Minnegang Creek Flood Study Report - Wollongong City Council

that handrails above these structures be modelled with a 100% bloc kage factor. In linewith this policy, the profiles and extents as presented in Section 6.1 reflect the worstcaseblockage scenario within the catchment.The culvert structures under Lake Heights Road, Northcliffe Drive and IllawarraYacht Club required a 100% blockage factor. As the clear diagonal opening under theJane Avenue bridge is greater than 6 m , the bridge was modelled with 25% blockage.The sections of the piped stormwater drainage system included in the MIKE 11 modelwere also modelled with a 100% blockage factor. This included the outlet to theBarina Park detention basin. Handrails in the catchment are located over the threeculverts and on the Jane Ave bridge. All these handrails were modelled as 100%blocked.For the Minnegang Creek catchment, the worst-case blockage scenario occurs whenall structures within the creek are blocked in accordance with the Blockage Policy.This results in higher peak flood levels within Minnegang Creek than any othercombination of blocked and unblocked culverts.The impact of the blockage policy is shown in Figure 6-13 to 6-16. These figuresshow the difference in flood levels in Minnegang Creek between the no blockage andworst-case blockage scenarios for the 1% AEP flood event at each structure. Theimplications of blockage and the resulting flow diversions for each of the structureswithin the catchment are discussed below.6.5.1 Lake Heights Road culvertThe profile for the Lake Heights Road culvert indicates that some attenuation of flowsoccurs under normal flow conditions. When blocked, flow builds up behind the culvertuntil it overtops Lake Heights Road. This results in a higher peak flow over the weirthan the combined flow through the culvert and weir. The higher flow results in higherflood levels downstream of the culvert until the detention basin in Barina Park. Floodlevels are increased for a distance of approximately 100 m upstream of the culvert dueto water ponding behind the blocked culvert.The Lake Heights Road culvert is located at the low point of Lake Heights Road.Therefore, any flow that does not pass through the culvert, will build up behind theculvert until the road is overtopped. Flow will then follow the alignment of the culvertto pass over the road and rejoin Minnegang Creek. Therefore, no diversions of flowoccur to adjacent to sub-catchments.6.5.2 Barina Park detention basinThe capacity of the Barina Park detention basin is inadequate to detain flood flows inall design events. However, blockage of the outlet leads to higher flows passing overthe basin embankment than for the unblocked outlet. This leads to increased floodlevels downstream of the basin. Flood levels upstream of the basin are not increasedsignificantly over the unblocked outlet levels as the levels upstream of theembankment in the unblocked case are limited by the level of the embankment.Flow diversions to adjacent sub-catchments do not occur due to blockage of the outletto the basin. The flows leave the basin over an increased width of the embankment andthen follow a similar alignment to flows resulting from the unblocked case. It isSV8507-D0-001 Rev 2 6-5418/10/02