DESIGN HYDROLOGY MANUAL

Transcription

1 DESIGN HYDROLOGY MANUAL 2010 ERRATA PAGES VENTURA COUNTY WATERSHED PROTECTION DISTRICT VENTURA COUNTY, CALIFORNIA JUNE, 2011

2 DISTRICT CONTACTS: Design Hydrologist: VCRat Programmer Reviewer: District Director: Mark Bandurraga, Engineer IV, Hydrology Section Scott Holder, Hydrologist IV, Hydrology Section Bruce Rindahl, Manager, Watershed Resources and Technology Division Norma Camacho, Ventura County Watershed Protection District

3 SECTIONTHREE Modified Rational Method and VCRat For the design storm, rainfall intensities are reduced if computations for other than the fourth day (maximum day) rainfall are specified. The reduction factor is 0.10, 0.40, 0.35, and 1.00 for first through fourth day, respectively Rainfall-Runoff Relationships The system linearly interpolates runoff coefficient curve data as a function of intensity entered as data input in VCRAIN.DAT and constructs a system storage table with up to 14 points, with one curve for each soil type. Infiltration rate (loss rate) data may be specified as data input as an alternative to runoff coefficient curve data. Through use of this table, runoff rate may be computed for a specified rainfall intensity by either (a) runoff coefficient times rainfall intensity or (b) rainfall rate minus infiltration rate (loss rate). The watershed acreage, effective impervious area of the watershed, and time of concentration data plus systems tables provide the information necessary for the system to compute watershed discharge using the rational formula. The following equations are used to compute runoff rates: Q = CIA R = I-f C = R/I = (I-f)/I (No Impervious Area) Cimp= P(0.95) + C(1-P) Q = [P(0.95)I + (1-P)R]A where: A = Watershed area in acres. C = Runoff coefficient. f = Infiltration rate in inches per hour; f varies with intensity for each soil. I = Rainfall intensity in inches per hour, assumed to be constant P = Percent imperviousness/100. Q = Flow rate in cubic feet per second. R = Runoff rate in inches per hour Effective Impervious Values Effective imperviousness is less than total impervious area because some of the impervious area runoff passes through pervious areas where some infiltration can occur. For example, roof drains that discharge onto a lawn may see a reduction in runoff due to infiltration. Roof drain discharge onto a driveway provides no opportunity for infiltration and thus the effective impervious area would be the same as the total impervious area. Compaction is also generally done uniformly across the development site, decreasing the amount of infiltration that can occur, even in a pervious area. These and other applicable factors should be evaluated by the engineer prior to performing the runoff calculations. VCWPD Design Hydrology Manual Errata Pages June 2011 Page 3-16

4 EXHIBIT 6A. RUNOFF COEFFICIENT CURVE- SOIL NUMBER 1 (NRCS TYPE D) VCWPD Design Hydrology Manual Errata Pages June 2011 Page A-8

5 EXHIBIT 6B. RUNOFF COEFFICIENT CURVE- SOIL NUMBER 2 (NRCS TYPE C) VCWPD Design Hydrology Manual Errata Pages June 2011 Page A-9

6 EXHIBIT 6C. RUNOFF COEFFICIENT CURVE- SOIL NUMBER 3 (NRCS TYPE C) VCWPD Design Hydrology Manual Errata Pages June 2011 Page A-10

7 EXHIBIT 6D. RUNOFF COEFFICIENT CURVE- SOIL NUMBER 4 (NRCS TYPE B) VCWPD Design Hydrology Manual Errata Pages June 2011 Page A-11

8 EXHIBIT 6E. RUNOFF COEFFICIENT CURVE- SOIL NUMBER 5 (NRCS TYPE B) VCWPD Design Hydrology Manual Errata Pages June 2011 Page A-12

9 EXHIBIT 6F. RUNOFF COEFFICIENT CURVE- SOIL NUMBER 6 (NRCS TYPE A) VCWPD Design Hydrology Manual Errata Pages June 2011 Page A-13

10 EXHIBIT 6G. RUNOFF COEFFICIENT CURVE- SOIL NUMBER 7 (NRCS TYPE A) VCWPD Design Hydrology Manual Errata Pages June 2011 Page A-14