1.4 Addressing current and future flood challenges
Addressing current issues
You now know that many of the parameters used to design and operate Folsom dam have changed since its completion in 1955, including the:
- Historically estimated discharge associated with the design 200-year flood;
- Ability of downstream channels to safely convey the objective release; and
- Flood control space necessary to attenuate the design flood.
While some steps have already been taken to address these issues, Folsom Dam no longer provides the level of protection to which it was originally designed. Several projects are currently underway at the dam site and downstream, which are aimed to reduce flood risk in the American River Basin. One of the largest of these is the American River Watershed Project, comprised of the following three sub-projects:
- American River Common Feature - levee improvements on the American River and Sacramento River, as well as in the Natomas area.
- Folsom Dam Modifications – increase floodwater release capacity from 35,000 cfs to 115,000 cfs earlier during flood events and investigate the feasibility of incorporating an “Advance Release” before a storm arrives.
- Folsom Dam Raise – raise the existing dam approximately 7 feet, which will add 95,000 acre-feet of flood control space to the lake’s current 977,000 acre-foot capacity. The project also includes: enlargement of the spillway at L.L. Anderson Dam; ecosystem restoration and habitat improvements; and building a permanent bridge for Folsom Dam.
After these modifications are complete, Folsom should be able to attenuate just over a 200-year flood magnitude. Furthermore, the combination of these improvements is intended to reduce the risk of downstream flood damage to a 1-in-130 chance in any one year.
Facing future challenges
While the management actions above will lessen present day flood risk, projected hydrologic impacts due to climate change, as well as increased development in flood prone areas, present major challenges to managing future flood risk. Unfortunately, we currently lack both reliable climate projections at the temporal and spatial resolution required for flood frequency analysis as well as methods to incorporate multiple, uncertain future scenarios into flood frequency analysis. Figure 1.4a. below shows a sample of flood frequency curves developed from downscaled Global Circulation Model (GCM) output, which was then used to force a hydrologic model of the American River Basin. Output from six different GCMs, each run under the A2 and B1 emissions scenarios are displayed.
Figure 1.4a. Log-Pearson Type III flood frequency curves fitted to historical gauge data (1905-2012) and future projections (2000-2100).
One of the major challenges water managers face in incorporating climate projections is the wide span of projections using difference GCMs and emissions scenarios.
Based on the flood frequency curves above, what is the range of 200-year floods projected with the climate models (GCMs)?
If the initial plans for Folsom intended the dam to protect against a flood with a 3-day average discharge of 164,000 cfs, based on the updated LP3 curve (dashed blue line), what is the range of return year intervals associated with this magnitude flood?
Climate change projections such as those above are currently not systematically incorporated into flood management planning due to the:
- Wide range of highly uncertain future climate projections which may deviate in both directions from the current conditions;
- Lack of reliable streamflow estimates at the temporal and spatial resolution required for flood frequency analysis; and
- Lack of methods to incorporate multiple, uncertain future scenarios into flood frequency analysis.
In regard to the first two bullet points, new generations of General Circulation Models (GCMs), Regional Climate Models (RCMs), and statistical downscaling techniques all promise to reduce, but by no means eliminate, uncertainty in future climate projections. When these improved datasets become available, the academic and professional communities will need new methods and tools to incorporate future scenarios along with historical data into flood frequency analysis to address the third bullet point.