SECTION IV - WinStorm
To evaluate storm sewer systems using the GIS, a storm sewer analysis program (WinStorm) was used. WinStorm was developed by the Texas Department of Transportation and used by Thompson to evaluate existing and proposed storm sewer systems, using data from ArcGIS. Results from the analysis were used to determine the hydraulic adequacy of each storm sewer system. If improvements were indicated, the program results would include the approximate parameters for improvements to result in a system meeting the design criteria.
WinStorm considers storm sewer system characteristics (pipe size, flowline elevations, etc.), watershed and subwatershed conditions (drainage areas, impervious cover, etc.), various rainfall frequencies, and the tailwater condition at the outfall of the system. The program computes a peak discharge and hydraulic gradients for storm sewer systems flowing full or partially full.
1. Rational Method
The program uses the Rational Method to compute peak discharges and evaluates storm sewer systems for a given rainfall frequency. The Rational Method equation is written as:
Where:
Q = Peak discharge at an analysis point (cubic feet/second, cfs)
C = Watershed coefficient related to the impervious area in the watershed
A = Drainage area (acres)
I = Average intensity of rainfall (inches/hour)
S (CA) = Summation of watershed coefficient times area for all drainage areas upstream of the analysis point
The watershed coefficient C is determined by relating C to the percentage of impervious area within the watershed drainage area. The equation for this relationship is written as:
where Ia is the percentage of impervious cover. WinStorm combines the drainage areas contributing to a storm sewer system and computes a composite impervious factor based on drainage area and land use type. The program then converts the percent of impervious cover to a watershed coefficient C. The following table lists the assumed percent impervious cover and C value for each of the land use types.
| Category (Code 87) |
Land-Use Description |
Ia% | C |
|---|---|---|---|
| 1 | Single-Family | 41.7 | 0.45 |
| 2 | Commercial | 100 | 0.80 |
| 3 | Multi-Family | 87.5 | 0.725 |
| 4 | Industrial | 83.3 | 0.70 |
| 5 | Public/Institutional | 83.3 | 0.70 |
| 6 | Park Green Space | 0 | 0.2 |
| 7 | Undeveloped | 0 | 0.2 |
| 12 | Agricultural | 0 | 0.2 |
| 13 | Rangeland | 0 | 0.2 |
| 14 | Forest | 0 | 0.2 |
| 15 | Water | 100 | 1.0 |
| 16 | Wetland | 0 | 0.2 |
| 17 | Barren | 0 | 0.2 |
| 18 | Transportations/Utilities | 90 | 0.74 |
| 20 | Street | 90 | 0.74 |
In the computation of peak runoff, the rainfall intensity is a function of the time of concentration (TC). The initial TC at the upstream manholes is given by the equation:
This equation computes the initial time it takes for the runoff to reach the inlet. After reaching the inlet, the time of concentration is increased by the travel time in the conduit, as determined by Manning’s equation velocities. The rainfall intensity, a function of the time of concentration, is given by the Steel Equation:
Where:
I = Rainfall intensity (inches/hour)
b, d, e = Numeric constants dependent on rainfall frequency
TC = Time of concentration (minutes)
|
Rainfall Frequency |
b |
d |
e |
|
2-year |
75.01 |
16.2 |
0.8315 |
|
3-year |
77.27 |
17.1 |
0.8075 |
|
5-year |
84.14 |
17.8 |
0.7881 |
|
10-year |
93.53 |
18.9 |
0.7742 |
|
25-year |
115.9 |
21.2 |
0.7808 |
|
100-year |
125.4 |
21.8 |
0.7500 |
Hydraulic gradients are computed using Manning’s equation, written as:
Where:
S = Energy slope (ft/ft)
Q = Runoff (cfs)
n = Manning’s roughness coefficient (dimensionless)
A = Area of flow (sq ft)
R = Hydraulic radius (Area/[Wetted Perimeter]) (ft)
WinStorm defaults to Manning’s roughness coefficients based on the sewer pipe construction material type, as entered in the input data.
For City of Houston storm sewer design criteria, minor losses through manholes are not required to be considered in sizing storm sewer pipe systems. Even though minor losses are generally not considered in the southeast Texas region, WinStorm has the capability to consider minor losses when analyzing storm sewer systems. Minor losses through manholes can be considered with the use of the equation written as:
Where:
Kent = Entrance loss coefficient
Kex = Exit loss coefficient
V1 = Flow velocity upstream of manhole (ft/sec)
V2 = Flow velocity downstream of manhole (ft/sec)
g = Gravitational constant: 32.2 (ft/sec2)
1. Analysis
WinStorm will execute with or without support of ArcGIS. In most cases, the program user will be running the program using data that is supported by ArcGIS. In some cases, however, a storm sewer system may not be in the storm sewer GIS, but will need to be modeled. In this case, the physical parameters of the system can be input into the program, and the system can be executed as a stand-alone application directly from Windows.
Beyond this distinction, the user-input forms and calculations are the same. The user has the ability to choose the starting tailwater elevation or the design frequency, alter the time of concentration to the inlets, and alter the roughness values used.
2. Hydraulic Calculations
As discussed previously, the Rational Method is used for calculation of peak flows for the analysis. WinStorm calculates the peak flow and hydraulic grade line elevation at each point in the system, based on system configuration and user-input parameters. Once the user selects the outfall system or systems to analyze and begins the analysis, the calculations are performed automatically according to the following procedures:
1. WinStorm compiles data from the drainage area, pipe, and manhole databases for each system outfall number.
2. Pipe connectivity is established according to pipe identifiers, and a level and order is assigned to each pipe. Drainage areas are assigned to the proper manholes.
3. Rational method calculations begin at the most hydraulically distant manhole, and proceed downstream according to the connectivity established.
4. The hydraulic grade line calculations start at the outfall and proceed upstream according to the level and order of the pipes. Friction losses are calculated using Manning’s equation, and minor losses, if requested by the user, are added at each manhole.
3. Program Constraints
WinStorm can represent a branched network, but cannot represent looped or interconnected systems, reverse flow, or multiple outfalls from a single system. In special cases such as these, the system can either be separated into multiple systems or analyzed with specialized software. In cases where systems flood, the program only indicates that the hydraulic grade line is above the manhole rim elevation. It does not determine the depth of ponding.
There are some pipe segments where storm sewer elevations (flow line and rim) were based on different datum adjustment. This problem can sometimes result with flow line elevations for a downstream segment to be higher than the elevations for the upstream pipe segments. When this occurs, the hydraulic gradient calculations may indicate that the storm sewer system is being flooded, or that levels are above the manhole rim elevation.
WinStorm was not used to evaluate pipe segments that have been classified as inlet leads, nor to determine the capacity of inlets, although the program does possess this capability.
4. Profiles
Profiles can be both displayed on screen and sent to the printer. The program uses the GIS databases to draw profiles of storm sewer pipes, hydraulic grade line elevations, and manhole rim elevations.