When Rain Turns to Flood: Identifying External Causes with Forensic Expertise
Forensic Engineering Perspectives on Urban Flooding and Public Systems
When a home or business floods during a rain event, the cause may be found inside or outside of the property line. While privately owned drainage elements such as French drains and sewer laterals are common culprits, this blog will focus on issues that may be found outside the property line, in the public infrastructure.
Understanding Pluvial Flooding
When multiple adjacent properties all experience similar flooding, this suggests that issues outside the individual properties are to blame.
Most urban and suburban flooding is “pluvial flooding”, meaning rain-driven and independent of an overflowing water body. Coastal and riverside communities can also be subjected to floods from a river “jumping its banks” (fluvial flooding), or by a storm surge or extreme high tide from an ocean or bay, but those are topics for another blog post.
Pluvial flooding is a result of the interaction between rainfall and the watershed. Urban and suburban watersheds consist largely of man-made drainage infrastructure, including:
individual roof gutters and downspouts
communal or municipal drain inlets
ditches and open channels
reservoirs and detention ponds
sewers
Sewers are classified as storm, sanitary, or combined, depending on the type of water they are intended to convey. A separate drainage system is one with two sets of sewer pipes: storm sewers to convey rainfall runoff, and sanitary sewers to convey wastewater. Separate storm sewer systems exist to convey rainfall runoff and may be completely dry during extended dry periods.
Separate sanitary sewer systems, while not designed to convey stormwater, become part of the stormwater conveyance system due to inflow and infiltration (I&I) of runoff through cracks, joints, and access openings.
In combined sewer systems that are common on the East Coast of the United States and in some older cities in the Midwest and West Coast (e.g. Chicago, San Francisco, Seattle), the sanitary sewer system and the storm sewer system are one and the same. Many combined sewer systems include active or passive control structures designed to switch from “dry weather mode” to “wet weather mode”, as the sewer systems have to function very differently when conveying large volumes of stormwater than when carrying relatively small amounts of sanitary sewage.
Identifying the Source of Flooding
In a jurisdiction with separate storm and sanitary sewers, pluvial flooding may come from either the storm or the sanitary system, since a sanitary sewer system typically fills up during a storm due to I&I. Potential causes of flooding differ depending on whether a sanitary or storm system is implicated; therefore, the first order of business is to determine whether the flood water is sanitary in nature. Toilet paper, fecal matter, and congealed grease indicate sanitary sewage. Combined sewer system flooding has a similar appearance to flooding from a sanitary sewer system overwhelmed by I&I.
Common Causes of Pluvial Flooding from Public Infrastructure
Understanding the root causes of pluvial flooding from stormwater, sanitary, or combined sewer systems is essential for effective mitigation. Causes of pluvial flooding from a stormwater, sanitary, or combined sewer system can be many:
Inadequate sizing: a pipe, culvert, channel, or other conveyance structure in the immediate vicinity of the flood location may have been undersized for the design rainfall.
Flow chokes: a downstream conveyance structure is smaller than the upstream conveyance, without change in slope or material. A deliberate choke can be part of appropriate design, if storage is provided upstream of the choke.
Slope transitions: Transition from steep to flat slope, without an increase in the cross-sectional size of the conveyance structure. Flow equals area times velocity; therefore, as the water slows down in response to a milder slope, it must occupy a larger cross-sectional area in order to maintain the same flow rate. Thus a slope change can act as a flow choke without a decrease in pipe size.
Hydraulic Jumps, Tunnel Bores and Shock Waves: transient and unsteady hydraulic events, which can form in response to flow chokes and slope transitions as well as other conditions, are beyond the scope of this blog post but can cause tremendous damage including but not limited to flooding.
Lack of air venting or obstructed or damaged vents: This keeps air entrapped and entrained in the sewer system, which decreases the capacity available for water.
Improperly phased capacity improvements: upstream conveyance capacity was increased without an increase to the downstream system. This can include intentional capacity enhancement such as upsizing of pipes, or rehabilitation such as lining which may unintentionally increase capacity by decreasing roughness.
Changes in the watershed: such as an increase in paved area resulting in higher runoff, without adaptations in infrastructure.
Structural damage to the conveyance structure that decreases cross-sectional area and/or increases roughness. Examples: deformation, collapse, “bellies”, offset joints.
Root intrusion into pipes
Buildup of fat, oil and grease (FOG, present in sanitary or combined sewers), cleansing wipes, soil, debris, or large objects decreases cross-sectional area and increases roughness.
Temporary flow barriers accidentally left in place after construction (bulkheads, bricks, sandbags)
Moving parts such as pumps, sluice gates and backflow valves not operating as designed.
Storm surge or high tides can exacerbate pluvial flooding by restricting the ability of storm sewer outfalls to discharge into receiving waters, even if the storm surge or tide itself does not cause flooding.
Lack of backflow preventers: If the flood occurred in part of a building that is below street grade, backflow preventers may be required.
Design Capacity and Extreme Events
Of course, some rain events simply exceed the design capacity of the infrastructure and would cause flooding even if the stormwater management system is perfectly designed, maintained and operated.
It is uneconomical to build a stormwater system to handle an infinite amount of rain. So, the standard practice is to design and size stormwater infrastructure to prevent flooding during rain events up to a defined severity level.
This design criteria is typically represented as a “return period”, which is the reciprocal, in years, of the probability that the design event or a more severe one will occur in any given year. Return period calculations are based on peak rainfall intensity in inches per hour over a defined duration, and/or total accumulation in inches.
Most stormwater infrastructure is designed for a 10-year to 100-year return period, meaning that the system should prevent flooding during rain events with a 1-in-10 to 1-in-100 probability of being exceeded in any given year. Therefore, knowing the return period of the rain event under consideration is important. If the return period of the event exceeds the infrastructure design criteria, it’s possible that nobody except mother nature can be blamed for flooding experienced on a property.
Some, but not all, locations in a watershed can be expected to flood when the design rainfall is exceeded. Suboptimal design, maintenance, and operation of the stormwater system can increase both the reach and severity of flooding beyond what would be expected based on the design criteria exceedance. Therefore, due diligence includes checking for signs of exacerbating factors in the infrastructure even when the rain event exceeds design criteria.
Key Data for Assessing Flood Events
The following information and data, if available, assists in assessing the return period of the rain event, whether the stormwater infrastructure was operating as designed, and where the problem areas, if any, are located:
Rain data from the nearest weather station or rain gage
Map of the stormwater conveyance system
Public resources on design criteria of the stormwater infrastructure
Historical flood records or accounts: is this a new issue, or has flooding occurred at this location in the past during comparable rain events?
Drawings or dimensions of upstream and downstream sewers or other conveyance or storage (canals, ditches, detention ponds or tanks)
Hydrologic and hydraulic modeling studies
Flow meter and level gauge data
Sewer maintenance records, in the immediate vicinity and downstream of the flood location
Sewer inspection records and/or videos, in the immediate vicinity and downstream of the flood location
Information on any recent construction, rehabilitation or upgrade work done on the stormwater conveyance system, including upstream and downstream of the flood area
Tide level data, if the storm water from the flood location is conveyed to a marine outfall
Collecting and analyzing this information is key to understanding the root causes of flooding events. Whether it's for resolving insurance claims, planning mitigation efforts, or pursuing legal action, having a comprehensive picture helps stakeholders make informed decisions.
I hope that this blog post did not flood you with too much information. If you feel out of your depth, EDT’s civil engineers can cut through the FOG and build a cause analysis that holds water.
About the Author
Megan E. Abadie, P.E. is a Consulting Engineer at Engineering Design & Testing Corp. (EDT) with a background in hydrology and water resources. Licensed as a Civil Engineer in multiple states including California, Arizona, Colorado, Nevada, Oregon, New Mexico, and Utah, Megan specializes in root cause investigations and scope of damage evaluations related to flooding, water intrusion, and infrastructure performance.