2. Pentwater Inlet, Michigan . 



Pentwater Inlet is an example of a Great Lakes inlet controlled by 

 vertical-walled jetties along the entire 2,000-foot channel (Fig. 5). 

 Meteorologically generated seiches of Lake Michigan are the primary water 

 level fluctuations causing reversing currents in the inlet. A model of 

 Pentwater will be calibrated and used to estimate hydraulic response of 

 the inlet to simultaneous lake seiching and river inflow. The procedures 

 used in this modeling are: 



(a) A hydrographic survey of the inlet is used to describe 

 the inlet geometry. 



(b) The inlet is modeled using one channel and six cross 

 sections . 



(c) The bay surface area, measured from a hydrographic 

 chart, is 1.81 x 10^ square feet. The bay area does not 

 change with bay water level because the bay has steep-sided 

 slopes, so 3 = 0. 



(d) Lake Michigan water level measurements used to force 

 the model were taken at 5-minute intervals on a tower located 

 adjacent to Pentwater Inlet. 



(e) The model time step used is: 



2000 

 At = —i====^= = 90 seconds 

 /32.2 X 15 



(f) Loss coefficients were specified as CDE = CDF = 1.0, 

 and Manning's n was estimated by n = 0.03777 - 0.000667 D 

 (recommended for depths greater than 4 feet and less than 30 

 feet) . 



(g) A preliminary run showed that temporal acceleration is 

 an important term in the inlet equation of motion for Pentwater 

 Inlet (Fig. 6). Therefore, several forcing cycles of model 

 operation before the time of interest are necessary to eliminate 

 transient terms due to startup conditions. 



(h) The model is calibrated by using Lake Michigan levels 

 to force the model. An initial run showed that predicted bay 

 level fluctuations adequately modeled observed levels (Fig. 7) . 



(i) The model was not verified. 



(j) The model was used to predict inlet velocities, dis- 

 charge, and bay levels for a 2-hour forcing wave with an 



15 



