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− | [[File:Riser.gif|thumb|A two stage riser with low flow controlled with an orifice and high flow controlled with a weir.]] | + | [[File:Riser.gif|thumb|500px|A two stage riser with low flow controlled with an orifice and high flow controlled with a weir.]] |
| + | |
| + | There are many configurations for multistage risers according to the flow control requirements. |
| + | It is easiest to begin downstream and design with the the highest return period required in mind. |
| + | e.g. start by specifying the pipe/culvert component to control the 100 year flow. |
| + | Then to work backwards to size the lip or weir at the top of the structure to meet the next return period flow. |
| + | |
| + | The riser is then designed with flow through one or more orifices. |
| + | |
| + | <math> |
| + | Q= |
| + | \left\{\begin{matrix} |
| + | 0, & Z\leq Z_{0} \\ |
| + | C_{w}L_{w}\left ( Z-Z_{0} \right )^{1.5}, & Z_{0}\leq Z\leq Z_{0}+H_{0}\\ |
| + | C_{d}A_{0}\sqrt{2g\left (Z-Z_{0}-0.5H_{0}\right)}, & Z_{0}+H_{0}\leq Z\leq Z_{1}\\ |
| + | C_{w}L_{w}\left ( Z-Z_{1} \right )^{1.5}+C_{d}A_{o}\sqrt{2g\left (Z-Z_{1}\right)}, & Z_{1}<Z |
| + | \end{matrix}\right. |
| + | </math> |
| + | |
| + | {{Plainlist|1=Where: |
| + | *''Z'' = the depth of ponded water in m, |
| + | *''Z<sub>0</sub>'' = the elevation of the bottom of the orifice in m, |
| + | *''H<sub>0</sub>'' = the height of the orifice in m, |
| + | *''Z<sub>1</sub>'' = the elevation of the weir in m, |
| + | *''C<sub>w</sub>'' = the weir coefficient, |
| + | *''L<sub>w</sub>'' = the length of the weir in m, |
| + | *''C<sub>d</sub>'' = the discharge coefficient of the orifice (often defaulted to 0.6 for a sharp edged orifice), |
| + | *''A<sub>o</sub>'' = the cross-sectional area of the orifice in m<sup>2</sup>, |
| + | *}} |
| + | |
| + | ==See also== |
| + | *[[Detention basins]] |
| + | *[[Flow control]] |
| + | |
| + | ==For review== |
| + | https://www.hydrologystudio.com/no-fail-detention-pond-design/ |