Agenda - 10-18-1983 - Laserfiche WebLink

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031 c416.NV..\ DESCRIPTION OF MODELS The Recursive Simulation Model was used to model the Eno River reservoir system as it is operated in 1983. Two model runs were made, one without con- servation measures and one including various methods to conserve the available water supply, Model #1 used current municipal withdrawal rates for Orange- Alamance and Hillsboro, plus a withdrawal made by Hillsborough and sold to Chapel Hill 1(0.W.A.S.A.), at a constant rate as long as the supply of water allowed such withdrawal. Model #2 used the same withdrawal rates, but imple- mented conservation measure based on the amount of water remaining in Lake Orange. Both models allowed Lake Orange to release water, in addition to over- the-spillway flows, in varying amounts through its outlet gates. Corporation Lake and Lake Ben Johnson, however, passed water only over their spillways. Neither Corporation Lake or Lake Ben Johnson currently maintain any minimum release from the impoundments for water quality purposes. Lake Orange, however, does usually release water for water quality maintenance. Both models reflect this release, with Lake Orange having the only required minimum release of the three impoundments. Table 1 summarizes the impoundment data of the two models. In order to properly evaluate the Eno River Reservoir system, a data base consisting of some suitable hydrologic data (such as rainfall or streamflow) needed to be selected so that naturally occurring streamflows could be adequate- ly reproduced. Rainfall records are lacking in the Eno River area so U.S.G.S. streamflow gaging station data appeared more suitable, Many of these gaging stations, however; are undergoing increasing amounts of regulation which in turn affected their usefulness. It was determined through examination that the Flat River at Bahama Gage (#02085500) was suitable to serve as a reasonable source of uni-egulated streamflow data. From this streamflow data, 50 years of natural streamflow was generated for the Eno River by simply applying a ratio of drainage areas to the Flat River streamflow data The study area was divided into three sub-basins corresponding to the dams at Lake Orange, Corporation Lake, and Lake Ben Johnson. This arrangement pro- vided maximum flexibility in determining the effects of regulation on the reser- voirs and onIthe flow passed from these reservoirs. Sediment deposition in the reservoirs was based on sediment loading of stream data from the North Carolina 208 Plan for Non-Point Sources. The amounts of sediment actually deposited in reservoirs and the probable distrilm tion of the deposited sediment were calculated based on existing procedures.'" The amount of sediment trapped by Lake Ben Johnson was adjusted to comply with the findings of the Engineering Analysis of Ben Johnson Lake and Dam report of January1583by Pridgen. The RecUrsive Simulation Model, a surface water model developed by Hai C. Tang, was used to mathematically track the daily fluctuations in reservoir performance. The simulated results from this model include the inflow from upstream subbasins, sub-basin drainage, water withdrawn, water impounded, and outflow,