Strong reactions are understandable as people face the uncertain consequences of more extreme storms, and then grapple with the risks, benefits, and costs of flood control dams. We would not expect people in the Coon Creek and West Fork Kickapoo valleys to be passive about a reversal in 60 years of flood protection strategy.

The 1996 study by William Krug, USGS, found that changes in agricultural practices reduced the 50-year peak discharge in Coon Valley by 57% between two periods of time: 1934/40 and 1978/81. The 14 flood control dams further reduced those early flows by an additional 7%.

Yes, land treatment improvements since the 1930’s have been the most influential factor in flood reduction at the lower end of the valleys. The big gains in land treatment have been attained by the agricultural community. Those gains must be maintained to protect the most substantial reductions in flood flows. Moving forward, practical improvements in land treatment are needed and encouraged, but the effect of those improvements would not be sufficient to replace the localized effects of a watershed dam.

The dams were designed to store and release runoff from a 50-year frequency rainfall event. The dams have minimal effect on larger storms because the runoff routes through open, vegetated spillways or overtops the dams. The influence of dams diminishes substantially moving downstream as more uncontrolled drainage basins contribute flow to the valleys.

Correct. On a watershed scale, the flood reduction benefits do not exceed the construction cost of replacing the dams due to the small acreage actually protected by the dams. The exception is Jersey Valley dam due to its recreational value.

We are recommending the decommissioning of dams now, because five dams recently failed, and the failures exposed serious vulnerabilities in the remaining dams. As we now evaluate the benefits and costs, the economic analysis does not support replacement of those failed dams.

We recommend the immediate decommission of high hazard dams due to the risk of failure and potential for loss of life. This recommendation is based on a total of 10 dam failures since 1978 in similar geography, and a detailed examination of the highly fractured, sandstone geology, and methodology used in the original dam construction. The remaining low hazard dams can be maintained until they fail or encounter repair costs that exceed their allocated benefits; then they should then be decommissioned.

The economic analysis does not speak to the emotional issues and stories of personal property loss and recovery after extreme storms and flooding. However, it does provide an objective assessment for making important public works decisions on a watershed scale.

A new normal in the frequency and magnitude of storms should not shift public policy toward funding projects that cost more than the benefits provided. However, a new normal does present environmental and social factors that must be evaluated prior to spending public funds, such as the potential threat to emergency access routes, the mitigation of flooded properties, the effect on fisheries, the effect on water quality and human health, etc. The public scoping process is a mechanism of the National Environmental Policy Act (NEPA) to uncover these types of factors under a new normal.

An economic analysis is the typical litmus test for a public works project, and it is embroidered into the legal framework, administrative rules, and assumptions for public funding. However, an economic analysis is only part of a Watershed Project Plan and environmental analysis which also evaluates potential non-monetary impacts, social concerns, and speaks to the new normal. We are squarely in the middle of this process and asking the public, “What are we missing?”

Federal interest in the original construction of PL-566 watershed dams is complete after 50-years, at which time, the counties have autonomy in the final disposition of the dams. Fifty years is also the economic life or service life of these watershed dams. After 50 years, the materials used in dam construction will require more frequent inspection and repair. Accumulated sediment will start to fill space behind the dam reserved for flood water retention. At some point, the dams will encounter repair costs that exceed their allocated benefits.

The Watershed Project Plan and environmental analyses being developed for Coon Creek and West Fork Kickapoo watersheds is likely to set the standard for re-planning expired dams in Southwest Wisconsin. Market conditions, construction costs, and climate have changed substantially since the dams were constructed in the 1960’s. Re-planning dams collectively will be a precursor to repairing a single dam with Federal and state funding. Several watersheds have multiple dams that were planned and constructed collectively in order to move the needle with regard to flood control, and as such, decisions should not be made on a dam-by-dam basis.

The benefit-cost ratio for watershed dams will increase if the storm precipitation depths increase. However, we do not want to bias the hydrologic model results by stacking conservatism upon conservatism, i.e. using the upper bounds of all data inputs.

A planning study focuses on a comparison of the mean predictions, and then develops a confidence interval based on the uncertainty in all of the factors used in the model. Uncertainty usually arises from errors during calibration, the design of model structure, and measurements of input data like precipitation amounts. We can also apply the upper and lower bounds of uncertainty to other factors in the flood model such as rainfall distribution patterns, soil infiltration rates, ground cover condition, flow path velocities, channel roughness, etc. The planning team has a quality assurance process that draws upon a number of techniques for determining the reliability of model predictions.

The question eludes to an uncertainty analysis which is an indispensable part of hydrologic study. It will be presented and discussed with the preferred alternative. An uncertainty analysis investigates the uncertainty of variables used in the decision-making process in order to assess confidence in the recommendations.

The table above is part of a large storm analysis by Daniel Wright, et. al. from the University of Wisconsin-Madison. To address the issue of climate change, the NRCS commissioned a special study to evaluate the intensity, duration, and frequency of large storms in the Coon Creek and West Fork Kickapoo watersheds over the last 18 years. Rainfall depths and distributions are typically sourced from NOAA Atlas 14 data, which are derived from historical rain gage records that span 30 years, but do not include data more recent than 2012.

This table compares the mean rainfall depths from Atlas 14 to the mean rainfall depths derived from the UW analysis. It shows that the 24-hour rainfall depths for the various storm probabilities have increased about 0.6 inch. Higher precipitation depths will increase the projected benefits of watershed dams, and the configuration of new dams.

The recommendation is to decommission the existing failed and remaining high hazard dams. A sudden breach of a high hazard dam presents a greater risk for loss of life than straight, uncontrolled floodwater. The response time is significantly less and the breach inundation area is larger than an uncontrolled floodplain.

Alternatively, homes and businesses within the breach inundation area could be removed to reclassify the dams as low hazard. The recommendation is to decommission low hazard dams after they fail or encounter a repair that exceeds the dam's allocated benefits. A sudden breach of a low hazard dam presents a greater risk for property damage than straight, uncontrolled floodwater. Breach inundation areas are zoned to prevent residential development so that dams can retain their low hazard classification.

Runoff curve numbers are used to model the volume of runoff from the land during a rainfall event. It is an important factor in storm hydrograph development. Runoff curve numbers were estimated using the EVAAL model developed by the DNR Water Quality Team (2014).

A runoff curve number is based on the following factors:

• Land cover, includes crop rotation and canopy cover

• Land treatment

• Hydrologic soil group (average infiltration rates)

• Antecedent moisture condition (soil moisture profile prior to a storm)

• Initial abstraction (amount of precipitation held in micro storage)

Land cover is downloaded directly from the USDA-National Ag Statistics Service. The last 5 years of USDA-NASS National Cropland Data Layers (CDL) were used to determine generalized crop rotations within the watersheds. The CDL is a raster, geo-referenced, crop-specific land cover data layer created annually for the continental United States using moderate-resolution satellite imagery and extensive validation. The EVAAL model combines land cover with hydrologic soil group value, derived from the USDA-NRCS Soil Survey Geographic Database (SSURGO), to determine the associated curve number in the NRCS Runoff Curve Number table.

Land treatment is difficult to assess without direct field observations, so the model outputs both a high estimate of runoff potential (high curve number) and a low estimate (low curve number) representing both worst-case and best-case infiltration scenarios. Land treatment includes the following descriptions:

• bare fallow,

• crop residue,

• straight row,

• straight row + crop residue,

• contoured,

• contoured + crop residue,

• contoured + terraced,

• contoured + crop residue + terraced.

The median value in the curve number range was used to represent a “fair” or average hydrologic condition for hydrograph development and comparative floodplain analysis of the “dam” and “no dam” scenarios. The low end of the curve number range was used to model “good” or improved land treatment, the infiltration equivalent of meeting tolerable soil loss conditions everywhere in the watershed, which in some locations would be land that is contoured + crop residue + terraces (or water and sediment control basins). This low end curve number was evaluated by local conservationists, Bob Micheel and Sam Skemp. They determined that the most practical land management strategies and improvements that could be attained in the watersheds with concerted effort would likely be represented by a curve number between “fair” and “good.”

Typical conditions and values were assumed for antecedent moisture condition, initial abstraction, and canopy cover.

Infiltration rate tests, alone, would not be sufficient to calibrate or affirm the runoff curve numbers. Runoff curve numbers were derived (back-calculated) from research in gauged watersheds where the hydrologic conditions were well-characterized or known. Runoff curve numbers can only be verified or cross-checked by comparing modeled runoff hydrographs to gauged flow.

The Watershed Project Plan will describe the flood control benefits of improving the hydrologic condition in the watersheds. The Plan will show the reduction in peak discharges, reduction in flooded acres, and increase in economic benefits that result from reducing the average runoff curve numbers. NRCS and county conservationists will need to prescribe practical land treatments and cover conditions (specific conservation practices), and target a percentage of watershed land use that is necessary to achieve those average runoff curve numbers.

Wetland plants and wildlife exist in the saturated sediment pools behind the failed and existing dams. These sites largely are considered constructed or artificial wetlands. When the dams are decommissioned, a natural stream channel will develop through the sediment pool. These sites will drain and revert to riparian buffers or floodplain wetlands.

The present character of these wetlands can be maintained with a grade stabilization structure (e.g. sheet pile weir). The natural stream channel would need to be trained and hardened to flow over this structure. Recent sheet pile weir installations for wetland restorations work in Wisconsin cost about $250,000 for construction and $25,000 for geotechnical investigation.

Constructed, in-stream wetlands would alter the natural geomorphic process in the valley. A grade stabilization structure in the floodway would be a barrier to aquatic organism passage and require significant operation and maintenance. Most wetland restorations, enhancements, or creations are completed in uplands or off-channel (in the floodplain fringe) to reduce costs, better manage water levels, and avoid sedimentation.

Agreed. Upland land treatment practices are responsible for reducing peak discharges in the valley by 50% or more compared to 1934-1940. The Project Plan will model the effects of further reductions in runoff curve number based on practical changes in land treatment and cover conditions. Upland land management strategies are critical because decommissioning dams will expose valley properties to the full brunt of uncontrolled, upland runoff.

In order to model “with dam” and “no dam” conditions, runoff curve numbers were selected to represent typical land use under “fair” hydrologic conditions. In order to model the effect of land treatment improvements, runoff curve numbers were selected to represent “good” hydrologic conditions, i.e. the most practical, long-term, land treatment and cover conditions that could be attained in the watersheds with concerted effort.

Runoff curve numbers were based on long-standing and accepted research values for the following land treatment combinations: straight row, contoured, terraced, crop residue, and fallow under “poor,” “fair,” or “good” conditions. Specific values were not assigned to no-till, cover crops, organic, or other soil health strategies.

Specific values were not assigned to no-till, cover crops, organic, or other soil health strategies. Developing runoff curve numbers for these prescriptive land management strategies is beyond the scope of this study. Runoff curve numbers for these strategies must be inferred from the NRCS Runoff Curve Number table based on whether they are generally facilitating or hindering infiltration.

A Watershed Project Plan and environmental analysis can leverage new resources by documenting a need that is environmentally, socially, and economically defensible. It does not include a financial plan for implementation of the preferred alternative for flood control, such as conservation workload analysis, real property acquisition, bond referendums, tax increment financing, Federal and State grants and cost-share, public-private partnerships, watershed appropriations and earmarks, recreational user fees, revenue taxes, etc.

The watershed planning study documents the first 7 Steps of Conservation Planning. Up to this point, we have been identifying problems and opportunities, evaluating resource data, and formulating alternatives. In Step 7, a decision will be made on the preferred alternative. The Watershed Project Plan document will provide a comparative analysis of the alternatives, and present the benefits, costs, environmental and socio-economic consequences of those alternatives.

The latest hydrologic study in Coon Valley was prepared by William Krug, USGS, 1996. Krug presented a table with a comparison of peak discharges in Coon Valley, with and without land treatment, with and without dams, for the full range of storm probabilities.

The findings of this study do not conflict with Krug’s conclusions. Krug reported a 5-10% reduction in peak discharges at Coon Valley as a result of the dams. A reduction in peak discharge does not necessary result in a 5-10% reduction in flooded acres or economic damages.

Yes, we are modeling the results of increased storm precipitation depths in the Coon Creek and West Fork Kickapoo watersheds based on a large storm analysis by Daniel Wright, UW-Madison. The Wright study creates a hydrologic record over the last 18 years. Rainfall depths and distributions are typically sourced from NOAA Atlas 14 data, which are derived from historical rain gage records that span 30 years, but does not include data more recent than 2012.

The Wright study indicates that the 24-hour rainfall depths for the various storm probabilities have increased about 0.6 inch. Higher precipitation depths will increase the projected benefits of watershed dams, and the configuration of new dams.

These observations are correct. The frequency and magnitude of flooding will increase without the dams, but the effect will diminish with distance down the valleys. However, the hydraulic and economic models show that the benefits of existing flood control protection do not exceed the costs of replacing the dams with safer construction.

The benefit-cost ratio is significantly higher for Jersey Valley Dam than for remaining dams in the two watersheds.

“Floodplain acres protected” provide a baseline for comparison: with and without dams; with and without land treatment improvements; with and without floodway improvements. This measure helps us define and estimate losses in agricultural production, recreational opportunities, buildings, roads, and utilities. It does not represent random, unique, or extraordinary property damages such as hay storage on cropland, or storage units with a concentration of high value items. Consistent, high value losses should be brought to our attention.

Damage recovery that occurred since 2007 was factored into the retrospective economic analysis if the model results show that the dams would have prevented the damages. The retrospective analysis cumulated benefits by multiplying flood damages (prevented) by the actual number of storm occurrences over the last 60 years. Likewise, the projected economic analysis accounts for flood damages (prevented) and the probability of flood occurrence using the entire storm recurrence series, i.e. 2-year, 5-year, 10-year, etc.

Road crossing failure is assumed when the hydraulic model indicates that a road overtops. When a dam prevents that same storm from overtopping a road, flood benefits accrue based on a typical replacement cost of the road crossing.

The loss of land to “aggressive” channel migration was not included in the analysis, because it is a random and unpredictable process; and channel migration eventually sets up an equal amount property gain and loss on each side. The channel has moved back and forth across the valley for hundreds of years, but the channel width remains the same.

Property damages caused by “aggressive” channel migration, such as buried utilities and fence lines, which occur during ordinary high-water conditions, were not included in the analysis, largely because it is a random and unpredictable process.

Property damages caused by large storms, such as overtopped roads and bridges, and stream habitat improvement projects, were included in the cumulation of benefits if the dams could have prevented the occurrence.

I believe it would be useful to look at the online map data with the FEMA Flood Hazard Zones (and maybe a couple of the other layers that are available like the cross-sections) from the FEMA public/NFHL map service (https://hazards.fema.gov/gis/nfhl/rest/services/public/NFHL/MapServer).

FIRM map overlays may provide useful context, but they may also distract from the focus of this study. Although very similar, the floodplain models will not meet the current regulatory standard (i.e. level of effort and administrative conditions) for a Flood Insurance Study and Flood Insurance Rate Maps (FIRMs). FIRMs provide detailed information about the base flood, or areas that have a 1% annual chance of flooding and a 26% chance over the life of a 30-year mortgage, according to FEMA. FIRM maps are calibrated to gauged watershed and therefore retrospective in nature.

This study models the existing and future hydrologic conditions in the watershed. Sub-basin hydrographs are developed and routed through the watershed, with and without dams, which were designed to control runoff from the 50-year storm.

Separate. The disposition of watershed dams may not affect the FEMA Flood Insurance Study and the resulting Flood Insurance Rate Maps (FIRMs), because the dams do not significantly affect the base flood elevation (100-year floodplain). The dams were designed to control runoff from the 50-year storm.

A sudden dam failure results in a breach inundation area (dam shadow). These areas are regulated or zoned from development in order to maintain low hazard dam classifications. If these areas are encroached by development, dams may be re-classified as high hazard and require expensive rehabilitation projects to increase their level of protection and reduce the potential for loss of life. If dams are decommissioned, zoning would likely be changed in the breach inundation areas only.