Alluvial Fan Flooding (1996) / Chapter Skim
Currently Skimming:
Introduction
Introduction
Pages 6-28
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 6... ...
For example, if a flood deposits large quantities of sediment on the channel bed in a reach, the conveyance capacity of the channel could be reduced drastically and the flow forced overbank at a lower discharge than would be predicted from prestorm surveys of the channel geometry. If overbank flooding causes erosion of a new channel or the reoccupation of an old channel, flood risk assessments based on the historical flow path would misrepresent the location and intensity of flooding downstream of the change.
|
From page 7... ...
In NFIP Regulations, CRF 44, §59. 1, when the form and position of the flow paths is so radically uncertain that the risk of flooding at a place cannot be estimated through traditional procedures, a characteristic that frequently is associated with alluvial fans, this type of flooding is called all~vialfan flooding Deviation from the traditional flood paradigm is further compounded on alluvial fans subject to debris flow hazard, that is, where the base flood is not caused by runoff but by a debris flow with triggering mechanisms, flow characteristics, and probability of occurrence that are completely different from those assumed in hydrological models of flood behavior.
|
From page 8... ...
Understanding these processes from a natural science perspective provides the committee's basis for evaluating how current NF1P practice might be improved to better characterize flood hazards and to more accurately delineate zones of flood risk. The committee's revised definition of alluvial fan flooding is presented in this chapter.
|
From page 9... ...
In its initial form (Dawdy, 1979) , the procedure assumes: That the peak flow-frequency has been estimated for the apex of the fan.
|
From page 10... ...
. Large volumes of sediment can be deposited by floodwater during the course of an alluvial fan flood event (bottom)
|
From page 11... ...
, which likened alluvial fan flooding to rolling balls down a cone. In this form, the procedure can be followed by anyone, even with little or no knowledge of alluvial fans and their flooding characteristics.
|
From page 12... ...
I) : Alluvial fan flooding means flooding occurring on the surface of an alluvial fan or similar landform which originates at the apex and is characterized by high-velocity flows; active processes of erosion, sediment transport, and deposition; and unpredictable flow paths.
|
From page 13... ...
An understanding of process issues is necessary for realistic and flexible regulatory practice and flood risk assessment in a range of environmental conditions. For example, the process approach to understanding what is distinctive about alluvial fan flooding at a particular location and the conclusion that both the process and the channel form on alluvial fans respond to the environmental history of the site allow a quick and simple mapping of process zones on alluvial fans in the manner demonstrated in Figure 1-2.
|
From page 14... ...
THE NFIP DEFINITION OF ALLUVIAL FAN FLOODING As noted earlier, the NF1P defines alluvial fan flooding as "flooding occurring on the surface of an alluvial fan or similar landform which originates at the apex and is characterized by high-velocity flows; active processes of erosion, sediment transport, and deposition; and unpredictable flow paths." This definition emphasizes a type of flooding, not a landform' and thus is inherently difficult to translate into the regulatory setting. Defining the hazard more explicitly in process terms emphasizes that a variety of flooding processes with varying distributions and levels of intensity occur on alluvial fans; because of the range of environmental conditions in which such floods occur, a degree of flexibility is needed in defining and quantizing them.
|
From page 15... ...
H is a surface which is subject to overbank flooding, channel shifting, or invasion from a distributary channel that might erupt from G and hence is the surface subject to alluvial fan flooding, as defined in this report.
|
From page 16... ...
After the 1993 Missouri River flood, sand deposits of 2 feet depth or greater covered 60,000 acres of adjacent farmland, causing damages in excess of more than $ ~ 00 million (interagency Floodplain Management Review Committee, 1994~. On the other hand, there are alluvial fans that have well defined channels and may not be subject to alluvial fan flooding as defined above.
|
From page 17... ...
Because of the convex cross-slope of many alluvial fans, floodwaters leaving the historical or perceived channel may follow a new direction and inundate areas distant from the channel. The alluvial fan flooding designation enables the inclusion of these
|
From page 18... ...
areas are at risk of being flooded even though they might be a meter or more higher than an adjacent channel. Because these regulatory difficulties exemplify specific weaknesses in the riverine flooding paradigm, they portray some of the essential elements that make alluvial fan flooding a distinct type of flooding and they will help formulate the committee's revised definition.
|
From page 19... ...
I o - o ._ ~: I o - o ._ to O
|
From page 20... ...
Figure I-Sb portrays the case of alluvial fan flooding where, during the base flood event, the channel might separate into two branches upstream of the cross-section, allowing a flow path to develop that invades the higher surface I After such a flow split occurs upstream, surface ~ may be flooded even during events smaller than the 100-year event depending on the specific behavior of a real sequence of floods.
|
From page 21... ...
21 ~ c ~ o - o c 11 5 si: o o = Q Q a: ._ c ._ o c o ._ o c o ._ Q At: _` CD ._ a)
|
From page 22... ...
The question raised by Figure I-5 is whether the presence of uncertainty in flood processes by itself constitutes alluvial fan flooding because failure to deal with channel changes by modeling scour, fill, and lateral movement (as the riverine flooding approach fails to do) results in grossly inaccurate delineation of flood hazard boundaries.
|
From page 23... ...
A FIRM showing alluvial fan flooding hazards mapped in this manner is an expression of uncertainly or the absence of knowledge about floods, however, rasher then an indication of how one might actually occur. Unlike a riverine FIRM, an alluvial fan flooding FIRM is of limited use for mitigation and management of flood hazards.
|
From page 25... ...
:P definition, the administrative concerns of the NF1P, and the criteria necessary to establish 100-year recurrence interval alluvial fan flooding as a distinct hazard: Alluvial fan f/oodin~ is a awe of Mood hazard' J ~ ~ ~ - For - -a ,~ ~ that occurs only on alluvial fans. It is characterized by flow path uncertainty so great that this uncertainty cannot be set asicle in realistic assessments of pooch risk or in the reliable mitigation of the hazard.
|
From page 26... ...
The potential for erosion and deposition, the related uncertainty in flow path behavior, and the imprudence of elevation on fill as a mitigation measure are joint and separate characteristics shared among many flood hazards on depositional environments other than alluvial fans, although not usually with the same intensity. It stands to reason that some of the same rules that apply to alluvial fan flooding should apply to this more inclusive type of flood hazard' termed uncertain flow path flooding.
|
From page 27... ...
In summary, the committee's revised definition limits alluvial fan flooding to flood hazard on alluvial fans. The committee recognizes that alluvial fan flooding is one type of flood hazard under the wider category of uncertain pow pathioo~ing.
|
From page 28... ...
1990. Entrenched channels and alluvial fan flooding.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.