10-15-2007 Planning Commission Packet

Home Browse Search

, , �� G . � . , . � Memorandum � �Bonestroo ., � 2335 Highway 36 W To: Tom Kellogg Project: 774 Tonkawa Rd. II Date: 6-27-07 St.Paul,MN 55113 From; Lance Hoff Client: Clty Of OfonO II Tel 651-636•4600 Fax 651-636-1311 Re: Drainage Review . File1�No: 139Gen www.bonestroo.com As requested,we've review the site against the requirements.and Iltandards of the City of Orono. Detaiis follow below: � � . . � . � We based our review on a site visit with Dan Hessberg(o, ner)and Peter Knaeble(engineer w/Terra)and the following information: �I ' o Plan sheet showing draintile system layout and typical cross-section(provided by the landscaper?). � o Drainage calculations provided by the owner's e�ngineer. o As-built survey provided by the owner s surveyo�(Demars-Gabriel). , � • As-built information for the draintile system was not provided. However,the system did match the • provided plan sheet per my inspection with the exceptior�that the northeast pop-up could not be . located. The northeast pop-up should be located and documented(pictures taken relative to the - home and adjacent retaining wall—submitted to the Ciry). Also,to function properly the system should be laid as flat as possible. The two draintile systems(located to the north and south of the house)are both located in relatively flat areas and should function properly. • A rule-of-thumb that is applicable when utilizing subsurface storage was used to check fhe adequacy of the draintile system. To meet 2-, 10-, and 100-yr peak flows and water quality requirements;the difference in runoff volume between the 2 areas of comparison(typically existing �and proposed conditions)during a 2-yr storm must be p�ovided in the subsurface storage. For this site,the areas for comparison are the approved impervious area versus the as-built impervious area. Results from our calculations are summarized in the table below. �� Runoff Volume from a Subsurface Storage Impervious Area* (SF) Z-yr S'torm(CF) (CF) Approved' As-Built !� Approved As-Built Required Provided'`* 75'-250' 250'-500' 75'-250' 250'-500' �� 2,563 395 2,857 454 691 �� 786 95 160 *Distances are measured from the lake. ' "`Assumes 40%voids in the rock. � � � • The City is concerned that the absence of the originally proposed and approved drainage swales following the lot lines will result in excess flows routed to the adjacent neighboring properties. The draintile system has been determined to convey and store runoff generated by storms up to a total rainfall depth of 0.8". For comparison purposes, 2-yr, 10-yr,and 100-yr storm depths are 2.8",4.2", and 6.0", respectively. When storm depths exceed 0.8",the draintile system will become overwhelmed and runoff generated from the entire site(impervious and peroious areas) � � i . i � I . ; ;