"Rivers erode in four ways:

Abrasion or corrasion - This is when large pieces of bedload material wear away the river banks and bed.

Attrition - This is when the bed load itself is eroded when sediment particles knock against the bed or each other and break, becoming more rounded and smaller.

Hydraulic Action - This is when the force of water erodes softer rock.

Solution or corrosion - This is when acidic water erodes rock."

So, there will be larger pieces of rock intake water that hit the bank of the spillway, slowly destroying it.

When engineering the spillway, you have to take that into account.

That means we have to accept that it will need repairs after use, just as a fire extinguisher needs it after use.