APPENDIX I: HYDROLOGY

PROJECT REPORT
FOR FLOOD CONTROL ON
OKABENA CREEK
WORTHINGTON, MINNESOTA
(LOCAL PROTECTION)

   1.   Climatology. The City of Worthington is in southwestern Minnesota, about ten miles north of the Iowa state line. The climate is temperate and well suited for agriculture. According to the 1930 Climatic Summary for southwestern Minnesota, the average annual snowfall amounts to 29.3 inches at Worthington, and 35.6 inches at Windom, Minnesota, which is about 30 miles to the northeast. The following tabulation lists the average annual precipitation and temperature, according to the 1952 Annual Summaries for Minnesota and Iowa, at three representative stations in the Worthington area.

 

	*Years of Record		Temperature Average Annual	Precipitation Average Annual
	Temperature	Precipitation
Worthington, Minnesota	58	61	44.4	27.10
Windom, Minnesota	26	29	45.0	25.49
Rock Rapids, Iowa	53	56	45.8	25.54
*Including 1952

 

   2.   Basin characteristics. West Okabena Lake receives the run-off from about 13.3 square miles of land drainage area, most of which is located north and west of the lake (plate 1). topography is gently rolling, but the short stream channels, notably Okabena Creek, have comparatively steep profiles and good flow capacity. Because of the gently rolling terrain, erosion and resultant downstream sedimentation do not appear to be an important problem. The city owns a dredge and since 1948 maintains a continuing lake dredging program amounting to 150,000 to 200,000 cubic yards annually with the purpose of eventually attaining a 20-foot depth in the lake. It is considered that a large part of this annual clean-out is comprised of the accumulation of years of sedimentation and that the annual deposition may not be more than 25,000 to 50,000 cubic yards.

   3.   Okabena Creek, which is the cause of the local flood problem, rises about 4 miles northwest of Worthington and drains an area of about 4.5 square miles, above the junction with Whiskey Ditch. Originally the basin contained about 6.5 or 7 square miles but within recent years drainage ditch systems in the upper portion of the basin have been revised to carry run-off westward into the Big Sioux River Basin.

   4.   No data on water loss rates during rainfall were available for the area. It appears, however, that the soil in the area is reasonably compact and cohesive, since the stream channels are not heavily eroded.

In general, the lands adjacent to stream courses are pastured and the upland areas are cultivated. Probably less than 15% of the land is timbered. It was considered that a run-off factor of 70% applied to the design rainfall pattern would be reasonably conservative.

   5.   Hydrology. The drainage area tributary to and including West Okabena Lake contains a total of 14.58 square miles. Since the sequence of inflow into the lake is an important factor in determining the maximum lake level resulting from the design flood, the drainage area was divided into six component parts designated areas A to F as shown on plate 1. Unit rainfall length was estimated as one or two hours, depending on the size of the areas, and synthetic unit graphs were computed for each basin subdivision (plate B-1 of this appendix). During the course of the study the one-hour unit graphs were changed to two-hour graphs.

   6.   The U.S. Weather Bureau co-operative station at Worthington has been operated for over sixty years. No attempt, however, was made to base design rainfalls on that record. Since data for Worthington have been included in the Yarnell rainfall frequency study (U.S. Department of Agriculture, Miscellaneous Publication No. 204), design floods were based on the Yarnell.

   7.   The primary flood problem in Worthington results from high flows originating in the Okabena Creek Basin. As shown on plate 1, discharge from the creek enters Whiskey Ditch at a point about 4,200 feet from West Okabena Lake, travels about 1,000 feet down that ditch and then, theoretically, is divided with part of the flow entering County Ditch No. 12 through a diversion dam, and the remainder continuing in Whiskey Ditch into the lake. Actually, the high flows of Okabena Creek overflow the left bank of the channel at the head of Whiskey Ditch and enter County Ditch No. 12 below the old diversion dam. Resultant high stages in County Ditch No 12 produce flow back-up in the storm sewers which discharge into that ditch and surface flooding in the newly developed residential area described in the main report.

   8.   Under the project plan, it is proposed to control flood flows from Okabena Creek by means of channel enlargement of Whiskey Ditch and by levees where necessary. About 25% of the flow would be diverted through uncontrolled openings into County Ditch No. 12 and 75% would enter the lake. Several factors were considered in establishing the flow-diversion ratio. First, the lake outlet structure built in 1939 as part of a Works Progress Administration project does not have adequate flow capacity to prevent a material rise in lake level when all the drainage area is contributing run-off under design storm conditions. Therefore, diversion of flood flows into the lake should be limited to some extent. Secondly, it was considered desirable, from the maintenance standpoint, that appreciable flow should be passed through County Ditch No. 12 occasionally. Such flows should be large enough to promote channel clean-out without causing any critical back-up effect in storm sewers. It appeared that a flow depth of 4.5 to 5 feet would fulfill this condition and, with the existing channel slopes, this depth corresponds to a computed flow of about 300 c.f.s. That discharge would amount to 25% of the Okabena Creek design flood crest, a discussion of which follows.

   9.   The criterion for design of the levee grade and channel sections along Whiskey Ditch is crest flow of the design flood. The flood based on a two-hour 100-year rainfall was computed and crest flow determined as 1,065 c.f.s. The 24-hour 100-year flow was then computed, based on rainfall patterned as shown on plate B-1, and crest flow determined as 1,200 c.f.s. Levee heights and channel sections upstream from the point of flow division were therefore based on a flow of 1,200 c.f.s. (24-hour, 100-year flow) and below the division point were based on 900 c.f.s.; see plates 2, 3, and 4.

   10. It was necessary to determine the maximum rise in lake level resulting from the design flow, and since run-off volume is the criterion, the 24-hour 100-year flow was adopted. Design floods were computed for each of the component tributary areas, and the total flood, including rainfall on the lake, was then routed through lake storage above the outlet structure (plate B-1). Maximum outflow from the lake was determined as 248 c.f.s. and maximum lake level at elevation 1,577.7, or 2.7 feet above outlet spillway crest elevation with the initial lake level at elevation 1,575. According to statements of the city officials there would be no damage along the lake shore at that stage unless strong winds occurred coincidentally with the rise in lake level. However, at present, flashboards one foot high maintain the lake level to about elevation 1,576 and the design storm occurrence would raise the lake level to about elevation 1,578.5. When informed of this fact local officials stated they would raise low-lying areas around the lake (spoil from lake dredging described in paragraph 2.) The lake storage volume above elevation 1,575 was estimated. The storage curve, lake outlet rating curve, diversion outlet rating curve, and several typical channel rating curves are shown on plate B-1.

   11. Standard project flood. According to the data contained in Hydrometeorological Section Report No. 23, the maximum possible 24-hour rainfall over 10 square miles in the Worthington area would be 24.4 inches. This amount was considered applicable to the 14.58 square miles of drainage area concerned in the project. Fifty percent of that rainfall, or 12.2 inches, was used as the standard project rainfall and patterned as discussed previously. Run-off would amount to about 8.5 inches and the resulting flows were determined for the component tributary areas. On Okabena Creek the crest flow was computed as 2,600 c.f.s. which is approximately equal to the design channel capacity plus levee freeboard at the head of Whiskey Ditch. However, since the capacities of the triple 8x5 foot box culvert plus the three 42-inch pipes of the diversion structure total about 1,800 c.f.s. at elevation 1,586 (levee crest near the structures), overtopping of the levee at higher flows would begin in the reach from about station 39 + 00 to station 43 + 60. The flood flows are higher than 1,800 c.f.s. for only 4½ hours and the overflow would range from 0 c.f.s. and back to 0 c.f.s. during that period of time. Experience has shown that if the back slope of the levee is well sodded and maintained there would be a possibility that the levee would not fail. Under the condition moderate flooding from the relatively small volume of overflow would result in the area near County Ditch No. 12. The remainder of the run-off from area A plus the run-off from the other areas would cause considerable flooding of the shoreline development. In case of levee failure it is estimated that flooding would occur over the area in the vicinity of County Ditch No. 12 to a depth of several feet, resulting in appreciable damage.

   12. Hydraulic design. Whiskey Ditch has been well maintained in the past and presumably will be well maintained in the future. As the section is uniform and the alignment straight, a Manning's "n" of 0.025 was indicated. The "n" in County Ditch No. 12 was chosen as 0.030. In determining the losses at the triple box culvert in Whiskey Ditch, the exit loss was the difference in velocity head in the triple box and in the exit channel. The "n" value used was 0.013 and the entrance loss as 15 percent of the velocity head in the triple box.

   13. The capacities of pipe culverts have been computed in accordance with the preliminary issue

of Chapter 2, Part CXVI of the Engineering Manual of Civil Works Construction. Entrance loss and friction loss coefficients for concrete and corrugated metal pipe were taken from the reports Hydraulic Tests on Concrete Culvert Pipes, technical papers No. 4 series B, and No. 5, service B, respectively, published by the St. Anthony Falls Hydraulic Laboratory, Minneapolis, Minnesota. A tabulation of the various culvert loss coefficients follows:

 

Pipe	"n"	Entrance Loss		Exit Loss
		Protruding	Flush
Concrete	0.010	0.15	-	1.00
Corrugated Metal	0.025	0.78	0.49	1.00

 

   14. The velocity in the culverts at the head of County Ditch No. 12 with design flow conditions is 10.05 feet per second, which requires a stilling basin. The relation between velocity and depth is such that there is no assurance that a hydraulic jump will operate successfully. Therefore, the stilling basin is a simple expansion designed to reduce the exit velocities to 4.0 feet per second. In accordance with data presented in the discussion by Mr. H.R. Henry to the article "Diffusion of Submerged Jets" in the 1950 Transaction of the American Society of Civil Engineers, near-uniform distribution of the velocities can be expected in a distance of 20 to 25 times the initial depth if the submergence is not great.

   15. Conclusions. The major portion of the hydrologic analysis was based on estimated or synthetic factors. In order to make due allowance for errors in judgment, all estimates were made on a more conservative basis than would be the case in a study where adequate data are available. In view of the fact that floods are of short duration and there is little danger of the levees becoming saturated, the run-off rate and levee freeboard used are considered to be conservative. Although the design storm is based on a theoretical 100-year rainfall, it is not considered that reliance should be placed on any frequency curves for recurrence intervals longer than the period of record. However, the project as based on the above analysis is adequate to protect the area against all but the most unusual rainfalls.

('69 Code, Ch. 7 App. Exhibit A)