HomeMy WebLinkAboutStorm water management assessment memorandum Revised July 30,2014
Storm Water Management System
Assessment Memorandum
— FOR —
Orono Truck Station
Hennepin County, Minnesota
Inspected by:Tom Christianson
Prepared by:Jessica Tello
Approved by: Brady Busselman,PE
Prepared By:
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763-476-6010
wwwsambatek.com
engineering surveying planning environmental 12800 Whitewater Dr,Suite 300,Minnetonka,MN 55343
Table of Contents
I. Introduction..........................................................................................................................................2
II. Site Description ...................................................................................................................................2
WetlandStatus........................................................................................................................... 6
III. Existing Conditions.............................................................................................................................7
Vegetation ................................................................................................................................. 7
DrainageArea............................................................................................................................ 8
InletStructure............................................................................................................................. 8
Outlet Control Structure............................................................................................................... 9
Survey....................................................................................................................................... 9
Evaluation................................................................................................................................ 12
IV. Recommendations.............................................................................................................................13
V. Appendix............................................................................................................................................14
Page 2 of 20
Storm Water Management System Assessment
Orono Truck Station
I. Introduction
Hennepin County Property Services retained MFRA, Inc. to conduct a storm water management system
assessment at the Orono Truck Station and outline recommended maintenance. This technical
memorandum details the findings of the inspection and outlines recommendations for repair and
maintenance. This facility was inspected on January 20�antl March 4th, 2014 under frozen winter
conditions. A follow-up inspection was completed on June 11th to observe those aspects of the storm water
management system that were under snow antl ice cover. A storm water management system
questionnaire was submitted to the facility's manager; however, as of the date of this report a response has
not been received (see blank questionnaire in the Appendix of this report).
II. Site Description
With the exception of the west pond topo survey, the following diagrams were gathered from a technical
memorandum furnishetl by Hennepin County Property Services, titled "Storm Water Pond Inspection" and
dated 11/4/2005. The Orono Truck Station contains two ponds; the east retention pontl was built in 2002,
the west pond was an existing wetland that is being used for storm water treatment. The 2005
memorantlum addresses the east pond conditions, but did not include a review of the west pontl. The
ponds receive runoff from the facility's buildings, landscaped areas and paved areas. Figure 1 shows the
locations of the ponds on site.
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Figure 1 —West and East Pond Locations
Page 3 of 20
Storm Water Management System Assessment
Orono Truck Station
East Pond
Figure 2 shows the grading plan for the east retention pond. The inlet structure is a 15-inch HDPE pipe that
drains into the retention pond from the north. The outlet control structure is an 18-inch corrugated metal
standpipe connectetl to an 18" carbon steel pipe that flows approximately 60 feet to the southwest and
discharges through a flared end section. Both the inlet antl the outlet drain onto rip rap for energy
dissipation. The design pond bottom is at an elevation of 933, and the top of the standpipe is at an
elevation of 938.65. No details were provided on the design of the standpipe, and it is unknown whether
the opening at the top of the pipe is the only outlet designed for this pond. The outlet control structure is
shown in Figure 3.
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Figure 2—East Retention Pond Grading Plan
Page 4 of 20
Storm Water Management System Assessment
Orono Truck Station
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Figure 3—Outlet Control Structure Profile
Page 5 of 20
Storm Water Management System Assessment
Orono Truck Station
West Pond
Figure 4 shows a topo survey of the west pond. Runoff from the truck station facility appears to discharges
to the west pond through an 18 inch flared end section. It also appears that runoff flows from the east pond
through a ditch on the south side of the facility and into a culvert which drains into the west pond. Runoff
also appears to flow overland from the facility directly into the south ditch before draining into the west
pond. The outlet from the west pond appears to be a 30 inch flared end section adjacent to the inlet from
the truck station facility.
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Page 6 of 20
Storm Water Management System Assessment
Orono Truck Station
Wetland Status
National Wetland Inventory (NWI) maps were reviewed to tletermine the jurisdictional status of the storm
water management system. Based on Figure 5, no wetlands are present in the vicinity of the East pond.
The west pond is a part of an existing wetlantl.
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Figure 5-National Wetland Inventory Map(1/9/14)
Page 7 of 20
Storm Water Management System Assessment
Orono Truck Station
III. Existing Conditions
The Orono Truck Station storm water management system was inspectetl on January 20th(east pond) and
March 4th (west pond). Skies were cloudy and temperatures were in the low 30's at the time of the first
inspection, antl skies were cloudy and temperatures were in the teens at the time of the second visit. A
follow-up inspection was performed on June 11th to observe those aspects of the storm water management
system that were untler snow and ice cover. The purpose of these inspections was to evaluate the existing
vegetation, runoff sources, inlet and outlet contlitions and pond condition (including a determination of the
depth of accumulated sediment). Photographs taken during the inspection are included in the Appendix of
this report.
Vegetation
Due to snow cover at the time of MFRA's first site visit, a thorough review of the vegetation could not be
completed. At the follow-up inspection, it was observed that the east pond area is almost completely
covered with cattails. Cattails typically do not grow in water deeper than 2 feet, and the maturity of these
plants intlicates that the permanent water level of the pond is consistently at or below 2 feet. The west pond
is also densely covered with cattails, which is not abnormal for a wetland with shallow standing water.
Buckthorn was noted in the west pond surround. Both cattails and buckthorn are considered an invasive
species and could be removed; however, their presence does not affect the ponds' performances.
Additionally, the "Storm Water Pond Inspection" prepared by HDR in 2005 noted the need for a vegetated
buffer to be reestablished near the northeast corner of the east pond. At the time of the inspection, it
appeared as though this maintenance issue had been resolved and a lack of vegetated buffer was not
observed. Table 1 lists the plant material that was visible at the time of the inspection, most of which is
appropriate in a storm water pond.
Page 8 of 20
Storm Water Management System Assessment
Orono Truck Station
Table 1 —Vegetation Onsite East Pond
Common Name Scientific Name
Smooth Sumac Rhus labra
Hone Locust Gleditsia triacanthos
American Elm Ulmus americana
Reed Canary Grass Phalaris arundinacea
Common Milkweed Asclepias syriaca
Canada Goldenrod Solida o canadensis
Cattail T ha S .
Common Burdock Arctium minus
Vi inia Cree er Parthenocissus a►vensis
Common Buckthom Rhamnus cafhartica
Smooth Brome Bromus inermis
Lea S u e Eu horbia esula
Box Elder Acer ne undo
Field Bindweed Convovulvus arvensis
Basswood Tilia americana
Riverbank Gra e V'�tas ri aria
Drainage Area
The retention ponds receive runoff from the facility's buildings, landscaped areas and paved areas. No new
impervious areas are known to have been added to the facility since the ponds'construction in 2002.
Inlet Structure
East Pond
The 2005 technical memorandum noted that a side bank near the inlet of the east pond had experienced
erosion problems. Vegetation has filled this area and the erosion issue is no longer apparent. Due to snow
cover at the time of MFRA's first site visit, an inspection of the both ponds' inlet structures could not be
completed.At the time of the second inspection, the 15 inch HDPE inlet pipe was being encroached upon
by vegetation. This vegetation should be cleared to allow for better flow and maintenance access. The inlet
pipe was in otherwise good condition and no erosion was noted.
West Pond
The west pond inlet structure appears to be an 18 inch flared end section that receives runoff from the truck
station facility(the west pond also appears to receive runoff from the east pond through a culvert crossing
under the road). At the time of the second inspection, the 18 inch flared end section and riprap were in
good condition. A small amount of vegetation has begun to grow inside the flared end section and should
be removed to prevent clogging of the system.
Page 9 of 20
Storm Water Management System Assessment
Orono Truck Station
Outlet Control Structure
East Pond
Due to snow cover at the time of MFRA's first site visit, an inspection of the outlet control structures for both
ponds could not be completed. The 2005 technical memorandum noted that a grate should be added to the
18 inch corrugated metal standpipe. At the time of the second inspection, it was observed that this
maintenance issue hatl been resolved and that the standpipe and surrountling riprap were in good
condition. The standpipe drains south to a ditch where runoff then flows overland to the west pond. The
downstream carbon steel pipe is in good condition and no erosion was evident in the ditch.
West Pond
The outlet structure of the west pond appears to be a 30 inch flared end section. This flared entl section is
adjacent to the 18 inch flared entl inlet and causes concern for short circuiting of the wet pond. At the time
of the inspection, the outlet was clear of vegetation and in good condition.
Survey
East Pond
A survey of the setliment depth was taken at two points in the east pond: between the inlet and outlet
structures, antl in the eastern section of the pond. A viable thirtl point coultl not be surveyed due to the
density of cattails. The coordinates of the two points have been provided so that their exact location may be
revisited. Survey point locations are shown in Figure 6.
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Figure 6—Survey Point Locations East Pond
Page 10 of 20
Storm Water Management System Assessment
Orono Truck Station
Table 3-Surve Measurements East Pond
Rod Readin A roximate Elevation
To of Stand i e 2.66 937.58
Invert of Stand i e" 5.21 935.03
Location:OR-1, Between Inlet and Outlet Structures
Coordinates: 152682.197,438536.391,850.432
To of Ice 6.02 934.22
To of Sediment from Ice 1.0 933.22
Bottom of Sediment from Ice 1.7 932.52
De th of Sediment 0.7 ft
Location:OR-2, East Section of Pond
Coordinates: 152708.701,438593.500,848.914
To of Ice 5.97 934.27
To of Sediment from Ice 0.7 933.6
Bottom of Sediment from Ice 0.5 932.8
De th of Sediment 0.8 ft
*Known elevation from 2005 report used as benchmark
It appears that an average of 0.75 feet of sediment has accumulated on the bottom of the basin.
Page 11 of 20
Storm Water Management System Assessment
Orono Truck Station
West Pond
A survey of the setliment depth was taken at two points in the west pond. The west pond area is dense with
cattails and a third point a distance from the first two was not feasible. The coordinates of the two points
have been provided so that their exact location may be revisited. Survey point locations are shown in
Figure 7.
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Page 12 of 20
Storm Water Management System Assessment
Orono Truck Station
Table 4—Survey Measurements West Pond
Rod Reading Approximate Elevation
Location:OR-1
152665.567,438197.848,838.386
Top of Sediment from Ice 1.0 NA
Bottom of Sediment from Ice 2.6 NA
Depth of Silt 1.6 ft
Location:OR-2
152674.771,438167.812,838.797
Top of Sediment from Ice 1.0 NA
Bottom of Sediment from Ice 1.8 NA
Depth of Silt 0.8 ft
Note: No benchmark elevation was available
It appears an average of 1.2 feet of sediment has accumulated on the bottom of the basin. No standing
water was observed in the west pond and the sediment deposition could be described as mud or slush like.
Evaluation
The East pond has been in operation for 12 years and during that time appears to have been functioning
properly. However, the maturity and prevalence of cattails in the east pond is an indication that the
permanent water level in the East pond is below 2 feet. The design permanent water level of the east pond
could not be determined from the 2005 technical memorandum or from the survey provided by Hennepin
County Property Services. The 2005 technical memorandum noted no sediment accumulation in the East
pond, and it is assumed no removal of sediment has occurred since the pond's construction in 2002. The
West pond was already in place at the time the East pond was constructed. It is unknown how long the
West pond has been in use. No record of maintenance activity in the West pond was available, and it is
assumed no removal of sediment has occurred in the West pond.
Page 13 of 20
Storm Water Management System Assessment
Orono Truck Station
IV. Recommendations
As previously stated, a follow-up site visit will be completed in spring 2014 to observe the inlet structures,
outlet control structures, vegetation, and sheet flow perimeters. A completed Stormwater Basin
Questionnaire has not yet been received from the facilities manager. Completion of this survey would allow
for greater understanding as to whether the ponds are functioning properly or if problems are arising with
inlet/outlet structures, vegetation, or erosion.
Based on the first inspection, the following actions are recommended:
�► Any vegetation encroaching on the inlet, outlet, or emergency overflow structures should be
removed. This maintains proper flow through the pond and ensures that all structures are
accessible for maintenance.
y Further analysis of the east pond outlet control structure design should be completetl to verify if the
permanent water level in the pond is at the design elevation. This analysis should include review of
adtlitional design documents (subject to availability) as well as visual inspection of the outlet control
structure, which will be completed as part of the follow-up inspection in spring 2014. If the
permanent water level is lower than the design elevation, further evaluation should be completed to
determine the cause of the low water level.
y Accumulated sediment should be removed from the east pond. A scaled drawing of the pontl was
not available, and no estimate of sediment to be removed has been calculated.
y Accumulated setliment should be removed from the west pond. Since the west pond is a wetland
area and there is no documentetl record of a stormwater maintenance plan, approval from the
Minnehaha Creek Watershed District will be required prior to performing maintenance activities in
the pond. A scaletl drawing of the pond was not available, and no estimate of sediment to be
removed has been calculated.
� The buck thorn near the west pond is an invasive species. It could be removed and the area
routinely mowed to prevent reestablishment, but its removal does not affect performance of the
pontl. As noted above, approval from the Minnehaha Creek Watershed District will be needetl prior
to performing maintenance activities in the west pond.
� We recommend that Hennepin County consider a 15-year cycle of sediment survey antl removal
for both ponds.
�► Please refer to the attached maintenance schedule.
When removing vegetation, care should be taken not to allow any removed vegetation to clog the outlet
structure, to re-stabilize bare areas to prevent erosion, and to avoid exposing the pond to hazardous
chemicals. A permit is not required to perform sediment removal in retention ponds. However, in June of
2012 the MPCA published Managing Stormwater Sediment Best Management Practice Guidance for
Municipalifies, which outlines how to efficiently test and remove sediment from retention ponds. Since the
purpose of the stormwater pond is to collect and retain pollutants, sediment should be tested according to
this guidance document and the sediment disposed of properly.
Page 14 of 20
Storm Water Management System Assessment
Orono Truck Station
V. Appendix
Photos
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Page 16 of 20
Storm Water Management System Assessment
Orono Truck Station
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Figure 12—Photograph of East Pond 15"HDPE Inlet with Encroaching Vegetation
Page 17 of 20
Storm Water Management System Assessment
Orono Truck Station
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Page 18 of 20
Storm Water Management System Assessment
Orono Truck Station
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Figure 15—Photograph of 18"FES Inlet on Left and 30"FES Outlet on Right
Page 19 of 20
Storm Water Management System Assessment
Orono Truck Station
Storm Water Pond Maintenance Schedule
Maintenance Procedure Frequency
Item
Wetland Monitor wetland vegetation health and composition and replant lost vegetation. Spring and Fall,additionally as
Vegetation Remove all wetland vegetation encroaching on inlet,outlet,and emergency needed to allow visual
overflow structures. ins ection of the ond
Non-native or The pond and pond surround should be inspected for the presence of non-native Spring and Fall,additionally as
Invasive and invasive vegetation. Identified species should be removed—through mowing needed to allow visual
Vegetation or other means recommended by the DNR.All vegetation on the berm which inspection of the pond
im edes access to ond should be removed.
Erosion Inspect pond and pond surround for bare or eroded soils,repair damaged areas. Spring and Fall,additionally as
needed after large storm
events or washouts occur
Debris Inspect pond and pond surround for debris;dispose of all debris properly. Spring and Fall,additionally as
debris build u is noticed
Inlet and Outlet Inspect inlet and outlet structures(including all corresponding grates and orifices) Spring and Fall,additionally as
Structures for clogging and damage.Remove sediment or debris from the structures. any problems with drainage are
Replace damaged grates/valves. noted
A report, including photos,should be made in conjunction with any cleanings or
repairs to document the pond's condition.Frequent removal of sediment from
outlets may be a sign of other unde�ying problems,such as erosion and loss of
sediment stora e de th in the ond.
Easily Inspect risers,barrels,embankments,and pipes for damage. Repair or replace Every one to three years,
Accessible any damaged components. additionally as problems with
Pond pond function arise
Component A report,induding photos,should be made in conjunction with any repairs or
re lacements to document the ond's condition.
Hard to Reach Remotely inspect(i.e.televise)components that are not easily accessible,such as Every five to ten years,
Pond under-drains,reverse slope pipes,or submerged/buried pipes.Repair or replace additionally as problems with
Components damaged components. pond function arise
A report,including photos,should be made in conjunction with any repairs or
replacements to document the ond's condition.
Sediment Visually monitor sediment depth during routine inspections.Complete survey of Monitor yearly, survey every 5
sediment depth and remove sediment when pond functionality is impaired. to 10 years,remove on a 5 to
10 or 15 year cycle based on
rate of sediment accrual
Normal The pond water elevation should be inspected to ensure that the normal water Annually,and more than 72
Water Level level is being maintained.Water levels below the outlet structure may signify a hours after large rainfall events
For Retention leak in the outlet system.Water levels that remain above the outlet structure for
Ponds lon er than 72 hours after a rainfall event ma si ni a clo in the s stem.
Mosquitoes If problems with mosquitoes arise,possible solutions are participating in a As needed
mosquito control program through the state or hiring a private contractor.
Presence of mosquitoes may be a sign of another underlying problem,such as
shallow onds in the pond embankment and sta nation.
Page 20 of 20
Storm Water Management System Assessment
Orono Truck Station
Stormwater Basin Questionnaire
Please complete the following survey and return it via fax, mail, or email.
1. Have any issues been noted during rain events, i.e.,water ponding in unwanted areas,
plugged outlets, or draining taking longer than 48 hours?
2. Have any changes in vegetation occurred, such as vegetation dying or the presence of a
new plant species?
3. Have any possible hazardous materials entered the basin from vehicles or agricultural
runoff?
4, Have any impervious surfaces been added to the basin's drainage area since its
installation?
5. Have any noticeable erosion or washout problems occurred?
6. Has any maintenance been performed on the basin/storm sewer system since the basin's
installation?
7. Do you have any additional comments about the basin's performance?
All questions can be addressed to Jessica Tello at 763-259-6679 or jtello .mfra.com . Thank you!
TECHNICAL MEMORANDUM
HDR ENGINEER/NG, INC.
STORM WATER POND INSPECTION
HENNEPIN COUNTY, MINNESOTA
Orono Truck Station Facility 11/4/2005
Reviewed by: Mike Johnson P.E. CFM, Dave Johnson
Prepared by: Foad Hussain
TABLE OF CONTENTS
INTRODUCTION................................................................................................................................ 2
PONDDESCRIPTION......................................................................................................................... 2
WetlandsStatus.................................................................................................................................3
PONDCONDITION ............................................................................................................................ 4
ExistingVegetation..............................................................................................................................4
RunoffSources.....................................................................................................................................4
InletStructure.......................................................................................................................................4
OutletStructure....................................................................................................................................4
SedimentElevation Survey..................................................................................................................4
OverallAssessment........................................................................................................................... 5
RECOMMENDATIONS...................................................................................................................... 5
APPENDIX—PHOTOGRAPHS.......................................................................................................... 8
11/4/2005
HDR PROJECT#32560 1 HENNEPIN COUNTY,MINNESOTA
ORONO TRUCK STATION FACILITY
STORM WATER POND EVALUATION
TECHNICAL MEMORANDUM
HDR ENG/NEER/NG, INC.
INTRODUCTION
HDR Engineering, Inc. (HDR) was retained by Hennepin County Property Services to perform an
inspection of the storm water pond and to make recommendations for long-term and short-term
maintenance of the pond at the Orono Truck Station Facility. This technical memorandum
describes the information collected during the pond inspection, documents the pond condition, and
provides recommendations for the maintenance of the pond.
HDR conducted an on-site inspection of the pond on October 19, 2005. At the time of the
inspection, the pond was not drained and was near the normal pool elevation.
POND DESCRIPTION
The Orono wet detention pond was built in 2002 at the east corner of the facility. It was designed to
receive storm water runoff from the Orono Truck Station site, which includes the facility building,
landscaped areas, and roads and paved parking lot. A plan view of the pond design contours is
provided in Figure 1.
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Figure 1 - Pond Layout Plan View
The Orono detention pond consists of one inlet and one outlet structure. The inlet structure
consists of a 15-inch polyethylene pipe and is located on north side of the pond. The outlet
structure consists of a corrugated metal standpipe, which has a diameter of 18 inches, and is
located on the west corner of the pond. The downstream end of the metal pipe consists of a
concrete flared end section without a trash guard/grate. Rock riprap was placed on the edge/foot of
the inlet and the outlet structures to prevent erosion.
Storm water enters the detention pond through the inlet culvert and is stored until it reaches the
maximum storage capacity of the pond. The approximate bottom elevation of the pond per the
design drawings is 933 ft as shown in Figure 1. The highest elevation up to which the water can be
stored in the pond is 938.65 ft (top of standpipe), as shown in Figure 2.
11/4/2005
HDR PRO.IECT#32560 2 HENNEPIN COUNTY,MINNESOTA
ORONO TRUCK STATION FACILITY
STORM WATER POND EVALUATION
TECHNICAL MEMORANDUM
HDR ENG/NEERING, INC.
56.D Lt iB" CS ?1PE + APRON.t� 1.01L � .
PROP. F1N[SHEq St1RFACE E%ISt1NG 5t1RFAC� �
T� OF t6' CMAP . � �
STANDPIP� = 938.65` � � •
FLCp'L1Mdf. ELEV = 935.00'�
FLr7WLIt� ELEY = 935.�7''
' PI,AC� APRON . .
Figure 2 - Pond Standpipe Culvert Profile
Wetlands Status
A review of the National Wetland Inventory (NWI) was performed to investigate the jurisdictionai
status of the pond. The Orono Truck Station pond does not appear to be part of the NWI as shown
in Figure 3.
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11/4/2005
HDR PRo,IECT#32560 3 HENNEPIN COUNTY,MINNESOTA
ORONO TRUCK STATION FACILITY
STORM WATER POND EVALUATION
TECHNICAL MEMORANDUM
HDR ENG/NEER/NG, INC.
POND CONDITION
A site visit was conducted by HDR to assess the existing condition of the Orono storm water
detention pond on October 19, 2005. The purpose of the pond site visit was to inspect the pond
condition, assess its current performance, and perform level surveying to determine the
approximate depth of the sediments deposited on the pond bottom. Photographs taken during the
site visit are included in the Appendix of this report.
Existing Vegetation
Almost the entire periphery of the Orono Pond was covered by medium size vegetation, except the
northeast side bank of the pond (area around/near inlet). However, no vegetation was observed
near the center of the pond. The pond periphery (except the northeast corner) supports a medium
population of reeds and sedges, which are extended into the pond. The assessment team was able
to walk through the vegetation without any problem. Overall, the vegetation supported by the pond
is normal and desirable for the pond operation. The northeast side bank of the pond lacks a
vegetative buffer strip and is experiencing erosion. The balance of the pond surface is open water.
Runoff Sources
The drainage area of the Orono wet detention pond consists of the paved parking lot, truck station
building, roads, landscape areas, and storage facility. The visual inspection of the Hennepin
County Orono wet detention pond revealed the pond site is moderately clean and no floating debris
was observed in the pond. However, some garbage/trash (bottles, cans, etc.) were noticed at the
side banks and near the outlet structure of the pond.
Inlet Structure
The inlet structure of the Orono wet detention pond appeared to be in good shape. The inlet
culvert/pipe discharging the runoff into the pond had no visible problems. However, no grate was
observed over the inlet culvert/pipe. Moreover, no sediment deposition in the inlet pipe was
observed during the inspection visit. Rock riprap was provided around inlet structure and at the foot
of the pipe to prevent erosion.
It was noticed that the side bank near the inlet of the Orono pond has experienced erosion
problems. Since no vegetation protection is available in that part of the pond, the soil has eroded
away and some cracks in the side bank were observed.
Outlet Structure
The outlet structure at the Orono wet detention pond was located and visually inspected. The metal
standpipe installed at the outlet structure had no visible problems. Rock riprap was placed around
the outlet structure and the bottom of the pipe to prevent erosion.
Sediment Elevation Survey
The purpose of the elevation survey was to take several measurements as close to the pond
bottom as possible to try to ascertain the amount of sediment deposited in the pond. The Orono
pond had vegetation around the perimeter and open water for most of the pond surface area.
Depth measurements were taken at points in the pond that could be safely accessed by the survey
team.
The invert of the downstream end of the outlet structure was at an elevation of 934.31 ft per the
design drawings and was used as a reference elevation. Elevation measurements were taken at
several locations within the pond (Table 1).
11/4/2005
HDR PRo.IECT#32560 4 HENNEPIN COUNTY,MINNESOTA
ORONO TRUCK STATION FACILITY
STORM WATER POND EVALUAT/ON
TECHN/CAL MEMORANDUM
HDR ENG/NEERING, INC.
Table 1- Summary of Pond Level Measurements
Staff Location �taff R�ading *HI , Ei�vs�an '
(�) ��� ���
"*Invert of the d/s end of the outlet structure 9.54 943.85 934.31
To of the d/s end of the outlet structure 8.00 943.85 935.85
Invert of stand i e 8.82 943.85 935.03
Invert of Inlet PE i e 8.36 943.85 935.49
To of Inlet PE i e 7.04 943.85 936.81
Ed e of water 8.63 943.85 935.22
Ed e of water 8.62 943.85 935.23
Bottom of ond 5 ft from outlet 11.22 943.85 932.63
Bottom of ond 8-10 ft from outlet 11.42 943.85 932.43
Ed e of water 8.72 943.85 935.13
Bottom of ond 5 ft from inlet 10.95 943.85 932.90
Bottom of ond 8-10 ft from inlet 10.98 943.85 932.87
'`HI=HEIGHT OF THE INSTRUMENT.
"'`INVERT OF THE DOWNSTREAM END OF OUTLET STRUCTURE WAS KNOWN(934.31 FT).
Based on the measurements taken during the survey, the amount of sediment accumulation in the
Orono pond appears to be negligible. Given that the water surFace elevation in the pond is not
above the design elevation, the pond outlet (standpipe) appears to be functioning and appropriately
draining the pond.
Overall Assessment
Given that the Orono wet detention pond was constructed in early 2002 and has been in operation
for approximately 2.5-years, it appears to be in good shape and is functioning as intended. The
presence of vegetation along the boundary of the pond indicates that influent storm water brings
plentiful amounts of nutrients and some percentage of these nutrients is being removed from the
storm water discharge flows. Although the side bank near the inlet structure has undergone erosion
and needs immediate attention, it appears that almost all of the storm water pond components are
present, intact, and functioning. The amount of sediment in the pond is negligible.
RECOMMENDATIONS
Based on HDR's inspection and assessment of the pond conditions, few recommendations are
proposed to improve or sustain the pond's performance. The recommendations are presented in a
tabular format in Table 2. Additional discussion relating to an action item is noted in the Comment
Item column and the related comments are provided following the table.
11/4/2005
HDR PRo.IECT#32560 5 HENNEPIN COUNTY,MINNESOTA
ORONO TRUCK STATION FACILITY
STORM WATER POND EVALUATION
TECHNICAL MEMORANDUM
HDR ENGINEERING, INC.
Table 2—Action Items
Action �tem �escriptian Recornmended Time ��rr�nt
to Comp�ate ltem
The side bank near the northeast corner of the pond As soon as possible. A
has experienced erosion and requires immediate repair
measures. The vegetative buffer strip should be
reestablished in that part of the pond to prevent further
deterioration of the side bank.
The grates should be installed over the inlet culvert and As soon as possible B
the outlet culvert (downstream end) of the Orono wet
detention pond.
The vegetation around the downstream end of the During normal, C
outlet structure of the pond should be trimmed where it scheduled maintenance
impedes access to the outlet structure requiring periodic and as needed.
inspection.
A periodic inspection of the pond should be scheduled As needed D
to keep the pond in operating and optimum condition.
Comment Item A
The northeast periphery of the Orono pond has undergone erosion and some cracks were observed
in the side bank during the inspection visit. It was also noticed that the northeast side bank consists
of the low cohesive soil with lots of small pebbles and rocks which might be due to construction of
the storage facility (constructed after the pond ) further north of the pond. The top soil of the
northeast side bank should be immediately replaced with the fertile cohesive soil and a seed mix
should be sprinkled to develop a vegetative buffer strip in this part of the pond. The vegetative
buffer strip will not only provide slope stability to the side bank but also improve the water quality
and pollutant removal efficiency of the pond.
Comment Item B
The grate over the inlet structure and outlet structure (d/s end) at the Orono wet detention pond was
not seen during the inspection visit. The provided design plan sheets do not include/show a grate
over the inlet structure and the downstream end of the outlet structure. It is recommended that the
grates should be installed over the inlet and outlet structure to trap the trash and debris and
improve the water quality of the Orono wet detention pond.
Comment Item C
The degree and density of the vegetation around and in the pond is appropriate at this time. If the
vegetation at the location of the inlet and outlet structures becomes excessive and impedes access
for visual inspections, it should be thinned as needed. The growth of large trees should only be
allowed where they will not impede the performance of the inlet and outlet structures and will not
compromise bank stability.
11/4/2005
HDR PRO,IECT#32560 6 HENNEPIN COUNTY,MINNESOTA
ORONO TRUCK STATION FACILITY
STORM WATER POND EVALUATION
TECHNICAL MEMORANDUM
HDR ENG/NEERING, INC.
Comment Item D
A periodic inspection schedule should be compiled and followed. Table 3 provides a suggested
maintenance and inspection schedule.
Table 3— Maintenance and Inspection Schedule
Maintenance Item Description Frequency
Remove accumulated trash and debris from the pond side Twice per year
slopes, embankments, and inflow and outflow structures.
Mosquito control As necessary
Storm damage/ Site spill incident As necessary, especially
after extreme weather
events or uncontained
material spill that may
flow to the pond.
Remove excessive vegetation growth that impedes access to As needed during the
pond structures. growing season to
maintain access.
Perform visual inspection of pond inlet and outlet structure. Twice per year.
Remove accumulated debris that may impede the inlet or outlet
flows. Perform or schedule repairs as needed.
Perform an annual inspection of the pond to make sure it is Annually
operating as desired. It is recommended that a digital photo
reference point should be established from which annual
maintenance assessment can be made and the pond condition
photo-documented. The pond should be drained and the
following items should be checked:
❖ embankment subsidence
❖ cracking or excessive tree growth on the embankment
❖ damage to the outlet standpipe
❖ sediment accumulation at the outlet
❖ sediment accumulation around the inlet
•3 inadequacy or failure of bank or other erosion controls
❖ changes to the pond configuration
❖ changes to the contributing watershed that may affect the
pond
Sediment removal 5-to 15-year cycle,
10 year interval
recommended
11/4/2005
HDR PRO,IECT#32560 7 HENNEPIN COUNTY,MINNESOTA
ORONO TRUCK STATION FACILITY
STORM WATER POND EVALUATION
TECHNICAL MEMORANDUM
HDR ENG/NEERING, INC.
APPENDIX - PHOTOGRAPHS
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Photo 1: Orono Pond View from East Boundary of the Pond
11/4/2005
HDR PRo�ECr#32560 S HENNEPIN COUNTY,MINNESOTA
ORONO TRUCK STATION FAC/UTY
STORM WATER POND EVALUATION
TECHNICAL MEMORANDUM
HDR ENG/NEERING, INC.
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Photo 2: Orono Pond View from West Boundary and the Outlet Structure
11/4/2005
HDR PRo,IECr#32560 9 HENNEPIN COUNTY,MINNESOTA
ORONO TRUCK STATION FACILITY
STORM WATER POND EVALUATION
TECHNICAL MEMORANDUM
HDR ENGINEER/NG, INC.
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Photo 3: Erosion at the Northeast Side Bank of the Orono Pond
11/4/2005
HDR PRo,IECr#32560 10 HENNEPIN COUNTY,MINNESOTA
ORONO TRUCK STATION FACILITY
STORM WATER POND EVALUATION
TECHN/CAL MEMORANDUM
HDR ENG/NEER/NG, INC.
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Photo 4: Storage Facility Situated North of the Orono Pond
11/4/2005
HDR PRo�ECr#32560 11 HENNEPIN COUNTY,MINNESOTA
ORONO TRUCK STATION FACILITY
STORM WATER POND EVALUATION
TECHNICAL MEMORANDUM
HDR ENG/NEERING, INC.
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Photo 5: Downstream End of the Outlet Structure of the Orono Pond
11/4/2005
HDR PRO,IECT#32560 12 HENNEPIN COUNTY,MINNESOTA
ORONO TRUCK STATION FACILITY
STORM WATER POND EVALUATION
Orono Truck Before Stormwater System After Stormwater System
Station Updates U�da#es
West Pond, southeast . �� d { , . � West Pond, southeast corner d , � >;�`�� ,..r, '
� . ��� �' ����» � �.� x � �. �.���'
corner at southwest '�a�,� Y f� rt t ��,. � � '�� � : at southwest corner of a ", �
corner of parking lot. ����, r+ �!��w + ��; _; ��';? parking lot. Note cast iron
,
Note open mouth (12 �_ ��� ��' , '� cover on gently sloped rough ___
inch plus diameter)of ' � surfaced concrete apron.
steeply sloped , �,;,. '� �� Stormwater flows into a � � ;�:;
corrugated steel drain �,a•..� precast concrete basin and
pipe—often hidden by � -� then horizontally onto rip rap � • ~� �`�
tall grass or snow. �,^ in photo below. � :_�"''�
,
- �� / �~i�;.. � �'&d�. . y'� •r
' � w. k
t�� '��, , . � � .
,. " �'f._..�` . . - �.�� T .. .. , .. . .
West Pond, southeast � '`' ' '* ��� : West Pond, southeast corner.
comer. Note steeply � , „ #"�,�aa."�` � . �_ � Note open rip rap for
sloped corrugated steel , �'»*� � � � � stormwater drainage, '� �' = :.�"
'''~ � �t`, � enclosed i e is one, rate �
drain pipe(12 inch plus � �.�` �.f�� ,�a+ �b�i p�p 9 g , , �� .', �
�� '"��+ in lace over o en mouth of -�!'"-.
diameter)terminating "`�' :, / p p �,. � / ' •
,�. ��tr�.v . . , !�'���ii ��� ,� , j / �.
underwater. Note open � ' � � �R . horizontal concrete "' . t�
mouth of horizontal =;• , �,,, �;��` ' �� stormwater pipe on left. �� � ; ��"�''�
concrete stormwater � '�� . �, .� �,+ � . �•
pipe on left. ; r ��. �,� '�i��`.. �s::... - ` s�
` t. s •.�•y�} .p ��_ 1 � � '...'�� . :�...�y�.r'
� � � ,� t '�. •.!b f�'.�}
.���.' �,.., i,4 2� �t tR '�����+������"����_ . '�
� �4
South edge of parking � �s<�� W F� � �' South edge of parking lot � `�'� "�; �
lot near center. Photo "& ��p¢�� 3� i"' near center. Photo shows rip �l�: a .
shows one of several � '��"���J��;�° �'•i'!, rap reinforced drainage area ;� �°� � � : �� ��`t�.
eroded drainage areas �e ���!, ��'� K leading down rip rap covered � r°� ��
leading to the east/west , . �;�,�`� �¢ surface into the east/west ���� -�` '` Y��� .� ;�.�� � .,���
ditch draining into Lake 1.; � ��' ditch draining into Lake �.; �y� �._q � ' '
Minnetonka. �,�� "� `'� ' ,�t,J :� ,� Minnetonka. Other eroded � ������_���r
� �-;, ° �;�'-,. '�.� • • areas have been seeded and
;,� ` '.� ��� _ . recontoured to direct
�;�� � � , , ���d��r: , stormwater to the rip rap area �p� `� + � ��
� � v �iop n ie� shown in phota , _..�� ��