Since the weir has to safely pass down excess flow, estimation of peak flow becomes a first step in design of tank weirs. Catchment area of tank is small and fan shaped, so in practice it is difficult to measure the discharge.
It is, therefore, estimated using empirical formulae. In north India Dicken’s formula is used, while for south India Ryve’s formula has been developed.
Ryves formula is stated as below:
Q = C.M2/3
where Q is peak flow in cumec
M is catchment area in sq km
C is a constant
= 6.74 for areas within 24 km from coast
= 8.45 for areas within 24-161 km from coast
= 10.1 for limited hilly areas.
This formula can be applied for a free catchment. For a tank in series or interconnected tanks the formula needs correction.
Modified formula is:
Q = CM2/3 – Cm. Mm2/3
where Mm is the catchment area in sq km intercepted by upstream tank
Cm is new coefficient which varies from 0.2 C to 0.33 C.
Waterway of the Weir:
The weirs constructed are generally broad crested weirs and the formula which gives discharge over broad crested weir can be used. Generally velocity of approach is neglected.
The discharge formula is in the form:
Q = C. L. H3/2
L = Q / (C.H3/2)
where L is length of weir in metres
H is design head over weir; it is given by (M.W.L – F.T.L)
C is a constant
= 1.84 for weir crests upto 0.9 m width.
= 1.66 for weir crests more than 0.9 m width.
= 1.66 for crests with dam stones
= 1.47 for crests with d/s sloping face.
Length of Apron:
Usually length of horizontal downstream apron is kept 2(D + H) from toe of body wall. Here D is height of the body wall above floor and H is maximum water head over the crest of the wall. A further factor of safety of 1.5 is provided when important areas lie below the surplus weir. Then length is kept 3(D + H).
Length of Stone Talus or Pitching:
Generally 3(D + H) to 5(D + H) length of stone pitching is provided on the downstream of apron in continuation. Greater length is provided for weaker foundation material.