Which countermeasure is designed to resist erosive forces without significantly altering flow?

Riprap is a countermeasure that consists of an arrangement of large stones or rock fragments placed on riverbanks, shores, or around bridge abutments to protect against erosion caused by water flow. It is specifically designed to absorb and deflect the energy of flowing water, which helps to resist erosive forces while maintaining the natural flow pattern and habitat of the watercourse.

The use of riprap is critical because it effectively stabilizes the soil underneath and around it while allowing water to flow over and through it. This means that it does not significantly alter the hydrological dynamics of the area; instead, it enhances the resilience of the structures or banks it protects.

In contrast, the other options introduce different interventions that may involve altering flow conditions or structure. Gabions can impede flow more than riprap as they are wire mesh containers filled with stone, which can change the hydrodynamic characteristics of the area. Spurs are strategically placed barriers extending into the water to redirect flow, which can impact sediment transport and patterns. Guide banks are structures that help direct flow but can also alter the natural course of the stream.

Overall, riprap stands out as the optimal choice for counteracting erosive forces without notably changing the surrounding flow dynamics.