Each case is configured as a network of
watersheds and treatment devices. Devices can be specified by the
user in any order, as long as a definite downstream order exists (i.e., no
feedback loops). The program checks for illegal networks. All
program output will appears in downstream order. At least one device
must be defined.
Devices are identified by name only (not by number as in the DOS version of P8). Because each watershed is referenced to a device, the device network should be specified before entering the watershed data. The device names must be unique (no duplicates). Device names can be of any length but should not contain commas (,) or quotes (' or ").
P8 does not keep track of precipitation falling directly on a device. In order to provide a complete water budget, watershed areas and weighted curve numbers should be specified to include device areas.
In simulating particle removal, each device
is assumed to be "completely mixed" (i.e., 1 stirred tank),
as opposed to plug flow (infinite stirred tanks). This
assumption may be conservative in some cases, particularly swales/buffer strips. To simulate more than
one stirred tank, place 2 or more devices in
series.
Device types
are summarized as follows:
|
Device Type |
Input Values |
Description |
Removes |
Infilt. |
Normal |
Spillway/ |
| Detention Pond |
Permanent & flood pool areas &
volumes |
configured as wet, dry, or extended detention |
x |
x |
x |
x |
| Infiltration Basin |
storage pool area & volume, infiltration rate; void fraction |
storage area with infiltration | x | x | x | |
| Swale or Buffer Strip |
length, slope, bottom width, berm side slope, overflow elev, Manning's n, infilt rate |
overland flow with hydraulics driven by Manning's equation or depth-dependent version thereof. |
x | x | x | |
| General Device |
area & discharge vs. elevation,3 outflow streams (normal, overflow, infiltration) |
user-defined hydraulics from independent model/analysis |
x | x | x | x |
| Pipe / Manhole | time of concentration (linear reservoir) |
collects watershed and/or device outflows and directs them to downstream device |
x | |||
| Flow Splitter | time of concentration, control elevation |
conditional routing based on water surface elevation of a downstream device |
x | x | ||
| Aquifer | time of concentration |
estimates stream base flow using a water budget; accounts for infiltration (watersheds & devices), evapotransp. and change in storage |
x |
Device Outlets
Up to three outlets are specified, depending on device type:
A destination must be specified for each outlet. Destinations can include a downstream device or out of the device network. The user interface constrains the outlet specifications to the correct outlet types for each device. The infiltration outflow can be routed to an aquifer device if groundwater flow and mass-balances are desired. Otherwise it would normally be routed out of the network.
The specified morphometric and hydraulic properties for each device are used to develop a stage/discharge table that specifies the area, volume, and each discharge rate through each outlet. In the case of a General Device, the stage/discharge table is entered directly. The results can be viewed using the 'List Inputs Stage/Discharge' procedure.
Select the special device named "*OUT*" to specify discharge out of the network from any outlet.
Define dimensions of bottom, permanent pool (dead storage; volume below the lowest pond outlet), and flood pool (temporary storage, volume below the normal outlet and spillway outlet. The bottom elevation is for user reference only, unless the device's pool elevation drives a flow splitter device.
The normal outlet drains the volume above the permanent pool, must be specified using one of five options:
* English units, see Bedient & Huber(1988), p.371.
See Hulsing, H., "Measurement of Peak Discharge at Dams by Indirect
Method", USGS, Techniques of Water Resources Investigations, Book 3, Chapter A5,
29 pp., 1967.
See Detention Pond Outlet Hydraulics.
Device
Type - Infiltration Basin
The bottom elevation is for user reference only, unless the device's pool elevation drives a splitter device used to simulate offline basin.
The storage pool area must be greater than bottom area. There is no permanent pool; however, a detention pond device can be used to simulate an infiltration basin with infiltration only from the flood pool.
VOID VOLUME % = normally = 100%. Some designs
(e.g.,trenches) include filling storage volume with coarse stones
(Schueler,1987). Adjust input accordingly.
INFILTRATION RATE refers to saturated soil
conditions (minimum value). The spillway (overflow) outlet is used when the
flood (temporary storage) pool is full.
To specify an offline infiltration basin (inflow stops when the storage pool is full), place a flow splitter upstream of the basin, referenced to the storage pool elevation of the infiltration basin.
The outflows (exfiltrate, overflow) can be routed to devices or out of the network. The exfiltrate can be routed to an AQUIFER DEVICE, if groundwater & baseflow simulations are desired (see sample input files )
Yousef et al. (1986) recommend assuming an infiltration rate of ~ 1.0 inches/hr in designing retention basins in sandy and sandy loam soils.
Typical infiltration rates:
| Reference | McCuen et al (1986) | Shaver(1986) | Musgrave (1955) |
| Sand | 4.64 | 8.27 | |
| Loamy Sand | 1.18 | 2.41 | |
| Sandy Loam | 0.43 | 1.02 | |
| Silt Loam | 0.26 | 0.27 | |
| Loam | 0.13 | 0.52 | |
| Silt Loam | 0.27 | ||
| Sandy Clay Loam | 0.06 | 0.17 | |
| Clay Loam | 0.04 | 0.09 | |
| Silty Clay Loam | 0.04 | 0.06 | |
| Sandy Clay | 0.03 | 0.05 | |
| Silty Clay | 0.02 | 0.04 | |
| Clay | 0.01 | 0.02 | |
| Hydro Soil Group A | 0.43 | 0.30 - 0.45 | |
| Hydro Soil Group B | 0.26 | 0.15 - 0.30 | |
| Hydro Soil Group C | 0.13 | 0.05 - 0.15 | |
| Hydro Soil Group D | 0.03 | 0.00 - 0.05 |
Device Type - Swale, Buffer , Emergent Wetland (Overland Flow Area)
Used to simulate devices for which flow hydraulics governed by Manning's equation for overland flow. Geometry is specified with a trapezoidal cross-section, fixed length, and maximum depth. See detailed description of swale/buffer hydraulics .
The bottom elevation refers to outlet invert. This is for user's reference only, unless device's elevation drives a flow splitter device.
The elevation/area/discharge table is estimated by applying Manning's equation to a trapezoidal swale. A buffer strip can be represented as a wide swale.
The model assumes overland sheet flow (uniformly distributed across surface with no channels). This requires careful design and regular maintenance to avoid channelization. and maintain vegetation.
The maximum depth refers to maximum depth at which Manning's equation applies. The spillway (overflow with infinite hydraulic capacity) outlet is activated if the maximum depth is exceeded. Overflow is directed to the same device as the normal outflow. The maximum depth should be set in consideration of the type and height of vegetation and BMP design guidelines. It is recommended that the device be sized so that overflows occur infrequently.
Model output (List Inputs Stage/Discharge or List Hydraulics) can be reviewed to compare device hydraulics with BMP design guidelines for the type of vegetation being used. Relevant hydraulic variables include maximum velocity, maximum depth, and frequency of dryout. In the Stage/Discharge table, a warning message is displayed if the computed velocity at any elevation exceeds 2 ft/sec, typical design criterion to avoid scouring. Note that simulated velocities may exceed the maximum values in the Stage/Discharge table if the device overflows. The maximum simulated velocity is listed in the Hydraulics table.
Manning's n reflects the roughness of the land surface & resistance to overland flow. Higher values will increase the depth & duration of flow in swales/buffers during & following storm events. Predicted particle removal efficiencies in swales/buffers are typically insensitive to Manning's n if infiltration rate = 0. Sensitivity increases with infiltration rate (Walker, 1990).
Hydraulic model options selected from a drop-down menu are as follows.
| Vegetation Retardance Categories | ||
| Avg
Grass Height (in) |
Good |
Fair Coverage |
| > 30 | A | B |
| 11-24 | B | C |
| 6-10 | C | D |
| 2-6 | D | D |
| < 2 | E | E |
| Typical Values for Manning's n | Value |
| Mc Cuen (1982) | |
| Light Turf | .2 |
| Dense Turf | .35 |
| Forest with dense grass understory | .8 |
| Bedient & Huber (1988) | |
| Pasture | .3 - .4 |
| Lawns | .2 - .3 |
| Bluegrass Sod | .2 - .5 |
| Short-grass prarie | .1 - .2 |
| Sparse vegetation | .05 - .13 |
| Bare clay-loam soil | .01 - .03 |
| Kadlec & Knight (1996) | |
| Treatment wetlands emergent vegetation | .5-1.5 |
Device Type -
General Device
Defines elevation,
area, discharge table for device with up to three outlets, labeled Infiltration,
Normal Outlet, Spillway. Similar input is required for hydrologic models (e.g.,
TR-20).
Elevation can be referenced to an arbitrary datum, unless device drives a Splitter device. Elevation values must be entered in increasing order. Blank rows at bottom of table are ignored.
Area and discharge must be specified in increasing order. The Spillway outlet is automatically activated when the water elevation reaches the maximum value specified in this table.
Prior to simulation, a similar elevation/area/discharge table is generated automatically for detention ponds, infiltration basins, and swales based upon the specified dimensions and outlet hydraulics. Listings of these tables are accessible via the program menu ('List Inputs').
Can be used to collect outflows from a number of watersheds and/or devices & discharge them to a specific device (or out of system) without change. This is analogous to the SWMM 'Manhole' (Dikinson & Huber, 1988)
To obtain graphic or statistical output for one or more watersheds, direct their outflows to a PIPE.
A pipe is modeled as a linear reservoir with a given TIME OF CONCENTRATION (hrs) (See Bedient & Huber (1988), p. 370-3). For TOC= 0,the device outflow responds immediately to inflows. Higher values will stretch the response out over longer times, while preserving water & mass balances. The magnitude of the peak flow is reduced, but the time of peak flow is not changed. Use this to simulate flow responses for large watersheds. The TOC is defined as the time required for 95% outflow response.
No particle removal occurs in a pipe, regardless of the TOC.
Pipes are useful for
obtaining program output for specific watersheds (without
treatment). To review output for a specific watershed, rout it to a pipe
device with TOC= 0.
Device Type -
Splitter
A flow splitter
can be used to direct flows to either of two devices, depending upon the water
surface elevation in one of them.
To simulate an offline infiltration basin, for example, place a splitter upstream of the infiltration basin, referenced to the basin's maximum storage pool elevation. The "normal outlet" for the splitter would be the infiltration basin. When the basin's storage pool is filled and the critical elevation is exceeded, the infiltration basin would be bypassed and the excess flows diverted to the splitter's "spillway" outlet.
A splitter is modeled as a linear reservoir with a given Time of Concentration (TOC, hrs) (Bedient & Huber (1988), p. 370-3). In a linear reservoir, the outflow rate is proportional to the storage volume (i.e. fixed hydraulic residence time). For TOC= 0, the device outflows respond immediately to inflows. Higher values will stretch the response out over longer times, while preserving water & mass balances. The magnitude of the peak flow is reduced, but the time of peak flow is not changed. TOC is defined as the time required for 95% outflow response. Particles are not removed in a splitter, regardless of TOC.
The normal outlet from
a flow splitter must be routed to another device (not out of network).
Device Type -
Aquifer
An Aquifer Device provides storage & discharge of
percolation from watershed areas and devices. The following water
budget is formulated:
Inflows = Percolation from
Watershed + Exfiltrate from Devices
Outflows =
Evapotranspiration + Baseflow
Baseflow = 2.303 Aquifer Volume / Time of Concentration
Change in
Storage = Inflows - Outflows
If TOC=0, the aquifer volume is always = 0 and Baseflow= Inflows - ET. Thatwould be an unrealistic case, however.
The maximum evapotranspiration (ET) rate is computed from the dailyair temperature & month. The ET rate is reduced if it would otherwise result in a negative aquifer volume.< /font >
The temporal response of aquifer outflow is modeled as a linear reservoir (i.e. the ouflow rate is proportional to the stored volume, Haith & Shoemaker, 1987). The Time of Concentration (TOC) is the ratio of the stored volume to the outflow rate. This is typically long (> 100 hours) and can be calibrated to watershed hydrographs.
The ET rate is set to 0.0 if it results in a negative aquifer volume. This will tend to happen in summer dry spells with low aquifer TOC. This may cause under-estimation of ET & over-estimation of base flow. This effect can be reduced by increasing Aquifer TOC and/or by increasing the number of passes through the storm file .
Device Water Surface Elevations
P8 keeps track of the water surface elevation in each device on a continuous basis. Discharge through each outlet is computed based upon water surface elevation relative to outlet control elevation.
P8 does not compare elevations across devices or account for backwater conditions.
Bottom elevations entered on input screens are for user reference only. Simulated water surface elevations (bottom elev + water depth) appear in program output . Bottom elevations are not used in computing discharge, however.
An exception to this is when a device is referenced by a flow splitter, in which case the bottom elevation must be consistent with the routing rule specified for the splitter.
Certain devices (pipes, splitters, aquifers) are modelled as linear reservoirs, each with a "Time of Concentration" expressed in hours. The linear reservoir model assumes that outflow at any time is proportional to the storage volume (Bedient & Huber, 1988). The TIME OF CONCENTRATION is used to compute the proportionality constant using the following equation:
K (1/hr) = 2.303/TOC (hr)
As used here, Time of Concentration (TOC) is defined as the time required for a 90% inflow/outflow response. This can be roughly equated to a hydrologic definition of watershed TOC stated by Bedient & Huber (1988), p.80: 'time of equilibrium of the watershed, where outflow is equal to net inflow'.
Since precip. data are input hourly, TOC values < 1 hr (typical of small urban watersheds) will have little impact on simulation results. TOC is more likely to be an important factor in simulating hydrographs for large watersheds. SCS methods (e.g., TR-55) can be used to estimate TOC values.
Higher TOC values will stretch the outflow hydrograph out over longer periods & decrease peak flow.
This factor adjusts the particle removal rates (settling velocities, first-order decay rates, second-order decay rates) for each device. Normally, it has a value of 1.0.
Other values can be used, for example, to account for effects of vegetation on particle removal rates. Theoretically, macrophytes can increase particle removal rates under a given hydraulic regime by increasing the effective surface area for settling (tray-settling concept), stabilizing bottom sediments, and/or through biological mechanisms. Design methodologies developed in Australia account for a ~5-30%% increase in sediment & phosphorus removal at a given hydraulic residence time in ponds with macrophytes vs. ponds without macrophytes (Phillips & Goyen, 1987; Lawrence, 1986). Their removal efficiency curves are consistent with 'Removal Scale Factors' of 2-3 for suspended solids & 3-6 for total phosphorus attributed to macrophyte presence in wet detention ponds.
Alternatively, values less than 1.0 can be assumed to account for poor hydraulic design (outlet next to inlet, promoting short-circuiting of inflows).