dq2 = Time Step * Area wtd * (HGL) / Link Length Qnew = (Qold – dq2 + dq3 + dq4) / ( 1 + dq1) when the force main is full dq3 and dq4 are zero and
dq4 = Time Step * Velocity * Velocity * (a2 – a1) / Link Length * Sigma the dq3 term in dynamic.c uses the current midpoint area (a function of the midpoint depth), the sigma factor and the midpoint velocity
dq3 = 2 * Velocity * ( Amid(current iteration) – Amid (last time step) * Sigma
dq1 = Time Step * RoughFactor / Rwtd^1.333 * |Velocity| The weighted area (Awtd) is used in the dq2 term of the St. Venant equation:
dq2 = Time Step * Awtd * (Head Downstream – Head Upstream) / Link Length
In this blog we show how the St Venant terms are used in SWMM5 as equations, table, graphs and units. We use a QA/QC version of SWMM 5 that lists many more link, node, system and Subcatchment variables than the default SWMM 5 GUI and engine. This also applies to #InfoSWMM and any software the uses the #SWMM5 engine.
SWMM5 is using is the most advanced equations as it takes into consideration the full dynamic (St. Venant) equations and not the more simplified kinematic wave / manning equations. The manning equation only considers the uniform flow conditions which represents a situation where the gravitational force on a column of water (due to the channel slope) balances out the frictional force. The full dynamic equations contains additional factors that affect the movement of water in a conduit or channel. These include the pressure force due to variation of depth along the length of the channel and the inertial (or convective acceleration) effect due to variation of flow area along the channel length. Because of these additional terms the flow/head relation you have in uniform flow conditions can be completely different according to the configuration of his network.