watervulnerability

Chequamegon-­‐Nicolet National Forest Watershed Vulnerability Assessment, Eastern Region (R9)
complete HUC-6s and seven were split watersheds. These were located in the moraines on the Medford
unit, the clay plain along Lake Superior, the southern half of Park Falls unit, southwest portion of Eagle
River/Florence RD, and central portion of Great Divide RD. The estimated average annual future and
historic potential groundwater recharge for these watersheds was 7.0 and 6.6 inches, resulting in an
average increase of 0.4 inches (Table 4). With a few exceptions, these watersheds will contain very few
surface waters that are substantially fed by groundwater and these will be the most susceptible to climate
change impacts. The exceptions are the split watersheds on the Bayfield Peninsula, which have low
groundwater recharge themselves but many of whose main streams are heavily fed by groundwater from
upslope groundwater recharge watersheds and an occasional isolated coldwater stream.
Infrastructure
While it is not possible at this time to predict changes in flood frequency and magnitude due to climate
change, the WICCI downscaled projections provide sufficient evidence that the frequency and intensity of
large precipitation events will increase and are likely to increase floods. The WICCI Stormwater Working
Group reported that more frequent and severe flooding in rural areas are likely from the projected
increases in rainfall and shifting precipitation patterns that favor more rain during periods of low
evapotranspiration and high soil moisture which result in lower infiltration rates (Potter et al. 2010).
Maintaining the current infrastructure, minimizing natural resource impacts, and reducing life cycle
maintenance costs will logically require road crossing designs that will last at least 75 and preferably 100
years. Structures installed in the near future must last until the late 21 st century and survive future climate
changes.
The CNNF 2004 Forest Plan revision included a guideline that all road and trail stream crossings be
designed to pass the 100-yr flood (USDA Forest Service 2004). Since 2004, the CNNF has attempted to
design all crossings to pass the 100-year flood with the headwater-to-depth (HW/D) ratio of less than 1
(i.e., water level below the top of the culvert) to prevent pressurized flow or surcharging in the structure
and to provide freeboard. In 2008, the US Forest Service published a guide for simulating stream
channels at road and trail stream crossings to maintain or restore ecological connectivity (USDA Forest
Service 2008). This design procedure also maximizes structure life and minimizes maintenance
requirements. Using this guide, a structure width is selected that will allow the construction of a channel
with bankfull width and stable banks, and a structure height is selected that will prevent pressurized flow
and maintain sediment transport.
In recent years, the CNNF has used two procedures to design road and trail stream crossings: no-slope
with tailwater control, and stream simulation. Both procedures consider bankfull width and pass the 100yr
flood with HW/D