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4.a)4) Staff Report-197th Street Erosion (Wenck) . L 1..: � I ._e�---� )� .__ ,� .`. .� � SCANDIA Staff Report Date of Meeting: July 9, 2013 To: City Council From: Kristina Handt, Administrator Re: 197`h St. Report Agenda Item #: 4.a)4) Background: At the May work session and on the May 21 road tour, the Council discussed with the Car-mar Watershed District the possibility of completing some improvements on 197'�' St to address erosion issues. Issue: Does the Council have any comments on the Memo from Wenck Associates regarding the proposed work on the ravine near 197`�' St? Proposal Details: Included in your packet is a copy of the technical memorandum from Wenck Associates, Inc. The Car- Mar Watershed District Board reviewed the report at their June meeting and is asking for any comments from the City of Scandia. The report describes two options for stabilizing the area. City staff is in agreement with the Wenck recommendation to pursue Option 2 so that the new pipe and overflow channel are located in the public right of way as opposed to private property. City staff would like to see the rain garden moved closer to the adjoining property and/or an agreement with the abutting property owner to maintain it. Fiscal Impact: At this time, City staff anticipates that all project costs would be covered by the Watershed and no City funds would be appropriated. If adjoining property owner is��ot responsible for rain garden,the estimated maintenance cost(per page 6 of the report)would be $500/year for the next 20 years. Options: 1) Option 1 described i��the Wenck report 2) Option 2 described in the Wenck report 3) Any additional changes/comments not noted in the Wenck report . t ^yy�� //''''�� ` Wenck Associates,Inc. _ '��r ■� �� 1802 Wooddale Drive r � � Suite 100 Woodbury,MN 55125-2937 Engineers � Scientist� (- (651)294-4580 �4�f�t��S� f '' �,.!{�SCE�r�C.d�� wanckSmP@wnckcom www.wenck.tom TECHNICAL MEMORANDUM TO: Mr.Jim Shaver,Administrator, Carnelian-Marine-St. Croix Watershed District Board of Managers, Carnelian-Marine-St. Croix Watershed District FROM: Ed Matthiesen, P.E. Todd Shoemaker, P.E.,C.F.M. DATE: June 7,2013 SUBJECT: 197`h Street Ravine Feasibility Study INTRODUCTION The Carnelian-Marine-St. Croix Watershed District retained Wenck Associates, Inc. (Wenck)to study stabilization methods to repair a severely eroded ravine located adjacent to 197`h Street in Scandia, MN (Figure 1).The purpose of this memorandum is to summarize our evaluation of the site and present recommendations to stabilize the ravine. BACKGROUND Wenck discussed the erosion with two residents and understands that the ravine erosion has been progressing for the last five to ten years and may have been exacerbated when a flared-end section was installed at the top of the slope in 2007 or 2008.According to a resident adjacent to the ravine,there are currently 13 properties served by 197`h Street. Of the 13 properties, approximately four contain "year-round"residents. City of Scandia staff reports that the number of year-round residents is expected to increase and may compel the City to expand or rehabilitate 197th Street to accommodate additional traffic. T:\2264_Carnelian-Marine-St.Croix Watershed District_Ndrive\02 197th St Ravine\Ol Feaslblllty Study\M-Admin&BoM re Feaslbility Study.docx Page 1 of 11 Technical Memo 197th Street Ravine Feasibility Study Carnelian-Marine-St. Croix Watershed District June 7, 2013 Table 1.Hydrology and hydraulic data for the 197`"Street ravine. Drainage area (ac) = 0.34 Impervious area (ac)= 0.11 Percent impervious= 33 1-yr runoff(cfs)= 0.26 2-yr runoff(cfs) = 0.39 10-yr runoff(cfs)= 0.93 100-yr runoff(cfs)= 1.7 Max velocity(fps)= 4 Max depth (ft) = 1 Ravine slope(ft/ft)= 0.55 Max shear stress (psf)= 34 Shear stress is the force of moving water against the bed of the ravine.Soil or rock begins to move when the drag force of flowing water is greater than the gravitational and frictional force holding the soil or rock in place.Shear stress in the 197"'Street ravine is very high,which indicates that moving water can easily erode and mobilize soil particles.Armoring the channel with rock or installing a pipe would counteract the erosive forces. Using the Wisconsin Natural Resource Conservation Service "Field Office Technical Guide"for streambank erosion,Wenck estimates the ravine exports approximately 8 tons per year of soil and 33 pounds per year of total phosphorus.These values were calculated assuming a rating of"severe" erosion (0.4 ft/yr),two foot average bank height, and dry soil density of SS pounds per cubic foot(silty soil). Wetland delineation and tree identification.Wenck visited the site again on May 10,2013 to delineate wetlands and identify trees for preservation and removal (Figure 2). One wetland was delineated along the base of the slope of the site.The wetland appears to be ground water fed and the western boundary is defined by side-hill seep hydrology.The wetland is a Type 7 wooded swamp dominated by black ash with marsh marigold,skunk cabbage and various sedge species in the herbaceous stratum.The quality of the wetland is relatively high based on the dominant species. Reed canary grass is present in a distinct area which is directly tied to the plume of sediment which has washed into the wetland from the upslope eroding ravine.Any proposed work in or adjacent to the ravine will likely have to be reviewed by the local Wetland Conservation Act authority to determine applicability of a wetland permit. Wenck identified individual trees of six inches or greater diameter-at-breast-height within the project area. (The table in Figure 2 lists the species present in the project area.)Some of these trees and smaller ones may have to be removed to open the canopy and encourage ground vegetation that can better hold soil in place. It may also be necessary to remove some trees for construction access. T:\2264_Carnellan-Marine-St.Crolz Watershed Dlstrlct_Ndrive\02 197th St Ravine\Ol Feaslbillty Study\M-Admin&BoM re Feaslbiliry Study.docx Page 4 of 11 Technical Memo 197`h Street Ravine Feasibility Study Carnelian-Marine-St. Croix Watershed District June 7, 2013 Secondary gully. During wetland delineation and tree identification, Wenck noted the head of a "secondary gully" approximately 125 feet south of the main erosion area (Figure 2).This gully is about ten feet wide and four feet deep at the most eroded spot and is also contributing to sediment deposition further down the slope. CMSCWD may want to consider stabilizing this gully at the same time the main ravine is stabilized. STABILIZATION OPTIONS Stabilization options include installation of a rain garden on top of the slope to slow water getting to the ravine and two options to stabilize the main ravine. Tree thinning.Tree thinning is necessary for construction access and to allow more sunlight to reach the forest floor for ground cover that can naturally stabilize the slope soils. Instead of removing from the site,thinned trees would be dropped perpendicular to the slope to serve as grade breaks and brush bundles. Early in final design, Wenck recommends a process that engages property owners and allows them to visualize which trees will be selected for removal.Trees can be flagged using red and pink surveying ribbon.The red ribbon indicates trees that are necessary to be removed to stabilize ravine banks, allow sunlight to reach the forest floor, and allow construction vehicle access.The pink ribbon indicates trees that are preferred to be removed to further stabilize the ravine, improve construction access, or both. Property owners are then given red and green ribbon.The red ribbon is to mark any additional trees they would like to have removed between the top of the slope and the bottom of the ravine, and the green ribbon is to wrap on trees marked with pink ribbon that they would like to preserve. Individual meetings with each property owner would be held to review why the tree was selected,why they would like to save the tree, and potentially negotiate a different tree for removal in exchange for the preserved tree. After tree removal negotiations are finalized,tree removals would be added to the design plans and a construction access route would be determined. Note that any tree removal must be approved by the National Park Service because of the St. Croix National Scenic Riverway designation. Rain garden.Wenck sized a rain garden (Figure 3)to fit within the existing site constraints: 197"'Street, topography,trees, and the existing inlet structure.The rain garden captures all runoff from a one-inch rainfall from the 197`h Street impervious surface. Rainfall events greater than one inch will continue to flow into the stormsewer as they do now.The area of the rain garden is approximately 700 square feet and six inches deep(below the catchbasin rim elevation).The estimated construction cost is listed in Table 2. T:\2264_Camelian-Marine-St Croix Watershed District_Ndrive\02 197th St Ravine\01 Feasibility Study\M-Admin&BoM re feasibility Scudy.docx Page 5 of 11 Technical Memo 197th Street Ravine Feasibility Study Carnelian-Marine-St. Croix Watershed District June 7, 2013 M � � +'u' � ,' � � � �a� , A`�%_"�` �w' ,��,-�r,,,..�. .�:;. �T� , �� i. ,�.. �... �. v 1 � r ^ ' k �II�� i � ` . _ ` ' . . , ... . . . . ,.. . , _. ...., r� . .;�^� , . � �j�y�� 'y''��°, ��k�: s �_ �T�.'�.'.� Y� �� s3�, . ,�� :-..�. ,�,;.�� •�,�.,., � ",�aai ��ti � .. � � _ — . - � � r � � � �� :. � �, .� • :� � ; � : �..�w• a= � �, � � � � � � - - � �� .�� :%c-.�.�ee: - - - ,i� � ' + � � : � � ,�' �° ; �, - ,,_ � . � � � ;. �; �-. .. I f 5 ^�.�_ ._ i ` � � ^ ' _ , -- I � . ' .�-'. . .�� . � . . "_ � - . _'.r� �9 � ' ,� ,k et Y�� _. �, � / �' � �, ' •,�w'' �� . .: T ; ,.� , i . . � .. , .. K' � ��" ��. � �' .. @{£'� �� - �� ^ . .�" '�"f ��1 §: �:. .,.. ... �"....a .. .. �� . '�_: y. .. ., : Figure 3.Proposed location of the 197th Street rain garden. Table 2.Estimated costs to construct the 197th Street rain garden. Bid Item Quantity Unit Unit Price Price Mobilization 1 LS $500 $500 Excavation & grading 50 CY $15 $750 Erosion control 1 LS $500 $500 Plantings 50 Each $5 $250 Mulch 1 LS $300 $300 Engineering and Construction Observation $3,000 20% contingency $1,060 Total $6,360 Wenck used the computer model P8 to estimate pollutant loading to and removal by the proposed rain garden. P8 predicts annual total suspended solids(TSS) and total phosphorus(TP) loads of 219 pounds per year and 0.71 pounds per year, respectively.The rain garden will remove approximately 192 pounds of TSS per year and 0.52 pounds of TP per year. Annual maintenance of the rain garden will likely consist of sediment and weed removal and occasionally installing new plants.The present value of annual maintenance ($500 per year for 20 years) is$7,400. Assuming a total cost(design, construction and maintenance)of$13,760,the dollars per pound of pollutant removal over 20 years is$3.58 for TSS and $1,323 for TP. T:\2264 Carnelian-Marine-St.Croix Watershed Dlstrict_Ndrive\02 197th St Ravine\Ol Feasihiliry Study\M-Admin&BoM re feasiblllty Study.doa - Page 6 of 11 Technical Memo 197`h Street Ravine Feasibility Study Carnelian-Marine-St. Croix Watershed District June 7, 2013 Ravine Option 1-existing channel alignment. Option 1 uses the existing ravine alignment to stabilize the ravine and reduce erosion.The greatest disadvantage to Option 1 is that much of the work occurs on private property. Therefore, easement or recorded agreements between the City, CMSCWD and property owners would be needed for construction and future maintenance.The estimated construction cost for Option 1 is listed in Table 3. Figure 4 shows the tasks needed to complete Option 1: 1. Thin trees for construction access and ground cover growth. 2. Install a new manhole at the end of the existing pipe to connect a new pipe. 3. Install the new pipe at the bottom of the existing ravine alignment with some contour smoothing. 4. Relocate the existing flared-end section to the end of the new pipe. 5. Backfill the existing ravine using soil from the sediment delta at the bottom of the slope or the stilling basin. 6. Place a rock overflow channel on top of the backfill.A potential cross-section of the stabilized ravine is shown in Figure 5. 7. Grade a "stilling" basin at the discharge location of the new pipe to reduce erosion potential at the outlet pipe and minimize a "point" discharge into the wetland. Table 3.Estimated costs to stabilize the 197th Street Ravine using Option 1. Bid Item Quantity Unit Unit Price Price Mobilization 1 LS $500 $1,200 Erosion control 1 LS $500 $500 Tree thinning 15 Each $200 $3,000 Manhole (4 foot) 1 Each $2,000 $2,000 12" HDPE pipe 125 LF $30 $3,750 Ravine backfill material 140 CY $12 $1,680 Geotextile 110 SY $3 $330 Rock overflow channel 30 Ton $100 $3,000 Excavation & grading for stilling basin 40 CY $12 $480 Plantings 100 Each $5 $500 Engineering and Construction Obsenration $15,000 20% contingency $6,288 Total $37,728 T:\2264_Carnelian-Marine-St.Croix Watershed Dis(rict_Ndrive\02 197th SS Ravine\OS Feasi6ility Study\M-Admin&BoM re Feasiblllry S[udy.doa Page 7 of 11 �o�ie�o�_��_es� � n�oaw �x I �m�aes�a�o a� oo � e �� t ap �� I �I�:�� I �+�-_vloz�aoa�� !�"„ i i 1 9' �� f- � i�� € € i ��-#�!#��¢� �¢f�4��#�f �#I�� I��#I a�� ' �a ' ' � I � � I� I�� --=1:--�� ' � i �� `Y' � e� e�1 � � � � E � � I � e I a!: aa � a� a 3 0 a�9�! ��'aa � �3�� 9�s� �9 �' i,��14s �� 6i�f.}4�s�4,�l��irc` 4y ��99ymlNyy�y�d�m�y��yx 'mEy� m��Jy yr�ymm��Jyyy�m� N � pi., .. _ I_a,...u'�_'w`�. ,.�aa.�..'�..,'�,u �xI..A�Iv.:� s saYlwhr�ar�,a:��,�xiem'c...:.,M4�xaxc^:�..r�r�,_ex:.a a, ��a��e�..�s�m.mra.xo aA_s:a'�'� Z Q� � .� .. . . . � �,.. 1; , . �.. � � � ., . �„ _ � j,L s�X*` fi � P� ,��` � ` � y:. � � � �'�Y�" �' , `.#� „ ,..�.. .,�,iyr :x`" '� �,��'�";`T�-',=; 'R""�,.•, u�' � ai, �s� �� ' ., r ��Y �° - � .. � �_.: � � �`�..�� , :� ��f � ��t��. �"f vr� � �� '`� '� *f�`�� .;�°�^' �.r'!.. �' i'T e �� *� � f �ry � ,y{e�-. _''� wA4r �, w ?� ; : � ��s -;. �a �� ..,. �,.�r.y�;. � . .�. 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O a C 5' � a '� y1 � � a n 3 m p �' I & 3� v r j o e _ c ' �w„ ,� � � g? a e � ` Y � � ' � � o EF C a U o � y d i- .- � av ��' � N �� y _ � ` N k � a J� J � � � � s - H :..�K 79�"".. � a x ..�y,.#k� f u .,.,e�F � � � Technical Memo 197`h Street Ravine Feasibility Study Carnelian-Marine-St. Croix Watershed District June 7,2013 BACKFILL ROCK VOIDS WITH SOIL FROM CLASS III RIPRAP KEYED INTO CHANNEL AT LEAST 6" SEDIMENT DELTA OR STILLING BASIN CLASS I RIPRAP PER MNDOT SPEC 3601.A2 ��, }�� ` EXISTING RAVINE BOTTOM SEED EACH SIDE ' �%, % - ,;;� �j'�,�--- WITH BWSR U9 �,'��,;;'�'��,;%' �i�.�,'; �i� ,,-��, � AND 12'WIDE ,ij/%��'j j , FILL FROM SEDIMENT DELTA OR EROSION CONTROL STILLING BASIN BLANKET +/-10 FEET 12" HDPE PLACED IN CHANNEL BOTTOM Figure 5.Typical cross-section of the stabilized ravine. Option 2-197`h Street right-of-way alignment.Option 2 uses the 197`h Street right-of-way south of the existing ravine to stabilize the ravine and reduce erosion.This option would not require a permanent easement or recorded agreement with property owners for maintenance but would likely require a temporary construction easement for work in the existing ravine and additional tree removal due to two work areas. Figure 6 shows the tasks needed to complete Option 2: 1. Thin trees for construction access and ground cover growth. 2. Install a new manhole structure. 3. Connect a new pipe to the new manhole. 4. Trench the new pipe into the 197th Street right-of-way. S. Salvage a portion of the existing pipe and relocate the flared-end section to the end of the new pipe. 6. Backfill the existing ravine using soil from the sediment delta at the bottom of the slope or the stilling basin. 7. Place a rock overflow channel on top of the pipe trench backfill.A cross-section of the stabilized "new" ravine would be the same as shown in Figure 5. 8. Grade a "stilling" basin at the discharge location of the new pipe to reduce erosion potential at the outlet pipe and minimize a "point" discharge into the wetland. The estimated construction cost for Option 2 is listed in Table 4.The increase in cost for Option 2 is due to additional tree removal and plantings. "Plantings" refers to species planted adjacent to the existing ravine, new pipe trench,and stilling basin. It is includect for budgetary purposes for both options;the desire for and exact number and species of plantings can be better defined during final design. 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L a' � ' I Z N r ,..�.� + �m � w ; ! zQ � � ...�.d6-Y ,�,/� . .,y � _ Q C' U o — - � '�� " p. d � � LL o � 4 � a � � �' o ',� u � ' e � � m y o'� 3 d ,�� re „a�,� �� 0 3� � � L o z � � -'�»., '� �;� � 8a � r N p '� I 9 E y � d = «`d, � � ;� � ':'�� ,��' � - N F C d U O y m o r � � rv , &^ d E +/; a m 7 �� ' � o «,� . � � .3 r . Technical Memo 1972h Street Ravine Feasibility Study Carnelian-Marine-St. Croix Watershed District June 7, 2013 Table 4. Estimated costs to stabili2e the 197`h Street Ravine using Option 2. Bid Item Quantity Unit Unit Price Price Mobilization 1 LS $500 $1,800 Erosion control 1 LS $500 $500 Tree thinning 30 Each $200 $6,000 Manhole structure 1 Each $2,000 $2,000 12" HDPE pipe 125 LF $30 $3,750 Ra�ine backfill material 140 CY $12 $1,680 Geotextile 110 SY $3 $330 Rock overflow channel 30 Ton $100 $3,000 Excavation & grading for stilling basin 40 CY $12 $480 Plantings 200 Each $5 $1,000 Engineering and Construction Observation $15,000 20% contingency $7,'108 Total $42,648 RECOMMENDATIONS Wenck evaluated two options to stabilize the 197`h Street ravine.Wenck recommends CMSCWD proceed with final design of the rain garden and Option 2 because the new pipe and overflow channel are relocated into the 197`h Street right-of-way. Prior to final design of these features,Wenck recommends a topographic survey using a total station. Wenck collected topographic data for this feasibility study using GPS technology but was not able to establish a consistent signal to ensure sufficient accuracy at some locations on the slope and in the woods. If the property owner and CMSCWD are interested, survey data could also be collected on the secondary gully. T:\2264_Camelian-Marine3t.Croix Watershed DisVict_Ndrive\02 197th St Ravine\Ol Feasibility Study\M-Admin&BoM re Feasibility Study.docx Page 11 of 11