>Flooding and Floodplain Access

Flooding and Floodplain Access


For healthy rivers, flood-waters have contact with flood-plains. In developed areas, natural flood-plains have been replaced by agricultural and urban areas protected by levees. Levee setbacks and extensive bypasses provide an artificial and partial solution. Ideally, rivers would have access to the flood-plain along much of its length.

QT: Indicators

What is it?

A floodplain is an area adjacent to a river or stream that is periodically inundated with water when the river level rises over the banks. The total floodplain area that is available to a river is a measure of how well a watershed is able to handle a flood and support floodplain vegetation (e.g., cottonwood trees) and aquatic (e.g., fish) communities. Two indicators are used to determine how capable a watershed is at buffering the effects of a flood to downstream areas. The first is informed by two metrics: 1) how much floodplain the river has access to (i.e., what proportion of the area adjacent to a river can be flooded) and 2) what length of the channel is confined by levees. The second indicator is the severity of a 50-year flood event. Fifty-year flood events are when water levels are high enough to significantly flood a river’s banks and pose risks to communities.

Why is it important?

Riparian floodplain systems are among the most dynamic and complex habitats on earth (Feyrer et al. 2006a). The health of a river and/or stream is very much dependent on the degree of connectivity it has with its floodplain and the number of regular flood events. Flood events provide important hydrologic and geomorphic functions that result in sediment deposition, ground water recharge, point bar formation, and scour events which are necessary for the maintenance, recruitment, and growth of riparian forests (Scott et al. 1997), as well as directly contributing to local productivity and biotic interactions (Lehman et al. 2008). For example, many plant and invertebrate species have developed life history adaptations which make them dependent on flood events for reproduction and/or enable them to exploit seasonal floodplain habitats (Opperman 2008; Stella et al. 2006; Feyrer et al. 2006a). The Sacramento River and tributary streams throughout the basin provide critical spawning and rearing habitat for salmon, trout and other resident fish species.

At the same time it is important to take into consideration the effects that flooding can have on human communities and activities. Towns and cities have traditionally been constructed in the lower reaches of the watershed, where fertile soils for farming are located, but where flooding presents a hazard to human life. For example, Yuba City, Marysville and Oroville have developed parts of the floodplain without adequate planning which has lead to drainage problems and localized flooding of streets and low lying properties. Future growth of cities and towns in the floodplain will put further pressure on floodplains by increasing the height of levees to minimize the risk of flood events, as well as building new levees in currently unconstrained floodplains. Rather than keeping floodwaters out of the floodplain, communities should explore ways to encourage flooding in areas that have habitat benefits for floodplain dependent species and which are not currently heavily developed.

Target or Desired Condition

The desired watershed condition is to have sufficient floodplain access to: 1) minimize the risk of flood events to communities; and 2) provide floodplain habitat to aquatic and terrestrial species that are dependent on floodplains for their survival. In addition, low levels of development and infrastructure should be located in areas that are prone to flooding so that channel alteration and construction of levees can be minimized. Given that each watershed has an existing level of development in the floodplain area desired conditions will vary by watershed. The target for the Report Card was likely historic or natural conditions. Identification of a desired condition and/or target is something that communities should decide through the use of zoning and future development plans.

What can influence or stress condition?

The present day floodplain has been greatly modified through the use of flood control mechanisms including levees, dams, river channelization, and rip-rapping. As a result of these flood control mechanisms, floodplain dependent species in many subbasin watersheds have been impacted or even eliminated resulting in a loss of aquatic and riparian biodiverisity. For example, the cumulative effecst of the Oroville and Shasta dams are thought to be the reason why the Tiger beetle is no longer found in the Lower Feather River Watershed. Because of the dams, the sandy edge river habitat and periods of prolonged flooding that the Tiger beetle needs to survive no longer exist (Knisley and Fenster 2005). In addition, the construction of levees results in the isolation of floodplains from their waterways, which in turn negatively impacts river ecosystem health. Lack of organic material and debris transport from the floodplain into the river channel decreases the available food and habitat for fish and aquatic organisms, thereby reducing fish populations and aquatic productivity. Dams significantly alter the frequency, intensity and duration of floodplain inundation leading to a simplification of the downstream and floodplain vegetation, animal, and fish communities.

Data sources

  • USGS 1:100,000 topographic maps
  • USGS gauges; Lower Feather: Thermalio (11406920), Oroville (11407000), Lower Yuba (11421000), Deer Creek (11418500), Lower Bear (11424000)
  • USFS and DFG meadow data
  • CA dam inventory
  • State and federal project levees positions were digitized from the Sacramento River LFPZ map (DWR, 2009)
  • Flood map for the Butte Basin, Lower Feather River Watershed, was obtained from USGS Open File Report 80-971, June 1981.

Data transformations

  • Data were projected to Teale Albers NAD83
  • LFPZ map was geo-referenced in ArcGIS using road-intersections from a geo-referenced USGS quad-map.
  • Areas permitted to flood were delineated using levees and boundaries of wildlife areas adjacent to rivers

Analyses

A score was created for each metric: a flood acre score, a levee score, and a flood frequency score. The overall score for a subwatershed is the average of the three metric scores (Table 2), which are evenly weighted. It is not possible to calculate confidence intervals for these metrics because the data are obtained from a single map in the case of levees and floodplain acres or from a single flow gauge in the case of flood severity, consequently there is no sampling error associated with them.

Levee score is the ratio of length of the channel (i.e., in “river-miles”) with levees on one or both sides of the river channel to the total length of the channel. Flood acre score is the ratio of the floodplain area that is still accessible to the channel relative to the historical floodplain area that was accessible. Flood severity score is a measure of the size and severity of a 50-year flood post dam relative to the size and severity of a 50-year flood pre-dam.

Levee score: To measure the area available for flooding in the lower watershed the number of river kilometers without levees was compared to the number of river kilometers with levees. The greater the value the fewer river kilometers there are
with levees.

Flood acre score: The percentage of the total area that was historically expected to flood that is made up of the area where regular flooding is permitted within the confines of flood management infrastructure (e.g., the entire bypass, the areas between levees and areas designated as wetlands or wildlife areas adjacent to the river).

Flood severity score: A measure of the size and severity of a 50-year flood post-dam construction relative to the size and severity of a 50-year flood pre-dam.

This indicator's data resource has not been uploaded yet.

What did we find out/How are we doing?

Combined scores for flooding and floodplain access for the lower elevation subwatersheds are given in Table 1 and shown in Figure 1. With regards to flood frequency, the analysis shows that the size of a 50-year flood was larger prior to the construction of dams in each of the subwatersheds or upstream portions. The explanation for this observation is that dams and reservoirs were built to attenuate floods by holding back large volumes of water in the upper watershed and slowly releasing this water over time. The effect of dam associated flood attenuation on 50-year flood size was most clearly seen in the Lower Feather River where Oroville Dam is located. The results for flood acres show that the lower Yuba has the greatest access to its floodplain and the fewest number of levied river kilometers.

Based on analyses, the Lower Yuba has the greatest ability to mitigate the disruptive effects of floods to downstream communities while also providing habitat for floodplain dependent species. Floods are not as strongly attenuated by dams in the Lower Yuba because there are other “natural” mechanisms (i.e., floodplain access) which help to mitigate the risks posed by floods. In other words, even though post-dam 50-year flood events are closer in size to pre-dam 50-year flood events, the lower Yuba has retained the ability to flood its banks in areas that do not pose risks to human communities, thus minimizing the risk of flooding in areas where communities are located. The high degree of floodplain connectivity in the Lower Yuba is an artifact of the hydraulic mining that occurred in the area, where the presence of the Yuba Goldfields made building levees downstream for flood control unnecessary. The river can pool and flood the Goldfields (i.e., fill all the interstitial spaces in the mining debris) which attenuated the flow of water downstream. In addition, hydraulic mining debris prevented the development of infrastructure in some parts of the floodplain, thereby minimizing risk to communities. It is important to note the presence of the Goldfields have ecological ramification with respect to aquatic connectivity and water quality. The Lower Feather ranked second overall, and the Lower Bear ranked last.

Temporal and spatial resolution

In the Feather River Watershed, data on floodplain access is only available for the lower portions of the watershed, consequently, commentary can only be made on floodplain access and flood risk for the lower part of the Feather River Watershed. In the upper watershed, local flooding will be desired to recharge groundwater (e.g., in montane meadows), improving summer base-flows. Spatial data for flood infrastructure allowed only a snapshot in time, prohibiting any kind of trend analysis.

How sure are we about our findings?

With regard to flood acre score and levee score, uncertainty is present because it is not known whether further development (i.e., levee construction) has occurred in the subwatersheds since the infrastructure was last mapped. With regard to flood severity, there is moderate confidence in the finding because natural flow data for each subwatershed are not available (i.e., flow data from before any dams were present). Consequently, the flood severity score is likely an overestimate (too high) because the size of the current 50-year flood is substantially smaller than the 50-year flood in the absence of any dams.

Sub-watershed
Area Code Score Trend Confidence
North Fork Feather NFF
East Branch North Fork Feather EBNFF
Lower Feather LF 43
North Yuba NY
Middle Yuba MY
South Yuba SY
Deer Creek DC
Lower Yuba LY 70
Upper Bear UB
Lower Bear LB 38