5125 SEAHURST AVE 2022-12-06 - Laserfiche WebLink

Erik Emerson From: Erik Emerson Sent: Thursday, June 27, 2019 4:22 PM To: Thad Newport Subject: 5125 Seahurst Stormwater Review Comments Attachments: Archived attachment Iist.txt Quick Look Thad, I have reviewed the submitted plans for the Seahurst SP project at 5125 Seahurst. I have the following comments: 1. Report Section 7,4th paragraph states that since only the frontage improvements are effective impervious area nd are less than 5,000 sf,then only MR 1-5 apply.This is not technically correct. Please revise this paragraph to state that MR 1-9 apply to the project, but due to the LID measures proposed the effective impervious area falls below the thresholds for MR 6 and MR7. 2. Report Section 9, please provide an analysis addressing the third criteria for determining if flow control is required.The project must not increase 100 year flows by more than 0.15 cfs vs existing conditions(DCSS Section 4-4.4 and SWMM MR 7 section.)The analysis should include all areas within the project.The proposed ,, 7lid facilities may be included as facilities or modeled in accordance with the modeling recommendations in the SWMM.The analysis should be performed for each TDA.Alternately, state that MR 7 is applicable and demonstrate that the performance standard is being met within a given TDA. 3. Please provide a figure documenting the area swap described in Section 9 showing the impervious areas N A accounted for. Note that the swapped areas must be within the same TDA. 111 4. The proposed landscaping infiltration trenches qualify as infiltration facilities.As such they must meet the r irements for infiltration trenches as defined in the SWMM.These requirements include compliance with the Site Suitability Criteria (SSC) in Vol III,Section 3.3.7 and the design criteria in Vol III,Section 3.3.11.As shown in the submittal the following do not appear to have been addressed: i. SSC 5 must be addressed.The minimum separation from ground water or an impermeable layer is 5 feet, or 3 feet with a mounding analysis.Also, the detail on Drawing No.4 should be changed to reflect this requirement. ii. SSC-7 must be addressed by a licensed geotechnical engineer. Specifically of concern is the potential for groundwater from the infiltration trenches to impact the foundations of the proposed homes down-gradient. iii. As shown in Figure 3.3.4 a protective layer of filter fabric should be included with a sacrificial layer of rock above it. iv. A monitoring well must be provided v. An overflow route must be defined.This can be piped or a surface route, but must be capable of conveying the unmitigated 100 year flow without erosion or impacts to private property or public infrastructure. Consider adding overflow structure and piping to additional CB's in 52nd St SW. If overflow routes cross parcels other than those on which they are located an easement complying with DCSS section 4-2.2(2) must be provided. 5. Since the proposed infiltration facility on Lot 6 will be mitigating water from other areas an easement is �F-equired.Technically the facility should probably be in a separate tract, but an easement should be sufficient in this case.The easement should be to the benefit of any lots which drain to it. It looks like lots 1 and 3 should be g-- 17- iA)