The primary purpose of flood control sizing criteria is to reduce peak discharge from storm events to reduce the impact of local and “catastrophic” flood events. These events can have considerable impacts on property, structures and human lives. Data from Sauer et al. (1983) suggest that at 50% impervious cover, the peak flow from the 100-year event can be two times the peak in an equivalent rural watershed. Techniques used to mitigate the impacts of these events including: controlling specific storms to predevelopment conditions and limiting development within the floodplain.
In this section, we present options and default sizing criteria for both overbank and extreme flood sizing criteria. We also discuss some analysis tools that can be used to size structures for overbank and extreme flood events.
Overbank Flood Control
Overbank Flood Control reduces the increase in the frequency and magnitude of out-of-bank flooding generated by urban development (i.e., flow events that exceed the bankfull capacity of the channel, and therefore must spill over into the floodplain). The primary purpose of overbank flood control of control is used primarily to protect property adjacent to the stream from frequent flooding.
Overbank flood control can include control of the peak runoff from the 2-year, 5-year, 10-year, or 25-year storm. Specific options include:
In this option, the peak from the overbank flood event is controlled to predeveloped levels.
A jurisdiction may alternatively require overcontrol of the design storm for flood control. For example, the 25-year event may be reduced to 60% of the predeveloped level. The purpose of overcontrol is to eliminate high peak flows resulting from coincident peaks that occur when many structures are used to control the design flood at different locations in a watershed.
A design manual should provide a table with rainfall depths associated with the design storm for overbank control.
Note that a jurisdiction may also choose to set forth specific criteria to determine the level of control required at a specific site, based on the site size, size relative to the watershed, and local watershed conditions. A later version of the Manual Builder will provide greater detail on how and when to conduct a downstream analysis.
Default Sizing Criteria for Overbank Flood Control
Reduce the peak from the 10-year storm event to predeveloped levels.
The following protocol must be followed when sizing stormwater structures for overbank control:
· The models TR-55 and TR-20 (or approved equivalent) are normally used for determining peak discharge rates.
· The standard for characterizing predevelopment land use for non-forested vegetated areas (including agriculture) should be meadow in good condition.
· Off-site areas should be modeled as "present condition."
· The length of overland flow used in tc calculations is limited to no more than 150 feet for predevelopment conditions and 100 feet for post development conditions.
Regardless of the specific method used to calculate peak runoff, the manual should specify exactly how the method is to be applied.
Specifying the predeveloped condition is particularly important because the peak runoff from some agricultural land uses may be as high as the peak runoff from residential land using NRCS Methods. Requiring that the predeveloped condition is meadow or forest is the most conservative option. Another alternative may be to specify a crop rotation for agricultural land.
Extreme Flood Requirements (Qf)
The intent of the extreme flood criteria is to (a) prevent flood damage from large but infrequent storm events, (b) maintain the boundaries of the predevelopment 100 year floodplain, and (c) protect the physical integrity of stormwater treatment practice control structure.
Basic Options for Extreme Flood Control
Although specific methodologies and applications may differ, a community may meet this objective through control of a large storm event, or by reserving the ultimate floodplain.
100 Year Control:
This option requires storage to attenuate the post development 100 year, 24 hour peak discharge rate (Qf) to predevelopment rates. The Qf is the most stringent and expensive level of flood control, and is generally not needed if the downstream development is located out of the 100 year floodplain. In addition, the conveyance system leading to a stormwater structure is often designed based on the discharge rate for the ten year storm (Qp10). In these situations, the conveyance systems may be the limiting hydrologic control.
100 Year Overcontrol:
This option requires control of the 100-year storm to less than predeveloped levels. The purpose of overcontrol is to eliminate high peak flows resulting from coincident peaks that occur when many structures are used to control the design flood at different locations in a watershed. One variation of this option is to alter flood control requirements at a site based on an analysis of downstream conditions.
Reserve Ultimate 100 Year Floodplain:
An alternative to 100 year control is to prohibit development within the ultimate 100 year floodplain (the floodplain assuming full build out),and ensure structures such as bridges and public utilities are adequately protected from the Qf storm. Often this can be done with a floodplain ordinance adopted by the community. (See Stream Buffer Ordinance for additional information on floodplain protection).
A design manual should provide a table with rainfall depths associated with the design storm for extreme flood control.
Note that a jurisdiction may also choose to set forth specific criteria to determine the level of control necessary at a specific site, based on the site size, size relative to the watershed, and local watershed conditions. A later version of the Manual Builder will provide greater detail on how and when to conduct a downstream analysis.
Default Sizing Criteria
Reduce the 100-year storm event only when the floodplain is adequately protected, and size structures to safely pass the runoff from 100-year storm event.
For more information on how to protect the floodplain, consult the Buffer Ordinance.
· The same hydrologic and hydraulic methods used for overbank flood control should be used to analyze Qf.
· In addition, off-site areas should be modeled as “ultimate condition” when the 100 year design storm event is analyzed.
Please note that the basis for design will vary between communities.
Tools of Analysis
A few options used to evaluate and route peak flows for overbank and extreme flood events are provided below. The first group (The rational formula and NRCS methods) are used to compute the peak flow from a specific storm event, and some of these can also be used to route storm events. The last (HEC-RAS) is used to determine the stream elevations for various flood events, and can be used to establish a floodplain, or evaluate downstream conditions.
NRCS Methods (TR-55 and TR-20)
Technical Release 55 (TR-55) presents simplified procedures to calculate storm runoff volume, peak rate of discharge, hydrographs, and storage volumes required for floodwater reservoirs. These procedures are applicable in small watersheds, especially urbanizing watersheds, in the United States. Limitations of this approach include NRCS type distributions, 24-hour duration rainfall, 10 subwatersheds, minimum 0.1 hour and maximum 10-hour time of concentration.
Technical Release No. 20: Computer Program for Project Formulation Hydrology (TR-20) is a physically based watershed scale runoff event model. It computes direct runoff and develops hydrographs resulting from any synthetic or natural rainstorm. Developed hydrographs are routed through stream and valley reaches as well as through reservoirs. Hydrographs are combined from tributaries with those on the main stream stem. Branching flow (diversions), and baseflow can also be accommodated.
The rational formula computes the peak flow only based on storm intensity, basin characteristics, and time of concentration.
The HEC-RAS system calculates water surface profiles for steady gradually varied flow. This program can be sued to identify the flood stage downstream that results from specific levels of control for overbank and extreme floods.