Hidraulica

Article 28
Chapter 1 from The Rapid Watershed Planning Handbook

Basic Concepts in Watershed Planning
his articleintroduces some of the basic water shed concepts that are at the heart of the rapid watershed planning approach. It is helpful to fully understand these concepts before embarking on a local watershed plan. Concept No. 1. There are many different watershed management units.Watershed and subwatershed units are most practical for local plans. Each watershed is composed of many individual subwatersheds that can have their own unique water resource objectives. A watershed plan is a comprehensive framework for applying management tools within each subwatershed in a manner that also achieves the water resources goals for the watershed as a whole. When developing a watershed plan,it is useful to consider how watersheds are configured. The term management unit is used to describe watersheds and their smaller segments. The two management units that will be focused upon in this handbook are the watershed and the subwatershed. A watershed can be defined as the land area that contributes runoff to a particular point along a waterway. A typical watershed can cover tens tohundreds of square miles and several jurisdictions. Watersheds are broken down into smaller geographic units called subwatersheds. Subwatersheds typically have a drainage area of two to 15 square miles

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with boundaries that include the land area draining to a point at or below the confluence of two second order streams and almost always within the limits of a third order stream. While managementunit size will vary among geographic regions and also as a function of slope, soils and degree of urbanization, this general definition provides a consistent and uniform basis for defining individual subwatershed boundaries within a larger watershed. The terms “watershed” and “subwatershed” are not interchangeable. The term watershed is used when referring to broader management issues across anentire watershed, while the term subwatershed is used to refer assessment level studies and specific projects within the smaller subwatershed units. There are other important management units to consider when developing a watershed plan. The largest watershed management unit is the basin. A basin drains to a major receiving water such as a large river, estuary or lake. Basin drainage areastypically exceed several thousand square miles and often include major portions of a single state or even a group of states. Within each basin are a group of sub-basins that extend over several hundred square miles. Sub-basins are a mosaic of many diverse land uses, including forest, agriculture, range, and urban areas. Sub-basins are composed of a group of watersheds, which, in turn, are composed of agroup of subwatersheds. Within subwatersheds are catchments, which are the smallest units in a watershed. A catchment is defined as the area that drains an individual development site to its first intersection with a stream.

T ab le 1: D e scrip tio n o f th e Vario u s W ate rshed Ma nag em en t Un it s W a tersh ed Man ag em en t U n it Catc hment S ubwaters hed W aters hed Subbas in B asin Typ ical A rea (sq u are m ile s) 0.05 to 0.50 1 to 10 10 to 100 100 to 1,000 1,000 to 10,000 In flu en ce o f Im p ervio u s Co ver very strong strong m oder ate w eak very w eak Sa m p le M ana gem en t M ea su re s pr actices and site des ign stream c lass ification and m anagem ent waters hed-bas ed zoning basin plan ning basin plan ning

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Table 1 describes the various management unitsand provides a comparison of impervious cover influences and possible management measures. Figure 1 illustrates how watershed management units nest together within the drainage system. A local watershed may have dozens of individual subwatersheds within its boundaries. A watershed plan tracks the planning and management within individual subwatersheds. Figure 2 illustrates this concept of...