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2008-March; Prairie Run Study Report- Prepared for Iverson Reuvers � �;;, . � ' - �i �. ,-. #: � .�_.', �_ � �� ` �`' � �. _; , s _ �. r= ��a��ie �'�� ►��e�� .�'��a��°t � . �: ��e�tvil��, 1�i��e��a �. �. - Prepared for� . Iverson I�euvers, ,�4ttor�rte,}�s at�,aw �� : }� 1t2'arch 2Q08 ��: �{ � - �� $ r�. �� ;� ,�, i � �- s >� .: < s=: �=� ,� - � �. � ys �� Y: i t I i x � � �'i f q" I � �I I,1 'i I � � 4 �.. EXHl81T s: � � '� . PYaiYie Run Study Report Albertville, Mitznesota Prepared for Iverson Reuvers, Attorneys at Law ?Vlarch 2008 4700�Yest 77t1i Street Minneapolis,MN 5�435 B�` � Phonc:(95Z)832-2G00 0 Fas:{952)532-2601 -� Prairie Run Study Report Albertville, Minnesota Table of Contents 1.0 Executive Summary._...,....,. ............................................. ............................................................... I 1.1 Findings and Opinions................... .........................•----................-�--•--............_...----............ 1 2.0 Introduction............................... � ...................•--.................._......----.... ............................................... .l Study Area............ ................................................................ .............................................. 7 2.2 Purpose of This Report.................... � ............ 3.0 Background............ ................................................ ................................................................. ........ .1 Desi�n Standards................. .................................................... ..... 8 .......................•�-�---......._ �.I.1 Levef-of-Sesvice........................................ ....................................................... ..-•-- �.I.2 L:vei-of-Protection................................................................................................8 �-Z City ofAlbertville Desi�n Standards for Level-of-Protec,ion.................... .. . ....................9 4.0 Modeling................................... ............................................ ......_ 12 ................................................• 4.1 Previous Modeiing Efforts................... ........................................................................... 12 4.2 Barr's Detailed XP-SWMivI Corr►puter Model.......................... . l� ...................................... 4.2.1 Hydrologic Modeling....................... ....................................................................]4 4.2.1.1 Watershed Data....................•-..-• ..14 ......................................................... 4.2,l_1.1 Watershed Area....................... ...f 4 4.2.l.l.2 Land Use and Impervious Data..................................................I S 4_2.1.1.3 Watershed Width and Slope.................. ._..,....,._�5 ......................... 4.2.1.1.4 Infiltratioa Data............... �6 ....................................................... . i 7 4.2.t.1.5 Depression Storage Data.................. .......................................... 4.2.1.i.6 Overlaad FEow Roughness Data....................... . ......1� . ....----�......... 4.2.i.2 Rainfali Daia....................................................................�-•--�• ----••---i 8 ..... . .1.2.1 2003 RainfaEi Event....................................................................18 42.1.2-2 100-Year Storm Event Rainfail Data...................... �g ................... . .2 Hydraulic Modeling............ . ..19 ............................................................................. . 4.2.2.1 Storm Sewer and Pond Network.......................... ....19 ............................. 42•2.2 Ditch and Overland �fow Network................ . .,.,.,�9 . ............................... .� Model Results............ .................................�--...... ........ ..................................................... 4.3.1 Model Verifcation.....................................................•--......._..............................20 43.2 Predicted 100-Year High Water Levels................ .. . ...,.....,._2p . .............................. . 4.3.3 Mitigation Measures....................... 2� .............�---.................................................. �.0 NPDES Permit Requirements for Prairie Run Development...................... .................................. 22 6.0 Commeats Relative Brian Mundstock's Revised and Supplemental Affidavits.................. --.......23 P:1Mpfs�3 MN1S6`�386066 Prairie Run Litigation\WorkFiteslPrairie Run Siudy Report Revised Current Version.DOC I List of Tabies Table 1. Land Use Impervious Fraction Assumptions for Hydrologic Modeling Table 2. Soris Input Data Table 3. Depression Storage and Overta*�d Flow Rouahness Input Parameters Table 4. 24-Hour Rainfal] at Albertville Tabfe 5. ?;p_gWMM Model Calibration Results for Matching Photographic Flood Leve! and Tim� Stamp Evidence of the June ?4-25, 2003 Storm Event Table 6. Modeled Flood Levels for the rut�e 24-25, 2003 and 100-year, 24-hour SCS Type II Storm Events at Key I,ocations. Tab(e 7. Prairie Run NPDES Reqttirements. ; ' i } 5 � P:1Mptsi23 MN186\2;8GOG6 Prairic Run i,itigationlWorkFilcs\Prairie Run Study Report Kevised Current Version.DOC fl List of Figures Figure l Wri;ht County Ditch 9 Study Area Fi�ure 2 Subwatershed Divides Fi�ure 3 Existing Land Use Pigure 4 Hydrolo�ic Soil Group Classification Figure 5 Rainfall Distribution Used in XP-SWMM Model Calibration (3une 2?-25, 2003 Storm Event at AlbertviIle, MN) FigUre 6 Calibration Results at Wright County Ditch 9 Upstream of CSAH 18 Figure 7 Calibration Resutts at Atbert Vilias Wetland Figure 8 Calibration Resuits at Pond SW of�2nd & Kah3 (Kahl Ave Pond} Figure 9 2007 Fload Mitigation Project—Phase 1, G:ading, Drair�age, and Erosion Control PIaR ' Figure ]0 Wright County Ditch 9 Study Area on USGS �uadran�te Map I Pigure 1 1 Watershed Comparison Map P:1Mpls\23 MN18b\2386066 Prairie P.un I,itiaationlWor[:Fiies\Prairie Run Study Report Revised Current Version_DOC ilt 1 .0 Executive Summary Tf�e prime foci�s of this study and ;eport was to perform a detailed revaew of the Prairie Run Develapment, its desi�n computatioiis and considerations relative to surface water and other availabfe data in order to deter:nine the adequacy of the development's designs and flood protection measures reiative to the City's ercfinances. Other purposes of this report were to (a) assess the water quality treatment facilities provided in the Prairie Run D�velopment relative to the Minnesota Pollution Controt Agency's (.'VIPCA's) I�TPDES stormwater permit requirements, (b) ass�ss t�e impacts of recently consTructed ponding basin downstream of Prairie �un that lies west of tl�e A(bert Viilas developrttent wou�d have on the flood levels in the study area including Prairie Run, {c) provide comments relative to the "Revised Aff`idavit of$rian Mundstock" and Supplementai Affidavit of Brian Mundstock for Court File No. 56-06-CV-4997; and (d) provide comments relative to information in the Hedlund deposition. '�.1 Findings and Opinions The major finding's of the work performed by Barr for this report are summarized beio«� and discussed in breater detai! in various sections of this report. 1. Barr En;ineering Company prepared a detailec� hydrologic and hydraulic XP-SWMM computer n�ode! of a major portion of the County Ditclt 9 (CD-9) watersited to analyze the 8.�-inch storm event that occurred on June 24-25, 2003 and the 100-year storm event to deterrrtine f�ood leve[ elevaiions in Prairie Run and efsewhere in the CD-9 watershed for both of those events. It is betieved that the Jur�e 24-2�, 200� storm produced the highest known water le��efs in Prairie Run. The estimated"hibhest known water leveJ"and the 100-year flood level are significantly izi;lier than those estimated and used by the developer's engineer for the Prairie Run Development, 9�2.i and 9499 feet, respectively. Therefore, the low floor and entry elevations already used or planned to be used for many of the lots in that deve(opmeni may not provide the necessary level-of-protection to meet the requirements of i the City's ordinances tllat were ctirrent at the time the developme�lt plans were pi-epared. 2. Since the Prairie Run Development �i�as under design during tI�e 3u�ie 24-2�, 2003 storm � event, and it� accordance ��ith City o;dinance at the time ofthe development, the"highest I known water levels"caused by tltat evenE could have easi1y been measared in tl�e I develop�nent by the design engineer/surveyor and should have been used as tl�e basis to P:1Mpls123 MN186�238606b Prairie Run LitigationlR'orkFilcs\Prairie [�un Study Report€tevised Current Version.DOC � . � esiablish the minimum buiidin�el:vation for the developmer�t. Barr's corrzputer model of the June 2003 storm predicts the flood level in Prairie Run to have bee❑ Elevation 9�2.L Had the developer's engineer surveyed tt�e debris line from tEiat storm, we believe it would have found the (ine to be at about Elevation 9�2.1, which is 4.6 feet hi�her than the elevation that was used as a basis for lov��floor and low entry elevations in the Prairie Run Development plans. Based on tl�is hibh water level aild the City ordinance in p(ace at the time the grading plan was approved, «�e believe the lowest floor and entry elevations should have been set at elevations no less than 9�4_i in the vicinity ofthe la�ge �i�et�and on the Prairie Run Development(the highest kno�vn water level (9�2.1)plus 2 feet, accordinb to Section l OQ0.9 (d) and Appendix A —Subdivision Regulations—Section A-600_4 of tlie City ordinances). 3. Assumin�the City's 2007 flood mitigation project (a stormwater detention basin south of the new St. Michael/AlbertvilIe Elementary School and ��est of the Albert Vittas development) is constructed as designed by Bolton & Me�i�, model predictions indicate a 1.0-foot reduction in the I00-yea;flood leve] and a I.3-foot reduction in the hi�hest known water (evef in the Prairie Run Developme��t. With the"highest known �vater level" being the control based an City oi-dinance, the low floor elevation of lots in Prairie Run could be lowered by l,3 feet to Elevation 952.8. lf the developer wants that level to be iowered to a different elevation (sucli as 2 feet above tE�e 1�0-year fload IeveI), we beiieve that would require a variance from the City's ordinance. 4. [3arr Enbineerin�reviewed the Prai:ie Run Development relative to conformance with the water quality requirements of a MPCA's NPDES permit. The MPCA's reqnirements are geared toward preventing sediments from ffushina or being re-entrained in tlae flows passing through water quality treatment basins. Regardless of whether or not sigttificant upstream water quality is provided, the MPCA requires the flows from upstreatn tributary areas be factored into the design of on-fine systems to help ensure flushing of settled sediments does not occur. Since tl�e north pond was desijned to route upstream off-site f7ows tf3rough the basin, the entire upstream �vatershed tributary to the north pond needed to be considered when sizing the permanent storage volume and outlet structure in accordance with the MPCA's NPDES permit requirements. Based on Barr's computations the north pond in ihe Prairie Rurt Development does not meet the NPDES desibn requireme�its. In order to meet the I�TPDES requirements, eitiler an additional 239,250 cubic feet of permanent ��a#er storage P:\Mplsl23 MN�g61238b066 Prairie Run [,itigalionlWorkFileslPrairie Rttn Study Rcport Revised Current Version.DOC 2 volume would need to be provided or the off-site fiows would need to be diverted around the north pond. 5. The City's ordinance that was current at the time specifically required (ow floor elevations to be established 2 feet above tlie "nibhes£known water elevation" as ther� was no determination of the 100-year flood level in the Prairie Fun Development area and there was sufficient data on the known high water level. 6. It is our opin:on that"sufficient data" was availab3e on the highest (:nown water level as the June 24-Z5,2043 storm occurred right when the plans were being developecl. And because "sufficieiit data" was avaiiable,the line of permanent aquatic vebetation should noI have been used to be in compiiance with the City's ordinance, 7. The ordinary hi�h �vater mark(OHW) is the line distinguishing the difference between wetland vegetation and upland vegetation, which is more a resuh of the affect of shaflow groundwater and regular f[ows and not rare and severe storm events. The OHW or tine of permanent aquatic vegetatioa� is not an indication of how high floodwat�rs �vill bounce in an area, and it is our opinion that regardless ofthe City's ordinance, the developer's engineer should have known this and not used the OHW in its development. 8. The Prairie Rui� developer's engineer, when preparing the development plans, should have performed basic dtfe diligeace to understand: (a) the waters}ied in which tlle Prairie Run iies; (b) understand how the County sized its culvert under CSAH 18 or County Road 19, and (c) proactively worked with the City to coordinate aa�d establish the appropriate efevatiotis to be used for E:ey deve(opment features. From our review of information provided, there is litTle indication that much, if any, of this basic due difigence or coordination was performed by the developer's engineer. 9. It is our opinion that regardless of whether or not the Ciry enoineer neglected to provide comment on the development pIans, the responsibilities for the Prairie Rui� Development design rests on tlie owner of the development and its engineer. 10. It is our opinion that the developer's enbineer should have surveyed the drift ar debris line after the June 24-25, 2003 storcn within the development and used that line to set !ow openings and low floors for the 52 lots within the development as it was sibnificanily higher thar� �vhat tl�e engineer used. P:1Mpls\23 MN\8fi�2386066 Prairie Run LitigationlVJorkFilcslPrairie Run Study Report Re��ised Curren!Version.DOC 3 I l. The City's ordinance �overning low #7oor eievations does not make a distinction on the rarity of a storm that occurred to create the "highest known water level,"and it does not mention the magnitude of a storm to be used to set the "hiDhest hnown water level." P�ather, it simFly says the 1lighest known water elevation is to be used. And we believe the June 24-25, 2003 storm created the"highest known water level" because sufficient data was available on it at the time the Prairie Run Development plans were prepared. So if the developer did not want to use that elevation, then it wouid have been incumbent vpon the developer to have app[ied for and obtained a variance from the City's ordinance to allow the use of a lower e[evation for setting key developmen� f atures. The City's ordinance was prescriptive and did not allow the developer or its engineer to arbitrarily pick the"iine of permanent aquatic vegetation"to be used as a basis to set low floors and entries in the deveIopment. 12. It is our opinion that had the developer's enbineer(Hedlund} performed a hasic revie�v of the ditcl� drainage system upstrearn, adjacent to, and downstream of t(�e Prairie Run Development, that sufficient concerns should have been raised by Hedlund to cause them to either analyze tl�e system and determine if tailwater was a problem, or to proactively work with the City to have the system tf�oroughly mode[ed so that a clear understanding of the poteatial flood leveis on the develo ment could be P obtained. We have received mi ' nimai, if any, infonnation indicating that �edlund m ade any svch requests of the City or that it tiad erform p ed even a basic revie�� to ob ' tain an under ' � standin�of the size of The watershed draining into the development, the discharges within County Ditch 9 adjacent to the development, and the potentiai taitwater effects the complex ditch system could pose on the develop�nent. Further, Wright County Soil and Water Conservation District sent the developer's en�ineer and the City a ietter cauTioning diat the hydrologic and hydraulic computations were serionsly lacking and that thcy had observed fEoodwaters backing up in the development. This notification occurred on October 1, 2004 well before the first home building permit was issued on February 4, 2005. Therefore, there was iime to revisit the hydraalic computations and modify the dra�vinbs befo;e construction of the ho�nes in the development started. l3. It is our opinion that because County Ditch 9 was immedia#ely adjacent to the Prairie Ran property and that County ditches have f�ows that typically exceed their banR capacity for events as smal! as a 1- or 2-year event, that to not take itito account the effect tl�e ditch might have on fTood leve[s in the Prairie Run Development was a major flaw in the design t':\Mp[s\23 MN\86�38G066 Prairie Run Litigation\t4�orkFiles\Prairie 12un Study Repori Revised Currcnt Vcrsion.DOC 4 — - - - - -� �j . considerations of the developer's enoineer for setting building and roadway elevations within the development. l4. The North Pond is a pond that was desibned to be "on-line"or right on the drainage channel. In other�vords, the pond was constructed so that the entire draina�e system upstream of thc pond would drain di:ectly through it. The MPCA's requi;ements specifically state that the pond volume to be provided for an on-line system needs to provide a pertnanent stora�e voiume of 1,800 cubic feet pre acre of drainage area that drains to the pond. Because the North Pond is on-Iine with the main ditclZ that passes through the P;airie Run Development, the I�iorth Pond by MPCA requirements needed to be sized for a tributary a�atershed of 209 acres(i89 acres of off-site drainage plus the 20 acres on-siEe) and not just the 35 acres for which it was desi�ned. 15. The City of Albertville's consultant, Shor; Elliot and Hendrick son I�ic. (SEH), ��as responsible for(a)reviewing the submittal infonnation provided by the de�e(oper and tl�e developer's e�igineer for the Prairie Run devcloptnent and (b) providing the developer and its enaineer �uid �. ance an � d support for determining design flows ai3ci fiood elevations for drainaQe D systems that cot�id impact the deveto me ° p nt. It is Barr s o in' p ion that SEH did not erform its P responsibilities. In particuiar, we believe SEH did not: a. Perforn3 the basic due diliaence ne , I b cessary to adequatei� review the submittal iiiformation prov�ded for the developmeni b, Recognize the comptex drainage system that passes throu;h ttte development a��d determine how that system couid impact hydraulics and f7ood level ' s in the develo ment F and inforrn the deve[oper's er�gineer of this matter c. Inform the City of Albertville of the importance of developinj a long-rarige comprehensive surface water management plan for the Wright County Ditch 9 watersi�ed because of t��e impeiiding planned deveto rnent of Prairie Ru P I1 d� Work to de � . . �elop interim measures for f[Qod contro[ a Q _ nd establ�sh�n� low f7oor elevations in the Prairie Run developenent untiI a Wriglit County Ditch 9 surface water marzagement plan could be prepared It is our opinion that had SEH performed the basic due diligence it w�as responsible to perforn3 and taken time!}�action to adequately address tl�e liydraulic and floodi��b issues with tf�e P�1Mp1s123 MN18612386066 Prairie Run Litigation;Wor3�Fi[eslPrairic Run St�dy Rcport Revised Current Version.DOC 5 � � . developer's engineer, it is likely that much if not all of the f7ooding and ponding issues ihe City currently faees could have been circumvented. P:\Mpls\23 MTV186l2386066 Prairie Rwl Litigation\1�'orkFi[es`,Prairie Run Study Report Rcvised Current Version.DOC f> 2.0 Introduction 2.1 Study Area The City of Albertvifle{City) is an e�terior rind suburb located about 23 mites west of Minneapolis in Wright County, as shown on Figure I. Aibertville has exper;enced significant growth in residential and commercial development over last decade. Much o;the residential arowth has occurred within the watershed tributary to Wright Coun#y Ditch 9. The Wrioht County Ditch 9 study area boundary �vas de�ned by the topooraphy of the area and City storm sewer information as shown in Fi�ure 1. 'The Wright Connty Ditch 9 study area (about 2,320 acres) includes agricultural, commercial businesses, and residential areas. The study area is roughly bounded by t-494 on the north, County Highway 19 on the east and Co�nty Road i 19 on the so�th (see Figure Z}. 2.2 Purpose of This Report The prime focus of this study and repo;t was to perform a detaiEed review of the Prairie Run Developmeat, its design cotnputations and considerations relative to surface tivater and other availabie data in order to determine the adequacy of the development's desib s and fiood protection ' measures re(ative to t13e City�s ordinances. Other purposes of this report were to (a)assess the water quality treattnent facilities provided in the Prairie Ru�� Development relative to the Minnesota Pollution Control Agency's {MPCA's) NPDES stormwater permit requirements, (b) assess the impacts of recentfy coi�structed ponding basin downstream of Prairie Run that lies ��est of the Aibert Vi]tas development wouid have on the flood leveis in tlie study area including Prairie Run, anci (c) provide comments relative to the "Revised Affidavit of Brian Mundstock" and Supplemental Affidavit of Brian Mundstock for Court File No. 86-Q6-CV-4997. i':1Mpls';23 MN\8G12386066 Prairie ttun LitiaationlWorkFileslPrairie Run Study�eport Re��ised Current Version.DOC 7 3.0 Background 3•1 Design Standards The desi;n standard for storm sewer systems in the early I900s �i�as significantly different than present-day enDineering design standards. In the early 1900s wheT� many county ditch systems were built, the ditches and,�ipes were desi�ned with litt[e understandin�of rainfall frequency and typicaEly could only handle flows from storms of vp to a �-year frequency rainfaif event. Today storm f7ows are typically ttandled in a system of pipes, ponds, curbs, autters, and overflows that are typically desi�ned to provide up to a I00-year level of protection from floodinJ homes and businesses. Althovgh ditcltes and pipe systems today are typically designed for �- or 10-year frequency events, ihe abo��egroui�d compor�ents of a storrn sewer system {the streets, curbs and gutters, swales, detention ponds, etc.)are designed to convey and manage the excess f7ows for storrns up to the 100-year frequency event so that l�omes and businesses are not flooded. In addition,at ponds and in low-lying areas it is commo❑ to add a Factor af safety in the form of added elevation (freeboard) above ti�e projected flood leve[ or extra volume. Tttis 100-year freeboard is typically 1 to 2 feet for ponding basins and is usually estabiisF►ed by City ordinance. 3•'I-1 Levei-of-Service Level-of-service is defined as the capacity provided by the storm sewer piping system to remove runoff and prevent frequent interference with normal daily transpo�-tation, comcnerce, or access that might result from a rainstorm. C'or example, under this [evel-of-service, street gutters might run fu[l, but when the runoff arrives at the storm sewer catch basins, it eniers the cateh basins and is carried away by the sEorm sewer pipes. Iatersections would not fEood, right-of-��ay would not be impacted, and public iiifrastructure ti�ould operate itorrnally. In ma��y communities, older storEn sewer piping and ditch systems ��ere desibned for a I-year or 2-year evet�t, which is small compared to modern criteria, wi�ich is typically a five- to ten-year tevei of service. 3.1.2 Level-of-Protection Level-of-protection is defined as the capacity provided by a draina�e system to prevent property damage and assure a reasonable de;ree of pubtic safety followinp a rainstorm or snowmeft. �'or example, runoff from Iarge storms miaht exceed the capaciTy of the storm sewer and bypass its catch basins and in turn col(ect in low-lying areas such as intersections and planned/designed ponding P:1�4p1s\23 MN\86�2356066 Prairic Run Litigation\WorkFiIcslPrairie Run Study Rcport Revised Current Versiwi.DOC $ arcas_ However, the flooding wonld not dama�e structures. Accumuiated �i�ater mi�ht temporarily i�iterfere widz traf#ic or access, but right-of-way sl�ould 6e undamaged and public infrastructure should operate nor►nally. Safety should not be significantly threatened, assuming persons use common sense and do not drive into the standinb water or try to walk or swim in fast-fiowing stormv��ater. The drainage system must have the tota[ capacity (in terms of pipe capacity, overland overfiow capacity, and detention pond capacity} to iimit tne fiood elevation to compty with the ahove criteria for an evenT representing the level-of-protection criteria. The modern standard of practice is that the "I00-year" frequency event is typically used as the fevel- of-protection design storm for the combination of pipes, cEirbs and gutters, overf7ow s�vales and po»ding basins systems. The l OQ-year frequency storm is a storm that lias a I percent probability of occurrence in any year. Federal and state programs use criteria based on an event witl� 1 percent probability(the 100-year storm) to define tlte #loodplaii� along rivers and streams and special flood insurance zones. Cities and other draina�e aut(�orities commonly extend this federal staFidard to over#lows and pondin�areas. However, tl�e criterio�� for teve;-of-protection l�as broader application—in addition to ponding areas, lakes, and streams, ti�is criterion can also be applied to locations served by the draina�e systems where there are depressed roadway intersections or other areas where the depth of flooding could pose a significant safety problem. In some instances, fIooding has occurred as a result of a storro breater than the 100-year event and a City may adopt an ordinance that requires the storm tlzat caused the hiQhest �:nown �vater [evel to be tlie leve[ of protection desigr� storm. 3.2 City of Aibertville Design Standards for �evel-of-Protection It is our understandit�g that between t 997 and December 2004 The City estabIislied the following criteria refative to the required level-of-protection. � Sectio❑ ]000.9 (d) of the A[bertville Zoning Ordinance o (d) I�ioh Water E[evation. No structure, except doci;s and retaining�valis shail be plaeed at a❑ elevation such that the lo�resT f7oor, includicig basement floor, is fess than two (2) feet above the highest known surface water Ievei or ordinary high �vater ievel OHW), or (ess than one (i) foot above the 100-year flood level, if determined, of any adjacent 1ake, pond, stream, �i�atercourse or�iretland. If sufficiei�t data on the knowii high water levels is not avai[able, the eievation of tl�e iine of permanec�t aquatic vegetation shall be used as the estima,ed higti water P:\Mpls\23 MN1SG123S60G6 Prairie Run Zitigationl�C�orkFileslI'rairie Run Stud}� Report Revised Curren[Version.DOC 9 (eveL 1t�hen fill is required to m�et tt�is elevation, the fi(I shalt be allo�ved to stabilize and construction slkall not be�in until the property has been inspected by the Buifding Jnspectar. • Appendia A—Subdivision Regulations—Section A-600.4 o (g) �rainage Courses. Lots abuiting a drainage course, wetiand, ponding area, drainage way, channel, or stream sha[i have additional depti� and width, as required uuder the provisions of provisions of the City Zoning Ordinance to assure building sites that are noi subject to ffooding. Lots abutting a lake, pond, stream, watercaurse or wetland shall contain adequate lot area tivhich is capab(e of accommodating a structure with i[s Iowest opening(windaw or door) no less than t��o (2) feet above the highest known water leve( or no less the two (2) feet above the 100-year fiood level. Tn determinin�the high water levels, Sectiot� ]0009 (d) of the City Zoning Ordina�ice shaIC be follawed. • Appendia A — Subdivision Regulations—Section A-700.6 o A-700.6 Draina�e. All surface and underground drainage systems shall be installed to adequately remove aEE natural drainage that accvmulates on tlie developed property. All such systems shall provide a permanent so[ution far the rerr�oval of drainage water. All property�vest of CSAH 19 that draiiis to Wribht County Ditch 9 shall not drain or discharge more than one-half(2) of the pre-development rate of runoff. It is our undersianding that after December 20, 2004 the above ordinanccs were revised to state the following: • Section 1000.9 (d)of the Albertville Zoning Ordinance o (d) High VVater Elevation. No structure, except docks and retaining walls shall be ptaced at an elevation such that the (o�vest buiIding opening is less tha�l two (2} feet above the highest known surface�vater Ieve1 or OHW or emergency overflow elevation, whichever is greater, if determined, of any adjacent pond ar watercourse. If sufficient data on the kno�vn high water levels is not available, tl�e elevation of the Iii1e of permanent aquatic ve;etation shall be used as the f':1Mpls\23 MN18b\23RG066 Prairie Run LitigationlWorkFilcs\Prairie Run Study Report Revised Currcnt Vcrsion.DOC 1 Q estimated higll «ater level. When fill is required to meet this elevation, the fill shalt be allowed to stabilize and construction shall not begin until the property ' has been inspected by the Building lnspector. ' • Appendix A— Subdivision Rebulations—SectionA-b00.4 o (g)Drainage Courses. Lots abuttinb a d:ainage course, ponding area, drainage way, or channel shall have additiona) depth and width, as required under the provisions of p,ovisions of the City Zonil�g Ordinance to assure building sites that are not subject to f3ooding. Lots abuttino a pond or watercourse stial! contain adequate loE area whieh is capable of accommodation a structure with its to«�est openin� (window or door) no less than two(2) feet above the highest k�iown water leve! or no less the two (2) feet above tlle 100-year flood level, or rivo(2) feet above the emergency overflo��v elevation, whichever is greater. In deterniining the I�iDh water levels, Section ]�00.4 (d) of the City Zoning Ordiaance shall he follo��ed. � Appendix A—Subdivision Regulations—Section A-700.6 o A-700.6 Drainage. All surface and underground drainage systems shail be instalied to adequately remove all ❑atural drainage that accumulates on the developed property. Ail sucli systems shall provide a perma«ent s�lution for the removal of drainage water. All �roperty west of CSAH 19 that drains to Wright County Ditch 9 shali not drain or discharge mo��e than one-half(1/2) of the pre-development rate of runoff. P:1Mpls\23 Mt�'\86\2386066 Prairie Run Litigation\WorkFileslPrairie Rwi Study Report Kevised Current Verslon.DOC 1 1 4.0 Modeling 4.1 Previous Modeling Efforts On June 24-25, ?003, tl�e City experienced an 8.5-inch rainfal[ cvent that resulted in significant flooding of low-lying areas. Followinb tl�at event the City commissioned the consultinb enoineerin� firm of SEH to perform a hydrotobic analysis of the event in 2003 to assess the reasons for floodina and identify remedia] measures for the 1.00-year event. SEH utilized the Soil Conservation Sen�ice {SCS) hydrograph method (curve number methodoloby) to generate runoff volumes a�id rates that were then hydraulically routed using a hydrolobic and hydraulic computer model know as XP-SWMM. The resulting June 23, 2004 I'lood Study Report concluded the existina drainage system can handie runoff fro�n the 100-year, 24-hour SCS Type II storm event. in the summer of 2005, runoff for storm events rangina between the 5- and �0-year evet�ts caused several storrnwater detention ponds to overflow their bank and encroach onto private property. On a fe�v occasions durin� 2005, the City installed temporary measures of sand bags and pumps to help protect a few homes (County Ditch #9 Flood Study; BoIton & Menk, 2006). Due to the floodin; concerns raised by the 2005 flooding, tl�e City commissioned Bolton &Menk Inc. to coriduct the 200ti County Ditch #4 rlood Study and subsequent Amendmet�t. Bolton & Menk also used the SCS curve number hydrograph method to benerate runoff votumes and rates. How�ever, rather than using the XP-SWMM model, Bolton & Menk developed a cfetai(ed HydroCAD modei to route ihe runoff throueh the stormwater conveyance network. Tf►ey utilized ne�v features in the recent version of H��droCAD tliat allo��s the user to specifically instruct the program to iook at downsiream water lcvels �i-hen computing discharge rates from upstream ponds and in specified instances allow flow reversals. The complexity of the draic�age system to Wright Cout�ty Ditch9 warranted tlzis type of model capability and the Xi'-SWMM and HydroCAD models can accorncnodate such conditions. The 2006 Amendment recommended adding storage vofume upstream of the Albert Villas development and ei�coura�ed tl�e City to work with School District No. 885 in order to develop the required fIood storabe that might lower flood tevels in tlte study area. Ultimately, ihe City acquired the necessary land in St. Michael and is currently constructing a Iarge flood level mitigation project adjacent to the new elementary schooE_ P:1MpIsF23 MIv`\86\2386066 Prairie Run LiYigalion\WorkFileslPrairie Run Study Repori Reviscd Current Version.DOC 12 4•Z Barr's Detailed XP-SWMM Computer Model A detailed analysis of the existing storm sewer system was comp(eted to determine the t00-year flood levels and the high water elevations at various locations in the CouF�ty Ditch 9 watershed and ensure thai the proposed revisions to the existing stormwater syste►n provide City desired flood relief. The input to the model was developed from daLa provided by the City, Botton & Menk, and SEH and additional #ielc! reconnaissance performed by Barr Engineering Company. Devetopin�a detailed XP-SWMM model for the entire ��atershed tributary to the County Road 1 i9 (also kaown as YS`" Street NE) crossing over Wright Cou►ity Ditch 9 involved the follo��i�tg major tasks in order to fully model the drainage system that could impact the flood tevels in the Prairie Run Development. 1. Corrtpiling all pertinent data ihe City, $olton �: Menk, and SEH provided Barr abou# the stormwater conveyance system in the study area. 2. Field delineate the major�vatershed boundary for the study area. 3. Delineating and enter into the computer model the sub«�atersl�eds needed to properly analyze t}le various drainage ways in the watersl�ed. This task utilized the detailed sub«�atershed divides developed in Bolton & Menk's 200b Wright County Ditch 4 Flood Study. 4. Devetoping and calibrating the XP-SWMM comp�iter model of the County Ditch 9 drainage area and existing stormwater conveyaace systents to (a) assess the validity of the computatioi�s used for desibn of the Prairie Run system componerits; (b) compare previons per#�ormed model results; (c)analyze the storm drainage systein for the SCS Type II ]00-vear, 24-hour storm event; and {d) deterrnine the hibt� water elevation in Prairie Run for tf�e June 24-25, 2003 storm event. 5. Modifying the XP-SWMM model to analyze the corrective option developed by Bolton & Menk to iower the flood levels in fEood- rone areas for p the SCS Type II 100-year, 24-hour storm event. 7'he existin c • g onditions niodel was modified to determine the � nfluence on flood levels in the study area of the recentfy constructed �onding area adjacent to the new St. AQichae(/Albertville Eiementary school as designed by Bolton & Menk. The United States Environmentai Protection Agency's Stortn Water Mariagement Mode1 (SWMM), with a computerized graphical interface provided by XP Software (XP-SWMM), was chosert as the P'�Mpls\23 MN1S61238GOb6 Prairie Run LiligationlR�orkFileslT'rairic Run STudy Report Revised Current Version.I�QC 13 surface water computer-modelinb package for tl�is study. XP-SWMM uses rainfall and watershed information to generate runoff that is routed simultan�ously through compiicated pipe, channel and o�-erland flow neri�orks. Simultaneous routing means that flow in the entire system is modeted for each time inerement of a storm simn(taneous�y, then the modet moves on to the next time increment durinc a storm, and so on (many other mode(s calculate runoff by subwatershed for the entire duration of the storm, before moving to the next sub�vatershed}. Simulta��eous routing allows the mode[ to account for flows in pipes, flows detained in ponding areas, the effects of back�vater conditions(such as downstream culvert or channel restrictions), and the complexity of routircg fiow I reversals ali of which do occur in tl�e study area. I The 1000-node version of XP-SWMM, Version 924, was used to simuttaneously modei tl�e siorm se�ver and overiand flow systems within the VVright County Ditch 9 study area. Data was input re�arding: (a) ditch and pipe locations, sizes, types, materials, and elevations; (b} stora;e basin elevation, volurne, and outflo��characteristics; {c) surface flow characteristics(overflow s��ale geometry, length and slope of gutters, etc.); and (d) rainfall amount and distribution, 4.2.1 Hydrologic Modeling T�vo major types of information are required by XP-SWM.M for hydrologic modelin� using S��MM's RunoffT�'on-linear Reservoir Mezhod: (I) watershed data and (2) climatic data (or precipitation). Tliese data are ttsed by XP-S WMM to generate runoff or inflo�� hydrograplis at various points to the stormwater conveyaace rtetworks. Barr utilized SWMM's Runoff Non-li�zear Reservoir Method to generate subwatershed runoff volumes and rates as a means to independeatly verify prior modeli�tb using SCS methodologies. 4•2.9.1 Watershed Data 4.2.�.9.1 Watershed Area Watershed drainage divides were delineated in the ArcMap beo;raphic information system (GIS) for ttie entire study area using 2-foot topographic data, field observations and detailed watershed divides developed by Boltoi� & Menk as part of its 2006 County Ditch 9 F(ood Study, including Bolton R: Me�ik's subwatershed narning conve�ition. Most of the Bolton & Menk's waterslied divides were verified usin�current 2-foot topographic data. The delineated watersheds within tlie study area are depicted in Fignre 2. P:1Mplst23 1�4�I\86\2386066 Prairie Run Litigation\WorkPilcsU'rairie Run Stud��Report Revised Current Versian.DOC E 4 4•2•1.1.2 Land Use and lmpervious Data All development practices witliin a watershed impact the quantity of runoff generated. Each la�id use generates a difierent quantity of runoff due primariJy to the arr►ount of impervious area within that watershed. The impervious areas i�iput into tlie XP-S WMM compater model to�enerate n.tnoff must be, by de�nition, hydraulically connected to the drainage systems being analyzed. This direct or connecte� impervious percentage includes driveways, roofrops and parkinb areas that are directly connected to the storm sewer system. Runoff from the portion of a rooftop draining onto adjacent pervious areas was not tceated as connected impervious areas. Pigure 3 shows the existi�tg land use plan usec! for the complete stady area. These land use data were based on Albertvi(!e's Zoning Map, the 2001 USGS I�ra�ionaf Land Cover Dataset and 2043 aeria!photographs. Tota1 imperviousness was estimated using satellite-derived (LandSat) data deveioped by the University of Minnesota for tl�e MPCA. Once the various laad use cate;ories were reclassified with a consistent land use system, the percent impervioF►sness by Iand use was deterinined by overlaying I�� the land use d�veloped for the study area with ihe 2000 LandSat-derived estimates of imperviousness I for the state. TE�e direct or connected impervious percentage w�re estimated using the aerial photobraphs and the total impervious snrface pertaini�i�to each land use type based on LandSat data. The impervious percentages used in this study are listed in �'able 1. Ti�ese percentages were applied to each �vatershed based on the respective amounts of the different Iand use types{see Fi�ure 3) within each watershed. The averaae directly connected percent impervious area over a11 subwatersheds was 14 percent of the total watershed area, with a minimum and maaimam of 0 and 36 percent, respectively. Ail data was digitized into ArcMap, version 9.2, a geobraphic information system (GIS), to create an electronic inforrrtatio , ❑ database that was used for the anal sis. Th y e land i use and im ervi p ousness assumptions were not modified throughout the XP-S`'VMM anaiyses due to !ack of d ata for caiibr ' , at�on of flo� �s. 4.2.1.1.3 Watershed Width and Slope Subwatershed width and slope have a signifcant irr�pact on the surface runoff timino, �eak fio�u rates (i.e., impact the time of concentration), and rur�off volume. "I'I�e US EPA's Storm Water Management Mode! User's Manuat suggests that a �ood estimate for the width can be obtained by first determining the maximum length of overland fIow aiid dividing the snb�vatersl�ed area by t}iis iength. Therefore, the subwatershed widths �vere computed using this sug�estcd methodoio�y. The seIected hydrologic methodolo�y generates runoff from each ofthe subwaterslteds by approximatit�g the�n as ►�on-Iinear reservoirs. TE�e runoffrates are computed as the product ofvelocity(from Manning's P:lMpls123 M]V\g6�386066 Prairie Ran Litigation\1§�orkP'iIcs\Prairie Ran Study Report Revised Current Vcrsion.DOC 1 5 . �, equation based on the difference betwcen total depth and depression storage and the average subtivatershed slope), and flow area(depth times width). If overland flow is visualized as runnir►g dotivn- slope off an ideaiized, rectancular subwatershed,then the width of the subwatershed is tl�e physical width of overiand flow. The tateral flow per unit width is computed and multip[ied by the width to obtain the runoff rate. The SWMM user's manua� (Starm Wa[er Management Model,- I�ersion � User's Manual }9S8) a�so indicates the width parameter shouid be used for peak runoff calibration. However,the widths were not modified for the model calibratio►� simulation because the computed flood ]evel closely matched the observPd eEevations. The subcatchment slope should reflect the average siope of the individual suhwatershed. Using simple �eometry, the calculation is simply the elevation difference divided by the length of fl�w, Using more corrtptex geometry, such as tl�ose used in modeling, several overland flow pathways are typically de(ineated, Eheir slopes determined, and a w-eighted s[ope computed using a path-length weighted average(Siorm Water Management Mode[; Version 4 User's Manual, EPA 1992). The average slope (ft/ft} ior each watershed was estimated using Arc View scripts deve]oped by Barr Engineering, spatial analyst, and a Digital E(evation Model (DEM) for the study area based on the 2-foot topobraphic data. The slopes �vere �ot modified for any of the historic event or design storm simuiations. 4.2.9.1.4 /nfiitrafion Data Wright County soil sarveys published by the USDA National Resource Conservation Service (USDA NRCS, previousiy known as the Soil Conservation Service—SCS) �i�ere used to determine the hydrofogic soil grvup(F-ISG) classifications of the soils within the study area. Figure 4 depicts the HSG classifcations throughout the study area. Tl�e predominatit soil type in Ehe study area is SCS Type B (sandy loam). [nf�ltration is the movement of�i�ater into The soil surface, For a given storm event, tlie infi[tration rate wil( tend to vary with time. At the beoinning of the storm, the initial infiltration rate is the maximum infiltration that can occur because the soit surface is typically drier and full of air spaces. As the storm event continues, tl�e inftltration rate will tend to gradually decrease as the soil air spaees fill with water. For long duration storn�s, the icifltration rate wilI eventually reacl� a constant ��alue, the Enirtimum infiltration rate. The Horton infiltration equation �i�as used to simulate tl�is variaTion of infiltration rate �vith time. P:1Mpls�23 MN\8G12336066 Prairic Run Li[igation\�[�orkl�i]es�F'rairie Run Study IZcpoR Revised Current Vcrsion.DpC 16 Horton infiltration parameters ��ere input into the model for each subwatershed. These parameters are used fo� the generation of runoff from the individnal subwatersheds. Horton in�itratian input pararneters include: Fc(minimum or nitimate value of infiltration capacity), Fo{maximum or initial value of infiltration capacity) and k (decay coefficient). Table 2 summarizes several XP-SWMM input parameters used for the County Ditch9 analysis. The table includes the Horton infiltration values for each hydrologic soil broup used to calculate camposite infiftration parameters for eac1� watershed. The Pc, Fo and k values were selected usina the guidelines established in the SWMM user's manuai. Composite Fe, Fo and k, values were calculated for each subwatershed in study area. These inputs were weighted to account for the pervious percentage and unconnected impervious percentage. Global databases v��ere set up, read into the models for every watersiled, and named the same as the respective watershed. 4.2-�•1.5 Depression Storage Data Depression storage inputs, the areas tl�at must be fiiled with H�ater prior to generating ru��off from both p�rvious and impervious areas, ��ere set within the genera( range of pub[ished values, lt represents the iniiial loss caused by such tl�ings as surface ponding, surface wetting, and inierception. The inodei hand[es depression storage differenily for pervious and impervious areas. 7'he imper��ious depression storage is replenisl�ed during dry simulation periods by evaporation. The �vater stored as pervious depressio�i stora�e is subject to both infiltration and evaporation. The pervious atld impervious depression storabe inputs are summarized in Table 3. 4.2.1.1.6 Qverland F/ow Roughness Data Overland flow is the surface runoff that occurs as sheet flow over land surfaces prior io concentrating into defined channels, ln order to estimate the overland flow or runoff rate a modified version of Mani�ing's equation is used by XP-SWMM. A key parameter il� the Manning's equatioii is the rougltness coefficient. Tl�e shallow flows typically associated��ith overland fiow result i►� substantial increases in s��rface friction. As a resu(t, the roughness coefficients typically used in o��en cnannel flow calculations are not applicable to overland flow estimates. These differences can be accounted for by using an effective rou�ltness parameter instead of the typical Matining's roug}�ness parameter. Typical values for the effective roughness parameter are pubtished �n the Army Corps of E�igineer's HEG1 User`s Manual, September ]990 and in Engineering Hydrology: Principles and Pf-actices P:\A4�Is\23 MN18612386066 Prairie Rua Lit3gationlWorkf'i(eslPrairie Run Study Report P.cvised Current Version.DQC 1 7 {Ponce, i 989). After revie�ving tlle above references in combination with the land use data, the pervious and impervious effective rougliness parameters iisted in Table 3 were selected for this study area. An area weighted perviovs roughness�vas determined for each subwate:shed in the study areas by weightinb the pervious area and unconnected impervious area. 4.2.1.2 Rainfail Data 4•2•�.2.9 2003 Rainfall Event Precipitation and flooding data from the June 24-25, 2003 rainfall event�vas available from SEH's previous modeling effort. However, after a review of tl�e SEH report, Barr determined that tf�e rainfali distribation used in the SLH analysis and subsequently in the Botton & Menk calibration originated from data coltected at the Crystal Airport, approximately 20 miles southeast of Albertvi[le. Since thunderstorms can vary considerably fron� one location to the next, even within a mi(e or two, Barr compared NEXRAD radar data available for the Crystal area to tl�at available for the Wrigi�t County Ditch 9 watersl�ed. The NEXRAD radar data available from the Nationai Climatic Data Ce►�ter{NCDC) for the June 24-25, 2003 storm at the Crys[al Airport was significantly different from tne data it had for what occurred ia the Wright County Diteh 9 �vatershed. Based on N�XRAD radar data available for the Nationa! Climatic Data Center(NCDC)the June 24-2�, 2003 storm ai the Crystal Airport iikely had a significantly different clistribution tizan what occurred at Albertville. Therefore, Barr Engineering colEected and utilized NEXRAD radar data from NOAA for the June 24-25, 2003 storm event to devetop a site-specific rainfall distribution for Albertville. NEXRAD collected rainfall intensity data at five-minute intervals for the J�ine 2003 storm. Therefore, the data pravide a very detailed look at the rainfall at Albertville. However, NEXRAD data indicatecf that the preci�itation total for the storm event was onty 6.3 inches (see Figure 5. Since Ioca! rain gage n�easuremertts in Albertville recorded 8.� inches of rain for the June 2003 event, the incremental rainfall totats measured by NEXRAD were adjusted to yield a storin total of 8.� incl�es. 4•2•1-2•2100-Year Storm Evenf Rainfall Data Rainfall a�naunts for The various storm durations and frequencies were obtained from the U.S. Department af Commerce's Technical Paper No. 40—Rainfatl Frequency Atlas of the Uiiited States (TP40). According lo TP40, �.8 inches of rainfall in a 24-hour period is considered a 100-year, 24-hour duration rainstonn for the Albertville area. Additional rainfatl totals for other storm ?:\MpIs123 MN18612386066 Prairie Run Litigationl��rorkFiles\Prairie Run S[udy Report Re�•fsed Current Version.DOC I g frequencies in tl�e Albertville area are iisted in Table 4, but the 100-year, 24-hour event was t��e prime focus for this study. 4•Z•2 Hydraulic Modeling Hydrographs �en�rated from hydrologic inodelinb in XP-SWMM were hydraulically routed from overiand conveyances and into tl�e storm sewer network. 4.2.2.1 Storm Sewer and Pond Network Data for the storm sewer, culverts, and ditch systems in the study area were taken from Boltort & Menk's HydroCAD modet. The available detetiTion volumes in wetland and ponds was also gathered from Bolton & Nenk's HydroCAD mode! and several of the storage areas were checked against the 2-foot topograph3� data for accuracy. The storm sewer data gathered from the HydroCAD mode[ include pipe inver-t elevations, len�ths, diameters, and material type. All e[evations entered inta the model are in feet above Mean Sea LeveE. Roughness coeffcients(Manning's "n") for all conerete and corruQateu meta( pipes were estimated as 0.012 and 0.024 respectively (the typical design values). Outlets from ponc�inb areas that may be in}et controlled were modeled in XP-SWMM assuminb a groove end projecting concrete pipe or corrugated metai mitered to the slope inlet condition. This allowed };P-SWMM to determine tlie controlling fIow condition in ±he outlet pipe (i.e., whether t}�e fiow i�i ti�e pipe is controlled by the irilet size, barrel capacity, or taitwater conditions) and accurately estimate the poad's �i�ater surface elevation. Prior to computing the hydrauiic grade !ine, al! ener�y [osses (minor iosses) in pipe runs and junctions tieeded to be estimated. In addition to the principal energy involved i» overcoining the fric,ion in each conduit run, energy (or head) is requirea to overcome chan�es in momentum or turbuience at outlets, inlets, bends, transitions,junetions, and access holes. The Urban D�'ainage Design Manual Hydr•aulic E�igineering Circzclar 22, Second Editrorr (U.S. DOT, Aubust 2001) was used to estirz�ate the numerous head Iosses in the study area stormwater conveyance system. 4.2.2.2 Ditch and Qverland Ffow Network Data for the ditch s}�stem and overland flow network were taken frocn ground elevations marked on developmer�t drawin�s and City's topograpl�ic data. Manning's "n" for tl�e surface flow chai�nels «�as genera[[y set equal to 0.03. P'\.rvlpls123 MT��\86123S66G6 Prairie Run LitigationllVorl�Fi►eslPrairic Run Study Report Revised Currcnt Versian.DOC 19 4.3 Model Results 4.3.1 Model Verification To help gauge �vhether the watershed computer modei prepared for the area would accuraYety modet the 1�iohest known water elevation in the Prairie Run Development and else where in Albem�iIle and the i00-year frequency event, attempts were made to duplicate reoorted f7ooding that resulted from the late-June 2003 storm which drop�ed 8S inches of rain in tl�e Albertville area. In order to accurately model the June 2003 storm, the XP-SWMM model was developed for the field conditions (lartd use, culverts, and storaae areas) present in June 2003. To assess tiie model's prediction of actual flooding, rnodef output for the June 24-25, 2003 storni modet was compared to observed/photograpi�ic flood elevation data presented as part of Bolton & Menk's flood study. The calibration results are summarized in Table S and on Fibures 5, 7, and A for tlyree locations in the sttidy area. Based on these calibration results Barr's XP-SWMM model appears to accurately reproduce the observed flood levels fron� the June 24-25, 2003 storm event. Therefore; the model should in turn accurately predict the flood levels resulting fro►n the ]00-year frequei�cy rainfafl event. Because the June 2003 storm dropped ttte I�ighest n�easured amount of rair�fall for a single stonn iTt AEbertville and because it occurred at the time, the development plans were prepared for the Prairie Run Developn�ent, it is assiEmed to have generated the l�ighest known water levels in ttie Prairie Run Development area. Therefore, tl�e June 2003 storm event was used both to generate the highest known water levels and to calibrate the model for generating the f 00-year f#ood levets. 4.3,2 Predicted 100-Year High Water Levels Once the modef was determined to accurately model the June 2003 event, Barr�nodi�ed ti�e calibrated XP-SWMM mode! to reflect recent improvements to severa! crossings atong Wright County Ditch 9 (CSAH 18 and Coanty Road t l9)and the Prairie Run Developineni and to ge�ierate 100-year flood levels respective of those itnprovernents. Table 6 is a summary of the modeled 100-year flood levels aT key locations in the study area. Based on these mode] resufts, there is general agreement between Barr's XP-SWMM model and E3olton & Men[:'s HydroCAD model. I3otl� the estimated "hi�hest known higl� water levef" and the 100-year flood levels are significantly hiplier than tl3ose estimated by the developer's enoineer for the Prairie Run Deve]opn�ent. Table 6 provides a comparison of the "hibhesi knawn water levef," the 100-year fTood level and the deve[oper's engineer estimated flood level. Therefore, tlle low floor and entry elevaiions used for that development may itot provide the ❑ecessary level-of-protection to meet tlie City's ordinances. P:1Mp[s123 MN;86\238G06G Prairie IZun l�itigationlWorkFiles\i'rairie Run Stud�� Report Revised Current Version.DOC 20 Sincc the Prairie Run Development�vas under design during the June 24-25, 2003 storm event, and in accordance wiTi� City ordinance at the time of the developmettt tne "hi�hest know water levefs" caused by ti�at event should have been measured by the design engineer/surveyor and should have been used to establish tt�e minimum building elevation for the developme►�t. That eievation would have likety been 9�2.1_ There was also pictorial information of flooding aiter that storm, as presented in Bolton & Menk's report at 5337 Kahler Drive, �vhich is located just upstream of the Prairie Run Development. A measured water surface elevation during that storm revealed that, at the time stamp on the photograph, the flood leve[ was 9�1.47. This �;�ater surface was lower than the hiahest poinT it had reacned at that location during tl�at storm. Our mode) matclied t13at water elevation for the time stamp at that location during that starrrc. Based on a flood levei of 9�2.1 at I-Iigh�vay i 8 and County Ditch 9, and the ordinance current at the time the developEnent plans �vere being prepared and modified, the low floor elevations for ti�e developmenT should have been set at 954.1. Since tf�e grading pians and other information supplied for this revicw for Prairie Run do not contain as-built or proposed low floor and entry elevations, individual lots were not reviewed. 4.3.3 Mifiigation Measures $arr modifec� the XP-SWMM model used to estimate the 100-year fiood levefs to incorporate the City's 2007 flood mitigation projeet adjacent t�the new etementary school in St. Michae] (see Figure 9}. [3oiton & Menk supplied Barr with tlte ;rading, draina�e and erosion contro) plan and other design information for the proposed project. Assztmin�;this project is constructed as designed, model predictions indicate that the �ood level of the Pond SW of�2nd & Kahl (Kah1 Ave Pond) wii! be reduced from 954.I to 951.4. Fn addition to reducing the flood leve[ at this location, the proposed mitigation project will reduce the Ip0-year flood level aiong VVrig[�t County Ditch 9 upstream of CSAH 18 by l.0 foot. The significant reduction in the Wright County Ditch 9 fiood level adjacent to the Prairie Run Devefopment is due to a reduction in water level dowtistream of CSAH 18 because of the additionai runoff beino detained in t]le ponding basin south of the St. Michaei/Albertville elementary school. P:\Mpls\Z3 M1v\b(1238G06G Prairie Run Litigation\�'�'orl:FileslPrairie Run Study Report Revised Current Vcrsion.DOC �1 5.0 NPDES Permit Requirements for Prairie Run — Development Barr Engineering reviewed the Prairie Run Development relative to conformance with the water quality requirements of the Mir�nesota Pollution Control Agency's NPDES stormwater permit. E3ecause the project disturbed more than 5 acres, an MPCA NPDES stormwater permit was required. The NPDES permit required: • Providing permanent (dead)storabe vo[ume(a volume of water below the pond's outlet control elevation) of 1,800 cubic feet per acre of��atershed tributary to the basin. • Tlie permanent storage volume must have a minimum depth of 3 feet and not exceed a depth of 10 feet. • The basins o�t(ef must be designed such that the «�ater yuality volurr�e is dischar�ed at a maxicnum rate of�.6b cubic feet per s�cond per acre of pond surface area. • The basins outlet m«st provide a skimming device to prevent floatable materials from passing downstream. The MPCA's requirements are geared toward preventin�sediments from f7ushing or being re-entrained in the flows through the basin. Regardtess of the upstream permanent storage vofume, the MPCA requires the flows from upstream tributary areas to be factored into the design of on-line systems to lielp ensure flusl�ing of seitled sediments does not occur, Table 7 compares the pond designed by the developer's engineer and the NPDES permit requirements for the Prairie Run Deveiopment. Since the Nort�i Pond was designed to route upstrearn off-site flows through tl�e basin, the entire upstream watersI�ed ttibutary to the pond needed to be considered when sizing tl�e permanent storabe volume and outlet structure in accordance witIl the MPCA's NPDES permit requirements. Based on T3arr's computatio�zs the north pond in tlie Prairie Run Development does not meet the NPDES design requirentents. In order to ilieet the NPDES requirements, either an additional 239,2�3 cubic feet of permane�t water storage voIume would need to be provided or the off-site flows �vould need to be diverted around the water quality pond. P.^,Mp1s123 MN186\23S606G Prairie Run Litigatian\�'�'orkFileslPrairie Run Study Report Revised Current Version.DQC 2? fi.0 Comments Relative Brian tUlundstock's Revised and Supplemenfiai Affidavifis Comments on Revised Affidavit of Brian Mundstock Court Fite #86-060-CV-4997 Paragraph No. 5: Mr. Mundstock states: "It is my professional opinion, based on my experience as a professional engineer, that in att situations, including platting and deveiopment of residential subdivisions that regional and area flood levels, incEading, but not Iimited to, the l OQ-year ftood level, are set by the appropriate�overnmental authority which could include the city, county, watershed distriet, or the Federal Emergency Management A�ency." Barr's comment: VVe generally concur with that statement. However, in areas where no fEood level is determinsd; we believe that the design engineer should perform basic due diligence to ❑nderstand the watershed in which their particular development lies and ascertain the flood level and the potential of Iligh-�vater f]ooding elevations. In This particular case, during the time the plans for the Prairie P.tin Development were under ' preparation, the June 24-2�, 2003 event occurred and generated a high-�vater elevation within ' tlte Prairie Run Developinent area. The City's ordinance That was current at the time specifically required [ow floor elevations to be established 2 feet above the "highest �:nown I water elevatiocz" as tl�ere was no determination of the 100-year flood level and tl�ere was sufficient data on the known high ��ater level. The etevation of the debris ditch line left from that storm should have been surveycd by the engineer for tlle Prairie Run Development, which in turn should have been used to comply with the City's ordinance. Paragraph No. 6: In paragraph No. 6 of the Mundstock affidavit, Mr. Mundstock indicated that typicalty the tocal Dovernment authority will tell the developer wl�at the ]00-year flood levels are, or if a comprehensive hydrologic study has not been canducted of the surroundin� watershed, tl�e loca( government authority will tell the deve[oper�a�hat to use as the estimated high-water levet. Sarr's comment: It is our opinion t[�at this is a moot point in this case as the June 24-25, 2003 storm and its resulting flood ieve[ in tl�e Prairie Run Development area, combined with the City's ordinance that was current at ihe time of the develo�ment clearly would i�ave establistled the "highest k�lown water elevation" and in turn tlie elevation of the lo«�est possible t�oor elevations in the Prairie Run Deveioprrtent. P:1Mp1s123 MN\8b\2386066 Prairie Run LiiigationlWork�ilcsiPrairie Run Study Report Revised Current Version.DOC 23 Paragraph No. 9: Para�aph I�'o. 9 of the Mundstock affidavit states: "In this case, the developmenT agreement and Zoning Orclinance l 000.9, Subdivision D specifically allow use of tlle line of permanent aquatic vegetation to estimate the high-water elevation for Ihe Pra[rie Run Development. "The high-water e(evation is the functionai equivalent of the ordinary hi�h water level but is to be distinguished from the highest known surface water leve! each of�vhich are distinct terms of art in the development agreement and derived from the ordinance. The high-water elevation or ordinary high water level for the Prairie Run Development was based on the line of permanent aquatic vegetation ar.d the grading plans relative to the preiiminary and final plat reflected this." Barr's comment: The City's ordinance that was current at the time the Prairie Run Developtnent plans were being prepared states "no structure, except docks and retainirig walls shail be placec! at an elevation such that the lowest floor, inclnding basemeitt floor, is less that t�vo(2) feet above the highest known surface water level or ordinary hibh water Ievel {OHW), or less than one- (i-) foot above the 100-year flood level, if determined, of any adjacent lake, pond, str�am, water course or wetland." The ordinance goes on to prescriptiveiy say, "If sufficient data on known hibh water levels is not available, the elevation of the line of permane��t aquatic vegefation shall be used as the estimated high �vater elevation." In this case, we believe that "sufficient data" was available on the highest know water ]evet as the June 24-25, 2003 storm occurred rigt�t when the plar3s were being developed. And, because"sufficient data"was available, then the [ine of permanent aquatic vegetation could not have beeil used to be in compliance with the City's ordinance. The June 24-25, 2003 storrn f7ood level in the development was certainly higher than the OHW and no 100-year flood level had been determined for that area at that time. Further, we understand the developer's en�ineer has been in the busi��ess of development design, inclpding detention basin designs, for at least 30 years and we, therefore, believe the developer's engineer should know that the OHW mark is the line distinguishing t)te difference between wetland vebetation and upland ve�etation, wllich is more a result of the affect of shallow groundwater and re�uiar snrfacc flows and not deterministic of flood elevations of rare and severe storm events. TIie OHW is not an indication of how high floodwaters will bounce in an area, and we beiieve that regardless of the City's ordinance; the developer's enaineer shou}d have known this and not used the OHW in its deve[oprnent Paragraph No. 10:Tlte Mundstock affdavit skates "In my experience, if tlie City engineer had any probleens or issues with the lirte of permanent aquatic vegetation being used as the estimated high- ��ater levei for Pi-airie Ruit Development, or if the Ciiy needed any additional information, or��anted P:1Mpls\2_i MN18612386066 f'rairie Run Litibation\WorkFileslPrairie Run Study Report Revised Curreni Vcrsion.pOC 24 the developer to use a different fibure as estimated high-��ater level, then the City would let tne deve(oper kno�v this very earty in the process. Given the City's compiete silence regardirtb the developer's use of the line of permanent apuatic vegetation to estimate tlte high-water elevations in the Prairie Run Development, and biven the fact that the development agreement and tlie re3evant ordinance specifically allowed ihe use of this method to estimate the hibh-water level, the developer was ia entitted to use it and rely upon it iii absence of information or instructions from the City to tlie contrary." Barr's comment: We do not agree that the developer was entitled io use ttte line of permanent aquatic vegetation to estimate the high-water elevations for the Prairie Run Developtnezit because: (a)the.�une 24-2�, 2003 stonn set flood level �recedence as it had occurred right wlien the development plans were being prepared;(b) the developer's engineer shou(d liave knowr� futty that the tine of permanent aquatic vegetation, regardless of it being aliowed to be used by City ordinance, should not have been used as it is not indicative of the f7ood bounce from a rare t 00-year type storm; and (c) basic due diligence shouid have been performed by the deveioper's engineer to uYi(ize the readily avaiiable USGS Quadrangle Topographic Maps, that clezrfy reveal a lar�e drainage area tt�at drains through and near the Prairie Run Developme��t, including the Iikelihood of a significant flood bounce that was higher t}ian tlie OHW. Figure [0 is a copy of ihe USGS Quadrangte Topograpttic Map showittg the drainage area in the vicinity of Prairie Run Deve[opment. It clearly reveals a si�nificant drainage system drains along the west boui�dary of il�e development. Figure t 1 sl�ows the comparison bet«-een the drai�iage area developed usin�tlie USGS Quadran�Ie Topogra�hic Map and simple fietd verifcation (925 acres)and the significantly smaller area that the developer's engineer used for tiis liydraEi[ic computations (39 acres}. The size ofihe calvert u,ider CSAH 14 slionld have been a further itidication that there �vas a significantly larger drair�age area, flow rate, and volume than was considered and ttiat ti�e line of permanent aquatic vebetation shouid not have been used. In our opinion: (1) a simple review of the USGS Quadrangle Tapography Map; (2) a measurement of the culvert that passes through CSAH 18; and (3) a ca[1 to the County's Highway Department to obtain its desi�n i�iformation for the cuiverts under Com3ty Road 9 artd CSAH I 8 by the developer's engineer should liave revea(ed this situation. We be[ieve P:'�Mplst23 MN`,86\23b6066 Prairic Run Litigation\WorkFiles\Prairie Run Study Report Revised Current Version.DOC 25 � the developer's engineer should have, in turn (a) performed flood level computaTions based ', on the sizeable drainage area, or(b) requested the Ciry to perform those computations, or II (c) closely worked wiit� tne City to establish �vhat elevations to be used for critica] � development features, such as low f7oors, roads, eieva,ions, eic. We believe that the ' developer's engineer, when preparing the developsnent plans, shouJd have performed tliis I basic due dili�ence and proactively and agaressively worked with the City to coordinate and establish the appropriate elevations io be used for key development features. From our I review of information provided, there is no indication that much, if any, of this basic due II diligence �vas performed by the developer's engineer. II Further, in an October 1, 2004 letter, SEH and Hedlund Engineering were clearty put on II noiice by Wright County Soil and Water Coi�servation District(SWCD) resource conservationist, Colleen Atlen that they were very concerned about the l�ydrolobic and hydraulic computations performed for the development. In the letter(Hedlund Deposition II Exhibit �3}, Ms. AIEen states: "The storm��uter calcularions pr•esented do not consider the �vater coming in from o_ff site II throz�gh the culvert under County Roc�d 19. This culvert drains a DNR-protected water I! and has the potential to inirocr'irce large amounts of water into the storn�water system. " "O�Te final, but ntost intportant note of caution. Our office remains concerned about the I 11ood potential tivitl� regards to Ditch 9. We have seen this wlrole area back up and flood I when Ditch 9 backs up. We think the City should do a con�prehensive lzyclrologic study of the Ditch 9, ii7cluding ererything that drains !o it, to deternzine flooding�otential and review possible opportzrnities for remedial measur-es. " In a letter to Todd Udvig of SEH, written by Randall Hediund of Hedlund Engineeriitg on October 13, 2004 in response to the Write County SWCD's October l, 2004 letter, Mr. li Hedlund wrote: I "Potential flooding fi•onr the Ditch 9 backup �-Us takerr inta account when designi�zg the plaf7. I tivo��ked tii�ith your St. Cloud office to come up with minimunt elevations for I buildings based on the 100-year flood elevatiofrs tivith Ditch 9. " ', At the time of the Wright County SWCD letter and the Hedlund Ietter there was still cliance II to chanbe building pad, roadway, and other elevations had the en�ineer a��d deveEoper taker� � these cautions seriously. Even though grading aetivities l�ad started, construction on the first ' house in ti�e devetopment did not begin until at least February 4, 200� as tl�at was ti�e date I P:1Mpis123 MN`�86t238606G Prairie Run LitigationlVJorl:FileslYrairie Run Study Report Revised Current Vcrsion.DOC 2b the first home buildin�permit for the development was issued, four months after Wright County's cat[tionary notification. Paragraph No.92: The Mundstock affidavit states: "In my professional eaperience, one of the nonnal and expected daties of the City Engineer is to revie�v ti�e preliminary and final plat subdivisions desigr.ed by a developer's engineer, including grading, drainage, and storm sewer plans to determine whetlter there are any engineering issues or deficiencies �vith those subdivisions, and to determine whether those subdivisions are in compliance with the pertinent City ordinances atld/or the development a�reement, as the case may be." In my professional experience, if the City engineer has any concerns about the p�at subdivisions, or nc�tices any deficiencies or if the submissions faii to comply �vith the City ordinances or ihe deveiopment agreement,the City engineer always communicates those concerns or deficiencies to the City Counci[, �vho in turn, either approves or denies the pfat, or requires the deve(oper to revise the plat submissions, based upon the comments of the City Enbineer. In my professio�iai experience, the Cify engineer atways conducts this revie�v priar to final plat approvaL In my professional experience, the faifure of the City engineer to conduct this review of the piat submissions is a breach of the duty of care and professional standards in the profession." Barr's comment: Tt is our opinion that there was a breech of duty on the part of hoth tl�e developer's engineer(Hedlund �ngineering} and the City's engineer{SEH). But ��egardless of whether or not the City enDineer neg{ected ta provide commerzt on the developmenT plans, we do not believe that tl�e developer's engineer performed the basic due diligence consistent with the standards of professional care for the Prairie Run Development Furtt�er, it is our experience that cities will perform basic cursary reviews, at3d wili nat bo tl�rou�h aitd check every aspect of a project design. We, in fact, have seen basic disctaimers by cities indicating that the city is not responsible for the desians it revie«�s and are not Iiable for flaws in the desi�ns. We believe that the respo«sibility for the accuracy or inaccuracy of the Prairie Run Development design rests on the owner of the devetopnient and its engineer. Paragrapi� No, 14:The Mundstock affidavit states: "It is my professionaf opinion based oz� my experience as a professional engineer and the infonnation provided to me that aEl of ti�e proposed structure elevations in the residential portion of the Prairie Run �rading p(an dated �/14/04 are accurately detertnined witlt respect to the line of permanent aquatic vegetation and are consistent witl� the de��elopme�Zt a;reeme��t and the relevant City ordinance that allo�i-s use of this �neihod to estimate high-water levels. I I�ave analyzed the S/14/04 grading p[an approved by the City Counci] i❑ P:\1�4p1s123 MM8G123S60G6['rairie Run Litigacion\WorkFilesU'r3irie Run Study Repor[ Revised Current Version.DOC 27 Jui�e 2004 and have determined that alf of the proposed structure (ow opening elevations for each of the 52 lots were at leasT 950.5 or three (3) feet above ttie ltigh-water level of 947.5 derived from the line of permanent aquatic ve;etation used to estimate hiah-water levels for the development." Barr's comment: We do not agree with the Mundstock affidavit where it indicates that the �rading plan dated S/14/04 is consistent with the relevant City ordinance that was current at that time. Drawing C-5.1 of Hediund deposition, Exhibit �0, indicates that the brading pIan for the development was initiatly prepared on June 6, 2003 and it was in turn modified at Ieast five times between June 6, 2003 and May 1�, 2004 wi�en the �rading p(an for the development was fnally approved. 1'he June 24-2�,2003 storm occurred during the time this plan was being worked upon. The revisions block on the drawing indicates that on August 4, 2Q03, the plan was modified ta adjust southerly lots and road widths. On November I4, 20Q3, the site was lowered 4 feet. On December 19, 2003, the lot lines of Lot 7 and Lot 8 of Block 2 were revised for mitiga[ion. On April 19, 2004, the pad eleva�ions for many of the lots were modifced and on May 1�, 2004 Sections A-A and B-B were modified. 1��e believe ihe City ordinance prescriptively required the "iiighest known water level"(the flood level for the June 24-25 storm) be used as "sufficient data"was available for that storm. We believe tl�e developer's engineer should have surveyed tt�e drift or debris line Ieft af[er the June 24-25, 2003 storm wiihin the development and used tliat line to set law ope�iings and low floors for the 52 Iots within the development. We believe t[�at had the developer surveyed that line based on Barr's modeling rest►]ts of the lune 24-25, 2003, tliat tl�e highest known water ievel established by that storm would have been about Clevation 9�2.1, approxitnately 4.6 feet higher than the elevation ased by tiie developer's engineer for laying oot the Prairie Run grading p(an. We do not have information on the lo�i� openings and Io�� floors for each of the 52 lots in the Prairie Run Development, but based on the highest known water elevation of 952.1 and information in the Mundstock affidavit, we suspect there are various lo�v f7oors and low entries that do not comply with the Cicy ordinance that was current at the tiine that the development was approved or the existing ordinance, that is current today. Paragr�ph No. 20:Tt�e Mundstock af�davit states: "It is my professional opiniort based on my caperience as a professional engineer that ttie a l 00-year storm would produce 5.9 inches of rain in 24 hours and tt�at such a storm has E out of 100 chance of accurring in any give year. A storm that produces 8.5 inches of rain in 4 hours could be akin to approximately a 5,000-year stonn and such a storm has a.02 percent chance of occurring in any gi��en year." P:1Mp1s123 MN186\238G06G Prairie R�n Litigationl��orkPileslPrairie Run Study Report Re��ised Curret�t Version.00C 2$ Barr's comment: We believe Mr. Munds#ock, in this paragraph, is referring to the June 24-2�, 2403 storm that occurred in Albertvitle where 8.5 inches of rain occurred in about 7 hours and presumably this paragraph was written to indicate that particular storm was of such a magnitude that it should not have been used for determiaing the "l�i�hest known water jevel" for the development. The City's ordinance does not make a distinction on the rarity of a storm that has occurred, nor does it mention the ma�nitude of a storm to be used to set tfie higEiest known water efevation. Rather, it simply says the highest known water elevation. Thus, if the developer did not want to use the fiood eievation o,`the June 24-25 storm in the Prairie Run area, then it would have been incumbent upon the developer to have applied for arad obtained a variance from the City's ordinance to a{low the use of a Iower etevation as a basis for setting key developme�it features. Paragraph No. 26: The Mundstock affidavit states: I have revietived City Engineer Bolton & Menk's engineering judgrnent that the besi,solr.�tion to the,flooding caused by County Ditch �39 is ro add storaoe volume upstrea�n of the Albert Villas development in the City of St. Michael thai ivould reduce high H�ater elevations by "approximately.0.7 ft. to 2.4 ft. in criticat areas of the lvatershed" tlzat include Prairie Ru�z. Although I have not yet been provided with Bolion c& Menk's HydroC.4D calculations for their flood mitioation plan, the data provided by Bolton & Menk shows that the additional storage volunze upstreana of the,4lbert T�illcts developn7ent in the City of St. Michael as pr-o��osed by Bolton c� Menk will lo��er high ��ater levels in the Prairie Run developnrent by approximately S.�1 i�zches dzn•ing cr 100 year flood event (Bc�M0082}. Assuniing the accuracy of f3olton c� Menk',s data, it is my professiorral opinion, based on nry e��ef�ience as a Prvfessionul Enoineer��rd tl�e information provided to nze, that the proposal to add storage volume upstream of tlie Albert Villas development ifz the City of St. Michael ti��ill elirninale the risk offlooding (i.e., u�ater ellter•ing tlze luw openino of a honzej on the Prairie ku�i development dur�ifzg a ]00 year flood evetzt. Barr's comment: The Mundstocfc affidavit appears to concur with 8olton & Menk's report that tlie stormwater flood mitigation detention facifity, which we understand is now designed and under construction in the City of St. Miehael, near the St. Michael-Albertville Eleme�itary School,just west of the Albert Viilas development, should be co►�structed. �3arr's computer model indicates that once the flood rt�itigatinn facility is constructed, the peak ]00-year flood level ��ould be reduced in Prairie Run by ap�roximately I foot. Also, rert►nning the Barr-modeled June 24-25, 2003 storm with the flood mitigation facility in- place, the peak flood level for tl�at stortn in the Prairie Run Develo�ment wouEd be reduced by 1.3 feet. Under both tl�e current City ordinance and the ordinance that was in place at the P:1Mpls\23 MN\86�386066 Prairie Run).,itigation\�'1�orkFi[es\Prairie Run S[udy Report Revised Currcnt\rcrsion.DOC 29 time�'rairie Run Development �i�as approved, the building elevations in Prairie Run need to �e set based on the"hi�hest known water elevation." Therefore, once construction of the flood mitigation facility is compieted and because it will alleviate the flooding for a storm of ma�nitude that resuited in the "liiahest known water level" by 1.3 feet, and based on the City's ordinance, we believe the elevation for the lowest floor in Prairie Run should be set 2 feet above the mitigation revised highest knawn flood level (Elevation 950.8 +2 =Elevation 9�2.5). If a lo�;�er building elevation is desired by the developer{such as 1-foot above the revised 100-year flood leve!); we believe the deveioper wouid need to apply for and obtain a variance to the City's ordinaace from the City. Supptemental Affidavit of Brian Mundstock Court Fife #$6-06-CV-4997 Paragraph No. 6.a. The Mundstock afttdavit states: "The existing site condition model �repared b l e � dlund d' , . Y id not take tailwater into account because the ta�lwater effect on die Prairie Run property was u�known to Hedlunc! at the time Hedlund prepared his grading and drainage plans for approval in lune 2004." Barr's comment: It is our o inion that h A ad Hedtund erformed a ba ' p sic review of the ditc h drainage system upstreain, adjacent to, and downstream of the Prairie Run Development, sufficient concerns should have bee�� raised bv Hedfund ., to either cause them to ana] �ze } the system and determine if tailwater was a problem or to proactively work with the City ta have the system t[3orou�hly modeied so that a clear understanding of the potential ffood levels on the developrnent could be obtained. We have not received information indicating that Hedlund made any such reqiiests of the City or that it had performed a.basic review to obtai�� an understandin� of(a) the size of tlie watershed draining into the developmer�t, (b) the discharges within the ditch adjacent to t(ie development, and(c) the potentiaE tailwater effects the complex ditch system conld pose on the development. Paragraph No. 6.b. The Mu�;dsiock affidavit states: "The area of the CouEity Ditch 9("the Ditch") watershed upstreaEn of the box culvert under County Road ]8 was not taken into account becaase it is adjacent to the Prairie Ru�z property and does not drain through Prairie Run under normal circumstances." Barr's commenf: It is our opinion that because County Ditch 9 was immediately adjacent to the Prairie Run property a�zd Lfiat County ditches ltave flows that typicaily exceed their ban[: P:1Mp1s123 MN\8612356066 Prairie Run Liti�ationl�l`UrkFileslPrairie Run Siudy Report i2eviscd Current Version.DOC �� capacity for events as small as a 1- or 2-year event, that to not taAe into account the effect the ditch might have on flood (evets in the Prairie Run Development was a major flaw in the design considerations of the devefoper's engineer for set#ing bui;ding elevations. Paragraph No. 7.a. The MundstacL; affdavit states: "I also note that Hedlund's 2004 hydrology model assumed free discharge at the downstream end of the Prairie Run property in the same way that F3olton & Menk's 2t�Ob hydrolo�y model assumed free discharge at the downstream end of their 2006 County Ditch 9 ffood stady. In snort, absent tailwater data, a free discharae assumpt;o�i is staadard practice in the industry." Barr's commenf: What the Mundstoc[: affidavit fails to recognize is that the slope of t(�e ditch system do�ynstream of tlle Prairie Run Development is significantly flatter than the slope of the ditch system where the �olton & Menk`s hydroloby model ended. We do not agree that absent tailwater data, a free discharge is or should be a standard �ractice in the industry. This is especially the case when dea�ia�g wit1� relativety flat channef slopes. Paragraph No. 7.d. Tbe Mundstock affidavit states: "The drainage facilities at Praine Run are compromised when the watersl�ed is experiencing f7ooding and County Ditch 9 is at peak �vater leveis." Barr's comment: We disagree with this statcment as«�e do not believe the drainage facilities at Prairie Run are compromised. Ratl�er we believe the draina�e facilities experience the natural impact of flo�vs ili a broader floodplain tl�an the simpie coi�fines of t}►e �nai�i c}zannel of County llitci� 9, When su fficient f7o�vs are hiQh enou h to rai e I � g s above the existing ditch banks the surface flows �iaturally spreads out into a n�uch Iarger floodplain I area, part of which ties within the Prairie Riin Development_ The onds wetlands P , etc_ o , n the Prairie Run Developr�lent are IargeIy all within the floodp(ain of County Ditch 9 that existed prior to Prairie Run Developmeilt. Par.agraph No. 8.a. �n this para�raph oftI�e Mundstock aff►davit, it is discussing the Minaesota Pollution Control Agency's NPDES stormwater permit for the developmeiit. fn this paragraph, Mr. Mundstock states: "Regardless the i�,►orth Pond was more than adequately sized to provide pennanent stora�e volume for a contributing waters}ted of 35 acres (10 acres off-site and 25 acres oa- site). P:1Mpls\23 MNi86123860Gb Prairie Ran L.itigationl��'orkF�lcs\Prairie Run Study Report Re�ised Carrenl Version.DOC .i 1 Barr's comment: The Nor�h Pond is a pond that was desiDned to be "on-[ine"or right on the drainage channel. tn other words, the pond �vas cor�structed so that the entire drainage system upstream of the pond would drain directly through it_ The MPCA's requires that the pond volume to be provided for an on-►ine system needs to provide a permanent storage volume of 1,800 cubic feet per acre of drainage area that drairts to the pond. $ecause the North Pond is on-line with the main ditch that passes throudh the Prairie Run Developn�ent, ihe MPCA's water quality treatment standards require the 1�Torth Pond to be sized for a tributary watershed of 2d9 acres (189 acres of off-site drainabe plus the 20 acres on-site)and not just the 35 acres for which is was desianed. Paragraph No. 8.b, Mr. Mundstock's affidavii states: "Accordingly, permanent storage volume is already provided for within #he l 89 acres af off-siTe draii�aae area described by Mr. Leichty in developments adjacent to tlie 3� acres of permanent storabe voluzne provided for by Prairie Run." Barr's comment: The MPCA's requirements for permanent dead storage volume in a particular pond (the volurne belov��the pond's outiet elevation) on a main drainage way were specifically written to consider the total drainage area to help prevent the possibiliTy of re- entrainino and flushin�sediment that may have settled out within the ponding basin dvring a previous eveilt. The Iligh flaws from the larger watershed can easily flush sediment that has settled witi�in a basin if ii is r�ot designed properly. Therefore, if there is ao other cl�oice than to piace a water quality treatme�lt basin on the main stem line of a drainage way, then that ponding basin needs to be designed for tlle entire draina�e ��ay to lielp �revent the potential for sediment flushing re�ardfess of whether or not additiona] water quality treatment basins I�ave been provided upstream. P:1Mp1s123 M7�1g6��3gb0G6 Prairie Run LitiQ�tionlWor[:Files\Prairie Run Stud�� Report Revised Curreat Version.DOC 3Z �. � -: �. : , • � _ ,. � �• - Kfr, - ^�,. n'. ' �5 ,r� � _ � �..,- " _ :_.Y " IS �� -7'abres � - � � - � � _ - � �� - �; a - ��_ � _ � - � � � - ��, - - � - - �� 4 � � - - - �� _ t ' ,* t �S F � 4 t . 5k `t _ t �! � �rh r 3; �6` �-._ S ' y,. -, c. .-- -. �L`y�, i t s � ' �� 'h f - _ " " ' - .< � *. 1. , 4 `; T �t � r ' �� °' ' � .� � �� h � _ _ �"t � x ��t � r � ' �� - _ F� � _ + �� f ' �� _ �y ' � �Sr�,r - `�- v� '2 _ ��s � t �.`+'�s - r.. �.�^� r � Nr� � � � l ' `.� _ ' ,.r - _ t - �y ' - a � _ ' E.� - �_.v� . _. , _.. . _.. _ _ ,� _ ��. ,_.._... �..,, ,._, .� : �,__ _ v ._.. _ ., _.. � _ . Table 9. Land iJse Impervious Fraction Assumptions for Hydrologic Modeling Directly Connected 7ota1 Impervious Impervious Percent" Land Use Percent % (%) Agricul#ural Rural 9 2 4 2 CuEtivated Crops 2.6 0.0 Deciduous Forest 4.8 a.0 Deve€oped High Intensity 76.1 71.1 Developed Medium intensity � 25 5 28 � Developed Open Space �2 6 7 6 Emerge�t Herbaceous Wetfands 100.0 10Q.0 Evergreen Forest 0.5 0.0 Golf Course 12.6 7.6 Grassland/Herbaceous 1.0 0.0 Highway Commercial 79.3 74.3 Limited Business 17.7 �2,� Low Density Single Family 22.6 20.1 � Pasture/Hay 5.3 Q.p Planned Unit Deveiopment 2g 2 23 2 Publicllnstitutiona! 29.5 24.5 Shrub/Scrub 2.2 a.0 Water 1 Q0.0 100.0 'Percent of watershed area Table 2. Soils fnput Data Fo �c K N drolo ic Soil Group in/hr) {in/hr) {1/sec) A 5 Q.38 Q.00115 B 3 Q23 0.00115 � 2 0.1 0.00115 C � 0.03 0.00115 Table 3. Depression Storage and Overiand Flow Roughness Input Parameters Depression Overland Flow Cover T e Stora e (inches Rou hness Pervious 0.17 0.24 Im ervious 0.06 0.015 P:1Mpls123 MN1Sbt2386066 Prairic Run Liti�ationlWorkfiles\Prairie Run Study Report Revisccl Current Version.DOC Table 4. 24-Hour Rainfall at Albertviile Re.urn ProbabiEity of Occurring 24-Hour Frequency in a Given Year Rainfatl Total % inches 1-Year 100 2.3 2-Year 5� 2.7 5-Year 20 3_� 10-Year 10 4.1 50-Year 2 5.3 100-Year � � 5 8 Table 5. XP-SWMM Model Calibration Results for MatcF�ing Photographic Flood Level and Time Stamp Evidence of the June 24-25, 2003 Storm Event BARR ' Pand Photo Phato XP-SWMM I Location Descri tion Elevation Time Results Observed Stain Line 954.62 6/25/20Q3 954.77 Pond SW of i0:03 AM I 52nd& Kahl Estimafed Water 953.73 6l25/2003 953.50 (Kahf Ave Level Based an 10:03 AM �'°��) Measure Down from Stain Line Albert Villas Observed Drift Line 952.02 6/25/2003 952.08 Wetland 6:59 AM CD#9 North Observed Water 951.47 j 6/25/2003 951.46 of CSAH 18 Level � 3:13 PM Table 6. Modeled �'lood �evels for the June 24-25, 2003 and 400-year, 24-hour SC5 Type If Storm Events at FCey Locations. BARR BARR Devefoper's Bolton�Menk SEH Report XP-SWMM XP-SWMM Engineer HydroCAD XP-SWMM June 2003 High 100-Year Estimated 100-Year 1Q0-Year Pond Location Water Levefs Results t00-Year Results Results Pond SW of 52nd & Kahl {Kahl Ave Pond) 954.77 954.9 949.'I 954.0 ( 950.0 Albert Villas Wetland 952.08 950.2 949.7 951.3 949.1 CD#9 North of CSAH 18' 952.06 949.9 947.0/947.52 949.9 950.5 Flood Levei Adjacent to fhe Prairie Run Developmeni 2 HWL iisted on Prairie Run grading plan for soufher� most NURP pond was 947.0. The line of permanent aquatic vegetation prior to development is reported to have been 947_5. P:\Mp1s123 MN\$612386066 Prairic Run LitigationlWorkt ileslPrairie Run Study Report�tevised Current Vcrsion.DOC i able 7. Prairie Run NPDES Requirements. Pond Location Northern Southern Pond Pond Tributary Area (acres) 35 4 a, rn � c� � c �;? � Permanent Pool Elevation 946.0 945.0 aw a � � Q � E � v Permanent Water Sforage x Volume (cubic fee#) 47i,'i47 13,939 � Tributary Area (acres) 228 4 I � � 'I � � •� •� � Permanent Pool Elevation 946.0 � 945.0 I�I a�i °' i � _ ' w � � Permanent Water Storage a � Vofume{cubEc feet) 410,400 7,200 z Deficiency {cubic feet) 239,253 None P:1Mpts123 MN!S6'Q3860G6 Prairie Run Liti�aEiort\WorkFileslPrairie Run Sh�dy RepoR Rcvised Current Vcrsion.pOC z. - �� S " �=P � _ �_ - ' �� " $�� ; � r s � f ' l . ' 1 �'5" i � T Z ` - • � 1 { ? j � . - f Y. } '-a t �L��VS .>. +F � _ y^ € �E�' .�' ; � 1 r^ ;' i s Fd � " ,'- ��..µ - - ; : i -� € � _ S '� _ f C ' � " � 1 { ' { �� .Y - - � S� r ,l . � . _ . �. � . .. . �!fi' 3 C `� �v` �* � 1 �� " F 4.,C _ � - � - - �y,� �.Y � _ � _ �� n _ _ - rt , r _ �° - _ � ' r+ g� �� . 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