hh := {1, 1, 1} . {q^2/2/g, p/(ρ g),  
          z}
hh

para = {g ->  9.8, ρ ->  1000}

example 9.6  p.235

para1 = {λ ->  0.02 , d ->  din,  
          aa ->  Pi/4 din^2, qq ->  20 10^-3} /. 
        din ->  0.05

paraa = {q -> 0  , p ->  3 10^5, z ->  0}; 
    hha = hh /. paraa

parab = {q -> 0, p ->  0 , z -> L}; 
    hhb = hh /. parab

hhL = λ/2  L/d u^2/g /. {u ->  qq/aa} /. 
        para1

Solve[hha == hhb + hhL /. para1 /. para, 
      L]

check

paraa = {q -> 0  , p ->  3 10^5, 
          z ->  0};
hha1 = hh /. paraa /. para

hhL1 = hhL /. para /. L ->  9.8

hha1 - hhL1