The detailed results are on file in the U. S. Army Engineer District,

Pittsburgh.

Test Results Summary is shown on Plate 20.

Unless specifically noted to the contrary, all tests reported in both Parts I and II were performed in accordance with the procedures given in the Engineer Manual EM 1110-2-1906, 30 November 1970, "Laboratory Soils Testing," Corps of Engineers.

APPENDIX 6

HYDROLOGY

Capacity Curves. The U. S. Geological Survey 7.5 minute series

topographic maps were used to derive the area curves for the ponding sites.

The scale of the topographic maps is 1 to 24,000 with 40-foot contour

intervals.

Values for the area curves were derived by planimetering the area of each contour. A computer program utilizing the conical method was used to determine the intermediate areas at one-foot intervals. The capacity curves were developed by averaging values at one-foot intervals by the endarea method. The curves showing elevation versus capacity for Youngstown Mines Corp., Island Creek Coal Guyan No. 5, and Amherst Coal Co. sediment ponds are shown on Plates 1, 2 and 3. No curves are shown for the Powellton Coal Co. pond on Rockhouse Creek, since it is a diversion pond with negligible tributary area. Total depth of flood impoundment will approximate the number of inches of storm precipitation. The three curves reflect the storage capacity of the natural valleys. The level of present silt deposition is indicated on the curves as well as the top of dam. These were obtained by recent field surveys. Total presently available storage potential lies between these two limits.

2. Probable Maximum Storm and Flood. The maximum probable flood used in this report for the areas tributary to the dams has been based on rainfall rates and duration from Hydrometeorological Report No. 33 (April 1965), "Seasonal Cariation of Probable Maximum Precipitation East of the 105th Meridian," prepared by the Hydrometeorological Section of the U. S. Weather Bureau. In development of this storm, consideration was also given to the Engineer Circular No. 1110-2-27, from the officer of the Chief of Engineers, ENGCW-EY, 1 August 1966.

The magnitude and intensity of rainfall shown for the month of August were most critical for the size of the areas above these dams. Storms at this time are caused by stagnant anti-cyclonic eddies in the air masses which carry potentially unstable moist currents over the upper Ohio Valley. Such storms usually occur in regions in which normal or less than normal precipitation has been occurring. An antecedent rain, however, could occur within a short span prior to the maximum storm rainfall.

3. Top of Embankment Determination. To minimize the possibility of embankment failures due to overtopping, storage of probable maximum storm runoff was presumed to occur after the maximum storage obtained by routing the 10 year, 6 hour storm through the various impoundments with recommended drainage facilities in place. This total storage was used to determine the minimum elevation needed to prevent overtopping of the embankment. The computed 24 hour rainfall for the design storm was 22 inches. Total losses were assumed as 3.0 inches with a resultant runoff of 19.0 inches. The following tabulation for the four dams gives a comparison of the lowest elevation on the top of the existing embankment and the minimum elevation which should be provided for storage without overtopping of assumed maximum runoff. The required embankment elevation does not include free board.

The required top elevation shown in the above tabulation is applicable for presently existing sediment conditions. When future settlement deposi

tion usurps a significant portion of storage capacity, the embankment must be raised to provide the capacity needed to retain maximum flood runoff. 4. Standard Project Flood. The Standard Project Flood is defined as one resulting from rainfall of high intensity which, although extremely rare, has a reasonably probable chance of occurrence in the local area. It is frequently considered to be of about half the magnitude of the maximum probable flood. Reservoir impoundment during this flood consequently would raise the water level to about 2/3 of total embankment height above the

bottom.

5. Discharge Outlet Control Requirements.

As a means of assuring passage in a safe manner of runoff from rainfall and snowmelt, a drop inlet type spillway can be provided; this type structure appears to offer the best solution to the problem for each dam. A drop inlet spillway is one in which the water enters over a horizontally positioned lip, drops through a vertical or sloping shaft, and then flows downstream through a horizontal or near horizontal conduit through the embankment. With this type of structure, sections of pipe could be added to the vertical or sloping shaft as the sediment accumulates and rises around it. This type of control has additional advantages as it provides automatic storage and release of inflow water without need of manual operation. The spillway must be dimensioned so that it will have a flow capacity which will enable impoundment and release of tributary inflow to be made without excessive storage

but with sufficient retention time for effective deposition of sediment during passage of the water.

Ten year, 6 hour, storm runoff conditions have been adopted for

normal operational design purposes. The size of the spillways were chosen so that they would be large enough to limit storage to about 10% of total runoff capacity during this flood and would balance outflow and storage so that maximum flow velocity through the reservoir would not exceed about .05 foot per second. At all other times when inflow has decreased, velocities through the impoundment would be less because of the lower storage level and smaller outflow rate and the greater ratio of total depth to flow rate resulting from normal pondage below the spillway lip. Average annual runoff in this area is about 1.2 cfs per square mile. An initial height of 4 feet has been allowed for top of spillway above the reservoir bottom. No allowance for area below the spillway level has been used in computation of flow rates through the reservoir as sedimentation could rise to near this height before the spillway lip was raised. The spillways so selected, as previously mentioned, were used in routing of the Probable Maximum and Standard Project Floods.

a. The 10-year 6-hour storm for each site was routed through storage using several sizes of drop inlets for each. This rainfall was determined from the Rainfall Intensity Frequency data of the U. S. Weather Bureau (Technical Report No. 40, "Rainfall Frequency Atlas of the United States," May 1961). The inflow for each impoundment was computed by use of the method as outlined in S.C.S. National Engineering Handbook No. 4, Chapter 21, with the rainfall so obtained. The 10-year 6-hour rainfall for the area under study is 3.00 inches. The loss rate will vary due to several factors