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Fast-moving city develops strategy to prevent flooding
Urban flooding is an issue faced by many cities around the country. Because of the fully developed nature of older cities, finding solutions to flooding problems is not an easy task. Many large cities have to choose between flood mitigation and acquiring houses for demolition when dealing with storm water management. The following strategies include an innovative approach to storage under residential streets. Some were used for a project in a Minneapolis residential neighborhood, and others may have applications in other communities.
Tough to catch
The project area is a 225-acre watershed that is predominantly residential with boulevards and backyard trees providing canopy cover. Streets average 30 ft wide with 6-in. curbs and varying slopes. Many of the catch basins are concentrated at the identified local low areas with six to eight catch basins surrounding the low intersection.
Preliminary analysis included setting up a hydrologic and hydraulic model of the watershed. Model results indicated that storm sewers lacked the required capacity to adequately convey peak flows during major storms, so there was significant street flooding. Additionally, a large portion of the flow that caused flooding was due to street conveyance and run-by.
Because of restrictions imposed by regulatory authorities concerning water quality and discharge rate, providing additional conveyance to drain the flooding areas was not possible. Also, the city government was reluctant to take houses to create storage ponds, which together with regulatory restrictions imposed rather constrictive limitations on the project. These limitations necessitated innovative approaches to solve the problem. The following section covers concepts that were considered, some of which were taken to full design and are being installed.
The thought behind installing large box culverts under the streets is that by providing storage under the street and installing catch basins along it, local runoff can be captured, stored and released to the main trunk sewer after the storm has passed.
A flap gate is installed at the connection of each storage structure to the main trunk sewer. The flap gates function as flow control, allowing the box culverts to operate as conveyance systems during the initial period of the storm. When the main trunk sewer is under surcharge condition, the flap gates close and the box culverts function as storage to contain the local runoff that would normally have flooded the area. When the main trunk sewer regains flow capacity, the flap gates gradually open and release the stored water.
The downside of underground storage is the cost and installation difficulty. Many utility concerns must be contended with when installing such large structures under city streets. The box culverts and excavation increase the project costs rather significantly. Also, inconvenience to the neighborhood during construction cannot be ignored.
By installing more catch basins and distributing them to a wider area rather than concentrating them at the low intersection, the effectiveness of each inlet is increased, reducing run-by.
Although this is a less complicated part of the project, installing catch basins along city streets also presents problems. Location of these catch basins has to account for fire hydrants, boulevard trees, safety concerns and connections to the main sewers.
In addition to the previously discussed approaches, additional strategies were considered for the Minneapolis project. For various reasons, these alternatives were not implemented but may have value for other cities facing flooding problems.
Alternative inlets and grates
Storm water can be directed into underutilized storm sewers by installing strip drains, low-point inlets or high-capacity inlets. Another alternative is to upgrade older, existing catch basin inlets with more efficient types such as the combined curb opening and grate.
Strip drains (a.k.a., multigrate, slotted vane drain or traverse drains) can be placed perpendicular to the curb on steep streets and intersections with little or no crown to prevent sheet flow from entering flood-prone areas.
Another configuration for strip drains is to place them parallel to the roadway center line in the curb, to capture run-by flows from entrances to parking lots and ramps that drain into streets. The grate should be configured to allow bicycles to safely traverse it.
Low-point inlet is a concrete curb-opening type with a width of 4 to 8 ft. These structures are primarily placed in vertical sags, but can be installed in series to capture high peak flows along streets. Care should be taken in designing and locating low-point inlets, because they attract children. The inlet opening height should be small enough to prevent entry by children.
High-capacity inlet, also known as a “jake grate,” is an upended box culvert covered with a simple metal grate. As the name implies, the benefit is that this type of inlet will handle very large flows and can function as an overflow inlet at a low intersection. A jake grate can be installed at a low intersection instead of adding many curb inlets to handle high surface flow rates. These grates should be designed for highway loading even if placed behind the curb.
Run-by flows can be detained or redirected by creating humps in the road. In locations where some street flooding can be tolerated, humps detain flow to allow the water to enter inlets sized for smaller rain events. The entire street section is raised to create the hump for water management. This strategy also can be used to send flood waters into open spaces or direct it into storm water ponds. Humps must be clearly marked to warn the public of their presence and in particular to warn emergency vehicles that may travel at higher speeds in response to a call for assistance.
Surface flows along streets and run-by between intersections significantly contribute to urban flooding. Strategies in this article were considered or implemented to solve flooding problems in a residential neighborhood. These strategies, though not entirely novel, show how city streets can be retrofitted for urban flood mitigation.