Hasn’t increased overland runoff from changes in surface topology contributed to the flooding?
Twin Lakes has risen approximately 10 ft over the 43 year period from 1974 to 2017. We know this because of these measurements:
| Date | Elevation | Note |
| 1974 | 959 | This measurement is included on a detailed topological map of Twin Lakes which was field-checked in 1974. |
| 2008 | 965 | Another measurement taken as part of a detailed study required by the DNR for certification of the Roberts water treatment plant documents the elevation of West Twin at 965.08 ft MSL in January of 2008. |
| 2017 | 969 | An unofficial elevation measurement taken with aviation grade altimeter equipment accurate to within +/- 2 ft documented the height of the West Twin to be 971 ft in August of 2017. Using the most conservative value from this variance range suggests the lake elevation should be gauged at approx 969 ft MSL for evaluation purposes. |
Annual change in lake elevation is simply calculated as inflow minus outflow. Inflow includes precipitation falling on the lake, surface runoff, wastewater, and groundwater inflow. Outflow includes evaporation from the lake, and groundwater outflow (seepage).
Annual change in lake elevation is then equal to:
(Precipitation + Surface runoff + Groundwater inflow + Wastewater) – (Evaporation + Seepage)
Or:
Surface runoff + Groundwater inflow – Seepage = Annual change in lake elevation + Evaporation – ( Precipitation + Wastewater)
Since Annual change in lake elevation, Evaporation, Precipitation, and Wastewater are all known, we can calculate the annual value for Surface runoff + Groundwater inflow – Seepage for the 43 year period from 1974 to 2016. This value is shown in the far right column of the attached chart and ranges from -6 inches in 2002 to -12 inches in 1988. This number is the difference between Surface runoff + Groundwater inflow and Seepage for every year since 1974.
In the attached charts, we assume that Surface-water runoff + Groundwater inflow is 5% of the total inflow to Twin Lakes, and Groundwater outflow is 25% of the total outflow from Twin Lakes. This is reasonably close to the 20% predicted by the 2009 USGS simulation of Twin Lakes (4).
| Type of source or sink | Inflow to Twin Lakes
(% of total) |
Outflow from Twin Lakes
(% of total) |
| Direct precipitation on the lakes | 85 | 0 |
| Direct evaporation from the lakes | 0 | 80 |
| Surface-water runoff or streamflow | 0 | 0 |
| Augmentation
(effluent from a wastewater-treatment plant) |
10 | 0 |
| Groundwater flow | 5 | 20 |
If we assume that Groundwater outflow is a constant 12 in/yr, then Surface runoff + Groundwater inflow ranges from a low of 0 inches in the dryest year of 1988 to a high of 6 inches in the wettest year of 2002.
| Year | Liquid precip (in) | Evaporation (in) | Surface runoff + Groundwater inflow – Seepage (in) | Seepage (in) | Surface runoff +Groundwater inflow (in) |
| 1988 | 34 | 39 | -12 | 12 | 0 |
| 2002 | 40 | 24 | -6 | 12 | 6 |
On average, Surface-water runoff + Groundwater inflow is 10% of the total inflow to Twin Lakes, and Groundwater outflow is 30% of the total outflow from Twin Lakes, if Surface runoff + Groundwater inflow is assumed to be a constant 12 in/yr. Adding 12 to the the far right column of the attached charts will show the value for Surface-water runoff + Groundwater inflow for each year.
In sum, whether Surface-water runoff + Groundwater inflow is assumed at 5% of the total inflow to Twin Lakes, or whether Groundwater outflow is assumed to be a constant 12 in/yr, the attached charts of the level of Twin Lakes for the 43-year period from 1974-2016 include overland runoff resulting from all changes in surface topology.