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HomeMy WebLinkAbout; San Diego County Method System Model; Hydrology Report; 1988-07-15ENGINEEAINO DEPT. LlBflclw Clty of Csrlsbad 2075 LU Palmas Drive CarlsbDd CA 92009-4859 Rational Method Hydrology Diego County Method System Model Copyright 1982-88 Advanced Engineering Sohare Release Date 7/15/88 Ver. 4.OA INTRODUCTION: The RATIONAL METHOD MASTER PLANNING program is a computer-aided design program where the user develops a node-link model of the watershed, and in this process estimates the conduit and channel sizes needed to accomodate the design storm peak flowrate. The study methodology is based on the well-known RATIONAL METHOD which estimates the peak flowrate (or Q in cfs) by the relation 0 = CIA where Q is the peak flowrate used for design purposes, C is a runoff coefficient and represents the simple ratio of runoff-to-rainfall, and A is the watershed area(acres) tributary to the study point of runoff concentration. For an I=1 inch/hour and an A = 1 acre, and a C = 1 .O, the Q = 1.008 cfs. Assuming that a uniform rainfall of constant intensity occurs over a watershed, then the peak flowrate will occur when the entire watershed is contributing runoff. This peak Q usually occurs when runoff from the most distant point on the watershed reaches the point of concentration. The time which it takes for the watershed runoff to reach the peak Q (from the beginning of the constant intensity storm) is called the time of concentration and is noted as Tc. Some of the basic assumptions used in the RATIONAL METHOD are: (1) the return frequency of the estimated Q is approximately the return frequency of rainfall; that is, to estimate a 25-year return frequency peak flowrate (a Q25), the I values are assumed to be of a 25-year return frequency; (2) rainfall intensities are assumed to be approximately uniform over the watershed; (3) the watershed runoff characteristics can be estimated sufficiently to be used in the runoff equation. SETTING UP THE PROBLEM: In order to develop a node-link model of the study watershed, the following steps are needed prior to beginning the study: (1) using a topographic map of the entire watershed, define the watershed boundaries and identify major streams and channels. (2) define the watershed boundary for each major stream or channel. These interior watersheds will be self-contained in that they can be modeled independently from the total watershed. Generally, the interior watersheds will merge with another interior watershed at a point of CONFLUENCE. (3) subdivide each interior watershed into SUBAREAS. Subarea size should be about 5 to 10 acres in the most upstream reaches, and may gradually increase as the study progresses downstream. (4) specify the runoff characteristics in each subarea. Basic information includes DEVELOPMENT TYPE such as commercial or agricultural; SCS (U.S. Soil Conservation Service) soil group, which is type A,B,C,or D where A is of low runoff potential(sands) and D is of high runoff potential(such as clay soil). (5) define a runoff coefficient C in each subarea. The computer program allows the user to specify a C-value or use the program C-curves which are a function of development type,soil-group, and the rainfall intensity. C-values will be between 0 and 1. (6) define nodal points along the major stream in each interior watershed. The study approach is to start at the most upstream point in an interior watershed and follow the main stream while runoff is accumulating and estimate channel sizes as the study progresses in the downstream direction. A method of node numbering is to use nodes 100.00 to 199.99 along stream #I, 200.00 to 299.99 along stream #2, and so forth, where node 100.00 is assigned to the most upstream point of interior watershed #I. (7) at a point of CONFLUENCE (where two or more major channels merge), define the nodal numbers to be used downstream of the confluence. Usually, one of the major streams will have significantly more runoff than the other streams and one may continue downstream with these numbers. STUDY APPROACH: The node-link model is developed by creating independent node-link models of each interior watershed and linking these submodels together at confluence points. Consider an interior watershed, say #2, which has been subdivided into SUBAREAS and NODE NUMBERS defined. Starting at the most upstream subarea (between node #201 and #202), the runoff is estimated by modeling an INITIAL SUBAREA. This model estimates a O based on the initial Tc, the corresponding I, the subarea A (usually less than 10 acres), and the runoff potential. The study continues downstream to node 203 by analyzing how long it takes for the initial subarea Q to reach point 203 by either (1 )street-flow,(2)pipeflow,or(3) channel flow. This TRAVEL TIME, Tt, is then added to the initial subarea Tc to estimate the next time of concentration by Tc(203) =Tc(202) +Tt. Using Tc(203), an incremental runoff addition, DO, to the main stream at node 203 is estimated by using the specified runoff potential (C) and rainfall intensity (1) corresponding to Tc(203). Then the DO = CIA where the C and I are based on Tc(203) and A is the area tributary to node 203. Thus O(203) = Q(202) + DQ. The study continues to the next downstream node #204 by estimating a new Tt, and so forth. COMPUTER INTERACTION: The program has been designed to be completely user friendly. The user-instruction manual is the program itself. All instructions and program options will be visible to you at the bottom of the screen. Simply type these instructions at any occasion and the program will respond. For example, type MAIN and you will return to the main menu of program options. Type EXIT and the program will protect the data files and properly finish the session. " . COMPUTER DESIGN INTERACTION: Because the analysis proceeds downstream along the main channel, design decisions can be easily made interactively. As the study progresses between two stream nodal points, the computer results are displayed showing peakflow information and channel flow data(such as depth and velocity). You will be requested to either accept the study results(in which case the subarea data is stored) or reject the results(in which case the program returns to the previous upstream point of concentration). The program has four OPERATING MODES: (1) CREATION. This mode is used to create a watershed data file containing all the subarea data entries and hydrology rainfall data. Two data banks are used: (i) HYDROLOGY CONTROL DATA. Includes the rainfall versus duration data(assumed to be a straight line on log-log paper), C-value options, return frequency, and a pipeflow friction slope reduction factor. (ii) SUBAREA DATA. A node-link model is made by defining successive subarea characteristics linked together by various flow hydraulic processes. (2) EXECUTION. This mode is used to generate study results in report form. Two options are available: (i) DETAILED REPORT. This provides the same results as displayed on the viewers screen during CREATION. (ii) SUMMARY REPORT. This summarizes the results into a tabular form. (3) EDITING. This mode allows the user to change, add, or delete subarea data and modify the node-link model. Additionally, the user can change the HYDROLOGY CONTROL DATA and generate a new master plan based on new rainfall or design criteria. (4) EXTEND. This options allows the user to return to the last entered link of the model and continue from that point in the CREATION mode. SUBAREA HYDROLOGIC PROCESSES CONFLUENCE: The CONFLUENCE model is the mechanism which allows the user to connect the interior watershed node-link models at a point of confluence. Up to 5 streams can be confluenced at a node. The stream entries must be made sequentially until all are entered. For example, suppose 4 streams merge at node #318. When the CONFLUENCE option is selected at the end of each interior watershed node-link model (at node 31 81, you will be requested to enter the TOTAL NUMBER OF STREAMS (which is 4) and which of the 4 streams you are confluencing (1,2,3, or 4). If you are confluencing the first stream (1 of 41, then the STREAM NUMBER is 1; likewise, if you are entering the second stream (2 of 4) for confluence, the STREAM NUMBER is 2; and so forth. After a stream is confluenced, the program returns you to the PROCESS MENU so that the next interior watershed node-link model can begin creation for eventual confluence at node 318. When the last (4 of 4) stream is confluenced, the confluence values are estimated and the study can continue downstream with the new values. The program allows only ONE POINT OF CONFLUENCE AT A TIME. This means that if 4 streams are for confluence, then until all 4 streams are entered no other points of confluence can be specified. After the 4 streams are entered, the confluence is modeled and the CONFLUENCE option is once again available for use. For a 2 stream confluence, the model is as follows: Let Qa,Ta,la correspond to the stream with the largest Tc and Qb,Tb,lb correspond to the other stream. If Ta=Tb, then the confluence time of concentration (Tp) is Tp =Ta and Q =Qa +Qb. If Qa,is larger than Ob, then Tp =Ta and Q=Qa+Qb(la/lb). If Qb is larger than Qa, Tp=Tb and Q=Qb+Qa(Tb/Ta). Should different confluence values be needed, accept the confluence model results and then use the USER-SPECIFIED INFORMATON AT A POINT option. INITIAL SUBAREA: Several methods for estimating an INITIAL SUBAREA Tc are reported in the Iiterature(e.g.,"Urban Stormwater Hydrology",Water Resources Monograph #7, American A.G.U.,1982). Because the INITIAL SUBAREA modeling procedure begins the watershed node-link model, this approximation may be the most critical. Consequently, the user needs to verify whether the approximation is reasonable. The program contains an INITIAL SUBAREA Tc approximation based on the Kirpich formula: TC = k(L*L*L/H)**.385 where L = watercourse length(feet1; H = drop in elevation(feet); .385 is an extrapolation exponent; and k is a function of development type(e.g. for Commercial development, k=.298. for agricultural k=1.246). Should the user prefer to use a specified TC Value at a node, then the USER-SPECIFIED INFORMATION AT A POINT option should be used. PIPEFLOW AND TRAPEZOIDAL TRAVEL TIME: Two options for modeling pipeflow are available:(l)let the computer estimate a buildable pipesize, and (2)the user specifies the pipesize. Both models assume no inflow into the pipe system as it connects the upstream and downstream nodes. Both models use the upstream node peak 0 and the computed gradient of the land between nodes to compute normal depth flow velocity. The velocity is used to estimate travel time, Tt, between nodes. The Tt is then added to the upstream Tc to estimate the Tc at the downstream node. Flow is assumed to be under pressure (full pipeflow) when the normal depth exceeds .82*(pipe diameter). The trapezoidal channel flow model is similar to the pipeflow model in that no inflow is assumed between nodes, and that Tt is estimated based on the upstream peak Q and the gradient of the land. STREET-FLOW ANALYSIS THRU SUBAREA: The streetflow model estimates the traveltime of the peak 0 between the upstream and downstream nodes. Since runoff generally accumulates in the street between nodes, the model estimates the average flow between nodes to analyze the streetflow characteristics. The model assumes a symmetrical cross-section with either a standard 6- or 8-inch curbface. The user specifies the arbitrary street halfwidth. Flow is modeled by two methods: (1 )all the flow is on one side of the street, in which case the flow may cross over the street crown and form "splitflow", and (2)the flow is evenly divided on both sides of the street. The model assumes all water outside of the curb as ponded, with zero flow. USER SPECIFIED INFORMATION AT A NODE: The user can specify the time of concentration(Tc,minutes); peak flowrate(Q,cfs); and total tributary area(A,acres) at a nodal point. These values will then be defined at the specified nodal point and will be used for any downstream calculations. The rainfall intensity will be based on the user specified Tc. This data will remain in effect unless modified by the user. ADDITION OF SUBAREA TO MAINLINE: As the study progresses in the downstream direction along the main stream or channel, runoff can be added to the peak flowrate at the Tc of the main stream. This model uses SUBAREA information of runoff potential and area, and uses the Tc of the main stream to estimate incremental runoff. Consequently, should the node-link model be changed upsream of the subject subarea, the node-link model automatically estimates the appropriate incremental runoff.