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Storm Water Drainage
Souder, Miller & Associate’s (SMA) storm water design and management experience includes site drainage analysis w/computer modeling software (predicting flow rate, water depth, sediment loads, flow velocity, flow depth, floodplain mapping, etc.), storm water detention and retention facilities, storm water routing studies, and flood and drainage structure design. SMA accomplishes these tasks by applying the following hydrologic and hydraulic analytical procedures.
Hydrologic Analysis
To begin any project, SMA determines which local, state, or federal agency’s design standards will be applied to the project analysis, (i.e. New Mexico State Highway and Transportation Department Drainage Manual Volume I & II, Hydrology 1995). SMA then determines the general shape of the ground surface using available contour information including digital elevation maps (DEM’s), USGS mapping, mapping from recent aerial topographic work, or mapping available through the local, state, or federal agencies. Utilizing the same mapping, SMA determines the size and slope of contributing drainage basin(s). Ground cover will be determined by field observations. With this information SMA models the various drainage basins according to the applicable design storm events. SMA develops a customized intensity-duration-frequency (IDF) curve for each project using information from the National Weather Service (NOAA) or other recognized data sources for rainfall information. Each project area’s percent imperviousness or runoff curve number (depending on the analysis method) will be analyzed using the current development density since it is assumed that all future developments within the drainage basin(s) will be required to contain developed storm water, (i.e. the increase in storm water runoff over and above the pre-development run off conditions per the Clean Water Act).
SMA engineers will determine the boundaries of contributing drainage basins on U.S.G.S. (DEM) quadrangle maps. These areas will be delineated in the (WMS) computer program along with calculating flow path lengths and time of concentration resulting along each flow path.
The Watershed Modeling Software (WMS) is a comprehensive software package for developing computer simulations of watersheds. The WMS software package has been used at hundreds of sites around the world to analyze watersheds. These sites include federal, state, private, and educational institutions. WMS is the new standard for watershed modeling. WMS uses Digital Elevation Models (DEM) from the USGS 7.5 minute quadrangle tiles to perform the following tasks: visualizing and delineating sub-basins in a watershed, computing hydrologic and geometric parameters for each sub-basin, defining hydrologic parameters that cannot be automatically computed, running several hydrologic models, including HEC-1, HMS, TR-20, TR-55, the Rational Method, and the National Flood Frequency Method, viewing the results of these hydrologic models, and importing and exporting Geographic Information System (GIS) data.
The TR-55 model is a storm event surface water hydrologic model based upon the Natural Resource Conservation Service (NRCS) hydrology techniques of unit hydrograph theory and runoff curve numbers method of calculating direct runoff from the rainfall occurring over specified areas (National Handbook, Part 630, Hydrology, NEH-630.10 and 630.16). This hydrologic model is limited to a maximum drainage area of 25 square miles and a maximum of 10 sub-areas. The model assists in the hydrologic evaluation of flood events for use in the analysis of water resource projects. Direct runoff is computed from watershed land areas resulting from natural rain events. The runoff is routed through channels and/or impoundments to the watershed outlet.
The TR-20 model is much the same as the TR-55 model but without the watershed basin and sub-area limitations.
The Simplified Peak Flow (SPF) method estimates the peak flow of runoff and runoff volume from small to medium size watersheds. Infiltration and other losses are estimated using the SCS Curve Number (CN) methodology. The Simplified Peak Flow method is limited to single basins less than 5 square miles in area, and should not be used when the time of concentration exceeds 8.0 hours.
Hydraulic Analysis
SMA’s engineers visit the project site to review existing drainage structures’ dimensions and condition, high water marks, and complete upstream and downstream investigations that will be used in modeling the drainage system properly. These investigations are used in the U.S. Army Corps of Engineer’s HEC-RAS program for scour analysis, or overtopping of existing or proposed drainage structures. SMA uses WMS with the HEC-RAS module to calculate the design storm flood elevations at the design point(s). Parameters for completing the model are collected by the SMA’s engineers and surveyors. For areas in which topographic mapping has not been completed, DEM and/or USGS topographic maps are used to supplement the topographic information.
The U.S. Army Corps of Engineer’s HEC-RAS Version 4.0 was designed to perform one-dimensional hydraulic calculations for a full network of natural and constructed channels. HEC-RAS’s key analytical elements are geometric data representation and common geometric and hydraulic computation routines. The basic computational procedure is based on the solution of the one-dimensional energy equation with energy losses calculated using Manning’s equation. HEC-RAS steady state flow system is designed for application in flood plain management and flood insurance studies to evaluate floodway encroachments.
The Haestad Culvert Master program is a tool available for solving culvert hydraulic problems. Incredibly flexible, fast, report-ready, and stress-free, CulvertMaster builds on the input you have on-hand while providing what you are missing. CulvertMaster analyzes systems with multiple barrels, different shapes and sizes, special tailwater considerations, and roadway overtopping considering watershed data, culvert characteristics, and even weir geometry. The software automatically compares and incorporates several design trials into complex hydraulic analyses and evaluate the results using the built-in Culvert Designer utility.
The Bentley FlowMaster program allows the engineer to design and analyze grate, curb, ditch, slotted, and combination inlets using calculations based on the FHWA Hydraulic Engineering Circular No. 12 and Circular No. 22 methodologies. FlowMaster contains inlet and gutter hydraulic options of in sag or on grade conditions with a continuously or locally depressed gutter and calculate water spread and gutter depth for a gutter or pavement section. The software has the ability to design and analyze channels, ditches, and free surface pipes of any shape including circular, box, elliptical, parabolic, and irregular channels. It utilizes uniform flow to solve for discharge, normal depth, channel dimensions, slope, or roughness, and compare the results using different friction methods.
SMA’s environmental clearances and permitting experience includes environmental impact statements (EIS), environmental assessments (EA), cultural resource surveys, threatened and endangered and/or protected species surveys, wetland delineation, wetland mitigation plans, US Fish and Wildlife surveys, cultural and protected species mitigation plans, 404 permitting, storm water pollution prevention plans (SWPPP) and management, and general regulatory compliance plans and documentation.