HYDRAULICS

2.3

HYDRAULICS

The OHWM is delineated on the basis of physical features present along the

stream channel. Rainfallrunoff models attempt to predict the hydrologic inputs

(discharge, volume, and duration of flow) entering the stream system from the

surrounding watershed but do not characterize the hydraulics of the flows (width,

depth, and velocity) that ultimately create the physical features associated with

the OHWM. Hydraulic models are therefore needed to characterize and predict

flow conditions in the stream system.

Types of Flows

Flow conditions in arid-region stream systems vary between channelized

flow, sheetfloods, and debris flows. Channelized flow, mostly ephemeral in the

Southwest, occurs in all settings but predominates over other flow types in valley

bottom settings. Perennial channelized flow occurs in very large watersheds that

extend into more humid climates (e.g., Colorado River) and in high-elevation

watersheds where snowmelt can extend flow into the hot, generally dryer

summer months (e.g., Colorado Rockies). Channelized flow is confined to well-

defined banks and, although potentially meandering or braided, is generally

unidirectional in nature (i.e., rapid expansions in flow width generally do not

occur).

Sheetfloods are defined as a sheet of unconfined flood water moving down a

slope (Hogg 1982). They occur in piedmont settings (the gently sloping areas

between the mountains and valley bottom) and in valleys during larger storm

events. Sheetfloods are generally very shallow and, unlike channelized flow,

experience rapid lateral flow expansion. Sheetflood zones and channelized

reaches occur together in downstream alternating sequences along discontinuous

ephemeral stream systems (Schumm and Hadley 1957, Bull 1997). Unique to

arid regions, discontinuous ephemeral streams are quite common in the south-

western U.S. and result from the inability of most storm events to transport

sediment through the entire stream system. Over time the position of sheetfloods

and channelized flow on discontinuous ephemeral streams will shift such that an

area of channelized flow can be transformed into a sheetflood zone over a few

years or decades. Such temporal and lateral variations in flow types complicate

hydraulic modeling of single storms and the long-term management of flood

hazards.

Debris flows, a viscous mixture of water, mud, and coarser particles (often

including large boulders), are largely restricted to mountain canyons and alluvial