1 - Alaska Energy Data Inventory

B. Losses in the Power Tunnel.(1) Unlined Tunnel - The unlined portion of the power conduit is a12 ft modified horseshoe tunnel. After reviewing photos of the Long Laketunnel and having discussions with engineers and geologists who had beenpresent during construction of the Long Lake tunnel it was decided toassume an average overbreak or roughnes~ 0, 4.5 inches, in the Crater Laketunnel. This overbreak was then used to calculate an equivalent diameterof 12.19 ft as well as other hydraulic properties of the tunnel.Mannings"n" was based on information summarized in HOC chart 224-1/5. An "n" valuewas obtained by averagillg the measured "n" values of those tunnels in HOC224-1/5 which were in the same range of cross sectional area as the CraterLake Tunnel.The selected value of .031 was slightly below the averageva 1 ue .032 obtained from the above procedure but was cons i aered a goodexpected resistance value. The 12 tunnels that were considered forresistance values ranged in area (design area) from 71 ft2 to 195 ft2.The l2-ft diameter horseshoe tunnel at Crater Lake wi 11of ;23.2 ft2 including an assumed overbreak of 4.5 inches.have a des i gn areaIt should bepointed out that the existing Long Lake Tunnel (originally designed as al4-ft diameter horseshoe tunnel) was designed with an expected average areaof 160.94 ft2 while the completed tunnel had a measured average crosssection area of 199.3 ft2 (ref. 8). HOC 224-1/5 shows that actual tunnelareas average about 15 pct greater than des i gn areas.If the percentageincrease in the driven tunnel area was 15 pct greater than the design area,the lin II value of the driven tunnel could be as high as .036 withoutreducing the design head. For minimum and maximum losses, values of .029and .0347 respectively were used for Mannings "n". Losses for benas,contractions, expansions, transitions, rock traps, and entrances are basedon various HOC data.(2) Lined Tunnel - There wi 11 be approximately 900 ft of 1 inedtunnel. It is estimated that there are 4 sections of lined tunnelincluding the gate structure. Losses in the lined tunnel will consist offriction losses and contractions and expansions. Friction losses are basedB3-5