t » -^ (4-68) 



where Li, is the distance to the farthest point and d^ is the average 

 bay distance. 



(c) The inlet channel depth is large compared to the ocean tidal 

 range, and the channel depth and width do not vary along the channel. 

 Hydraulic calculations may be made with a reasonable degree of confidence, 

 even if channel cross-section variations exist but are not too extreme. 

 For irregular jettied or unjettied channels, an effective channel 

 length, L* , which can be used in place of L , is given by 



^. = m 



(4-69) 



v^ere R and A^ are average values of the channel hydraulic radius and 

 cross-sectional area used in the hydraulic calculations and R^ and 

 A^^ are the hydraulic radius and cross-sectional area at each of n 

 sections of equal length Ax , spaced along the channel. For jettied 

 inlets the length may be taken as the distance along the channel axis from 

 the seaward end of the jetties to the section on the bayward end of the 

 channel where the flow velocity is diminished to a small percentage (e.g., 

 20 percent) of the average channel velocity. For unjettied inlets that are 

 not too irregular in cross section, the length may be taken as the 

 distance along the channel axis between the points on each end where the 

 velocity is, for example, 20 percent of the average velocity. 



(d) Bay walls are vertical over the bay tidal range. Hydraulic 

 calculations may be made with a reasonable degree of confidence if there 

 is no extensive flooding of tidal flats. 



(e) There are negligible density currents at the inlet and negligible 

 inflow to the bay from other sources (rivers, overland flow, precipita- 

 tion, etc. ) . 



The values for k , k , and f must be also established for 

 calculations to proceed. kg^ may be assumed equal to unity (kg^ = 1.0), and 

 kg^ will probably vary between approximately and 0.2 as the entrance 

 hydraulic efficiency decreases. A value of k = 0.2 is recommended for 

 most calculations. 



The friction factor f or Manning's n (n = 0.093R f ) depends on 

 the bed roughness and flow velocity. For a sandy channel bottom typical of 

 most inlets, f can vary between 0.01 and 0.07, depending on the peak 

 velocity and the phase of the tidal cycle. If no information is available to 

 estimate the friction factors, a value of f = 0.03 may be used. 



Losses caused by bridge piers, sills, channel bends, etc., must also be 



accounted for in hydraulic calculations by adding a loss coefficient similar 



to k and k in the equation defining F . Like k^^ and k^^ , this 



coefficient defines the number of velocity heads (V /2g) lost at a channel 

 disturbance. 



4-164 



