Hydraulic Grade Lines

Question

Hi Tom 
 Was wondering if you could please confirm my theory on HGL&rsquo;s? 
 We modelled a new subdivision (includes single family lots and condo blocks) based on flow test results at the nearest existing fire hydrant. Based on our modelling this resulted in a HGL of 340.04m at all nodes during the average day demand scenario. 
 Now we are designing the WM for one of the new condo blocks in the subdivision. In the condo block modelling we propose to use the reservoir method (instead of the hydrant flow test method) with an elevation of 340.04m based on the results of the subdivision modelling. 
 I would appreciate your thoughts on this. 
 Regards 
 
 Feel free to contact our office with any questions/concerns. 
 Regards, 
 
 Abe Harder 
 Senior Civil Designer

Tom Walski · Answer

Abe, 
 
 It depends on the head loss between the source and the place where you connect. If you are tying into a strong system with s 24 or 36 inch pipe, then 340 m will not change much. However, if you are tapping in to a 6 or 8 inch pipe, the HGL will drop as flow increases during a fire. It will be less than 340 m and that is why you conduct the flow test. You need to account for head loss in the supply system. 
 
 Tom

Jesse Dringoli · Answer

The following article has a comparison between the reservoir approach and the approach where a pump curve is used based on hydrant flow test data. I have improved the wording based on Tom's response here. 
 Modeling a Connection to an Existing System