that no water has entered them in recent years. There are indications that the flood waters might be heavily laden with silt at times. Certainly the evidence is enough to justify sampling of the flood waters to determine the silt content. If it is found that the silt settles rapidly when the velocity of the water is retarded, the uppermost basin of the spreading system might feasibly be utilized as a settling basin from which only desilted water would pass into the remaining spreading basins. On most southern California spreading areas, water is spread for replenishment of ground water and not for flood control purposes. In order to protect the spreading areas, silty water during flood flows is not diverted. As soon as the peak is passed and the flow recedes, -6- the water tends to become clean and diversions for spreading are started. This is particularly true where the basin system of spreading is used. To the Piru Creek spreading grounds no water is diverted until it has been determined by test that the silt content has dropped below 20 cubic feet per acre-foot of water. Even this amount settles in the uppermost basins and necessitates periodical removal. Here water is spread nearly every year and for many weeks continuously. At the Santa Ana River spreading grounds, flood waters are permitted to pass until observations show that the flow has clarified. Structures. The present structures of the Meadow Valley Wash spreading system are in a state of disrepair. The small weirs between basins have deteriorated and are in need of replacement or possibly repair in some cases. One dike has failed and must be worked over before spreading can be done. All other dikes should be carefully inspected throughout their length and strengthened where necessary. Earthen dikes of this size are subject to leaks caused by rodents and and general deterioration, and require constant maintenance during spreading operations. The present diversion works should be remodeled if spreading of flood flows is considered. There now exists a concrete weir across the main channel which originally had collapsible gates by which the water level could be raised and diversion made to the spreading grounds. In case of a major flood it was intended that the gates would drop automatically and permit the flood water to pass. These gates are now in poor condition and do not appear to be usable. There is no structure at present which would provide control of water entering the spreading grounds. It is believed highly desirable that such a structure be installed. It should contain gates or other provisions whereby full control of the water entering the spreading grounds could be maintained. If diversion were attempted with the works as they now exists, the entire stream flow, particularly in flood stage, might be thrown out of its natural channel and meander through the spreading area. If a control structure is added it would be desirable to have the crest slightly higher (say six inches) than the crest of the weir across the stream channel. This would permit the rolling bed load to pass over the weir and remain in the main stream channel instead of being diverted. A sluiceway on the end of the weir where diversion is made would be a desirable feature if the diversion weir is rebuilt or remodeled. Such a sluiceway would provide a means of passing the bed load and also tend to prevent deposition of debris in front of the gates leading to the spreading area during small flows when diversion is not made. The Corps of Engineers, U. S. Army, in its "Interim Report on Survey, Flood Control, Meadow Valley Wash and Lower Muddy River, Nevada", dated November 15, 1948, cited flood conditions in Meadow Valley Wash. Recommended were dams in Pine and Mathew Canyons above Caliente. Even with these dams built, it was estimated that a 16,000 sec. foot peak discharge would occur in Meadow Valley Wash at a frequency of once in 200 years. Therefore, any diversion works installed for the Meadow -8- Valley Wash spreading grounds would be subjected to severe floods and would have to be designed accordingly. Without the Army dams a maximum peak discharge of 27,000 sec. feet is expected at a frequency of once in 200 years, 21,500 sec. feet in 100 years and 16,200 sec. feet in 50 years. Usefulness of spreading water for control of floods. Although the usual purpose of spreading grounds is to recharge ground water supplies, they are often considered?in some cases erroneously?a means of controlling floods. In occasional instances they might have some merit for flood control. In southern California, spreading system construction was at many locations sponsored as flood control projects. For small floods they functioned satisfactorily, but in a major flood such as occurred in March 1938, many failures resulted, and in general the spreading grounds were ineffective. In some instances, the failures resulted from the fact that the diversion works were inadequate for the flows that occurred. In addition, under major flood conditions the movement of a large amount of debris, consisting of floating logs, boulders, cobbles, and sand made diversion of water impossible. Assuming that adequate diversion works were installed and control of the diverted water accomplished, the effectiveness of spreading for the reduction of flood peaks may be calculated as follows for Meadow Valley Wash: -Q- Area of spreading surface = 800 acres Surface storage (assuming aver. depth of four feet) = 3,200 acre-feet Percolation rate (estimated average) two feet per day Capacity of spreading area: First day, basins filling plus percolation = 4,800 acre-feet or 2,400 sec.-feet Second day and following days = 1,600 acre-feet or 800 sec.-feet In case of a major flood the 4,800 acre-feet or 2,400 sec.-feet for the first day could not be expected to alleviate, to any great extent, the damage that would occur. If spreading were done at the peak discharge of a 16,000 sec.-foot flood, with the basins empty at the starting time, the peak would be reduced to 13,600 sec.-feet?a disastrous flood discharge. If the basins were full when the peak flow occurred, then the peak could be reduced by only 800 sec.-feet. For the smaller floods (which occur more frequently) spreading would be of greater benefit. For a flood lasting more than one day, the reducing effect of spreading would be 800 sec.-feet on the basis of an estimated average percolation rate of two feet per day. The most desirable structure or other means of controlling floods would, of course, be automatic and function whether or not attended. In the case of Meadow Valley Wash spreading system, it would not be desirable to let gates, etc. be open to flood flows at all times. Therefore, attendance would be required during spreading, and any plans should take into account attendance, maintenance, and the provision of roads to the diversion and other structures, to which there should be access at all times. -10- California Wash California Wash drains a large area of desert lands of relatively low elevation. It is reported that damaging floods have occurred in lower Moapa Valley as a result of run off from this wash. It appeared that water could be spread at many places, but the effect of infiltration on floods might be of small consequence with what might be accomplished by surface storage. For example, if the run-off is of short duration (and most of the precipitation occurs here in brief, violent storms with a high percentage of run-off) the actual amount of water percolating into the soil during the storm would be small. Assuming a percolation rate of four acre-feet per day; then in four hours, 0.67 of a foot in depth would percolate into the soil. Therefore, adequate surface storage would have to be provided to store the flood flows temporarily. This surface storage could be obtained by construction of dams in the main wash or by numerous small dams in secondary tributary channels laid out in the form of spreading basins. Dissipation of water retarded by these dams could be by percolation into the soil, either in the reservoir area itself or by release into the main wash at a properly controlled rate. In view of the fact that the flood discharges can be estimated only approximately, the amount of water percolating into the soil during the flood flow would be far less than the permissible error such an estimation, and adequate storage on the surface would be required for complete control. Infiltration would be of value in emptying the surface-storage reservoir or basins. -11- Cottonwood Wash In the vicinity of the Bar Nothing Ranch and Oliver Ranch on the Cottonwood Wash drainage area, there are several springs of great value to the local ranches. The flow of some of these springs decreases during the summer. The brief examination made indicated that spreading of water in the wash and alluvial fans above the springs might benefit the flow. A spring in the canyon above the Bar Nothing Ranch is said to flow constantly throughout the year and from year to year, but the springs most needing improvement are two or three miles from steep bluffs from which torrential surface run-off occurs after storms. These flows are normally of short duration and run off rapidly in the main stream channel. It appeared that spreading basins located upstream from the springs would serve to retard the flash flows and allow time for the water to infiltrate into the soil. It is believed that any water thus put underground would eventually show up as spring water. Whether or not such a plan is feasible depends largely on the frequencies of the storms and the amount of run-off. As far as is known no actual records exist, but a variety of statements were received from local inhabitants regarding the frequencies and amounts of run-off. If one or two storms each year resulted in substantial run-off in the main channel of the wash, it is believed that spreading to increase the spring flow would be well-worth trying. No detailed examinations were made of the valley area to locate definite locations for spreading areas, but judging from the appearance of the general terrain, spreading could -12- be accomplished in the areas of catchment of the springs. As stated before, the success of such a venture would depend principally on the amount of water available for spreading. Water probably originates as precipitation both in winter-type and summer-type storms. The winter flows are probably largely absorbed naturally in the alluvial slopes the the mouths of canyons, but the summer flows result from brief, violent storms and produce a high percentage of run-off. Pomona, California May 9, 1949