I am preparing a drainage report for a client that is having flooding problems. His site existed prior to the development of an adjacent property. A broad swale previously flowed onto my clients property from undeveloped land. The land was developed and the stormwater from that development dicharges directly to my clients parking lot (no easement or any type of agreements)through two 6" pipes (side by side with matching inverts). Durring a large storm, water shoots out of the pipes like a fire hose and floods a large portion of his parking area (we have pictures showing this in action). He has recently renovated his building and had to treat for mold problems, as the flood water reaches his building. I have designed improvements on his site to solve the problem (not yet constructed) and am now generating a report to detail the cause of damage. Several Major "errors" were found on the plans for the adjacent property- apparrent modification of the existing grades to show that a much greater area flowed onto my clients site in the pre-developed condition is just one of the many crazy things.
The locality requires 50/10 detention- (i.e. detain 50yr post back to 10yr pre). The calcs on the plans for the adjacent development shows a computer output (program not listed or known) that used NRCS (TR-20) to generate the 50yr post developed peak flow from the pond. Setting aside the errors I noted in the data on these plans..... The only data shown for the 10-yr pre-developed flow is a small handwritten area on the plan stating a C-value, Tc (coorisponding I value), and site area. The Q-10 is listed below these values, and is simply Q=cia (i.e.-Rational Method)
My question is this- is it appropriate to compare the peak discharge from a pond that was calculated using TR-20 with a pre-development peak discharge from the site that was calculated using the Rational Method? The pre-developed peak is simply that- a peak flow from a site without regard to any "volume" or durration... its just that when preparing pre vs post calcs, I normally use the same method- as one value is being compared to the other (apples to apples sort of thing)- since the post is routed through a pond, the Rational Method isn't used at all- I typically use the NRCS method- which is accepted for all the surrounding localities.
The locality requires 50/10 detention- (i.e. detain 50yr post back to 10yr pre). The calcs on the plans for the adjacent development shows a computer output (program not listed or known) that used NRCS (TR-20) to generate the 50yr post developed peak flow from the pond. Setting aside the errors I noted in the data on these plans..... The only data shown for the 10-yr pre-developed flow is a small handwritten area on the plan stating a C-value, Tc (coorisponding I value), and site area. The Q-10 is listed below these values, and is simply Q=cia (i.e.-Rational Method)
My question is this- is it appropriate to compare the peak discharge from a pond that was calculated using TR-20 with a pre-development peak discharge from the site that was calculated using the Rational Method? The pre-developed peak is simply that- a peak flow from a site without regard to any "volume" or durration... its just that when preparing pre vs post calcs, I normally use the same method- as one value is being compared to the other (apples to apples sort of thing)- since the post is routed through a pond, the Rational Method isn't used at all- I typically use the NRCS method- which is accepted for all the surrounding localities.
