HYDRAULIC PROTECTIONS

Diagnosis of the current state

Quilmes’ present state of susceptibility to flood risk is threefold:

– Due to its low-lying nature, wind-driven ocean surges, termed sudestadas, can penetrate inland the coastline via overland flow and via backflow through natural gravity drainage creeks termed arroyos.

– The arroyos, which provide gravity drainage of precipitation runoff and untreated sewerage to La Plata river, have been anthropogenically altered to constrict their outflows to limited locations. The arroyos are highly channelized, disconnected across sub watersheds, and possess limited to no system storage capacity.

– It is well understood that riverine or surge flood risks may occur independently, but there also exists the possibility of “compound flood risk,” which may occur when both of the conditions noted above occur simultaneously. In this case, sudestada surge inflows may elevate the tailwater condition for the arroyos, which prevents gravity drainage from occurring and causes the arroyos to back up and overtop their banks.

A key geographic feature which plays a hand in both surge inflow behavior and precipitation runoff outflow behavior is the Au Dr. Ricardo Balbin Highway 1. This freeway rests at the historic interface of the estuarine marsh flood plain along the shoreline and the upland transition. Its man-made embankment provides both an unintended, partial protection against sudestada inflows, but also provides an impediment to precipitation runoff outflows since the arroyos must cross under or through the embankment via culverts or other hydrologic control structures. Additionally, the elevated areas within the marsh floodplain that were former garbage dumps provide further barriers to rainfall runoff outflows, yet like the highway, provide some impediment to sudestada inflows. Settlements on the seaward side of the highway in the marsh plain are low lying and susceptible to inundation via sudestadas.

General overview of proposal

Quilmes’ tri-fold exposure to flood risk will require a nuanced series of solutions capable of handling a range of conditions. For most of the time, rainfall runoff flooding is the dominant flood risk driver, whereas sudestada surge inflows or the joint coincidence of rainfall events during sudestadas are likely to be ephemeral in nature, lasting only on the order of hours to days. To defend against the spectrum of events, there are a few primary system components required:

– Rainfall runoff storage- to ensure that both rainfall and joint coincidence events may be handles by the system, internal storage capacity for runoff is required to be present inside the sudestada surge protection line of defense. Maximizing system storage is a widely accepted best practice for stormwater management. Retention and detention lagoons and facilities also allow for the alteration of the runoff hydrograph such that runoff volumes exceeding channel capacities may be temporarily stored and then released at a controlled rate. To accomplish this goal, a system of detention lagoons, enlarged channels, and outflow control structures are proposed. Renaturalization of existing channels and proposed channels will provide value beyond flood risk reduction. Wetland plant species can assist in nutrient removal from outflows, stabilize channel banks, and provide a higher aesthetic appeal to the community.

– Sudestada storm surge protection- a “leaking levee” system is required to allow precipitation drainage for the majority of the time, but which retains an ability to manage one-way outflow. To accomplish this task, flow regulation gates such as flap gates and sluices are proposed to allow rainfall runoff egress without allowing sudestada ingress. To accomplish this goal, it is proposed that the existing highway embankment be utilized for the dual purpose of flood protection. Locating the main line of flood protection along the highway will also make the best use of natural-based defense provided by the estuarine marshes located along the shoreline.

Riverine city considerations

There are many worldwide case studies for coastal city redevelopment plans. Most of these plans bring innovations in engineering, planning, and design to other coastal regions where robust water infrastructure is critical to survival and economic prosperity. It is recommended that for Quilmes, smart building practices such as requirements to construct either the building pad or first floor elevation of all buildings to be above 100- to 500-year (1% to 0.2% annual exceedance probability exceedance) water surface elevation, be instituted. Also, smart building practices should be followed such the at first floors may be floodproofed as future flood risk profiles change. Finally, the location and orientation of the proposed building pads should be oriented such that they do not constrict or limit the egress of storm water runoff from the arroyos.