LANGDON LAKE

MCWD H/H and Pollutant Loading Study – 2003 C-1 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Langdon Lake Table of Contents

C. Langdon Lake...... 3 C.1. General Description...... 3 C.2. Physical Features...... 6 C.2.a. Land Cover/Land Use...... 6 C.2.b. Geology ...... 10 C.2.c. Soils ...... 11 C.2.d. Groundwater...... 11 C.3. Water Quantity ...... 16 C.3.a. Watershed Hydrology...... 16 C.3.b. Watershed Hydraulics ...... 19 C.3.c. Water Quantity Findings and Discussion...... 19 C.3.d. Watershed Recommendations...... 24 C.3.e. Watershed References ...... 24 C.4. Scour and Erosion-Prone Areas ...... 26 C.4.a. Streams ...... 26 C.4.b. Lakeshore ...... 26 C.5. Water Quality ...... 27 C.5.a. Watershed Pollutant Load Analysis ...... 27 C.5.b. Lake Modeling and Associated Goals...... 32 C.5.c. MPCA Impaired Waters and Point Source Permits ...... 38 C.6. Recommendations ...... 39

MCWD H/H and Pollutant Loading Study – 2003 C-2 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

C. Langdon Lake

C.1. General Description

The Langdon Lake watershed is located along the western boundary of the MCWD and within the cities of Minnetrista and Mound (Figure IV.C.1-1). The watershed is 1055 acres (about 1.6 sq. miles), and includes five subwatershed units (designated LL-1 through LL-5). Figure IV.C.1- 2 shows the subwatersheds and their drainage configuration.

Langdon Lake (LL-5) is located at the downstream end of the watershed and is the last in a string of water bodies (Figure IV.C.1-2). On the western end of the watershed, runoff is first received by Black Lake (unofficial name, DNR ID 27-183W) (LL-1) before discharging into Saunders Lake. Saunders Lake (LL-3), receiving flow from both LL-1 and LL-2, is allowed to discharge through a berm into a small creek carrying flows towards Langdon Lake. The creek passes under West Edge Boulevard where runoff from subwatershed LL-4 is received before flowing into Langdon Lake just past the former Mound Wastewater Treatment lagoon. Langdon Lake outlets directly into Lost Lake before entering Cooks Bay of Lake Minnetonka.

MCWD H/H and Pollutant Loading Study – 2003 C-3 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion !"110

Minnetrista RT 8 RT 6 !"15

Black Lake Mound

!"110

Langdon Lake

!"125 Saunders Lake

!"44 !"110

Subwatershed Boundaries Major Watershed Boundary Major Roads City Boundaries Streams Lakes 0.2500.25Miles

N Figure IV.E.1-1 Langdon Lake Watershed Political Boundaries H&H Report/Basins/Projects/flow_030609_kl

MCWD H/H and Pollutant Loading Study – 2003 C-4 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Black Lake LL-4 LL-5

LL-1

LL-2 <

< To Cooks < Langdon Lake Bay via

Lost Lake <

<

Saunders Lake

LL-3

< Flow Direction Subwatershed Boundaries Major Watershed Boundary Streams

Lakes 0.2500.25Miles

N Figure IV.C.1-2 Langdon Lake Watershed Flow Direction H&H Report/Basins/Projects/flow_030609_kl

MCWD H/H and Pollutant Loading Study – 2003 C-5 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

C.2. Physical Features

The following sections detail the MLCCS, geology, soils, and groundwater of the Langdon Lake watershed.

C.2.a. Land Cover/Land Use

For comparison purposes, the various MLCCS land cover classifications have been combined into five impervious surface area categories and six vegetative cover type categories (Figures IV.C.2-1 and IV.C.2-2). Although not shown here, each of the impervious surface area categories was further broken down with respect to type of land use and vegetative cover found on non-impervious surface areas. A more detailed map showing MLCCS cover types to Level 3 for the entire MCWD is included in Appendix 3 (Figure IV.Appendix.3-1). A description of all MLCCS cover types is also included in Appendix 3 and is incorporated into the District's interactive GIS tool.

Land use in the watershed changes dramatically across the political boundary between the cities of Minnetrista and Mound. Open space in the form of woodlands, forests, grasslands and maintained natural areas dominates the western portion of the watershed that lies within the City of Minnetrista. The Dakota Rail line that divides the watershed into north and south sections hinders connectivity between natural areas in this portion of the watershed. The eastern part of the watershed is dominated almost entirely by residential land use types. Single family residential land use surrounds Langdon Lake to the south and north. To the east the lake is surrounded by commercial and institutional land use.

Currently in the Langdon Lake watershed, forests and woodlands, lakes and open water wetlands, and “26% to 50% impervious cover” dominates the landscape, with each of these categories making up approximately 20% of the landscape. Under 2020 land use conditions, lakes and open water wetlands is the most dominant category, with “11% to 25% impervious cover” (18%), and “26% to 50% impervious cover” (19%) close behind. The biggest percent

MCWD H/H and Pollutant Loading Study – 2003 C-6 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion increases (Table IV.C.2-1) were in the “0% to 10% impervious cover” and “11% to 25% impervious cover,” categories. The biggest percent decreases were found in agricultural land, forests and woodlands, and grasslands.

MCWD H/H and Pollutant Loading Study – 2003 C-7 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Major roads Subwatersheds Surface water DUTCHDutch

!"15

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-MLCCS analysis performed in 2002. Langdon -More detailed MLCCS information can be found in Appendix 3 of this volume.

!"12 5 Minnesota Land Cover Classification System 0% to 10% impervious cover 11% to 25% impervious cover 26% to 50% impervious cover !"110 51% to 75% impervious cover 76% to 100% impervious cover Agricultural Land Forests & Woodlands

44 Grasslands !" Lakes & Open Water Wetlands Maintained Natural Areas Wetlands 1000 0 1000 Feet Figure IV.C.2-1 N Langdon Lake Watershed Minnesota Land Cover Classification System H&H Report/Basins/painter_langdon.apr__030221_aps

MCWD H/H and Pollutant Loading Study – 2003 C-8 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Figure IV.C.2-2 Langdon Lake Watershed Land Cover

250 Existing Conditions 2020 Conditions 20% 200

15% 150

10% Area [acres] 100 Percent Watershed Area Watershed Percent

50 5%

0 0%

er er er er er d s s s s s v v v v v n d d d ea d o o o o o a n n an r n c c c c L la la l A la s s C l d s et l et us u u s s ra o as a o o o u ou u o r W r W i i i io i lt G r u v v v v v u W e at er er er r er c t p p p e p ri & a N p g s W d im im im m im st e I A e n in % % % r e a 0 5 0 % 0% o p t 1 2 5 5 0 F O in 7 1 a to to to o & t to s M % % e 0% 1 6 % k 1 2 1 6% a 5 7 L Land Cover Category

MCWD H/H and Pollutant Loading Study – 2003 C-9 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.C.2-1 Langdon Lake Watershed Land Cover Percent Change Percent Change Land Cover Category (from existing to 2020 conditions) 0% to 10% impervious cover 305% 11% to 25% impervious cover 81% 26% to 50% impervious cover 0% 51% to 75% impervious cover 0% 76% to 100% impervious cover 0% Agricultural Land -17% Forests & Woodlands -43% Grasslands -60% Lakes & Open Water Wetlands 0% Maintained Natural Areas 0% Wetlands 0%

C.2.b. Geology The St. Lawrence-Franconia Formation is the uppermost bedrock unit within most of Langdon Lake watershed. The Jordan Sandstone is present in the southeastern portion of the watershed, but is eroded away in the north and west.

The Quaternary deposits are associated with the Des Moines Lobe glaciation. They are composed primarily of loamy till in hummocky and irregular topography. Other deposits of peat and muck are present in or near marshlands and ponds throughout the watershed. Peat and muck is partially decomposed plant matter and fine-grained organic matter. Along the southern portion of the watershed (along Highway 110) there are ice-contact deposits. These sand and gravel deposits formed as meltwater from a Des Moines Lobe glacier flowed at or behind the ice margin.

(See Volume II: Framework and Methodology, D. Groundwater for a description of methodology for sections C.2.b through C.2.d.)

MCWD H/H and Pollutant Loading Study – 2003 C-10 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion C.2.c. Soils Soil Hydrologic Groups are shown on Figure IV.C.2-3. The predominant hydrologic groups are B (moderate infiltration rate when wet) and D (very slow infiltration rate). The group D soils are found in low-lying areas that have a seasonably high water table or in areas with clayey soils. Group B soils have moderately fine to moderately coarse texture. Some group C (slow infiltration rate) soils are present and are composed of fine-grained material. The soil figure should be used as a guide for further soil examination at the site of inquiry. The group A soils (high infiltration rate) are present in a few areas of the southern portion of the watershed where there are sandy soils.

C.2.d. Groundwater Water table elevation contours are shown on Figure IV.C.2-4. Shallow groundwater flow is generally from the west and northwest toward Langdon Lake in the east, and from there towards Lake Minnetonka. Figure IV.B.2-5 shows depth to the water table. Areas that have deeper readings on this map are associated with a rise in the topography as compared with the nearby wet areas.

Infiltration potential for the watershed is represented on Figure IV.C.2-6 and is generally medium to low. Much of the area is classified medium due to the presence of group B soils. Due to the variable composition of soils that have been classified as “Organic”, areas that contain these soils have variable infiltration potential as well. There are some areas of high and medium high infiltration potential in the southern portion of the watershed (along hwy 110). These are areas where sandy soils combine with sand & gravel deposits from the quaternary parent material to produce a high infiltration potential.

Further geology and groundwater discussion can be found in Volume V: Watershed Issues Integration.

MCWD H/H and Pollutant Loading Study – 2003 C-11 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Major roads Subwatersheds Surface water DUTCHDutch

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!"15

Langdon

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Soil hydrologic groups !"110 A B C !"44 D A/D B/D C/D 1000 0 1000 Feet

N Figure IV.C.2-3 Langdon Lake Watershed Soil Hydrologic Groups H&H Report/Basins/painter_langdon.apr__030221_aps

MCWD H/H and Pollutant Loading Study – 2003 C-12 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Major roads Subwatersheds DUTCHDutch Surface water

!"15

!"15

940 $ Langdon

!"12 5

!"110

Arrow indicates !"44 groundwater flow direction.

Elevation contour 1000 0 1000 Feet (10 ft interval)

N Figure IV.C.2-4 Langdon Lake Watershed Water Table Elevation Contour H&H Report/Basins/painter_langdon.apr__030221_aps

MCWD H/H and Pollutant Loading Study – 2003 C-13 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Major roads Subwatersheds Surface water DUTCHDutch

!"15

!"15

Langdon

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!"110 Depth to water table (ft) 1 - 9 10 - 19 !"44 20 - 29 30 - 39 >39 feet 1000 0 1000 Feet

N Figure IV.C.2-5 Langdon Lake Watershed Depth to Water Table H&H Report/Basins/painter_langdon.apr__030221_aps

MCWD H/H and Pollutant Loading Study – 2003 C-14 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Major roads Subwatersheds Surface water DUTCHDutch

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Langdon

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Infiltration potential "110 ! High Medium High Medium !"44 Medium Low Low Variable - Organic Variable - Bedrock 1000 0 1000 Feet

N Figure IV.C.2-6 Langdon Lake Watershed Infiltration Potential H&H Report/Basins/painter_langdon.apr__030221_aps

MCWD H/H and Pollutant Loading Study – 2003 C-15 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

C.3. Water Quantity

C.3.a. Watershed Hydrology

A description of the watershed morphology, drainage, land use, land cover, and soils exists under Langdon Lake watershed sections C.1. and C.2.. For labeling of modeled features and XP- SWMM diagram, refer to Langdon Lake Watershed Figure IV.Appendix.1-C1 in the Appendix.

Input Parameters:

The hydrology of the Langdon Lake watershed is influenced by wetland, lake large lot rural areas found in Minnetrista and denser urban development predominately located in the City of Mound. Hydrologic input parameters include: area, slope, width, percent impervious, depression storage, hydraulic conductivity, capillary suction, and initial soil moisture deficit. The methodology used to generate these parameters is described under the model methodology section (II.F.1). The input parameter values for Langdon Lake are shown on Table IV.Appendix.1-C1.

Subwatershed Boundaries:

The Langdon Lake watershed, which is tributary to “Lost Lake” DNR ID 27-180W (LST-1) and ultimately flows into Cooks Bay of Lake Minnetonka, was subdivided into a total of 5 sub- watersheds. The average sub-watershed size in the Langdon Lake watershed is 211 acres.

The largest subwatershed, with a total area of about 470 acres, contains Langdon Lake (LL-5), for which the watershed is named. The surface area of Langdon Lake, approximately 142 acres, makes up about 30 percent of the total area in the subwatershed. Langdon Lake outlets to Lost Lake through a culvert under CSAH 110.

MCWD H/H and Pollutant Loading Study – 2003 C-16 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Subwatersheds LL-3 and LL-1, both containing large open water wetlands, are the next largest in size at 314 and 120 acres respectively.

The subwatershed boundaries drawn were aided by the use of new MCWD topography (10 foot resolution, generated with 5 foot interpolated intervals), USGS 10 foot topography and stormsewer/structure locations from the City of Minnetrista and City of Mound. Hennepin County CSAH 110 road plans also added boundary definition along the 110 corridor. Where applicable and reasonable, subwatershed boundaries were matched with those already in use by others (i.e. city subwatershed boundaries). In general, boundary definitions were straight forward.

Boundary considerations: • LL-3: Only minor boundary ambiguities existed in this subwatershed along the railroad and CSAH 110 along the southern boarder. Additional field verification of drainage culverts and new topographic information from a proposed development adjacent to Saunders Lake helped resolve uncertainties.

Subwatershed growth:

The subwatershed area and percent impervious for all Langdon Lake subwatersheds (LL) for existing and 2020 conditions are listed in Table IV.C.3-1.

In general, the entire Langdon Lake watershed is predicted to have a significant increase of impervious areas (indicative of development). The overall predicted percent increase in year 2020 for the entire Langdon Lake watershed is 4.4 percent.

The last column of Table IV.C.3-1, highlights that the greatest increases in impervious surfaces are predicted to occur in subwatersheds LL-1, LL-2 and LL-3. These subwatersheds are currently dominated by rural land use and located mostly in the City of Minnetrista. Features making these subwatersheds desirable development locations include: • Easy access by CSAH 110 on the south and Lynwood Lane on the north

MCWD H/H and Pollutant Loading Study – 2003 C-17 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion • Nearby existing stormsewer in the City of Mound • Close proximity to Lake Minnetonka • Located on the urban fringe of development surrounding the Twin City Metro Area • LL1 and LL3 both contain large open water wetlands

As this area develops, care should be taken to ensure development does not negatively impact the watershed resources. Special attention should be given to subwatershed LL-1, LL-2 and LL-3 because they present an increasingly rare opportunity to implement low impact strategies for the District as the area converts from a rural to a more urban land use. Specific recommendations concerning development in Langdon Lake watershed are located in section C.6: Recommendations.

Table IV.C.3-1 Langdon Lake Growth by Subwatershed Percent 2020 Percent Change in Percent Subwatershed I.D. Area (acres) Impervious* (%) Impervious* (%) Impervious (%)

LL-1 120 28 31 4 LL-2 75 4 10 6 LL-3 314 27 34 7 LL-4 77 25 27 2 LL-5 470 56 60 3 Average ** 211 38.5 42.9 4.4 Total Area 1055 * Includes open water and saturated wetlands. ** Percent impervious average is weighted on area.

MCWD H/H and Pollutant Loading Study – 2003 C-18 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion C.3.b. Watershed Hydraulics

Input Parameters:

Table IV.Appendix.1-C2 in the Appendix shows a summary of the hydraulic input parameters for all the modeled links. More specific input information, for example cross section slopes, friction coefficients, or entrance/exit conduit losses (minor losses), can be found under the hydraulic mode of the XP-SWMM model.

Stage/area information for all the storage nodes modeled (lakes, wetlands, ponds, road crossings, etc.) are also available in the model.

Drainage Routing:

The Langdon Lake watershed drainage is characterized by a series of aging and poorly maintained culverts along the railroad and at the outlets of “Black Lake” DNR ID 27-183W and Saunders Lake DNR ID 27-185W. Despite significant capacity restrictions in several locations, drainage eventually makes its way into Langdon Lake at the downstream end of the watershed. The notable exceptions are the storm-sewered areas adjacent to (and draining to) Langdon Lake in the City of Mound (Figure C.1-1) and new development on the southeastern end of Saunders Lake.

C.3.c. Water Quantity Findings and Discussion

A summary of subwatershed findings and notes resulting from the modeling effort are compiled in Table IV.C.3-2. Additional modeling specific comments pertaining to the individual subwatershed basins are in Table IV.Appendix.1-C5.

MCWD H/H and Pollutant Loading Study – 2003 C-19 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.C.3-2 Summary of Water Quantity Findings for Langdon Lake Watershed Issues Flooding Issues Appendix.1-C5) Water Quantity Water Quantity Boundary Issues DNR jurisdiction Landlocked Issues Subwatershed I.D. Named water body Notes (see Table IV. IV. Table (see Notes Infrastructure notes notes Infrastructure Backwater conditions conditions Backwater Improvement Priority 100-year Critical Event Event Critical 100-year Flow Velocities/Erosion Flow Velocities/Erosion Significant 2020 Impacts 2020 Impacts Significant Additional H/H Modeling outlet structure damaged LL-1 Rainfall Black Lake 27-183W medium sediment impaired LL-2 Rainfall low debris impaired LL-3 Snowmelt Saunders Lake 27-185W medium sediment impaired LL-4 Rainfall medium

LL-5 Snowmelt Langdon Lake 27-182P high

MCWD H/H and Pollutant Loading Study – 2003 C-20 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Results Summary:

The normal water level (NWL), high water level (HWL), peak discharge, and peak velocities predicted for the 100-year events are listed in Table IV.Appendix.1-C3 and Table IV.Appendix.1-C4 of this volume. Hydrographs and time dependent stages and velocities for continuous simulations and other event runs can be found in the provided XP-SWMM models. Figure IV.Appendix.1-C1 in the Appendix shows the XP-SWMM model diagram depicting the names of the links and nodes representing the hydraulics (water routing) of the watershed.

Subwatershed Critical Event:

The modeled 100-year, 10-day snowmelt runoff event produced the “critical” or greater HWL in Saunders Lake (LL-3), the small creek flowing from Saunders to Langdon, and Langdon Lake (LL-5). The 100-year, 24-hour storm event produced greater HWLs in Black Lake and the modeled depressions on the north side of the railroad tracks (subwatersheds LL-1, LL-2, and LL- 4). Table IV.C.3-2 and Table IV.Appendix.1-C3 should be referenced for each subwatershed’s “critical” event and HWL.

2020 Impacts:

Areas of the watershed predicted to have a more significant impact on water resources resulting from 2020 land use changes are LL-2 and LL-4. Both subwatersheds are located on the northern sides of the railroad tracks bisecting the watershed. High water levels at the modeled depressions are predicted to increase from approximately 0.6 to 0.9 feet under 2020 conditions.

Additional impacts to the small creek running from Saunders Lake to Langdon Lake include an increase in discharge and velocity. Development plans for areas directly adjacent to this creek were used to aid model construction. Peak flows and volumes should be strictly regulated, especially in the portions of LL-5 draining directly to the creek, in order to minimize increased erosion and scour. Special efforts to minimize sediment transport to Langdon Lake are

MCWD H/H and Pollutant Loading Study – 2003 C-21 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion warranted to avoid exacerbating existing problems in Langdon Lake (water quality impacted by sediment transport). For further discussion of Langdon Lake water quality refer to section C.5.

Special Subwatershed Issues:

Multiple structures in the Langdon Lake watershed are in need of maintenance and/or repair. Culverts under the railroad receiving runoff from LL-2 and LL-4 are congested with sediment. The Black Lake outlet structure (DNR ID 27-183W) of subwatershed LL-1 is visibly cracked and shifted. The small culvert and berm controlling Saunders Lake, LL-3, is frequently impacted by debris.

Further evaluation/investigated of the structures controlling Black Lake and Saunders Lake is of particular importance due to the current and predicted future development pressures in the subwatersheds containing these lakes (LL-1 and LL-3). Alterations or repairs of the structures should be implemented prior to or in conjunction with area development. The issue of structure and berm integrity at the outlets of Black and Saunders Lakes has been identified in the City of Minnetrista SWMP.

The HWL of Black and Saunders Lakes are dependent on their outlet configuration. The HWL predicted for both lakes reflects current conditions. Any changes to their outlet structures should be evaluated to ensure the required level of flood protection is provided.

Particular consideration and emphasis should also be given to discharge rates in the small creek running from Saunders Lake to Langdon Lake in the western end of subwatershed LL-5. Efforts to minimize erosion and scour in the small creek will help reduce sediment and pollutant transport to Langdon Lake. Under existing conditions, direct discharge received by the creek is limited to runoff from undeveloped land and is also restricted by the small outlet culvert from Saunders Lake. Future development adjacent to the creek in LL-5 and/or work on the Saunders Lake outlet should maintain existing peak rates. Volume control implemented in adjacent and upstream subwatersheds will also help minimize sediment transport in the creek by reducing the time the creek bed must withstand peak discharge rates.

MCWD H/H and Pollutant Loading Study – 2003 C-22 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Several smaller wetlands exist in the subwatersheds that were not modeled explicitly. The wetland in LL-4, north of the railroad tracks and a small landlocked wetland in LL-5 located between Saunders Lake and Langdon Lake. Efforts should be made to preserve (or improve) the function and value of these wetlands.

Flow Velocities/Erosion Issues:

The following Table IV.C.3-3 highlights pipes showing high velocities for the modeled events. It is recommended that inlet and outlet erosion control measures or energy dissipation designs are implemented in these areas. Section C.4 provides additional information on erosion prone areas in the Landon Lake watershed.

Table IV.C.3-3 Significant Conduit Velocities in Langdon Lake Watershed Peak Velocity (ft/s) Link or Multi Link Description Existing 2020 Name Snow-melt Event 1.5-year, 100-year, 100-year, 100-year, 10-day 24-hour 24-hour 24-hour Railroad culvert LL-2 RR 9.5 12.0 12.7 11.1 Railroad culvert LL-4 RR 6.0 13.5 14.6 5.5

Where: Velocity Range 10 to 11.9 12 to 14.9 15+

Flooding Issues:

None of the modeled roads are predicted to overtop during the 100-year design events.

The 100-year water level at the Saunders Lake outlet berm (LL-3) and at County Road 110 between Langdon Lake (LL-5) and Lost Lake (LST-1) exceeds the minimum freeboard (2 feet)

MCWD H/H and Pollutant Loading Study – 2003 C-23 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion required by the district. This information can also be obtained using Table IV.Appendix.1-C3 and the road overtopping elevation found in the model as a multi-link.

C.3.d. Watershed Recommendations

Recommendations specific to water quantity issues can be found in C.6: Recommendations, along with recommendations relevant to other aspects of Langdon Lake water resources.

C.3.e. Watershed References

Pertinent information available to aid model construction and to compare and contrast XP- SWMM model results included: ∗ MCWD hydrodata ∗ MCWD Water Resource Plan TR-20 Model results ∗ MNDNR historical water elevations Langdon Lake (LL-5) ∗ MNDNR lake file data Portions of Saunders Lake Floodplain Study (July 1998) 2 foot topography for portions of Saunders Lake and adjacent land by Otto Associates, 1999. ∗ MNDNR Hydrographic Survey Reports Saunders Lake (27-185W) 4/26/1988 ∗ Surface Water Management Plans City of Minnetrista: Surface Water Management Plan (Draft 3, September 1998) City of Mound: Surface Water Management Plan (April 2000) ∗ Development Plans Between Saunders Lake and West Edge Boulevard. ∗ Mn/DOT / Hennepin County Road Plans Hennepin County Project No. 7921 (CSAH 110 at outlet from Langdon Lake to Lost Lake)

MCWD H/H and Pollutant Loading Study – 2003 C-24 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion ∗ USGS Quadrangle Maps ∗ Models by others Xp-swmm model (print) of Saunders Lake for Floodplain study MFR HydroCAD model for City of Minnetrista (1998)

This information was used to aid model construction and also for model validation where applicable. That engineering and modeling judgment was used to assess and resolve conflicting information. Additional field information was gathered when necessary to fill in gaps, update and/or resolve conflicting information.

The XP-SWMM model results were calibrated against measured data and compared to other models for general results validation.

MCWD H/H and Pollutant Loading Study – 2003 C-25 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion C.4. Scour and Erosion-Prone Areas

C.4.a. Streams

The Langdon Lake watershed does not contain one of the six main creeks selected for a more detailed erosion and scour analysis. However, the small creek running from Saunders Lake to Langdon Lake currently experiences erosion. Development in the watershed is predicted to increase peak discharge, velocity and volumes received by the creek. To reduce scour potential in the creek, it is important to emphasize rate control in those developing areas. Volume control is also important to avoid increases in time exposure to high flows in the creek. Several areas of high and medium infiltration potential exist in the watershed providing an opportunity to implement volume control practices.

C.4.b. Lakeshore

The identification of lakeshore erosion areas was conducted primarily at the Regional Team meetings, when participants were asked to locate any known erosion areas on a map of the area represented. The RT 6 and 8 meetings did not identify any locations in the Langdon Lake watershed. This, however, does not necessarily mean that none exist; rather, it indicates that the members have not seen specific problems. The District should remain vigilant in locating lakeshore erosion because of the direct threat that these problems present through sediment delivery into lakes.

MCWD H/H and Pollutant Loading Study – 2003 C-26 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion C.5. Water Quality

C.5.a. Watershed Pollutant Load Analysis

The pollutant loads (lbs/ac-yr) for the Langdon Lake watershed are illustrated in Figure IV.C.5- 1 through -3. The remaining model results, including runoff volume and pollutant loads (lbs/yr), are listed in Appendix 2 of this volume.

As the Langdon Lake watershed develops, increases are expected in the pollutant loads generated from the changing land uses, as summarized in Table IV.C.5-1. The majority of the southwest quadrant of the watershed is predicted to change from vacant/agricultural land to single family residential. In order to maintain current pollutant loading rates, about 78 lbs. per year of phosphorus will need to be removed in the watershed. Similar relative increases in total nitrogen and total suspended solids will also have to be eliminated. These load reduction targets should be spread out over the entire watershed to avoid the detrimental cumulative effects of development.

Table IV.C.5-1 Langdon Lake Watershed Pollutant Load Summary TP load (lbs/yr) TN load (lbs/yr) TSS load (lbs/yr) % % % Existing 2020 Increase Existing 2020 Increase Existing 2020 Increase Increase Increase Increase 148 226 78 52% 1389 1909 520 37% 39293 58912 19619 50%

The highest TP loads (per unit area) in the Langdon Lake watershed originate from the eastern portion of the watershed, located in Mound, that contains mostly single family residential land use (Figure IV.C.5-1). This area is more developed than the rest of the watershed, and therefore has higher levels of imperviousness. The southwestern portion of the watershed is predicted to show the greatest increase in phosphorus loading from existing to 2020 conditions. This area is presently classified as “vacant/agricultural,” and is predicted to be mostly single family

MCWD H/H and Pollutant Loading Study – 2003 C-27 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion residential by 2020. The acreage of single family residential land use surrounding Langdon Lake is also predicted to increase by 2020, leading to a predicted increase in pollutant loadings due to future planned development from that region as well.

The lowest loading rates are in the less developed areas of the western portion of the watershed. Phosphorus loads in the northwest corner are not predicted to substantially increase.

The TN and TSS loads follow a similar pattern (Figures IV.C.5-2 and IV.C.5-3), with higher loads surrounding Langdon Lake, and predicted increases around the lake and in the southwestern portion of the watershed. Management implications for these modeling results are discussed in Section C.6: Recommendations.

MCWD H/H and Pollutant Loading Study – 2003 C-28 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Existing Conditions 2020 Conditions

Watershed Boundary Lakes Watershed TP Loads (lbs/ac-yr): 0.0 - 0.13 0.5 0 0.5 Miles 0.13 - 0.2 0.2 - 0.28 Figure IV.C.5-1 N 0.28 - 0.35 Langdon Lake Watershed 0.35- 1.0 Total Phosphorus Loads

H&H Report/Basins/Projects/pload_030220_kl

MCWD H/H and Pollutant Loading Study – 2003 C-29 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Existing Conditions 2020 Conditions

Watershed Boundary Lakes Watershed TN Loads (lbs/ac-yr): 0.0 - 0.9 0.5 0 0.5 Miles 0.9 - 1.2 1.2 - 1.5 N 1.5 - 2.0 Figure IV.D.5-2 Langdon Lake Watershed 2.0 -7.8 Total Nitrogen Loads H&H Report/Basins/Projects/pload_030220_kl

MCWD H/H and Pollutant Loading Study – 2003 C-30 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Existing Conditions 2020 Conditions

Watershed Boundary Lakes Watershed TSS Loads (lbs/ac-yr): 0 - 40 0.5 0 0.5 Miles 40 - 60 60 - 80 Figure IV.C.5-3 N 80 - 110 Langdon Lake Watershed 110 - 445 Total Suspended Solids Loads H&H Report/Basins/Projects/pload_030221_kl

MCWD H/H and Pollutant Loading Study – 2003 C-31 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion C.5.b. Lake Modeling and Associated Goals

This section summarizes the lake modeling results for Langdon Lake. WiLMS input parameters (see Volume II: Framework and Methodology) are presented in Table IV.C.5-2, and the lake modeling results are presented in Table IV.C.5-3.

In the case of Langdon Lake, the watershed pollutant load estimate did not accurately predict the current in-lake concentration. Therefore the load to the lake is derived from the observed in-lake concentration, along with lake and watershed characteristics (see Volume II. Framework and Methodology, Section F for a complete explanation). The mean annual runoff used for the lake modeling was 4.4 inches for existing conditions and 4.9 inches for 2020 conditions, the same values used in the watershed pollutant loading model. The runoff volume increases as development fills in currently undeveloped parts of the watershed.

Table IV.C.5-2 WiLMS Input Parameters Mean Watershed TP Load Lake Area Volume Drainage Lake Depth to Lake (lbs./year) (acres) (ac-ft) Area (ac) (ft) Existing 2020 Langdon 144 1195 8.3 911 296 457

Table IV.C.5-3 Lake Modeling Results Observed (µg/L) Predicted 2020 TP (µg/L)

Observed Based on modeled Lake “Adjusted 2020” TP (June- Years averaged watershed estimate** Sept mean) loadings

Langdon 87* 1998 - 2000 -- 111 *TP average of 1997 - 2000 is 115 µg/l. The 1998 - 2000 estimate is more reflective of current conditions since there was an alum treatment applied to Langdon Lake in 1998. **See methodology, Volume II, section F.2: Modeling, water quality.

MCWD H/H and Pollutant Loading Study – 2003 C-32 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Langdon Lake is a shallow lake with a mean depth of 8.3 feet. Historically, TP concentrations in the lake have been extremely high. This is thought primarily to be due to the abandoned Mound Wastewater Treatment Plant that discharged wastewater into the Mound Wastewater Treatment Lagoon. The capacity of this plant, at 1.25 MGD, is two to three times higher than the other six wastewater treatment plants that operated on Lake Minnetonka until 1974. TP concentrations began to decline after the WWTP was closed, and in 1998, the lake was treated with alum and TP concentrations were further reduced. However, Langdon Lake still scores a D+ on the 2000 MCWD report card. Figure IV.C.5-4 shows phosphorus concentrations and secchi transparency for Langdon Lake from 1997 to 2000. By the year 2020, watershed loading is estimated to increase by 52%, resulting in in-lake P concentrations reaching 111µg/L.

MCWD H/H and Pollutant Loading Study – 2003 C-33 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion Figure IV.C.5-4 Langdon Lake Historic Summer TP and Transparency

300 Alum treatment in 1998

250

RT 6 Proposed RT 8 Proposed 200 TP goal 70 µg/L TP goal 55 µg/L

150

100 Surface TP [µg/L] Surface

50

0 3-Aug-96 29-Jul-97 24-Jul-98 19-Jul-99 13-Jul-00 8-Jul-01

3-Aug-96 29-Jul-97 24-Jul-98 19-Jul-99 13-Jul-00 8-Jul-01 0

0.5

1

Secchi (m) Secchi 1.5

2

Alum treatment in 1998

2.5

MCWD H/H and Pollutant Loading Study – 2003 C-34 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

Table IV.C.5-4 reviews the lake goals recommended by Regional Teams 6 and 8 and identifies the loads to each lake that correspond to the RT recommendations. The lake itself is located in RT6; however, parts of the watershed are located in RT8. These two teams recommended different goals for Langdon Lake, and both will be presented and discussed here, in addition to the percent load reduction necessary to achieve each desired goal . (More information regarding the RT6 and 8 goal recommendations can be found in Volume III: Public Involvement, F. Regional Team 6 and H. Regional Team 8.)

Table IV.C.5-4 Langdon Lake Watershed Lakes Goals and Target Loads TP Goal (µg/L) TP Load to Lake (lbs/yr) Required % Current Load Load Proposed Reduction in Lake MCWD (calculated from either Goal Regional Load 1997 PLOAD estimate or (calculated Team TP (current vs. goal) Goal observed in-lake from RT Goal concentration) goal) Langdon 50 70 (RT6) 296 221 25 Langdon 50 55 (RT8) 296 148 50

Modeled phosphorus loads to Langdon Lake currently can not explain the high phosphorus concentration of 89 µg/L within the lake. This is indicative of phosphorus entering the lake from a source other than particular landuse practices associated with the upstream watersheds. The likely sources for the excess P are: 1) historic discharges from the Mound Wastewater Treatment Lagoon, 2) P-export from the cattail marsh at the west end of the lake, and 3) P export from wetlands in the watershed. The “unknown” phosphorus sources to Langdon Lake total approximately 296 lbs./yr.

In order to reach the Langdon Lake goal of 70 µg/l (RT6), the following load reduction allocation is proposed.

MCWD H/H and Pollutant Loading Study – 2003 C-35 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion 1) The source of excess P needs to be identified and reduced by 50%, from 296 lbs./yr. to 148 lbs./yr. If the source is identified to be internal loading, consider an alum injection system, only if adequate data are collected to show that this is a viable, cost- effective solution. 2) Current loadings within the subwatersheds LL-1 through LL-4 will need to be reduced by 50%. The phosphorus loads in the upper 10% of subwatershed LL-5 will also need to be reduced by 50%. These areas comprise the approximate watershed of a potential BMP site, explained in (4) (Figure IV.C.5-5). 3) P loads in the remainder of LL-5 will need to be reduced by 10%. 4) Conversion of potential BMP site (Figure IV.C.5-5) to a wetland/bioretention system would be designed to maximize uptake of dissolved P form LL-1 through LL-4 and the upper 10% of LL5.

In order for the more ambitious goal of 55 µg/l (RT8) to be met, (3) in the above list will have to be altered to:

3) P loads in the remainder of LL-5 will need to be reduced by 50%.

MCWD H/H and Pollutant Loading Study – 2003 C-36 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

LL-1 LL-4 LL-2 < LL-5

Langdon <

< ÚÊ Lake <

<

Saunders Lake LL-3 Langdon Lake Watershed Boundary Subwatersheds < Flow Arrows ÚÊ Potential BMP Site 0.2500.25Miles

N Figure IV.C.5-5 Langdon Lake Watershed Potential BMP Site H&H Report/Basins/Projects/pload_030506_kl

MCWD H/H and Pollutant Loading Study – 2003 C-37 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion

C.5.c. MPCA Impaired Waters and Point Source Permits

Within the Langdon Lake watershed, there are no water bodies on the MPCA’s 303(d) list of impaired waters.

Today, very few point source discharges of any treated material occur in the Minnehaha Creek Watershed. Of the six discharges that currently exist, none are located in the Langdon Lake watershed.

MCWD H/H and Pollutant Loading Study – 2003 C-38 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion C.6. Recommendations

To address the load reduction needs identified in Table IV.C.5-4 and to incorporate the management alternatives in Tables III.F-4 and III.H-3 (Volume III. Public Involvement, F. Regional Team 6 and H. Regional Team 8 ), the management scheme outlined in Table IV.C.6-1 is proposed for the Langdon Lake watershed. Details of the recommendations follow the table. Recommendations applicable to the entire MCWD are discussed in Volume V: Watershed Issues Integration.

Table IV.C.6-1 Langdon Lake Watershed Recommended Actions Category

Receiving Recommended Action Water Body Priority Maintenance Maintenance Responsible Party* Party* Responsible Capital Improvement Improvement Capital Information /Education /Education Information Permitting/Enforcement Permitting/Enforcement Monitoring/Investigation 1) Minimize sediment Langdon High X X A,B,E transport in creek Lake 2) Enhance or replace Langdon High X A,B,E Saunders Lake outlet Lake 3) Investigate and remediate Langdon A, B, former Mound Wastewater High X X Lake F Treatment Plant 4) Retrofit new stormwater Langdon practices into redeveloping High X X X A, B Lake areas of Mound 5) Achieve a no net Langdon A, B, increase in phosphorus for High X X X X Lake D, E new development 6) Roadway Reconstruction Langdon A, B, High X X Stormwater Plan Lake D 7) Repair or rebuild Black Saunders Medium X A,B,E Lake outlet Lake 8) Preserve small wetlands Langdon Medium X A,B,E in LL-4 and LL-5 Lake

MCWD H/H and Pollutant Loading Study – 2003 C-39 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion 9) Water quality monitoring Langdon of Langdon Lake to assess Lake Medium X A phosphorus loading to Lake Minnetonka 10) Work with Langdon Lake property owners to Langdon A, B, Medium X X implement stormwater Lake E management practices 11) Maintenance and Saunders energy dissipation at Lake & Low X X A,B culverts under railroad (LL- Langdon 2 & LL-4) Lake *Responsible party: A – MCWD, B – City, C – Three Rivers Park District, D – Mn/DOT, E – Private Landowners, F – Met Council

1) Minimize sediment transport in creek from Saunders to Langdon Lake. Development is predicted to occur within LL-5 which is directly adjacent to the small creek flowing from Saunders to Langdon Lake. Erosion and scour potential should be minimized to reduce sediment transport to Langdon Lake by emphasizing rate control. Volume control will also help minimize erosion potential.

2) Enhance or replace Saunders Lake outlet. The Saunders Lake outlet is prone to frequent debris clogging leading to fluctuating water levels, and the integrity of the outlet berm to withstand such fluctuation is questionable. A debris screen should, at a minimum, be retrofit to the upstream side of the outlet structure to prevent clogging. Preferably, a new outlet should be designed such that protection of downstream resources are preserved (minimizing peak flows to the outlet creek flowing to Langdon), and a more reliable flood protection elevation is established. Existing and future development pressure adjacent to Saunders Lake (LL-3) make the outlet assessment a timely need.

3) Investigate and remediate former Mound Wastewater Treatment Plant. An assessment should be made of the former Mound Wastewater Treatment Facility to determine if phosphorus export is occurring into adjacent wetlands and Langdon Lake.

MCWD H/H and Pollutant Loading Study – 2003 C-40 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion This analysis should also include an evaluation of Langdon Lake sediments to assess the potential for internal loading.

4) Retrofit new stormwater practices into redeveloping areas of Mound. This redevelopment should be designed to lower the rate and volume of stormwater runoff. Additionally, the existing urban areas of Mound should incorporate stormwater management BMPs.

5) Achieve a no net increase in phosphorus for new development. New development should not result in a net increase in stormwater rate, volume or pollutant loading. Developments should be planned to provide on-site treatment of stormwater with the objective to avoid or minimize additional stormwater runoff to the extent possible. Offsite mitigation, where necessary, should be provided concurrent with the new development.

6) Roadway Reconstruction Stormwater Plan. A “Roadway Reconstruction Stormwater Management Plan” should be developed (with the Shoreland District of Lake Minnetonka Cities) that incorporates stormwater management improvements and design into roads as they are maintained, upgraded, or newly constructed. CSAH 15 is proposed to be upgraded and could serve as a pilot for this effort.

7) Repair or rebuild Black Lake outlet. The Black Lake outlet appears to function but is visibly cracked and in an obvious state of disrepair. Repair or rebuild outlet prior to development of property adjacent to the lake. Assess downstream culvert under railroad tracks in conjuncture with any outlet work at Black Lake.

8) Preserve small wetlands in LL-4 and LL-5. A couple of smaller wetlands not explicitly modeled exist in LL-4 and LL-5. Their function and value should be preserved or enhanced as development pressure increases.

MCWD H/H and Pollutant Loading Study – 2003 C-41 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion The wetland in LL-5 (between Saunders and Langdon) is landlocked and should remain as such. Special attention to preservation of a healthy hydroperiod and volume control should be emphasized for areas draining this and all landlocked pockets.

9) Water quality monitoring of Langdon Lake to assess phosphorus loading to Lake Minnetonka. Monitor water quality of Langdon Lake to better characterize the role that internal loading plays with the lake’s phosphorus budget. This effort should be completed in conjunction with the investigation into the old Mound Wastewater Treatment Plant.

10) Work with Langdon Lake property owners to implement stormwater management practices. Work with shoreline residents of Langdon Lake to create riparian buffers, rain gardens and, where appropriate, infiltration swales. Where runoff from several areas converges, treatment ponds should also be considered.

11) Maintenance and energy dissipation at culverts under railroad (LL-2 & LL-4). Modeling indicates high pipe velocities at pipes outletting LL-2 and LL-4 under the railroad tracks. Field investigation of these same pipes revealed significant sediment restrictions.

These recommendations emerged out of discussions as part of the Regional Team 6 and Regional Team 8 public involvement process. Additional issues and management recommendations were identified as part of this process. A complete presentation of the recommendations can be found in Volume III: Public Involvement, Regional Team 6 and Regional Team 8, which includes information regarding the priority of each issue, who would be responsible for undertaking each suggested management approach, and a recommendation of when the approach should be undertaken.

MCWD H/H and Pollutant Loading Study – 2003 C-42 Emmons and Olivier Resources, Inc. Volume IV: Watershed Modeling and Discussion