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~TNJT Mn/DOT - OFFICE OF BRIDGES AND STRUCTURES - Room 610 EXHIBIT III-C, I of 3 STATE OF 'UNNESOTA OFFICE MEMORANDUM TO: Fi Ie DATE: May 12. 1988 FIOIII Li sa K. Sayler 1'IIIIlE: 296-08:26 _EeT: Bridge .~327 Scour Analysis Bridge .~327 over the Red Lake River, Pennington County US 59, In Thief River Falls Evaluation of the scour equations from the Technical Advisory shows the potential for scour problems at the piers of this bridge. The superstructure of this bridge was just replaced in 198~. so it is unlikely the bridge will b. replaced any time soon. The inspection record does not indicate any problems from scour, soundings at the piers have periodically been taken during the last few years. The bridge sits in the backwater of a dam located about 1/2 mile downstream. The dam increases the depth at the bridge, but decreases the velocity. The scour calculations are based on a 1987 HEC-2 run made by the USGS as part of a flood insurance study. The "a" value lJsed for the abutment scour equati ons is an equi val ent val ue determine-i fr'om the are", of the abut,l"allls pn;.j.,<.;ling into the approach flow. Q.oo Water surface elevation Thalweg elevation Mean bed elevation Approach Velocity Contraction scour 1~,400 cfs 1118.9 ft 1098.7 ft 1104.3 ft 4.:5 fps 0.3 ft East abutment Abut.ent scour Total scour Bed elevation Scour elevation Bottom of footing 12.1 ft 12.4 ft 1113.0 ft 1100.6 ft 1101.0 ft West abutment Abutment scour Total scour Bed e1evati on Scour elevation Bottom cri footing 9.1 ft 9.4 ft 1114.0 ft 1104.6 ft 1101.0 ft 8-12 I I .or- ~... '(.1 I I I I I I ( I I I I I I I I I I