4.5 Water Quantity

Goal 5 - Water Quantity

Maintain or reduce existing flows from drainage within the watershed to decrease the negative effects of stormwater runoff and bounce from existing and proposed development as well as provide low flow augmentation to surface waters.

Discussion

Development and the associated creation of new impervious surface increases the volume of stormwater runoff from the landscape, changes the rates and times to peak runoff flow, and decreases the amount of stormwater that naturally percolates into the soil to recharge groundwater.  The District’s long term goal in the Langdon Lake subwatershed is to achieve no increase in the volume of stormwater discharged from the subwatershed into Lost Lake.  Implementation strategies will include minimizing new runoff volumes from development, and encouraging infiltration and groundwater recharge.

A key strategy to achieve this goal is the adoption of a volume management standard for new development and redevelopment that requires the abstraction (removal from runoff through infiltration, capture and reuse, evapotranspiration, etc.) of one inch of rainfall.    Approximately 70 percent of annual runoff volume in Minnesota results from precipitation events of 1” or less (MPCA, 2000).  Requiring new development and redevelopment to abstract (retain on site through infiltration, evapotranspiration, or capture and reuse) runoff from small rain event would significantly reduce new volumes of runoff flowing downstream and help reduce future erosion in streams and channels; minimize new pollutant loading that would have been conveyed by that stormwater; and help maintain groundwater levels, preserving wetlands.  The additional new volume could be mitigated through construction of regional infiltration basins, restoring drained wetlands, reforestation and revegetation, or other means.

Modeled annual volume of runoff in the Langdon Lake subwatershed, and estimated reductions resulting from application of a proposed 1” abstraction rule for new development and redevelopment (acre-feet).

2000 modeled annual subwatershed runoff volume

245

2020 modeled annual subwatershed runoff volume

290

Ultimate Development modeled annual subwatershed runoff volume

290

Increase between 2000 and Ultimate development

45

Estimated volume abstracted by 1” rule

32

New volume to be abstracted through other means such as capital projects, wetland restorations, reforestation and revegetation, etc.

13

Desired Outcomes: Management of new annual water volume discharged from the subwatershed.

Metrics:  Acre-feet volume abstraction.

Goal 5.1 - Langdon Lake

Reduce volume of stormwater runoff from new development and redevelopment and maintain or reduce existing water volumes discharged from the subwatershed into Lost Lake.

Actions

  1. Amend existing or establish new District rules requiring abstraction of the first one inch of rainfall on new permitted development and redevelopment.
  2. Track volumes abstracted and new volumes created resulting from permitted development.
  3. Provide assistance to LGUs and developers to foster low impact development and redevelopment that minimizes new stormwater volumes.
    1. Develop and distribute model ordinances and design standards that incorporate low impact design principles.
    2. Sponsor educational opportunities for LGU staff, developers, elected and appointed officials and other interested parties to provide practical information and opportunities for sharing experiences.
  4. Encourage the development and maintenance of depressional storage within the subwatershed.

Goal 5.2 - Langdon Lake

Manage water volumes and velocities in the channel between Saunders and Langdon Lakes to prevent further erosion.

Actions

  1. Inspect the channel at least annually to assess its condition.
  2. Work cooperatively with the Cities of Minnetrista and Mound to reduce volumes as necessary.

Goal 5.3 - Langdon Lake

 Require public stormwater conveyance and control structures in the subwatershed to be sized and maintained properly to convey current and ultimate stormwater flows to minimize erosion and flooding potential.

Actions

  1. The HHPLS noted that culverts under the railroad embankment from LL-2 and LL-4 (see Figure 2) were in need of repair, as were the outlet structures from Black Saunders Lakes.  Local plans should identify these locations and set forth a plan for repair or improvement.  Assist LGUS in determining specific impacts and potential improvements.
  2. Require LGUs to provide to the District a copy of their annual NPDES report.