The SBUH Method requires input of parameters that describe physical drainage basin characteristics. These parameters provide the basis from which the runoff hydrograph is developed. This section describes the three key parameters (contributing drainage basin areas, runoff CN, and runoff time of concentration) that, when combined with the rainfall hyetograph in the SBUH Method, develop the runoff hydrograph.
The proper selection and delineation of the contributing drainage basin areas to the BMP or structure of interest is required in the hydrograph analysis. The contributing basin area(s) used should be relatively homogeneous in land use and soil type. If the entire contributing basin is similar in these aspects, the basin can be analyzed as a single area. If significant differences exist within a given contributing drainage basin, it must be divided into subbasin areas of similar land use and soil characteristics. Hydrographs should then be computed for each subbasin area and summed to form the total runoff hydrograph for the basin. Contributing drainage basins larger than 100 acres shall be divided into subbasins. By dividing large basins into smaller subbasins and then combining calculated flows, the timing aspect of the generated hydrograph can be made more accurate.
2-7.2.1 Curve Numbers
The NRCS has conducted studies into the runoff characteristics of various land types. The NRCS developed relationships between land use, soil type, vegetation cover, interception, infiltration, surface storage, and runoff. The relationships have been characterized by a single runoff coefficient called a curve number. CNs are chosen to depict average conditions—neither dry nor saturated. The PEO shall use the CNs listed in the Highway Runoff Manual, the NRCS website, or the GIS Workbench.
The factors that contribute to the CN value are known as the soil-cover complex. The soil-cover complexes have been assigned to one of four hydrologic soil groups according to their runoff characteristics. These soil groups are labeled Types A, B, C, and D, with Type A generating the least amount of runoff and Type D generating the most. The Highway Runoff Manual shows the hydrologic soil groups of most soils in Washington State. The different soil groups can be described as follows:
• Type A: Soils having high infiltration rates, even when thoroughly wetted, and consisting chiefly of deep, well drained to excessively drained sands or gravels. These soils have a high rate of water transmission.
• Type B: Soils having moderate infiltration rates when thoroughly wetted and consisting chiefly of moderately fine to moderately coarse textures. These soils have a moderate rate of water transmission.
• Type C: Soils having slow infiltration rates when thoroughly wetted and consisting chiefly of soils with a layer that impedes downward movement of water or soils with moderately fine to fine textures. These soils have a slow rate of water transmission.
• Type D: Soils having very slow infiltration rates when thoroughly wetted and consisting chiefly of clay soils with a high swelling potential, soils with a permanent high water table, soils with a hardpan or clay layer at or near the surface, and shallow soils over bedrock or other nearly impervious material. These soils have a very slow rate of water transmission and comprise areas such as wetlands.
The HQ Materials Laboratory can also perform a soil analysis to determine the soil group for the project site. This should be done only if an NRCS soils map cannot be located for the county in which the site is located, the available SCS map does not characterize the soils at the site (many NRCS maps show “urban land” in highway ROWs and other heavily urbanized areas where the soil properties are uncertain), or there is reason to doubt the accuracy of the information on the NRCS map for the particular site.
When performing an SBUH analysis for a basin, it is common to encounter more than one soil type. If the soil types are similar (within 20 CN points), a weighted average can be used. If the soil types are significantly different, the basin should be separated into smaller subbasins (previously described for different land uses). Pervious ground cover and impervious ground cover should always be analyzed separately. If the computer program StormShed3D is used for the analysis, pervious and impervious land segments will automatically be separated, but the PEO will have to combine and manually weigh similar pervious soil types for a basin.
2-7.2.2 Antecedent Moisture Condition
The moisture condition in a soil at the onset of a storm event, referred to as the antecedent moisture condition (AMC), has a significant effect on both the volume and rate of runoff.
Recognizing this, the SCS developed three AMCs as described below:
• AMC I: soils are dry but not to the wilting point
• AMC II: average conditions
• AMC III: heavy rainfall, or light rainfall and low temperatures, has occurred within the last 5 days, and soil is near saturated or saturated
Table 2-5 gives seasonal rainfall limits for the three AMCs. These derive from the amount of rainfall in any 5 days.
The CN values generally listed are for AMC II; if the AMC falls into either group I or III, the CN value will need to be modified to represent project site conditions. The Highway Runoff Manual provides further information regarding when the AMC should be considered and conversions for the CN for different AMCs for the case of Ia = 0.2S. For other conversions, see the National Engineering Handbook (NRCS 2010).
2-7.2.3 Time of Concentration
Time of concentration (Tc) is defined as the time for runoff to travel from the hydraulically most distant point of the watershed to a point of interest in the watershed. Travel time (Tt) is the time water takes to travel from one location to another in a watershed. Tt is a component of Tc, which is computed by summing all the travel times for consecutive components of the drainage flow path. While this section starts the same as Section 2-6.3, the analysis described in this section is more detailed because water traveling through a basin is classified by flow type.
The different flow types include sheet flow; shallow, concentrated flow; open-channel flow; or some combination of these. Classifying flow type is best determined by field inspection and using the parameters described below:
• Sheet flow is flow over plane surfaces. It usually occurs in the headwater areas of streams and for short distances on evenly graded slopes. With sheet flow, the friction value (ns, which is a modified Manning’s roughness coefficient) is used. These ns values are for shallow flow depths up to about 0.1 foot and are used only for travel lengths up to 150 feet on impervious surfaces without curb and 100 feet on pervious surfaces. The Highway Runoff Manual provides the Manning’s n values for sheet flow at various surface conditions.
For sheet flow of up to 100 feet, use Manning’s kinematic solution (Equation 2-8) to directly compute Tt:
Tt = (0.42 (nsL)0.8)/((P2)0.527(So)0.4)
where:
Tt = travel time (minutes)
ns = sheet flow Manning’s coefficient (dimensionless) L = flow length (feet)
P2 = 2-year, 24-hour rainfall (inches)
So = slope of hydraulic grade line (land slope, feet vertical/1 foot horizontal [ft/ft])
• Shallow flow: After the maximum sheet flow length, sheet flow is assumed to become shallow concentrated flow. The average velocity for this flow can be calculated using the ks values from the
Highway Runoff Manual. Average velocity is a function of watercourse slope and type of channel. After computing the average velocity using the velocity equation (Equation 2-9), the travel time (Tt) for the shallow concentrated flow segment can be computed by dividing the length of the segment by the average velocity.
• Open channels are assumed to begin where surveyed cross-section information has been obtained, where channels are visible on aerial photographs, or where lines indicate that streams appear on USGS quadrangle maps. For developed drainage systems, the travel time of flow in a pipe is also represented as an open channel. The kc values from the Highway Runoff Manual used in the velocity equation can be used to estimate average flow velocity. Average flow velocity is usually determined for bankfull conditions. After average velocity is computed, the travel time (Tt) for the channel segment can be computed by dividing the length of the channel segment by the average velocity.
A commonly used method of computing average velocity of flow, once it has measurable depth, is the following velocity equation:
• Manning’s kinematic solution should not be used for sheet flow longer than 300 feet.
• The equations given here to calculate velocity were developed by empirical means; therefore, English units (such as inches) must be used for all input variables for the equation to yield a correct answer. Once the velocity is calculated, it can be converted to metric units to finish the travel time calculations in the case of shallow concentrated flow and channel flow.
The Highway Runoff Manual shows suggested n and k values for various land covers to be used in travel time calculations. Stormshed3G will calculate time of concentration with inputs of slope and the appropriate coefficient. For small basins, a minimum time of concentration of 5 minutes shall be entered
