Greg and Shahab,

 

Thank you for taking the time earlier today to help us work through some of the issues for the subject plan and the related plat.  Here is a summary of our findings and follow-ups:

 

·         High Mesa Consulting Group (HMCG) will revise the note on the plat to identify the remaining blanket drainage easement as being the responsibility of the Owner of Tract B, as opposed to former Lot 2.

·         Our Building Permit Plans will include a berm and sediment/nuisance flow trap to prevent cross-lot drainage from Tract A to Tract B, thereby eliminating the need for an easement.

·         HMCG will provide you with supplemental calculations to support the pipe sizing and the feasibility of collecting flows in a cul-de-sac.

·         HMCG will take a closer look at the overflow condition and provide curb cuts downstream of the turn lane to allow any street flow to enter the ditch.

·         HMCG will propose to the Design Team that the sidewalk along Central be asphalt pavement seeing how it will likely be removed in the near future with storm drain construction.

 

As a follow-up, we present the following information:

 

·         Review of the existing intersection reveals that the high point is set at 100.64, slightly higher than the overflow point on the top of the sidewalk culvert (see attached excerpt from topo).  As such, the initial overflow after the 18” pipe and sidewalk culvert capacity is indeed to the ditch.  Further review of our topo and from Google Earth confirms that there is actually a downstream curb cut before the intersection at Unser.  Based on these findings, it does not appear that the overflow to street flow issue is as significant as we thought earlier, but still does exist.  I will show this to the Design Team tomorrow and recommend that a curb opening and rundown be added to the building permit plans. 

·         Attached herewith are supplemental calculations that show the open channel gravity flow of 97 cfs (291/3) at a depth of 3.6 ft in a 48” pipe.  Also attached are pressure calculations that start with an additional 1.85 ft of head above the 3.6 ft normal depth to simulate the head loss for a 90 degree bend which is conservative compared to a 90 degree manhole turn.  The pressure calculations show that the required head at the cattle guard results in a hydraulic grade line right at the flowline or grate elevation in the cul-de-sac (no popping manhole lids or backing up).

·         I have seen many ways to look at inlet capacities.  In this case, we will have some flow velocity, but effectively a sump due to the curb at the cul-de-sac.  Assuming the grates are a submerged orifice, it would take 16.2 standard city grates to accept 291 cfs.  The cul-de-sac at the point beyond the car wash entrance is about 50 ft wide, allowing 15 grates.  If you add the existing single “C”, and the proposed double “C” we propose to add at the low spot in the entrance (where the sidewalk culver is), you get 18 grates, total.  This would result in no street flow.

 

Based on the above, I believe your questions and comments can be addressed for infrastructure list approval (3-48” pipes can carry the flow and a cattle-guard type of structure could intercept the flow).  If you concur, please let me know and copy Stephen Woodall who has requested an “endorsement” of the infrastructure list from Hydrology before he approved our estimate.

 

Thanks again,

 

Let me know if you have any questions.

 

Graeme